COMPARATIVE VERTEBRATE MORPHOGENESIS ZOO COMPARATIVE VERTEBRATE MORPHOGENESIS ZOO 4690 4690

FLORIDA ATLANTIC UNIVERSITY @ DAVIE BROWARD FLORIDA ATLANTIC UNIVERSITY @ DAVIE BROWARD CAMPUSCAMPUS

Professor: James Kumi-Diaka (BSc. DVM, MSc, PhD. Vee-dip)Professor: James Kumi-Diaka (BSc. DVM, MSc, PhD. Vee-dip)Office: Science & Education Bldg. Room 278 Phone: (954)236-1135Office: Science & Education Bldg. Room 278 Phone: (954)236-1135

Course Objective: Course Objective: 1.1. To expose students to the concept and principles of To expose students to the concept and principles of

developmental biology (embryology and gross anatomy) with developmental biology (embryology and gross anatomy) with regard to structural and functional physiological development in regard to structural and functional physiological development in vertebrates.vertebrates.

Course Outline:Course Outline:1.1. Introduction: historical perspective; phylum chordata; its Introduction: historical perspective; phylum chordata; its

characteristic features; concepts of homology and analogy.characteristic features; concepts of homology and analogy.2.2. Vertebrate phylogeny: classification.Vertebrate phylogeny: classification.3.3. Gametogenesis: definitions; nuclear and cytoplasmic; cell Gametogenesis: definitions; nuclear and cytoplasmic; cell

divisions; purpose of.divisions; purpose of.4.4. Fertilization: What is it?; purpose of; the process of; end-product Fertilization: What is it?; purpose of; the process of; end-product

of; parthenogenesis.of; parthenogenesis.5.5. Early development: cleavage/segmentation holoblastic and Early development: cleavage/segmentation holoblastic and

meroblastic cleavages; formation of morula and blastula.meroblastic cleavages; formation of morula and blastula.6.6. Gastrulation: the formation of gastrula; the process involved; Gastrulation: the formation of gastrula; the process involved;

formation of primary germ layers; extraembryonic membranes formation of primary germ layers; extraembryonic membranes and their role in development.and their role in development.

Course Outline cont’dCourse Outline cont’d7.7. Induction: principle of; physiological significance; classification of.Induction: principle of; physiological significance; classification of.8.8. Placentation: amniotes and anamniotes; classification; functions of.Placentation: amniotes and anamniotes; classification; functions of.9.9. The integument: components of: structure and derivatives of.The integument: components of: structure and derivatives of.10.10. Skeletal system I: components of; skull development; morphology.Skeletal system I: components of; skull development; morphology.11.11. Skeletal system II: origin and developmental anatomy of vertebrae.Skeletal system II: origin and developmental anatomy of vertebrae.12.12. Skeletal system III: appendicular skeleton, origin, development and Skeletal system III: appendicular skeleton, origin, development and

morphology.morphology.13.13. Musculature I: muscle origin; fate of myotomes: classification; histoanatomy Musculature I: muscle origin; fate of myotomes: classification; histoanatomy

and topography.and topography.14.14. Muscular system II: development of brachiomeric & appendicular muscles.Muscular system II: development of brachiomeric & appendicular muscles.15.15. Development of coelomic cavities, and mesenteries.Development of coelomic cavities, and mesenteries.16.16. Respiratory system: evolution, morphogenesis. and derivatives of; Respiratory system: evolution, morphogenesis. and derivatives of;

classification.classification.17.17. Digestive system: evolution and morphogenesis, derivatives of, classification Digestive system: evolution and morphogenesis, derivatives of, classification

of.of.18.18. Urogenital system I: kidney structure; morphogenesis; classification; Urogenital system I: kidney structure; morphogenesis; classification;

primordial gonads.primordial gonads.19.19. Urogenital system II: reproductive system; embryogenesis and Urogenital system II: reproductive system; embryogenesis and

morphoanatomy.morphoanatomy.20.20. Cardiovascular system I: circulatory system; morphogenesis of Cardiovascular system I: circulatory system; morphogenesis of

extraembryonic circulation.extraembryonic circulation.21.21. Cardiovascular system II: aortic arches and their derivatives.Cardiovascular system II: aortic arches and their derivatives.22.22. Cardiovascular system III: the heart - its embryonic origin and development.Cardiovascular system III: the heart - its embryonic origin and development.23.23. Cardiovascular system IV: arterial and venous circulation; evolution of. Cardiovascular system IV: arterial and venous circulation; evolution of.

Course Outline cont’dCourse Outline cont’d

24.24. Nervous system I: morphogenesis and histoanatomy of Nervous system I: morphogenesis and histoanatomy of neural tube (brain and spinal cord).neural tube (brain and spinal cord).

25.25. Nervous system II: development of autonomic nervous Nervous system II: development of autonomic nervous system, spinal nerves, spinal cord. system, spinal nerves, spinal cord.

26.26. Nervous system llI: cranial nerves, special senses, olfactory, Nervous system llI: cranial nerves, special senses, olfactory, optic, otic, etc.optic, otic, etc.

TEXT BOOK(S)TEXT BOOK(S):: Any of the following text books will be OKAny of the following text books will be OK

1. Analysis of Biological Development –Klaus Kalthoff, 2nd 1. Analysis of Biological Development –Klaus Kalthoff, 2nd Ed.Ed.

2. Patten’s Foundations of Embryology 6th Edit, Bruce M 2. Patten’s Foundations of Embryology 6th Edit, Bruce M CarlsonCarlson

3. Principles of Development 2nd Edit, Lewis Wolpert3. Principles of Development 2nd Edit, Lewis Wolpert

EXAMINATIONS;EXAMINATIONS; Exam I = 02/04/08 100%Exam I = 02/04/08 100%

Exam II = 03/12/08Exam II = 03/12/08 100% 100% Exam III = 04/21/08 100%Exam III = 04/21/08 100%

EVALUATIONS:EVALUATIONS: A A >>90%; B = 80-89%; C = 70-79%: D = 60 – 69%: F = < 60 90%; B = 80-89%; C = 70-79%: D = 60 – 69%: F = < 60

PLEASE NOTE:PLEASE NOTE: i) All make-ups exams will draw i) All make-ups exams will draw 15% penalty15% penalty ii) Exam results will NOT be given over the phone (do ii) Exam results will NOT be given over the phone (do

not).not). III) THERE ARE NO EXTRA-CREDITSIII) THERE ARE NO EXTRA-CREDITS IV) IV) NO CELL PHONES ALLOWED DURING EXAMS NO CELL PHONES ALLOWED DURING EXAMS – – 25% PENALTY25% PENALTY

Zygote - all higher animals start their lives from a single cell zygote - Zygote - all higher animals start their lives from a single cell zygote - dual origin from two gametes – spermatozoon + ovum dual origin from two gametes – spermatozoon + ovum

Ontogeny - The time of fertilization represents the starting point in the Ontogeny - The time of fertilization represents the starting point in the

life historylife history

- Ontogeny refers to the individual’s entire life span- Ontogeny refers to the individual’s entire life span..

German biologist August Weismann (1834-1914) made the important German biologist August Weismann (1834-1914) made the important distinction between the soma (body) and the germ-cell line (gametes).distinction between the soma (body) and the germ-cell line (gametes).

- thought that the germ-cell line was all-important for perpetuation of - thought that the germ-cell line was all-important for perpetuation of the the

species, and that the soma was primarily a vehicle for protecting species, and that the soma was primarily a vehicle for protecting and and

perpetuating the germ plasm. perpetuating the germ plasm. This viewpoint provides a convenient framework for looking at the This viewpoint provides a convenient framework for looking at the

perpetuation of life (Fig. 1-2).perpetuation of life (Fig. 1-2).

Once an individual has passed the reproductive Once an individual has passed the reproductive years, the remainder of its ontogeny does not years, the remainder of its ontogeny does not provide direct physical input into the generative provide direct physical input into the generative process strictly.process strictly.

EMBRYOLOGYEMBRYOLOGY

Historical perspectiveHistorical perspective

The origin of the embryo: Aristotle’ TheoriesThe origin of the embryo: Aristotle’ Theories

i)i) Preformationist conceptPreformationist concept- everything in the embryo was preformed from - everything in the embryo was preformed from

the the very beginning and dimply got bigger during very beginning and dimply got bigger during developmentdevelopment

- the sperm contained the embryo (homunculus – - the sperm contained the embryo (homunculus – tiny tiny

human in the head of the sperm)human in the head of the sperm)

ii) Epigenesis (means ‘upon formation’)ii) Epigenesis (means ‘upon formation’)- new structures arose progressively during - new structures arose progressively during

developmentdevelopment

Cell Division and the Cell CycleCell Division and the Cell Cycle Is the fundamental properties of living systems, of vital importance Is the fundamental properties of living systems, of vital importance

Cell division in vertebrates takes two forms—mitosis and meiosis.Cell division in vertebrates takes two forms—mitosis and meiosis.

Mitosis (for review, see Fig. 1-14) is the standard form of cell division in Mitosis (for review, see Fig. 1-14) is the standard form of cell division in somatic cells and it results in two genetically equal daughter cells. somatic cells and it results in two genetically equal daughter cells.

Meiosis (Fig. 3-7) is confined to certain stages of development of Meiosis (Fig. 3-7) is confined to certain stages of development of gametes gametes

Mitosis is one component of the cell cycle. Mitosis is one component of the cell cycle.

The life history of a cell can be divided into four periods (Fig. 1-15). The life history of a cell can be divided into four periods (Fig. 1-15). Immediately after mitosis and the separation of the dividing cell into Immediately after mitosis and the separation of the dividing cell into daughter cells, the G1 (gap I) period, often called the interphase daughter cells, the G1 (gap I) period, often called the interphase commences. Its length is extremely variable. In rapidly cleaving embryos commences. Its length is extremely variable. In rapidly cleaving embryos just after fertilization, the G1 phase is very short and sometimes may not just after fertilization, the G1 phase is very short and sometimes may not even exist.even exist.

At the other extreme, the G1 phase of mature neurons persists as the G0 At the other extreme, the G1 phase of mature neurons persists as the G0 phase throughout the remainder of the life of the cell because further cell phase throughout the remainder of the life of the cell because further cell division does no occur. division does no occur.

FUNDAMENTAL PROCESSES AND FUNDAMENTAL PROCESSES AND CONCEPTS IN DEVELOPMENTCONCEPTS IN DEVELOPMENT

Protoplasm:Protoplasm: The complex substance (organic and The complex substance (organic and

inorganic) of which all cells are composed.inorganic) of which all cells are composed. Protoplasm exists solely in the form of Protoplasm exists solely in the form of

organisms and could only be spoken of in term organisms and could only be spoken of in term of living things.of living things.

The essence of the principle of evolution The essence of the principle of evolution states that all organisms have arisen from states that all organisms have arisen from common ancestry through a gradual process common ancestry through a gradual process of change and diversification.of change and diversification.

However, it is generally held that the physical However, it is generally held that the physical and chemical composition of the earth’s and chemical composition of the earth’s surface and atmosphere are no longer surface and atmosphere are no longer amenable to the de novo creation of life, it amenable to the de novo creation of life, it therefore follows that all present day organism therefore follows that all present day organism have arisen from preexisting organisms.have arisen from preexisting organisms.

LIVE - THE ORGANISMLIVE - THE ORGANISM

Individuality:Individuality: Vast differences between individuals in any given Vast differences between individuals in any given

population.population. Differences are due mostly to genetic constitution: Differences are due mostly to genetic constitution: that is, among the individuals of a given that is, among the individuals of a given

population, two or more population, two or more allelesalleles occur at a large occur at a large proportion of the gene lociproportion of the gene loci

i.e., although same numbers and kinds of genes in i.e., although same numbers and kinds of genes in their chromosomes, the genes may occur in many their chromosomes, the genes may occur in many alternate forms. alternate forms.

Variations residing in the gene pool of a population Variations residing in the gene pool of a population provide the potential for evolutionary change.provide the potential for evolutionary change.

The Gene:The Gene: Consists of DNA, an organic molecule composed of two long, Consists of DNA, an organic molecule composed of two long,

twisted chains of structural units called nucleotide. twisted chains of structural units called nucleotide.

Each nucleotide unit is composed of the sugar deoxyribose, Each nucleotide unit is composed of the sugar deoxyribose, phosphoric acid, and a nitrogenous base of which there are four phosphoric acid, and a nitrogenous base of which there are four different kinds; viz: adenine, thymine, cytosine, and guanine different kinds; viz: adenine, thymine, cytosine, and guanine (A,T,C,G). (A,T,C,G).

Therefore there are four different kinds of nucleotide.Therefore there are four different kinds of nucleotide.

The nitrogenous bases in the nucleotide can fit only in four The nitrogenous bases in the nucleotide can fit only in four combinations: A-T; T-A; G-C; C-G. combinations: A-T; T-A; G-C; C-G.

The order of these base pairs varies, and the specificity of any The order of these base pairs varies, and the specificity of any part of the DNA molecule, depends upon this order.part of the DNA molecule, depends upon this order.

Definitions

Chromatin- replicated DNA and assoc. proteins Chromatids- condensed chromatin Chromosomes- two sister chromatids attached by centromere Kinetochore- protein band along chromosome

around centromere Mitotic spindles-

Kinetocore- fibers from centrosome attaching to kinetocore Polar- fibers extending centrosome to opposite centrosome Astral- fibers radiate outward from centrosome

Mutation: Mutation: Any changes in the code of nucleotide pairs in the DNA Any changes in the code of nucleotide pairs in the DNA

molecule will result in a corresponding change in the protein molecule will result in a corresponding change in the protein it codes and, by extension, in the cells and even the it codes and, by extension, in the cells and even the organisms of which the protein is a part.organisms of which the protein is a part.

Such alteration of the DNA molecule constitutes Such alteration of the DNA molecule constitutes mutation.mutation.

* * The number of alleles at a gene locus is increased only The number of alleles at a gene locus is increased only through mutation. through mutation.

Mutations are the sole source of new genetic variability and Mutations are the sole source of new genetic variability and therefore lie at the root of evolutionary potential. It is genetic therefore lie at the root of evolutionary potential. It is genetic variability within the gene pool that is the raw material of variability within the gene pool that is the raw material of evolution. evolution.

For instance, fertilization, which is the union of reproductive For instance, fertilization, which is the union of reproductive cells, provides for a new and unique combination of genetic cells, provides for a new and unique combination of genetic propensity.propensity.

REF: REF: glossaryglossary – back of the text book: – back of the text book:

**EmbryologyEmbryology – the study of the embryo. – the study of the embryo.

**Ontogeny Ontogeny – The developmental events or history of an – The developmental events or history of an individual that bring it into being: the totality of developmental individual that bring it into being: the totality of developmental operations.operations.

**Embryogeny Embryogeny – origin of the embryo: the segment of ontogeny – origin of the embryo: the segment of ontogeny that precedes hatching or birth.that precedes hatching or birth.

**PhylogenyPhylogeny – the succession of forms that culminates in any – the succession of forms that culminates in any given structural entity is the evolutionary history or phylogeny given structural entity is the evolutionary history or phylogeny of that entity. The term applies to the total body form or any of that entity. The term applies to the total body form or any part thereof.part thereof.

Primary Terms in Embryogenesis:Primary Terms in Embryogenesis:

**HomologousHomologous and and AnalogousAnalogous – refer to structural – refer to structural and functional similarities:and functional similarities:

HomologousHomologous – equivalent parts in the sense of – equivalent parts in the sense of having a common ancestry and regardless of structure having a common ancestry and regardless of structure and function are said to be homologous, or are and function are said to be homologous, or are homologues.homologues.

Analogous Analogous – parts having similar functions whether – parts having similar functions whether homologous or not, are said to be analogous, or are homologous or not, are said to be analogous, or are analogues.analogues.

Early embryonic stages of vertebrates reveals close Early embryonic stages of vertebrates reveals close likeness to fully formed fishes, amphibia and reptiles. likeness to fully formed fishes, amphibia and reptiles. However, as development proceeds the embryos of However, as development proceeds the embryos of these different animals become more and more these different animals become more and more dissimilardissimilar

The VertebrateThe Vertebrate

Evolution and DevelopmentEvolution and Development

Modifications of development Modifications of development new features specific to new features specific to classes/speciesclasses/species

THE VERTEBRATETHE VERTEBRATE

THE CHORDATETHE CHORDATE - features: - features: All vertebrates are chordates; but not all chordates are All vertebrates are chordates; but not all chordates are

vertebrates.vertebrates.

Vertebrates share certain features with some non-Vertebrates share certain features with some non-vertebrates - features possessed by all chordates:vertebrates - features possessed by all chordates:

1.1. NotochordNotochord – lies along the dorsal midline; provides axis – lies along the dorsal midline; provides axis support for the body; present in embryos of all chordates: support for the body; present in embryos of all chordates: persists throughout life in some forms: In most chordates, persists throughout life in some forms: In most chordates, notochord is transient & is replaced by the vertebral notochord is transient & is replaced by the vertebral column.column.

2.2. Hollow neural tubeHollow neural tube – dorsal and above the notochord; – dorsal and above the notochord; the central nervous system of chordates.the central nervous system of chordates.

3.3. Pharyngeal cleftsPharyngeal clefts (slits) (slits) – passageways from the – passageways from the pharyngeal portion of the digestive tract to the exterior: pharyngeal portion of the digestive tract to the exterior:

- respiratory organs (gills)- respiratory organs (gills)carries over to adult life in carries over to adult life in aquaticaquatic

chordates, and serves aschordates, and serves as - Only transient existence in non-terrestrial chordates - Only transient existence in non-terrestrial chordates

4.4. Sub-pharyngeal glandSub-pharyngeal gland – not as salient a feature as (1-3); – not as salient a feature as (1-3); - Located ventrally in the pharynx; - Located ventrally in the pharynx; - binds iodine and related substance: - binds iodine and related substance: - called endostyle in primitive chordates:- called endostyle in primitive chordates: - in vertebrates, its homologue is the - in vertebrates, its homologue is the thyroid glandthyroid gland..

In addition to these exclusively diagnostic In addition to these exclusively diagnostic characteristics, most chordates also exhibit the characteristics, most chordates also exhibit the following featuresfollowing features::

5.5. TailTail – rearward extension of the terminal opening of the – rearward extension of the terminal opening of the digestive digestive

tract.tract.

6.6. LiverLiver – lies ventral to the digestive tract, and kidneys – lies ventral to the digestive tract, and kidneys (distinct tubular unit) rest dorsal to the digestive tract(distinct tubular unit) rest dorsal to the digestive tract

7.7. HeartHeart – ventral to the digestive tract; pumping blood via – ventral to the digestive tract; pumping blood via a closed system of vessels.a closed system of vessels.

8.8. Internal skeletonInternal skeleton (over and beyond the notochord) – (over and beyond the notochord) – provides support and protectionprovides support and protection..

Other features shared with other Other features shared with other animal phyla and are thus animal phyla and are thus not not exclusively diagnostic:exclusively diagnostic:

9.9. Bilaterally symmetricalBilaterally symmetrical body body – right and left are mirror – right and left are mirror

images of each otherimages of each other..

10.10. MetamerismMetamerism or segmentation – serial or segmentation – serial repetition of such structures as nerves, blood repetition of such structures as nerves, blood vessels, muscles and certain other parts.vessels, muscles and certain other parts.

1111 CoelomCoelom – all chordates have a – all chordates have a true body true body cavity, cavity, or or

coelom.coelom.

Fig 2-2 reveals the diagnostic characteristics of Fig 2-2 reveals the diagnostic characteristics of a true vertebrate (FIG-2).a true vertebrate (FIG-2).

* All of the non-vertebrate chordates are * All of the non-vertebrate chordates are commonly referred to as commonly referred to as PROTOCHORDATES.PROTOCHORDATES.

A. Mammalian A. Mammalian

-muscular diaphragm for breathing; -muscular diaphragm for breathing;

-a four-chambered heart; -a four-chambered heart;

-a single aorta; -a single aorta;

-two occipital condyles; -two occipital condyles;

-nourish young by lactation -nourish young by lactation -has functional placenta (-has functional placenta (except in the egg-laying except in the egg-laying

mammals) to nourish the fetus and remove waste products from mammals) to nourish the fetus and remove waste products from it.it.

CLASSIFICATION OF VERTEBRATESCLASSIFICATION OF VERTEBRATES

B.B. Aves Aves - 4-chambered heart- 4-chambered heart

- endotherm (warm-blooded) - endotherm (warm-blooded)

- attainment of high and constant metabolic rate - attainment of high and constant metabolic rate

- internal thermogenesis - internal thermogenesis heat is derived from within heat is derived from within the body, the body,

- characterized by feathers and adaptations for flight- characterized by feathers and adaptations for flight- - uses yolk in eggs to nourish the young/embryo- uses yolk in eggs to nourish the young/embryo

C) Amphibia C) Amphibia - - for the most part semi-aquatic; for the most part semi-aquatic;

- are ectothermic.- are ectothermic.

- mostly egg-laying- mostly egg-laying

- Includes: frogs, toads, salamanders.- Includes: frogs, toads, salamanders.

D.D. PiscesPisces – – various classes of fishesvarious classes of fishes

- all built around a common blueprint as a superclass - all built around a common blueprint as a superclass Pisces.Pisces.

4 major classes of the Pisces:4 major classes of the Pisces:

1.1. Agnatha - jaw1ss ; extinct except for the lampreys and Agnatha - jaw1ss ; extinct except for the lampreys and hagfheshagfhes

2.2. Placodermi – armored , jawed fishes;Placodermi – armored , jawed fishes;

3.3. Chondricthyes – modern fishes, the sharks, skates and Chondricthyes – modern fishes, the sharks, skates and rays;rays;

4.4. Osteichthyes – advanced bony fishes which constitute Osteichthyes – advanced bony fishes which constitute half the living species of vertebrates.half the living species of vertebrates.

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THE VERTEBRATE: Pisces amphibia reptilian aves

mammalia Are chordates –> identical anatomic features Same/common developmental process

Since all animals are related in some degree, it is Since all animals are related in some degree, it is possible to sketch an outline of the early stages of possible to sketch an outline of the early stages of development that applies to all classes of development that applies to all classes of vertebrates.vertebrates.

The basic events involved in development are:The basic events involved in development are:

a)a) Gamete maturationGamete maturation –gametogenesis –gametogenesis

b)b) FertilizationFertilization

c)c) CleavageCleavage

d)d) GastrulationGastrulatione)e) Organogenesis Organogenesis organ system organ system

organismorganism

EMBRYOGENYEMBRYOGENY

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a)a) Gamete maturationGamete maturation : : - gametogenesis - gametogenesis spermatozoa + oocytes (both haploid) spermatozoa + oocytes (both haploid) - a period of chromosomal reduction from diploid to - a period of chromosomal reduction from diploid to haploid in both spermatozoa and ova.haploid in both spermatozoa and ova.

b)b) FertilizationFertilization - gametes - gametes zygote (unicellular/diploid) zygote (unicellular/diploid)

fertilization initiates two events viz:fertilization initiates two events viz:i.i. Activation Activation of the ovum to start dividing i.e. initiates of the ovum to start dividing i.e. initiates

development, anddevelopment, andii.ii. Establishment Establishment of diploidy of diploidy - union of the two haploid - union of the two haploid

gametesgametes..

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c) c) CleavageCleavage

- a period of segmentation of the zygote - a period of segmentation of the zygote

- series of mitotic divisions - series of mitotic divisions blastula (multi-cellular)blastula (multi-cellular)

d) d) GastrulationGastrulation

- further mitotic divisions of blastula - further mitotic divisions of blastula gastrula gastrula

establish primary germs layers:establish primary germs layers:

ectoderm (outer);ectoderm (outer);

endoderm (inner);endoderm (inner);

and mesoderm (middle).and mesoderm (middle).

e) e) OrganogenesisOrganogenesis organ system formation organ system formation

EMBRYONIC ORGANIZATION:EMBRYONIC ORGANIZATION: The EctodermThe Ectoderm - - the outer strata of the embryo the outer strata of the embryo

represents the embryonic skinrepresents the embryonic skin.. The EndodermThe Endoderm - - inner stratum is the future lining of inner stratum is the future lining of

the the

GIT;GIT;

the space it encompasses is the cavity of the GIT, the the space it encompasses is the cavity of the GIT, the archenteron.archenteron.

The MesodermThe Mesoderm - - between skin and embryonic gut;between skin and embryonic gut;

Neural TubeNeural Tube - -ectodermal in origin; along dorsal axis of the ectodermal in origin; along dorsal axis of the embryo; embryo;

formed by invagination of the neural plate and joining of formed by invagination of the neural plate and joining of the neural folds in the midline;the neural folds in the midline;

; anterior portion will form the brain; the remainder will ; anterior portion will form the brain; the remainder will form form

the spinal cord of the adult.the spinal cord of the adult.

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The neural crest The neural crest - - arise from cells within the neural folds; arise from cells within the neural folds;

- and occupies either side of the neural tube;- and occupies either side of the neural tube;

- derivatives include - ganglia of the cranial and spinal nerves: - derivatives include - ganglia of the cranial and spinal nerves:

- some of the cells migrate to parts of the body to form - some of the cells migrate to parts of the body to form different tissues different tissues

including the sympathetic nervous system (SNS), sensory including the sympathetic nervous system (SNS), sensory ganglia of ganglia of

cranial nerves V, VII, IX and X; adrenal medulla, pigment cells; cranial nerves V, VII, IX and X; adrenal medulla, pigment cells; teeth teeth

parts of the skull.parts of the skull.

The neural crestThe neural crest Is Ectodermal in originIs Ectodermal in origin Adult derivatives:Adult derivatives:

- ganglia of the spinal nerves- ganglia of the spinal nerves

- sympathetic nervous system (SNS), - sympathetic nervous system (SNS),

- sensory ganglia of cranial nerves V, VII, IX and X- sensory ganglia of cranial nerves V, VII, IX and X

- adrenal medulla- adrenal medulla

- teeth - teeth

- parts of the skull.- parts of the skull.

- pigment cells- pigment cells

The Notochord and Mesoderm The Notochord and Mesoderm - notochord is derived - notochord is derived from mesodermal cellsfrom mesodermal cells that that

segregate fromsegregate from

median dorsal mesoderm; median dorsal mesoderm; lies between the gut floor and lies between the gut floor and the the

developing neural tube above.developing neural tube above.

The lateral mesoderm spreads ventrolaterally and the spreading The lateral mesoderm spreads ventrolaterally and the spreading sheets on either side meet in the ventral midline. sheets on either side meet in the ventral midline.

At same time these mesodermal sheets split into two layers:At same time these mesodermal sheets split into two layers:

a)a) splanchnicsplanchnic mesoderm and mesoderm and

b)b) somaticsomatic mesoderm mesoderm

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The cavity between them is the coelom - a true cavity The cavity between them is the coelom - a true cavity bounded by an epithelium of mesodermal origin.bounded by an epithelium of mesodermal origin.

The splanchnic mesoderm unites with adjacent endoderm The splanchnic mesoderm unites with adjacent endoderm to form the to form the splanchnopleuresplanchnopleure..

The somatic mesoderm unites with ectoderm to form the The somatic mesoderm unites with ectoderm to form the

somatopleuresomatopleure..

Dorsoventral division of the Dorsoventral division of the mesoderm into:mesoderm into:

i.i. EpimereEpimere - most dorsal and flanks the neural - most dorsal and flanks the neural tube and notochord; gives the following tube and notochord; gives the following derivatives – sclerotome, dermatome and derivatives – sclerotome, dermatome and myotome; myotome;

i.i. MesomereMesomere - narrow; between the epimere and - narrow; between the epimere and hypomere;hypomere;

ii.ii. HypomereHypomere - most ventral and flanks the gut - most ventral and flanks the gut..

Note:Note: When we speak of an organ or part of it being derived from a When we speak of an organ or part of it being derived from a

given germ laver, we have in mind only the intrinsic given germ laver, we have in mind only the intrinsic functional portion of that organ; for there is no structure in functional portion of that organ; for there is no structure in the animal body that is the product of a single germ layer the animal body that is the product of a single germ layer exclusively. exclusively.

For illustration:For illustration:

a)a) The intestine is considered to be endodermal in origin, yet The intestine is considered to be endodermal in origin, yet only its secreting and absorbing interior lining is so derived. only its secreting and absorbing interior lining is so derived.

b) The muscles, peritoneum, connective tissues, blood vessels, b) The muscles, peritoneum, connective tissues, blood vessels, and nerves, which constitute the bulk of the GIT, are derived and nerves, which constitute the bulk of the GIT, are derived

from mesoderm and ectodermfrom mesoderm and ectoderm..

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a)a) In normal development there are instances in which the In normal development there are instances in which the cells of one germ layer give rise to structures customarily cells of one germ layer give rise to structures customarily farmed by those of other germ layers. farmed by those of other germ layers.

b)b) The developing tail of vertebrates is illustrative. The developing tail of vertebrates is illustrative.

c)c) The muscles of the tail are a product of ectoderm, The muscles of the tail are a product of ectoderm, whereas muscles in general are mesodermal in character. whereas muscles in general are mesodermal in character.

d)d) Further, the skeleton is ordinarily derived from Further, the skeleton is ordinarily derived from mesodermal connective tissue, mesodermal connective tissue,

e)e) yet the gill skeleton is produced by neural crest cells yet the gill skeleton is produced by neural crest cells originally associated with the nervous system.originally associated with the nervous system.

* Yet the organ-germ layer concept is still useful as a * Yet the organ-germ layer concept is still useful as a descriptive scheme of classification and identification. descriptive scheme of classification and identification. Furthermore, there is a high degree of homology between Furthermore, there is a high degree of homology between these layers and the products provided by them.these layers and the products provided by them.

What are the Components of DevelopmentWhat are the Components of Development??

Development Development - - a series of changes culminating in the formation of a a series of changes culminating in the formation of a

complex complex

organism from relatively small and simple, one cell organism from relatively small and simple, one cell organism. Includes:organism. Includes:

i) i) GrowthGrowth; ii) ; ii) morphogenesismorphogenesis; iii) ; iii) differentiationdifferentiation

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i.i. GrowthGrowth - - developmental increase in mass (synthesis developmental increase in mass (synthesis of new protoplasm - cytoplasm, nucleic and cytoplasmic of new protoplasm - cytoplasm, nucleic and cytoplasmic products) accompanied by cell division which is products) accompanied by cell division which is characteristic of growth.characteristic of growth.

ii.ii. MorphogenesisMorphogenesis - - generation of new form; the generation of new form; the organized movements of multiplying cells and the organized movements of multiplying cells and the attendant physical reshaping of areas. A sheet of cells may attendant physical reshaping of areas. A sheet of cells may undergo delamination (splitting) into two or more layers; a undergo delamination (splitting) into two or more layers; a cord may hollow out (undergo cavitation); there may be cord may hollow out (undergo cavitation); there may be folding, invagination (inpocketing), evagination folding, invagination (inpocketing), evagination (outpocketing).(outpocketing).

iii.iii. DifferentiationDifferentiation - - encompasses a host of encompasses a host of operations culminating in increasing diversification of form operations culminating in increasing diversification of form and function. Events by which cells and other parts become and function. Events by which cells and other parts become different from one another and also different from what they different from one another and also different from what they were originally.were originally.

* Morphological differentiation* Morphological differentiation - - as as they multiply, cells become structurally different from other they multiply, cells become structurally different from other cells. E.g. from a general ectoderm --> nerve cells and cells. E.g. from a general ectoderm --> nerve cells and epidermal cells acquire distinguishing features of size, epidermal cells acquire distinguishing features of size, shape, and internal architecture.shape, and internal architecture.

* Behavioral differentiation* Behavioral differentiation - - acquisition of special functional capabilities: e.g. nerve cells acquisition of special functional capabilities: e.g. nerve cells come to transport electrical disturbances; muscles to come to transport electrical disturbances; muscles to contract; gland cells to secrete special products. etc.contract; gland cells to secrete special products. etc.

* Chemical differentiation* Chemical differentiation - - as an as an illustration, consider the egg. An egg has complex chemical illustration, consider the egg. An egg has complex chemical composition. As it cleaves and progressively become a composition. As it cleaves and progressively become a blastula, gastrula and embryo, individual cells and areas of blastula, gastrula and embryo, individual cells and areas of cells become biochemically different from one another. The cells become biochemically different from one another. The process by which this fixity of developments prospect is process by which this fixity of developments prospect is acquired is called acquired is called DETERMINATION.DETERMINATION.

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DifferentiationDifferentiation - - could therefore be redefined could therefore be redefined as the series of events by which materials become as the series of events by which materials become chemically determined and then assume distinctive form chemically determined and then assume distinctive form and function. and function.

THE GAMETESTHE GAMETES: : In all vertebrates reproduction is effected by In all vertebrates reproduction is effected by

sexual means with the use of specialized sexual means with the use of specialized germ cells called gametesgerm cells called gametes - - spermatozoa in the in spermatozoa in the in

male and ova in the femalemale and ova in the female.. The Primordial Germ cellsThe Primordial Germ cells: : There are different concepts There are different concepts

about the origin of the germ cells found in the adult gonads.about the origin of the germ cells found in the adult gonads.

i.i. One doctrine states that the PGCsOne doctrine states that the PGCs are merely special are merely special endodermal wandering cells, having nothing to do with endodermal wandering cells, having nothing to do with reproduction;reproduction;

and that gamete formations is a matter of and that gamete formations is a matter of differentiation of somatic cells within the gonads.differentiation of somatic cells within the gonads.

ii.ii. Another doctrine:Another doctrine: - - PGCs are extragonadal cellsPGCs are extragonadal cells gonads gonads gametes.gametes.

In vertebrates, identification of the PGCs comes later in In vertebrates, identification of the PGCs comes later in development ; development ; are external to prospective gonads are external to prospective gonads and external to embryo when first recognized and external to embryo when first recognized

Most recent notion:Most recent notion: - PGCs are the sole progenitors of the gametes.- PGCs are the sole progenitors of the gametes.

How do the PGCs reach the presumptive gonads?How do the PGCs reach the presumptive gonads?i.i. Extra-vascular/mesenteric route:Extra-vascular/mesenteric route:

The PGCs may migrate, following cell surface signals The PGCs may migrate, following cell surface signals such as such as

fibronectin in the dorsal mesentery and peritoneum fibronectin in the dorsal mesentery and peritoneum into the into the

region of the developing gonads (genital ridge).region of the developing gonads (genital ridge).

ii.ii. The vascular routeThe vascular route::

PGCs may go through the vascular system to the PGCs may go through the vascular system to the genital ridge. genital ridge.

- - birds and reptilesbirds and reptiles

A.A. EmbryogenesisEmbryogenesis Stages of embryogenesisStages of embryogenesis::

fertilization fertilization cleavage cleavage gastrulation gastrulation organogenesis organogenesis

fertilization fertilization histogenesis histogenesis (differentiated tissues)(differentiated tissues)

Developmental Period StagesDevelopmental Period Stages

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1.1. FertilizationFertilization - union of egg and sperm. - union of egg and sperm. Classification of egg: Classification of egg: micro-, meso-, and macro-micro-, meso-, and macro-

lecithallecithal.. One polarity axis - One polarity axis - animal-vegetal axisanimal-vegetal axis..

Fertilization establishes:Fertilization establishes: Stimulation of eggStimulation of egg Diploidy (2n) Fig 1.6Diploidy (2n) Fig 1.6

2.2. CleavageCleavage DefinitionDefinition ClassificationClassification BlastomeresBlastomeres End result of cleavage – Fig. 1.7End result of cleavage – Fig. 1.7

3.3. GastrulationGastrulation - involves movement of cells.- involves movement of cells. - Morphogenetic movements - cells rearrange; migrate; - Morphogenetic movements - cells rearrange; migrate;

spread; spread; bend and fold forming a bend and fold forming a

gastrula.gastrula. - End-product of gastruIation - End-product of gastruIation 3 germ layers. 3 germ layers. ectodermectoderm

mesodermmesoderm endodermendoderm

READING – text book:Chap 12

- migration of the germ cells

- fertilization- gametogenesis spermatogenesis +

oogenesis

The totality of preparatory events The totality of preparatory events leading to the formation of the haploid leading to the formation of the haploid gametes from diploid gonials through gametes from diploid gonials through the process of mitotic and meiotic the process of mitotic and meiotic (reduction) divisions.(reduction) divisions.

GametogenesisGametogenesis

SpermatogenesisSpermatogenesis Occurs in the

seminiferous tubules in the testis

A.A. Spermatogenesis:Spermatogenesis: i i SpermatocytogenesisSpermatocytogenesis - involving mitotic and - involving mitotic and

meiotic divisions of the spermatogonia through meiotic divisions of the spermatogonia through formation of spermatocytes and spermatids.formation of spermatocytes and spermatids.

ii ii SpermiogenesisSpermiogenesis (spermateliosis) - (spermateliosis) - morphogenesis of the spermatid to yield the morphogenesis of the spermatid to yield the characteristic spermatozoa ; no cell division characteristic spermatozoa ; no cell division involved.involved.

iii iii SpermiationSpermiation - release of spermatozoa into the - release of spermatozoa into the lumen of the seminiferous tubuleslumen of the seminiferous tubules

iv iv EmissionEmission V EjaculationV Ejaculation

spermatogenesis

Spermatogonia

Spermatocytes

–primary

- secondary

Spermatids

- round

-elongated

Spermatozoa

The SpermatozoaThe Spermatozoa Sperm - great morphological diversity between kinds of Sperm - great morphological diversity between kinds of

vertebrates.vertebrates.

Head and tail (neck-midpiece-principal piece/tail-endpiece).Head and tail (neck-midpiece-principal piece/tail-endpiece).

Head - spheroidal in teleosts; rod or lance-shaped in Head - spheroidal in teleosts; rod or lance-shaped in amphibia; spirally twisted in passerine birds; spoon-shaped amphibia; spirally twisted in passerine birds; spoon-shaped in humans and many other mammals; hooked in mouse and in humans and many other mammals; hooked in mouse and rats.rats.

Serves two functionsServes two functions: genetic (in the DNA), and activating: genetic (in the DNA), and activating..

Mitosis and MeiosisMitosis and Meiosis

SpermatogenesisSpermatogenesis

SpermatocytogeneSpermatocytogenesissis

SpermiogenesisSpermiogenesis SpermiationSpermiation EmissionEmission EjaculationEjaculation

SpermatogenesisSpermatogenesis

Spermatogenesis Spermatogenesis starts in testes in starts in testes in seminiferous seminiferous tubulestubules

Spermatogenesis Spermatogenesis starts in outermost starts in outermost layer of tube and layer of tube and daughter cells daughter cells move towards move towards lumen. lumen.

Male Reproductive TractMale Reproductive Tract

Sperm pass throughSperm pass through

Seminiferous Seminiferous

tubulestubules EpididymisEpididymis Vas DeferensVas Deferens Ejaculatory DuctEjaculatory Duct UrethraUrethra Penis (within)Penis (within)

Seminiferous TubulesSeminiferous Tubules

In testesIn testes Very convoluted: if Very convoluted: if

stretched, 1.5 miles! (in stretched, 1.5 miles! (in one testicle only)one testicle only)

Sertoli cells= Sertoli cells= Sustentacular cells: Sustentacular cells: sustain spermsustain sperm

Leydig cells= Interstitial Leydig cells= Interstitial cells: make androgenscells: make androgens

Functions of Sustentacular Functions of Sustentacular CellsCells

Secretion of MSecretion of Müllerian-inhibiting factorüllerian-inhibiting factor Support of mitosis and meiosisSupport of mitosis and meiosis Support of spermiogenesisSupport of spermiogenesis Secretion of androgen-binding proteinSecretion of androgen-binding protein Secretion of inhibinSecretion of inhibin Maintenance of blood-testis barrierMaintenance of blood-testis barrier

Seminiferous TubulesSeminiferous Tubules

SpermatozoonSpermatozoon

HeadHead NeckNeck TailTail Lacking many Lacking many

intracellular intracellular structuresstructures

At this step, can’t At this step, can’t fertilize or move fertilize or move coordinatelycoordinately

B.B. Oogenesis:Oogenesis: The totality of the processes involving The totality of the processes involving

mitotic and meiotic divisions of the diploid mitotic and meiotic divisions of the diploid oogonia, culminating in the formation oogonia, culminating in the formation haploid ovum, and nonfunctional polar haploid ovum, and nonfunctional polar bodies.bodies.

The Vertebrate Egg:The Vertebrate Egg: The nutritive substance in all eggs is the The nutritive substance in all eggs is the

yolk. Yolk is primarily produced in the liver, yolk. Yolk is primarily produced in the liver, and is transported in a soluble form via the and is transported in a soluble form via the bloodstream to the ovary.bloodstream to the ovary.

In the ovary, the yolk is transferred by In the ovary, the yolk is transferred by ovarian cells that surround the oocyte and ovarian cells that surround the oocyte and is deposited as platelets or granules. The is deposited as platelets or granules. The vertebrate is classified according to vertebrate is classified according to amount of yolk in the cytoplasm.amount of yolk in the cytoplasm.

Oogenesis

Step 1Step 1

OogoniaOogonia are small cells are small cells that increase in number that increase in number by mitosisby mitosisWhen they enlarge When they enlarge slightly, they are slightly, they are considered considered primary primary oocytesoocytes

Primary FollicleOocyte enclosed by Follicle

cellsPrimary oocytes become Primary oocytes become enclosed by a single layer of enclosed by a single layer of follicle cellsfollicle cells

Follicle cells from the Follicle cells from the surface layer of the surface layer of the ovaryovary

Primary folliclePrimary follicle is the is the structure including a primary structure including a primary oocyte enclosed in follicle oocyte enclosed in follicle cellscells

Oogenesis

At puberty, the primary follicles enlarge rapidly and accumulate yolk caused by the

follicle stimulating hormone (FSH) from the pituitary

gland

As the yolk increases, the oocyte is pushed to one side as a small disc

The disc is called the blastodisc

The blastodisc contains the enlarged nucleus of the primary oocyte which is called the

germinal vesicle

YOLK

BLASTODISC

Oogenesis

First meiotic division of the primary oocyte occurs just before ovulation

The result is the first polar body pinching off and the secondary oocyte

The secondary oocyte contains most of the yolk and the blastodisc

Fertilization

The The second meiotic second meiotic divisiondivision begins but stops begins but stops at metaphaseat metaphaseThe secondary oocyte The secondary oocyte (sometimes called the (sometimes called the ovum at this stage) is ovum at this stage) is mature enough to be mature enough to be fertilized andfertilized andOvulation occursOvulation occurs

Oogenesis/Fertilization The secondary oocyte

(ovum) is released from the ovary as the follicle ruptures

It enters the infundibulum of the oviduct where it will encounter sperm cells

As sperm penetrates the blastodisc, meiosis begins again and the result is the second polar body and the mature ovum

READING – text bookChapter 8 - fertilization

- cleavage amphioxus xenopus chick sea urchin

- gastrulation

Cleavage and Blastulation

Cleavage- Occurs in eggs activated by fertilization. Cleavage is the subdivision of eggs into cells called blastomeres. The blastomeres divide quantitatively into smaller units until they are of such a size that they can readily undergo the subsequent events of blastulation, gastrulation, and interaction that are involved in formation of tissues and organs

Cleavage & Blastulation The blastodisc initiates

cleavage First cleavage occurs in

the zygote by way of mitosis and the result is two cells called blastomeres

Cleavage occurs in the cytoplasm, not the yolk

Second cleavage = 4 blastomeres

Cleavage (mitosis) continues until the blastula forms

Blastula

Blastulation•The formation of a segmentation cavity or blastocoele within a mass of cleaving blastomeres. Rearrangement of blastomeres around this cavity forms the type of definitive blastula, characteristic of each species•The definitive blastula is thought to terminate cleavage stages•The wall of the blastula is a mosaic of cellular areas, each of which will normally produce a certain structure during subsequent development. In other words, each area of cells in the wall of the blastula has a certain prospective fate which will be realized in normal development.

The Vertebrate The Vertebrate Egg:Egg:

Nutritive substance in all eggs is the yolk. Nutritive substance in all eggs is the yolk.

Yolk is produced in the liver Yolk is produced in the liver transported in a soluble form via transported in a soluble form via the bloodstream the bloodstream to the ovary.to the ovary.

In the ovary, the yolk is transferred by ovarian cells that In the ovary, the yolk is transferred by ovarian cells that surround the oocyte surround the oocyte is deposited as platelets or granules. is deposited as platelets or granules.

Classification of eggClassification of egg::

The vertebrate is classified according to amount of yolk in the The vertebrate is classified according to amount of yolk in the cytoplasm: cytoplasm:

microlecithal: mesolecithal: macrolecithalmicrolecithal: mesolecithal: macrolecithal

Eggs:Eggs:i.i. MicrolecithalMicrolecithal - small amount of yolk, scattered - small amount of yolk, scattered

evenly in cytoplasm (Amphioxus; tunicates; evenly in cytoplasm (Amphioxus; tunicates; eutherian mammals)eutherian mammals)

ii.ii. MesolecithalMesolecithal - relatively larger amount of yolk - relatively larger amount of yolk in cytoplasm; not quite evenly scattered in cytoplasm; not quite evenly scattered (Amphihia; dipnoi; Petromyzontia).(Amphihia; dipnoi; Petromyzontia).

iii.iii. Macrolecithal Macrolecithal - enormous amount of yolk as - enormous amount of yolk as food reserve (Myxinoidea; Chondrichtyes; food reserve (Myxinoidea; Chondrichtyes;

Osteichthyes;reptiles; and birds).Osteichthyes;reptiles; and birds).

* * TelolecithalTelolecithal eggs (mesolecithal and eggs (mesolecithal and macrolecithal) - where yolk is concentrated in one macrolecithal) - where yolk is concentrated in one hemispherehemisphere the vegetal pole of the egg the vegetal pole of the egg (greatest concentration of yolk), (greatest concentration of yolk),

than the animal pole (smallest amount of yolk).than the animal pole (smallest amount of yolk).

* The amount and distribution of yolk influences * The amount and distribution of yolk influences the pattern of cleavage and movement of cells the pattern of cleavage and movement of cells and tissues during gastrulation.and tissues during gastrulation.

Union of the ovum and spermatozoon to form the Union of the ovum and spermatozoon to form the

zygotezygote.. Occurs in fluid mediumOccurs in fluid medium - - All the events involved in All the events involved in

fertilization (internal fertilization (internal

and external) take place in a fluid medium.and external) take place in a fluid medium.

Barriers encounteredBarriers encountered by the spermatozoa include by the spermatozoa include: : cumulus oophorus; corona radiata (encasing the ova); zona cumulus oophorus; corona radiata (encasing the ova); zona

pellucida; pellucida;

the vitelline membrane; mucus in the genitalia (internal the vitelline membrane; mucus in the genitalia (internal fertilization).fertilization).

Fertilization Fertilization

In some vertebrates (mouse) the zona pellucida contains In some vertebrates (mouse) the zona pellucida contains glycoproteinglycoprotein (zona glycoprotein) which acts as receptor for (zona glycoprotein) which acts as receptor for the binding protein of the sperm membrane.the binding protein of the sperm membrane.

The acrosome produces The acrosome produces hyaluronidase (lytic enzymehyaluronidase (lytic enzyme) which ) which dissolves the egg barriers.dissolves the egg barriers.

The acrosome of many different kinds of sperm have been The acrosome of many different kinds of sperm have been shown to produce a shown to produce a specific protein called specific protein called bindinbindin,, which which reacts with bindin receptors (glycoprotein) on the ovum’s reacts with bindin receptors (glycoprotein) on the ovum’s vitelline membrane on contact, thus attaching the tip of the vitelline membrane on contact, thus attaching the tip of the acrosome to the membrane.acrosome to the membrane.

Acrosomal Reaction:Acrosomal Reaction: Morphological changes and biochemical events undertaken by Morphological changes and biochemical events undertaken by

the spermatozoa in order to acquire fertilizing capability.the spermatozoa in order to acquire fertilizing capability.

Identified in several vertebrates and invertebrates.Identified in several vertebrates and invertebrates.

Capacitation:Capacitation: Sperm of several species require a period of Sperm of several species require a period of exposure to compounds - glycosaminoglycans (GAGs) in the exposure to compounds - glycosaminoglycans (GAGs) in the female genitalia before the bindin reaction can occur. female genitalia before the bindin reaction can occur.

This preparation is part of what is known as This preparation is part of what is known as sperm sperm

capacitationcapacitation..

Steps in AcR Steps in AcR a)a) Disappearance of the Acrosomal membrane at the Disappearance of the Acrosomal membrane at the

apical region.apical region.

b)b) The Acrosomal and sperm membranes become The Acrosomal and sperm membranes become continuous (Fig 5-2A).continuous (Fig 5-2A).

c)c) At the same time, disintegration of Acrosomal At the same time, disintegration of Acrosomal granule, as does the portion of the egg membrane granule, as does the portion of the egg membrane exposed to the granule (signifying release of lytic exposed to the granule (signifying release of lytic enzymes).enzymes).

d)d) Indentation of that region of the acrosomal membrane Indentation of that region of the acrosomal membrane abutting abutting

the nucleus (arrow in 5-2A).the nucleus (arrow in 5-2A).

e) Elongation and deepening of the indentation, thus creating a e) Elongation and deepening of the indentation, thus creating a

steady elongating Acrosomal filament that traverses the egg steady elongating Acrosomal filament that traverses the egg envelope and finally approaches the plasma membrane of the envelope and finally approaches the plasma membrane of the ovum (5-3C).ovum (5-3C).

f) Zygote Formation:f) Zygote Formation:

Establishes definitive contact between the two gametes (with Establishes definitive contact between the two gametes (with the meeting of the sperm and egg plasma membranes - one the meeting of the sperm and egg plasma membranes - one continuous membrane. The contents are now essentially continuous membrane. The contents are now essentially components of one cell - the zygotecomponents of one cell - the zygote..

g)g) Activation of : Activation of : Contact of the acrosome with the plasma membrane of the Contact of the acrosome with the plasma membrane of the

egg egg activates the otherwise quiescent egg into action:activates the otherwise quiescent egg into action:

i.i. Polyspermy: Definition of: Polyspermy: Definition of: i.i. Significance of :Significance of :

Blockage of polyspermy:Blockage of polyspermy:Block polyspermy:Block polyspermy:

a) Fast block to polyspermy:a) Fast block to polyspermy: - contact between acrosome and ovum membrane results in - contact between acrosome and ovum membrane results in

electricalelectrical charges charges change in membrane potential across the change in membrane potential across the surface of the surface of the egg, egg, immediately preventing further sperm immediately preventing further sperm penetration of the egg.penetration of the egg.

b) Slow block to polyspermy:b) Slow block to polyspermy: -- cortical reaction; I cortical reaction; I cortical vesicles cortical vesicles discharge cortical granules discharge cortical granules to fuse with to fuse with

egg plasma membrane egg plasma membrane chemical alteration in the membrane I chemical alteration in the membrane I inactivating its bindin receptors inactivating its bindin receptors releasing any other sperm that releasing any other sperm that had attached to it had attached to it thus preventing further fertilization. thus preventing further fertilization.

This constitutes the slow block to polyspermy.This constitutes the slow block to polyspermy.

ii.ii. Consequences of sperm penetration:Consequences of sperm penetration:

a) Establishment of the plane of bilateral symmetrya) Establishment of the plane of bilateral symmetry sperm penetration of ovum sperm penetration of ovum rearrangement of its rearrangement of its

internal internal constituents constituents leading to the establishment of the leading to the establishment of the

plane of plane of bilateral symmetry of the embryo to come.bilateral symmetry of the embryo to come.

b) Alterations in the physiological properties of the egg; b) Alterations in the physiological properties of the egg; changes in permeability ;changes in permeability ; and alterations in the rate of metabolism; and alterations in the rate of metabolism; ovum becomes a truly dynamic entity and embarks on ovum becomes a truly dynamic entity and embarks on

physicochemical transformations that lead into the physicochemical transformations that lead into the

period of period of cleavage.cleavage.

Establishment of Diploidy Establishment of Diploidy -- union of the haploid nuclei:union of the haploid nuclei: Fertilization establishes/restores the diploid number of chromosomes for Fertilization establishes/restores the diploid number of chromosomes for

the new organism.the new organism. Fertilization restores diploidyFertilization restores diploidy which represents a new combination of which represents a new combination of

hereditary prospects.hereditary prospects. In mammals and most other vertebrates, each pronucleus (male and In mammals and most other vertebrates, each pronucleus (male and

female) loses its membrane, and concomitantly its chromatin resolves female) loses its membrane, and concomitantly its chromatin resolves into a haploid set of chromosomes. into a haploid set of chromosomes.

The two set of chromosomes arrange themselves across the spindle. The two set of chromosomes arrange themselves across the spindle. This arrangement, marking the readiness for the first cleavage division, This arrangement, marking the readiness for the first cleavage division,

represents completion of the process of fertilizationrepresents completion of the process of fertilization.. * Fertilization is completed with formation of the zygote and * Fertilization is completed with formation of the zygote and

initiation of cleavageinitiation of cleavage..

Stages in the Events of Stages in the Events of FertilizationFertilization

Fertilization-sequelae - SummaryFertilization-sequelae - Summary

1. Prevention of polyspermy via:1. Prevention of polyspermy via:- fast: changes in electrical membrane potential- fast: changes in electrical membrane potential

- slow: cortical reaction (granules fusing with egg - slow: cortical reaction (granules fusing with egg membranemembrane

2. Establishment of diploidy2. Establishment of diploidy

3. Establishment of plane of bilateral symmetry in presumptive 3. Establishment of plane of bilateral symmetry in presumptive

embryoembryo

4. Establishment of dynamic entity of the ovum 4. Establishment of dynamic entity of the ovum physiological physiological

transformation transformation initiation of cleavageinitiation of cleavage

Fertilization of the ovum to the zygote without spermatozoa.Fertilization of the ovum to the zygote without spermatozoa.

This indicates that activation and nuclear phases of fertilization are This indicates that activation and nuclear phases of fertilization are essentially distinct; that development does not absolutely require a essentially distinct; that development does not absolutely require a union of egg and sperm.union of egg and sperm.

The eggs of every animal group, including mammals, can be started The eggs of every animal group, including mammals, can be started on a course of development, often to the completion of a new on a course of development, often to the completion of a new individual, by a host of chemical and physical agents. individual, by a host of chemical and physical agents.

This strongly suggests that the ovum possesses in itself all the This strongly suggests that the ovum possesses in itself all the capacities to form an embryo, needing only some agents to trigger capacities to form an embryo, needing only some agents to trigger the action.the action.

PARTHENOGENESISPARTHENOGENESIS

In normal development, the spindle for the first cleavage division In normal development, the spindle for the first cleavage division is assembled around the centriole brought by the sperm.is assembled around the centriole brought by the sperm.

What is the source of the centriole in parthenogenesis? This may What is the source of the centriole in parthenogenesis? This may reside in the egg. In some cases, spindle appear to arise reside in the egg. In some cases, spindle appear to arise spontaneously.spontaneously.

The easiest way to induce parthenogenicity in the frog is to stab The easiest way to induce parthenogenicity in the frog is to stab the egg with a very fine needle. A method that is effective only if the egg with a very fine needle. A method that is effective only if the needle carries the needle carries

some foreign protein into the egg. The protein provides a center some foreign protein into the egg. The protein provides a center around which the cleavage spindle arisesaround which the cleavage spindle arises

Although parthenogenesis occurs naturally in some vertebrates (some Although parthenogenesis occurs naturally in some vertebrates (some birds and amphibians), it rarely occurs in others such as the whip-tailed birds and amphibians), it rarely occurs in others such as the whip-tailed lizards (lizards (Cnemidophorus uniparensCnemidophorus uniparens), all the adults of which are females.), all the adults of which are females.

Recent experiments have shown that elements of both male and Recent experiments have shown that elements of both male and female genomes are necessary for full development in mammals.female genomes are necessary for full development in mammals.

Expectedly, the chromosome numbers in parthenogenetic individuals Expectedly, the chromosome numbers in parthenogenetic individuals are haploidare haploid..

In some cases, there is tendency for the restoration of diploidy via: In some cases, there is tendency for the restoration of diploidy via: fusion of haploid cleavage nuclei prior to cell division; fusion of egg fusion of haploid cleavage nuclei prior to cell division; fusion of egg nucleus with a polar body nucleus before cleavage begins.nucleus with a polar body nucleus before cleavage begins.

Advanced or complete development occurs only if regulation to Advanced or complete development occurs only if regulation to diploidy is accomplished: haploid individuals generally succumb in diploidy is accomplished: haploid individuals generally succumb in early embyonic stages.early embyonic stages.

Eggs – Types:Eggs – Types: IsolecithalIsolecithal - small amount of yolk evenly - small amount of yolk evenly

distributed; mammals.distributed; mammals. MesolecithalMesolecithal - moderate yolk in vegetal pole; - moderate yolk in vegetal pole;

amphibians.amphibians. MacrolecithalMacrolecithal – large amount of yolk in the – large amount of yolk in the

eggegg

oror TelolecithalTelolecithal - large yolk filling almost entire - large yolk filling almost entire

egg; fish, reptiles, birds.egg; fish, reptiles, birds. CentrolecithalCentrolecithal - yolk concentrated in the - yolk concentrated in the

central cytoplasm; insects, arthropodscentral cytoplasm; insects, arthropods..

CleavageCleavage

HoloblasticHoloblastic - - entire egg cleaves during entire egg cleaves during cytokinesis.cytokinesis.– Complete HoloblasticComplete Holoblastic blastomere. blastomere.

– Partial Holoblastic Partial Holoblastic micromeres + micromeres + macromeresmacromeres

MeroblasticMeroblastic - - characterized by partial cytokinesis; characterized by partial cytokinesis;

two types:two types: Discoidal cleavageDiscoidal cleavage - - restricted to animal pole, restricted to animal pole,

yolk-free area.yolk-free area.

*Vegetal pole remains uncleaved in telolecithal eggs.*Vegetal pole remains uncleaved in telolecithal eggs.

Cleavage PatternCleavage Pattern

From 128 cell stage = blastulaFrom 128 cell stage = blastula Several layers thick; Several layers thick; Larger blastomeres in vegetal pole – Larger blastomeres in vegetal pole – macromeresmacromeres.. Smaller blastomeres at animal pole – Smaller blastomeres at animal pole – micromeresmicromeres..

Amphibian CleavageAmphibian Cleavage

Mammalian CleavageMammalian Cleavage

Cleavage Cellular MechanismCleavage Cellular Mechanism

Experimental evidence:Experimental evidence: In In isolecithal eggsisolecithal eggs - mitotic spindle is centered - - mitotic spindle is centered -

cleavage furrows form all around the cleavage furrows form all around the circumference.circumference.

In In mesolecithal eggsmesolecithal eggs - mitotic spindle is displaced - mitotic spindle is displaced to the animal pole - the furrow first appears in to the animal pole - the furrow first appears in animal pole then cuts towards the vegetal pole.animal pole then cuts towards the vegetal pole.

In In telolecithal eggstelolecithal eggs - mitotic spindle is eccentric - - mitotic spindle is eccentric - the cleavage furrow forms only near the mitotic the cleavage furrow forms only near the mitotic apparatus.apparatus.

Cleavage Cellular MechanismCleavage Cellular Mechanism

CleavageCleavage - general consideration: - general consideration: Series of a regular rhythm of mitotic divisions of the egg following Series of a regular rhythm of mitotic divisions of the egg following

activation either by the spermatozoon or by parthenogenetic agent, activation either by the spermatozoon or by parthenogenetic agent, and ending with formation of a blastula.and ending with formation of a blastula.

The individual cells during the process of segmentation are called The individual cells during the process of segmentation are called blastomeres.blastomeres.

During the period of segmentation there is no increase in cytoplasmic During the period of segmentation there is no increase in cytoplasmic massmass (embryo does not grow); segmentation leads to increase in the (embryo does not grow); segmentation leads to increase in the number of blastomeres but not the size; number of blastomeres but not the size;

; ; there is no growth during these mitotic divisions as the G1 and G2 there is no growth during these mitotic divisions as the G1 and G2 phases of the cell cycle immediately before and after DNA synthesis phases of the cell cycle immediately before and after DNA synthesis are eliminated.are eliminated.

Cleavage & GastrulationCleavage & Gastrulation

With each cell division there is additional DNA synthesis With each cell division there is additional DNA synthesis and protein, to provide for the increased nuclei, and protein, to provide for the increased nuclei,

at the expense of cytoplasmic reserve at the expense of cytoplasmic reserve nuclear gain is nuclear gain is balanced by cytoplasmic loss: balanced by cytoplasmic loss:

leading to numerous leading to numerous small blastomeres each with its small blastomeres each with its nucleus and cytoplasm.nucleus and cytoplasm.

Since the area of constriction that divides the cytoplasm Since the area of constriction that divides the cytoplasm ((cytokinesiscytokinesis) is organized by the ) is organized by the centriolecentriole of the cell, the of the cell, the plane of cell division is always at right angle to the long axis plane of cell division is always at right angle to the long axis of the spindleof the spindle

Variations in Cleavage Pattern:Variations in Cleavage Pattern:a.a. Amount of yolk in egg cytoplasmAmount of yolk in egg cytoplasm Influences the rate and pattern of cleavage Influences the rate and pattern of cleavage

division; the greater the amount of yolk and the division; the greater the amount of yolk and the more unevenly distributed the yolk, the more the more unevenly distributed the yolk, the more the mitotic spindle in the egg and later in the mitotic spindle in the egg and later in the blastomeres are displaced from the center of the blastomeres are displaced from the center of the cell toward the less yolky ends; and upon division, cell toward the less yolky ends; and upon division, blastomeres of unequal size result. In an egg with blastomeres of unequal size result. In an egg with most heavily laden yolk (birds), segmentation is most heavily laden yolk (birds), segmentation is confined to a small region (blastodisc) at the confined to a small region (blastodisc) at the animal pole.animal pole.

b.b. Cytoplasmic organizationCytoplasmic organization Exemplified by the spiral cleavage in annelids Exemplified by the spiral cleavage in annelids

and mollusks - where the rotational movement and mollusks - where the rotational movement of blastomeres resulting from alternately tilted of blastomeres resulting from alternately tilted obliquely to the right and left, so that obliquely to the right and left, so that successive generations of blastomeres are successive generations of blastomeres are oriented in a twisted fashion.oriented in a twisted fashion.

Summary – cleavage patternSummary – cleavage pattern

1.1. Holoblastic (total) CleavageHoloblastic (total) Cleavage:: The whole egg divides as do the blastomeres:The whole egg divides as do the blastomeres:

a.a. Equal Equal holobalsticholobalstic: In microlecithal eggs (mammals) : In microlecithal eggs (mammals) - blastomeres of equal sizes- blastomeres of equal sizes

b.b. Unequal Unequal holoblasticholoblastic: In mesolecithal eggs : In mesolecithal eggs (amphibians) – blastomeres of unequal sizes(amphibians) – blastomeres of unequal sizes

2.2. Partial CleavagePartial Cleavage: (Teleost fish) part of egg : (Teleost fish) part of egg remains undivided; in moderately macrolecithal remains undivided; in moderately macrolecithal eggseggs

3.3. Meroblastic CleavageMeroblastic Cleavage:: (reptiles; bird & (reptiles; bird & primitive mammals) - extremely macrolecithal primitive mammals) - extremely macrolecithal eggs –segmentation is confined to small area at eggs –segmentation is confined to small area at

the animal polethe animal pole..

GASTRULATION – GASTRULATION – commences at the conclusion of commences at the conclusion of segmentation & formation of the blastulasegmentation & formation of the blastula

Gastrulation -Gastrulation -

GASTRULATIONGASTRULATION is by a process of rapid cell multiplication by which the blastula is by a process of rapid cell multiplication by which the blastula

is molded into a stratified structure - is molded into a stratified structure - gastrulagastrula

involves flattening and invagination (inward folding) of theinvolves flattening and invagination (inward folding) of the

prospective endodermal plate into the blastocoelprospective endodermal plate into the blastocoel

the new cavity is called gastrocoel (archenteron) which opens the new cavity is called gastrocoel (archenteron) which opens to the exterior by the blastoporeto the exterior by the blastopore

further migration of cells and elongation of the further migration of cells and elongation of the gastrula establishes two layers: a) an outer gastrula establishes two layers: a) an outer ectoderm, neural and epidermal, and b) inner cell ectoderm, neural and epidermal, and b) inner cell mass encompassing prospective notochord, mass encompassing prospective notochord, mesoderm and endodermmesoderm and endoderm

the organs of the body are all derivatives of these the organs of the body are all derivatives of these

three primary germ layersthree primary germ layers..

Note:Note: In amphoxius, the mesodermal pouches are hollow from the beginning In amphoxius, the mesodermal pouches are hollow from the beginning

which is to say that they contain a coelum. which is to say that they contain a coelum.

Thus the Amphioxus features an enterocoel, (the coelomic spaces Thus the Amphioxus features an enterocoel, (the coelomic spaces trace their origin to the cavity of the primitive gut).trace their origin to the cavity of the primitive gut).

The vertebrates feature a schizophoel - a coelom formed by the The vertebrates feature a schizophoel - a coelom formed by the splitting of an originally solid layer of mesoderm.splitting of an originally solid layer of mesoderm.

In each case the end result is the same: somatic mesoderm associated In each case the end result is the same: somatic mesoderm associated with epidermal ectoderm to form somatopleure; and splanchnic with epidermal ectoderm to form somatopleure; and splanchnic mesoderm associated with endoderm (endodermal gut) to form mesoderm associated with endoderm (endodermal gut) to form splanchnopleure.splanchnopleure.

AMPHIBIAN - FROGAMPHIBIAN - FROG eggs are mesolecithal - moderate amount of yolk present in eggs are mesolecithal - moderate amount of yolk present in

form of oval granules scattered throughout the cytoplasm, form of oval granules scattered throughout the cytoplasm, but show gradual concentration to one pole;but show gradual concentration to one pole;

telolecithal - greater concentration of yolk located at the telolecithal - greater concentration of yolk located at the vegetal pole.vegetal pole.

Cleavage: follows the pattern seen in the amphioxus with Cleavage: follows the pattern seen in the amphioxus with minor differences:minor differences:

Gastrulation (cont’d)Gastrulation (cont’d)

involves invagination, involution and stretching and spreading involves invagination, involution and stretching and spreading of cells of the blastula to form a spherical gastrula surrounded of cells of the blastula to form a spherical gastrula surrounded externally by ectoderm and containing endodermal and externally by ectoderm and containing endodermal and mesodermal components internally, comparable to the situation mesodermal components internally, comparable to the situation in amphioxus.in amphioxus.

throughout gastrulation the embryo retains its spherical shape throughout gastrulation the embryo retains its spherical shape and a uniform size.and a uniform size.

endodermal ridges meet in the dorsal midline and unite to endodermal ridges meet in the dorsal midline and unite to create a tubular endodermal gut.create a tubular endodermal gut.

mesoderm in the dorsal midline is set off as a definitive mesoderm in the dorsal midline is set off as a definitive notochord, separate from the presumptive epimere, notochord, separate from the presumptive epimere, mesomere and hypomere the forerunners of other organs.mesomere and hypomere the forerunners of other organs.

the overlying ectoderm would give rise to neural tube.the overlying ectoderm would give rise to neural tube.

AvianAvian first cleavage division is an irregular shallow furrow in the first cleavage division is an irregular shallow furrow in the

central portion of the blastodisc.central portion of the blastodisc.

this furrow does not cut completely through the depth of the this furrow does not cut completely through the depth of the disc; and this results in two incompletely separated disc; and this results in two incompletely separated blastomeres.blastomeres.

the second division occurs at right angle to the first but does the second division occurs at right angle to the first but does not cut entirely through the depth of the disc.not cut entirely through the depth of the disc.

further incomplete divisions result in irregular pattern of further incomplete divisions result in irregular pattern of cleavage, creating two groups of blastomeres: a) cleavage, creating two groups of blastomeres: a) completely bounded central blastomeres and b) completely bounded central blastomeres and b) incompletely bounded marginal blastomeres.incompletely bounded marginal blastomeres.

further divisions of the blastomeres create a stratification of further divisions of the blastomeres create a stratification of cells. This results in a compact plate several cells thick; cells. This results in a compact plate several cells thick; which is called the blastoderm.which is called the blastoderm.

the bulk of the yolk is external to the blastoderm (which still the bulk of the yolk is external to the blastoderm (which still contains some yolk cells).contains some yolk cells).

The bilaminar blastoderm represents the blastula.; the epiblast The bilaminar blastoderm represents the blastula.; the epiblast represents the animal half; and hypoblast (endoderm) represents the animal half; and hypoblast (endoderm) represents the vegetal half; and the intervening space, represents the vegetal half; and the intervening space, represent the blastocoel.represent the blastocoel.

GastrulationGastrulation the hypoblast (endoderm) is already present in part at the hypoblast (endoderm) is already present in part at

beginning of gastrulation.beginning of gastrulation.

Major differences in the fate of embryonic cells of avian and Major differences in the fate of embryonic cells of avian and amphibian?amphibian?

i) formation of extraembryonic organs from the blastoderm i) formation of extraembryonic organs from the blastoderm to support embryo prior to hatchingto support embryo prior to hatching

The epiblast of the entire area opaca is destined to provide The epiblast of the entire area opaca is destined to provide extraembryonic ectoderm.extraembryonic ectoderm.

The epiblast (upper layer) is the presumptive ectoderm,The epiblast (upper layer) is the presumptive ectoderm, mesoderm, and intraembryonic endoderm.mesoderm, and intraembryonic endoderm.

The anterior two-thirds of the area pellucida is prospective The anterior two-thirds of the area pellucida is prospective ectoderm.ectoderm.

The greater part of the ectodermal area is prospectiveThe greater part of the ectodermal area is prospective dermis and the bulk of this is extraembryonic.dermis and the bulk of this is extraembryonic.

ii) Invagination in the frog involves folding and ii) Invagination in the frog involves folding and retraction of a sheet of cells into the retraction of a sheet of cells into the blastocoel, formation of archenteron and a blastocoel, formation of archenteron and a blastopore: in the avian, the separation of blastopore: in the avian, the separation of these layers from the ectoderm involves the these layers from the ectoderm involves the process of convergence and ingression.process of convergence and ingression.

the process of convergence and ingression initiate the the process of convergence and ingression initiate the arrangement of mesodermal and endodermal tissues arrangement of mesodermal and endodermal tissues between the epiblast and hypoblastbetween the epiblast and hypoblast

the final stages of gastrulation in the avian embryo involve the final stages of gastrulation in the avian embryo involve a continuation of convergence and ingression of cells a continuation of convergence and ingression of cells culminating in the establishment of the germ layers and culminating in the establishment of the germ layers and other presumptive organs- somite and epimere, notochord other presumptive organs- somite and epimere, notochord etc. etc.

Placenta formation in amniotes

**The four extraembryonic membranes areThe four extraembryonic membranes are:: i) i) amnionamnion - protective - protective iiii)) chorion chorion - respiratory and nutritive - respiratory and nutritive Iii) Iii) allantoisallantois - respiratory and nutritive - respiratory and nutritive iv) iv) yolk sacyolk sac - nutrition (source of albumin) - nutrition (source of albumin)

Salient points:Salient points: a) Amniota: - amnion-possessing vertebrate such as: a) Amniota: - amnion-possessing vertebrate such as:

mammals: birds and some reptilesmammals: birds and some reptiles..

b) b) ANAMNIOTA:ANAMNIOTA: - cyclostome, fishes, and amphibian only - cyclostome, fishes, and amphibian only EEM is the yolk sac (lacking other membranes including the EEM is the yolk sac (lacking other membranes including the amnion.amnion.

c) c) OVIPAROUSOVIPAROUS:- external deposition of eggs and young :- external deposition of eggs and young developing independent of parents.developing independent of parents.

all birds; most fishes, amphibians and reptilesall birds; most fishes, amphibians and reptiles

d) d) VIVIPAROUS.VIVIPAROUS. - retention of egg by a parent (in female - retention of egg by a parent (in female reprod. tract); or specially designed pouch in male or reprod. tract); or specially designed pouch in male or female,) ; and young are “born alive”.female,) ; and young are “born alive”.

THE REPRODUCTIVE CYCLETHE REPRODUCTIVE CYCLE

The ovarian cycles in mammals The ovarian cycles in mammals shedding of blood and cellular debris from the uterus at montly shedding of blood and cellular debris from the uterus at montly

intervals. The menstrual cycle represents the integration of three intervals. The menstrual cycle represents the integration of three very different activities: very different activities:

(1) (1) the ovarian cyclethe ovarian cycle, the function of which is to mature and , the function of which is to mature and release an release an

oocyce,oocyce,

(2)(2) the uterine cyclethe uterine cycle, the function of which is to provide the , the function of which is to provide the appropriate appropriate

environment to the developing blastocyst to implant environment to the developing blastocyst to implant

(3)(3) the cervical cyclethe cervical cycle, the function of which is to allow sperm to , the function of which is to allow sperm to enter the enter the

female reproductive tract only at the appropriate time. female reproductive tract only at the appropriate time.

These three functions are integrated through the hormones of the pituitary, These three functions are integrated through the hormones of the pituitary,

hypothalamus, and ovary.hypothalamus, and ovary.

The majority of the oocytes within the adult human ovary are The majority of the oocytes within the adult human ovary are mainly in the prolonged diplotene stage of the first meiotic mainly in the prolonged diplotene stage of the first meiotic prophase (oftenprophase (often

referred to as the dictyate state. Each oocyte is envelopped by a referred to as the dictyate state. Each oocyte is envelopped by a primordial follicle consisting of a single layer of epithelial primordial follicle consisting of a single layer of epithelial granulosa cells and a less-organized layer of mesenchymal thecal granulosa cells and a less-organized layer of mesenchymal thecal cellscells

Periodically, a group of primordial follicles enters a stage Periodically, a group of primordial follicles enters a stage of follicular growth. During this time, the oocyte undergoes a 500-of follicular growth. During this time, the oocyte undergoes a 500-

fold increase in volume (corresponding to an increase in oocyte fold increase in volume (corresponding to an increase in oocyte diameter from 10 micro-meter in a primordial follicle to 80 micro-diameter from 10 micro-meter in a primordial follicle to 80 micro-meter to a fully developed oocyte.meter to a fully developed oocyte.

the secondary stage of follicle development is marked by the the secondary stage of follicle development is marked by the appearance of the zona between the granulosa cells and the ovum.appearance of the zona between the granulosa cells and the ovum.

antrumantrum: a follicular cavity is formed by creation of a cleft within the : a follicular cavity is formed by creation of a cleft within the granulosa cells; this is called antrumgranulosa cells; this is called antrum

tertiary follicletertiary follicle; a follicle with an antrum is called tertiary follicle; a follicle with an antrum is called tertiary follicle further increase in size -, antrum enlarges, oocyte still attaches to further increase in size -, antrum enlarges, oocyte still attaches to

granulosa on one size.granulosa on one size. cumulus - the granulosa investing the ovum is now called cumulus - the granulosa investing the ovum is now called cumulus.cumulus.

theca cells - as the follicle grow, a framework of connective theca cells - as the follicle grow, a framework of connective tissue is laid around the ovum, outside the granulosa cells: tissue is laid around the ovum, outside the granulosa cells: this is the theca layer outside the parietal granulosa.this is the theca layer outside the parietal granulosa.

oocyte, still encased in the zona pellucida and remnants of oocyte, still encased in the zona pellucida and remnants of the cumulus, stretches and floats in the antral fluid; the the cumulus, stretches and floats in the antral fluid; the release of the cumulus from the theca-granulosa causes release of the cumulus from the theca-granulosa causes them to extend out into the follicular fluid in a mass of them to extend out into the follicular fluid in a mass of projections called corona radiata.projections called corona radiata.

Ovulation:Ovulation: the mature follicle, called the Graafian follicle, finally bursts open to the mature follicle, called the Graafian follicle, finally bursts open to

release the oocyte. This process is called ovulation.release the oocyte. This process is called ovulation.

in most mammals ovulation is followed by completion of the first in most mammals ovulation is followed by completion of the first meiotic division and extrusion of the first polar body.meiotic division and extrusion of the first polar body.

in uniparous mammals, such as the human female, only one follicle in uniparous mammals, such as the human female, only one follicle develops into maturity and only one follicle ovulate, generally.develops into maturity and only one follicle ovulate, generally.

in litter-bearing mammals (pluriparous) several folliclesin litter-bearing mammals (pluriparous) several follicles attain maturity, and several ova are eleased at ovulation.attain maturity, and several ova are eleased at ovulation.

* * Considerable variability in timing of ovulation in mammalsConsiderable variability in timing of ovulation in mammals may occur hours before or after heat: in human female may occur hours before or after heat: in human female

ovulation may occasionally occur almost a any time in the estrus ovulation may occasionally occur almost a any time in the estrus cycle.cycle.

Ovulation is not spontaneous in all mammals. In rabbit, cat, mink Ovulation is not spontaneous in all mammals. In rabbit, cat, mink ovulation takes place only after nervous stimulation derived from ovulation takes place only after nervous stimulation derived from the act of copulation. the act of copulation.

- these are called induced-ovulators- these are called induced-ovulators

The Estrus CycleThe Estrus Cycle:: the breeding pattern and behavior among the vertebrate are the breeding pattern and behavior among the vertebrate are

remarkably variable.remarkably variable.

this is dependent on whether or not the breeding cycle is seasonal or this is dependent on whether or not the breeding cycle is seasonal or intra-seasonal.intra-seasonal.

Some mammals mate periodically throughout the year;Some mammals mate periodically throughout the year;

others mate only during a restricted breeding season. others mate only during a restricted breeding season.

except the higher primates, all female mammals permit mating only except the higher primates, all female mammals permit mating only at definite times called “heat” or estrus.at definite times called “heat” or estrus.

EstrusEstrus = a period in the estrus cycle during which the female mammal = a period in the estrus cycle during which the female mammal is in a physiological and psychological readiness for mating, and hence is in a physiological and psychological readiness for mating, and hence sexually accepts the male.sexually accepts the male.

Repetition of the estrus period at a set interval constitutes the estrus Repetition of the estrus period at a set interval constitutes the estrus cycle.cycle.

Estrus cycle lengthEstrus cycle length; - varies remarkably in different vertebrate groups:; - varies remarkably in different vertebrate groups:

Rat and Mouse - 4 to 5 day intervalRat and Mouse - 4 to 5 day interval

Dogs - every 6 months (twice yearly)Dogs - every 6 months (twice yearly)

Domestic cow and pig - 3 weeks (18 - 25 days)Domestic cow and pig - 3 weeks (18 - 25 days)

Deer family and wild sheep - mating only once a year. Deer family and wild sheep - mating only once a year.

Phases of the Estrus’s cycle Phases of the Estrus’s cycle a) a) Estrus Estrus = a period of sexual desire and acceptance of mating= a period of sexual desire and acceptance of mating b) b) MetestrusMetestrus = a period of preparation for pregnancy - increasing P4 = a period of preparation for pregnancy - increasing P4

from from developing CL developing CL anestrus anestrus proestrus proestrus estrus etc estrus etc