Gamete production

The process of sperm or ovum production that occur in gonads directly through meiosis

The specific type of meiosis that forms sperm is called spermatogenesis, while the formation of egg cells, or ova, is called oogenesis

SPERMATOGENESIS Spermatogenesis is the process of producing sperm with

half the number of chromosomes (haploid) as somatic cells

 The germ cells progress first from the diploid to haploid state and then change shape to become spermatozoa

Spermatogenesis occurs in medullary sex cords known as seminiferous tubules

Seminiferous tubules are part of the male gonad or testes

Cells involved in spermatogenesis1. Sertoli cell                        Support for germ cells  Environment for germ cells to develop and mature Substances initiating meiosis or the reduction from diploid

to haploid cells

2.    Leydig cells  Produce testosterone Located adjacent to seminiferous tubules.

   Spermatogenesis can be divided into three parts:

I.  Spermatocytogenesis—proliferative phase II. Meiosis—production of the haploid gamete III.Spermiogenesis        -Spermatids mature into spermatozoa

(sperm)

process:

1.         Spermatogonia divide

Located near outer surface of seminiferous tubule Originate at puberty One or two divisions of spermatogonia occur to maintain

their population in a stem cell pool These divisions are mitotic Spermatogonia proliferate several times and undergo 1 to 5

stages of division and differentiation After the last division, the resulting cells are termed

primary spermatocytes

2.  The primary spermatocytes then undergo the first of the two divisions that constitute meiosis

The first meiotic division produces two secondary spermatocytes

Division of the secondary spermatocytes completes meiosis and produces the spermatids

3.  Stem cell spermatogonia remain dormant for a time and then join a new proliferation of spermatogonia

This new wave of spermatogonial divisions does not wait for the previous generation of cells to complete spermatogenesis

The purpose of this phenomenon is to ensure a residual population of spermatogonia

The time required for one spermatogonium to divide and form spermatozoa requires about 4.5 to 5 times that time span between divisions of the stem cell spermatogonia

OOGENESISOogenesis is the process of producing ovum with half the

number of chromosomes (haploid) as somatic cells In mammals, oogenesis occurs in the ovarian follicle of the

ovary

Process: Oogonia ( 2n ) divide by mitosis to form primary oocytes

( 2n ) surrounded by follicular cells The primary oocytes begin to undergo meoisis but the

process halts and does not resume until puberty After puberty, a primary oocyte give rise to a large,

secondary oocytes ( 2n ) and a first polar body A secondary follicle that contains secondary oocyte is

pushed to one- side of fluid-filled cavity A process called ovulation releases the secondary oocyte

from the surface of the ovary If the oocyte is not fertilized shortly after its release, it will

degenarate

If a secondary oocyte is fertilized, it enters the second meiotic division

Meanwhile, the empty follicle is developing into corpus luteum ; if pregnancy does not occur, the corpus luteum begins to degenarate in about 10 days

A second meiotic division will give rise to an ootid and second polar bodies

The ootid develops into a functional ovum, while the non-functional polar bodies will be disintegrate

The mature ovum has haploid ( n) number of chromosomes In human, meiosis 2 is completed only when the ovum is

penetrated by a spermatozoa

A STEP OF FERTILIZATION IN HUMAN

a) Arrival of spermDuring sexual intercose, the male deposits semen containing sperm cells in the vagina near the cervix.The sperms swim from the vagina into the cervical canal propelled by the whiplike movements of their tails.An average human ejaculate contains over one hundred million sperm, but only a few dozen complete the journey. And of these, only one will succeed in fertilizing the egg.

CAPACITATION

b) Capacitation

There is a series of functional changes that cause the tail of the sperm to beat even more vigorously and prepare its plasma membrane to fuse with the plasma membrane of the oocyte.During capacitation, the sperm are acted upon by secretions in the female reproductive tract that result in the removal of cholestrol, glycoproteins, and proteins from the plasma membrane around the acrosome of the sperm.

ACROSOME REACTION

c) Acrosomal Reaction

When the sperm head comes in contact with the egg’s jelly coat, the acrosomes ruptures and releases its hydrolytic enzymes which begin digesting the jelly.The rupture of the acrosome is accomplished through the union of the acrosomal membrane with the sperm plasma membrane. Thus, the inner surface of the acrosomal membrane becomes the outer surface of the new plasma membrane and its binding molecules are on the outside now.

A pool of actin monomers originally behind the acrosome begins to polymerize to form thin microfilaments that push the former acrosomal membrane away from the sperm head to make an acrosomal process with a microfilament core.When the enzymes finish hydrolyzing through the jelly the acrosomal process reaches the vitelline membrane.

d) Fusion

The vitelline membrane ruptures and the sperm plasma membrane joins with the egg plasma membrane becoming one membrane.The plasma membranes of the sperm and egg fuse. The sperm nucleus enter the egg. Fusion causes the cortical reaction.

e) Cortical reaction

The union of oocyte and sperm cell membranes triggers cortical granules to release enzymes that harden the zona pellucida.This reduces the chance that the other sperm cells will penetrate, and it forms a protective layer around the newly formed fertilized egg cell.

f) Mitosis

The sperm nucleus enters the secondary oocyte’s cytoplasm and swells.The oocyte divides into mature ovum and a smaller second polar body which then disintegrates.Then, the nuclei of the male and female cells unite, producing a single diploid nucleus that contains 23 chromosomes from each nucleus, completing the process of fertilization.The fertilized ovum called zygote is now ready for the first cleavage.

Cleavage

Cleavage

In embryology, cleavage is the division of cells in the early embryo.

The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote.

The different cells derived from cleavage are called blastomeres and form a compact mass called the morula.

Cleavage ends with the formation of the blastula.

Cleavage pattern1. Amount and distribution of yolk in their egg.

Term Yolk distribution Example

Isolechital Egg that have small amount of evenly distributed yolk in the cytoplasm.

In most echinoderms, molluscs, ascidians and mammals.

Mesolechital

Egg with moderate amount of yolk present mostly in vegetal hemisphere.

In amphibian egg.

Telolechital

Egg with a large amount of yolk filling the entire egg except for a small area near the animal pore.

In most fish, reptiles and bird egg.

Centrolechital

Egg in which the yolk is concentrated in the central cytoplasm.

In egg of all insects and many other arthropods.

Cleavage pattern2. Polarity of egg

Site Animal pole Vegetal pole

Amount of yolk Relatively yolk free Yolk free

Portion of embryo Housing primary tissue

Site containing “yolk”

Cleavage rate Faster Slower

Types of cleavage1. Holoblastic cleavage.Type Radial Bilateral Rotational Spiral

Characteristic

Radial cleavage in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.

The first cleavage results in bisection of the zygote into left and right halves. The following cleavage planes are centered on this axis and result in the two halves being mirror images of one another

One of the daughter cells divides meridionally, whilst the other divides equatorially.

The cleavage plane are oriented obliquely to the polar axis of oocyte.

Egg cell Mesolecithal

Isolecithal Isolecithal Isolecithal

Example Amphibian, Sea urchin

Ascidians Mammals Molluscs, Annelids

Types of cleavage2. Mesoblastic cleavage

Discoidal cleavage Type Superficial cleavage

The cleavage furrows is limited to a disc of cytoplasm at animal pole.

Characteristic

Cytokinesis is limited to a surface layer of clear cytoplasm.

Large amount of yolk-rich cytoplasm at the vegetal pole remain uncleaved.

Site that remain uncleaved

Yolk-rich inner cytoplasm remains uncleaved.

Birds, Reptile, fish Occurs in Insects, antropods which have centrolecithal egg cells.

Telolecithal Egg cell Centrolecithal

Spiral

Stage 1The development in mammals occurs inside

the body of the female. The eggs are small with very little or

practically no yolk. So they are called microlecithal and

isolecithal.

Stage 2Segmentation starts at about 14 - 15 hours

after fertilisation as the egg moves down through the fallopian tube.

Mammalian zygote undergoes holoblastic and equal cleavage.

This means that the egg is completely divided.

Stage 3Cleavage division in humans, result in the

complete division of the zygote and the subsequent cells, the blastomers.

This type of cleavage is known as holoblastic cleavage.

Stage 4The first cleavage furrow is vertical passing

through the imaginary axis that runs through the animal and vegetal pole.

The 2 daughter cells which are thus formed are called as blastomeres.

The second cleavage is also vertical but at right angles to the first one forming 4 blastomeres of equal size. Subsequent divisions occur one after the other in an orderly fashion.

The divisions are rapid and the blastomeres become progressively smaller.

During cleavage, the number of cells in the embryo increases but the size of the embryo remains unchanged.

GastrulationTissue formation-7-11

daysBlastocyst immediately

prior to gastrulation.Propose of gastrulationi. Create anterior-posterior

orientation of embryo and archanteron (gut embryo)

ii.Creates three germ layer; ectoderm, mesoderm and endoderm.

gastrulationGastrulation occur at

the primitive streakThe inner cell mass

delaminates hypoblast cells that line the blastocoel, forming the extraembryonic endoderm of the primitive yolk sac and a two-layered (epiblast and hypoblast)

gastrulationthe epiblast splits into

the amnionic ectoderm (which encircles the amnionic cavity) and the embryonic epiblast.

Cells of ectoderm invaginate and involute at primitive streak.

Epiblast cells converge at the midline and ingress at primitive streak to produce mesoderm.

gastrulationIngression of these

cells result in formation of mesoderm and replacement of some of hypoblast cells to produce endoderm.

gastrulationDuring gastrulation,

extraembryonic mesoderm forms within the hypoblast or embryonic mesoderm and migrates out to form the blood vessels of the chorion and connect the chorion to the embryo through the umbilical cord.

• A part of organogenesis in vertebrate embryos.

• Results in the formation of the neural tube, which is the precursor of the brain and spinal cord.

• In human, neurulation occurs through two distinct phases: Initially, primary neurulation takes place followed by secondary neurulation.

PRIMARY NEURULATION

The cells of primitive neural plate form a cup and then fuse to form a tube.

Involves:1. Formation of neural plate2. Shaping of the neural plate3. Bending of the neural plate4. fusion of the neural fold

STAGES OF PRIMARY NEURULATION

PICTURES OF PRIMARY NEURULATION

Neural plate stage

Formation of neural folds

Lateral bending with convergence of the neural

folds

Neural folds fusion with dysjunction of the overlying

ectoderm.

Primary neurulation forms brain, and spinal cord as far caudal as the second sacral level (S2).

Neural plate shaping is due to both directed cell division as well as changes in the shape of neural plate cells (converting neural plate cells from cuboidal to columnar through the action of microtubules).

Neural plate bending involves four separate actions: 1.Formation of a midline furrow, the neural groove (or

median hingepoint) directly overlying the notochord (and induced by the notochord)

2.Elevation of the laterally paired neural folds surrounding the neural groove

3.Formation of bilaterally paired lateral hingepoints and convergence of the neural folds toward the midline

4.Fusion of the neural folds to one another, and separation of the cutaneous from neuroectoderm (dysjunction)

SECONDARY NEURULATION

Begins after completion of primary neurulation.

Cells form a cord structure and migrate to make the tube hollow.

Involves the making of a medullary cord & its subsequent hollowing into a neural tube.

Occurs in the presence of an intact cutaneous ectoderm overlying the caudal cell mass.

Creates the lowest portion of the spinal cord including most of the sacral and all the coccygeal regions.

During this process, the neural tube is produced by the tail bud, a mass of stem cells representing the remnant of the primitive streak that are located at the caudal end of the embryo.

These stem cells undergo proliferation and condensation followed by cavitation and fusion with the central canal of the neural tube formed by primary neurulation.

COMPARISON BETWEEN PRIMARY AND SECONDARY NEURULATION

ORGANOGENESIS

•Process by which the ectoderm, endoderm and mesoderm develop into the internal organs of the organism.

•The germ layers in organogenesis differ by three processes: folds, splits, and condensation.

•Vertebrates develop a neural crest that differentiates into many structures, including some bones, muscles, and components of the peripheral nervous system.

PRODUCTS FORMED BY THE THREE GERM LAYERS

SUMMARY: