Principles of Development Chapter 8. Early Concepts: Preformation vs Epigenesis The question of how...

Principles of Development

Chapter 8

Early Concepts: Preformation vs Epigenesis

The question of how a zygote becomes an animal has been asked for centuries.

As recently as the 18th century, the prevailing theory was a notion called preformation – the idea that the egg or sperm contains an embryo.A preformed miniature infant, or

“homunculus,” that simply becomes larger during development.

Early Concepts: Preformation vs Epigenesis

Kaspar Friederich Wolff (1759) demonstrated there was no preformed chick in the early egg.Undifferentiated granular material became

arranged into layers.The layers thickened, thinned, and folded to

produce the embryo.

Early Concepts: Preformation vs Epigenesis

Epigenesis is the concept that the fertilized egg contains building materials only, somehow assembled by an unknown directing force.

Although current ideas of development are essentially epigenetic in concept, far more is known about what directs growth and differentiation.

Key Events in Development

Development describes the changes in an organism from its earliest beginnings through maturity.Search for

commonalities.

Key Events in Development

Specialization of cell types occurs as a hierarchy of developmental decisions.Cell types arise from conditions created in

preceding stages.Interactions become increasingly restrictive.

With each new stage:Each stage limits developmental fate.Cells lose option to become something

differentSaid to be determined.

Key Events in Development

The two basic processes responsible for this progressive subdivision:Cytoplasmic localization Induction

FertilizationFertilizationFertilization is the initial event in

development in sexual reproduction.Union of male and female gametesProvides for recombination of paternal and

maternal genes.Restores the diploid number.

Activates the egg to begin development.

FertilizationFertilizationOocyte Maturation

Egg grows in size by accumulating yolk.Contains much mRNA, ribosomes, tRNA and

elements for protein synthesis.Morphogenetic determinants direct the

activation and repression of specific genes later in post-fertilization development.

Egg nucleus grows in size, bloated with RNA.Now called the germinal vesicle.

FertilizationMost of these preparations in the egg occur

during the prolonged prophase I.In mammals

Oocyte now has a highly structured system.After fertilization it will support nutritional

requirements of the embryo and direct its development through cleavage.

After meiosis resumes, the egg is ready to fuse its nucleus with the sperm nucleus.

FertilizationA century of

research has been conducted on marine invertebrates.Especially sea

urchins

Contact Between Sperm & Egg

Broadcast spawners often release a chemotactic factor that attracts sperm to eggs. Species specific

Sperm enter the jelly layer.

Egg-recognition proteins on the acrosomal process bind to species-specific sperm receptors on the vitelline envelope.

Fertilization in Sea UrchinsPrevention of

polyspermy – only one sperm can enter.Fast block

Depolarization of membrane

Slow blockCortical reaction

resulting in fertilization membrane

Fertilization in Sea UrchinsThe cortical reaction follows the fusion

of thousands of enzyme-rich cortical granules with the egg membrane. Cortical granules release contents between

the membrane and vitelline envelope.Creates an osmotic gradient

Water rushes into space Elevates the envelope Lifts away all bound sperm except the one

sperm that has successfully fused with the egg plasma membrane.

Fertilization in Sea Urchins

Fertilization in Sea UrchinsOne cortical granule

enzyme causes the vitelline envelope to harden.Now called the

fertilization membrane.

Block to polyspermy is now complete.

Similar process occurs in mammals.

Fertilization in Sea UrchinsThe increased

Ca2+ concentration in the egg after the cortical reaction results in an increase in the rates of cellular respiration and protein synthesis.The egg is

activated.

Fusion of PronucleiAfter sperm and egg membranes fuse, the

sperm loses its flagellum.

Enlarged sperm nucleus is the male pronucleus and migrates inward to contact the female pronucleus.

Fusion of male and female pronuclei forms a diploid zygote nucleus.

Cleavage

Cleavage – rapid cell divisions following fertilization.Very little growth

occurs.Each cell called a

blastomere.Morula – solid

ball of cells. First 5-7 divisions.

PolarityThe eggs and zygotes of many animals (not

mammals) have a definite polarity.

The polarity is defined by the distribution of yolk.The vegetal pole has the most yolk and the

animal pole has the least.

Body Axes

The development of body axes in frogs is influenced by the polarity of the egg.

At fertilization, the pigmented cortex slides over the underlying cytoplasm toward the point of sperm entry. This rotation (red arrow) exposes a region of lighter-colored cytoplasm, the gray crescent, which is a marker of the dorsal side.

The polarity of the egg determines the anterior-posterior axis before fertilization.

The first cleavage division bisects the gray crescent. Once the anterior-posterior and dorsal-ventral axes are defined, so is the left-right axis.

Amount of YolkDifferent types of

animals have different amounts of yolk in their eggs.Isolecithal – very little

yolk, even distribution.Mesolecithal –

moderate amount of yolk concentrated at vegetal pole.

Telolecithal – Lots of yolk at vegetal pole.

Centrolecithal – lots of yolk, centrally located.

Cleavage in FrogsCleavage planes usually

follow a specific pattern that is relative to the animal and vegetal poles of the zygote. Animal pole blastomeres

are smaller. Blastocoel in animal

hemisphere. Little yolk, cleavage

furrows complete. Holoblastic cleavage

Cleavage in BirdsMeroblastic

cleavage, incomplete division of the egg.Occurs in species

with yolk-rich eggs, such as reptiles and birds.

Blastoderm – cap of cells on top of yolk.

Direct vs. Indirect Development

When lots of nourishing yolk is present, embryos develop into a miniature adult.Direct development

When little yolk is present, young develop into larval stages that can feed.Indirect development

Mammals have little yolk, but nourish the embryo via the placenta.

BlastulaA fluid filled cavity, the blastocoel,

forms within the embryo – a hollow ball of cells now called a blastula.

GastrulationThe morphogenetic

process called gastrulation rearranges the cells of a blastula into a three-layered (triploblastic) embryo, called a gastrula, that has a primitive gut.Diploblastic

organisms have two germ layers.

GastrulationThe three tissue layers produced by

gastrulation are called embryonic germ layers.The ectoderm forms the outer layer of the

gastrula.Outer surfaces, neural tissue

The endoderm lines the embryonic digestive tract.

The mesoderm partly fills the space between the endoderm and ectoderm.Muscles, reproductive system

Gastrulation – Sea UrchinGastrulation in a sea urchin produces an

embryo with a primitive gut (archenteron) and three germ layers.

Blastopore – open end of gut, becomes anus in deuterostomes.

Gastrulation - FrogResult – embryo with gut & 3 germ

layers.

More complicated:Yolk laden cells in vegetal hemisphere.Blastula wall more than one cell thick.

Gastrulation - ChickGastrulation in the chick is affected by the

large amounts of yolk in the egg.

Primitive streak – a groove on the surface along the future anterior-posterior axis.Functionally equivalent to blastopore lip in

frog.

Gastrulation - ChickBlastoderm consists

of two layers:Epiblast and

hypoblast Layers separated

by a blastocoel Epiblast forms

endoderm and mesoderm.

Cells on surface of embryo form ectoderm.

Gastrulation - Mouse

In mammals the blastula is called a blastocyst.Inner cell mass will become the embryo while

trophoblast becomes part of the placenta.

Notice that the gastrula is similar to that of the chick.

Suites of Developmental Characters

Two major groups of triploblastic animals:ProtostomesDeuterostomes

Differentiated by:Spiral vs. radial cleavageRegulative vs. mosaic cleavageBlastopore becomes mouth vs. anusSchizocoelous vs. enterocoelous coelom

formation.

Deuterostome Development

Deuterostomes include echinoderms (sea urchins, sea stars etc) and chordates.Radial cleavage

Deuterostome Development

Regulative development – the fate of a cell depends on its interactions with neighbors, not what piece of cytoplasm it has. A blastomere isolated early in cleavage is able to from a whole individual.

Deuterostome Development

Deuterostome means second mouth.

The blastopore becomes the anus and the mouth develops as the second opening.

Deuterostome Development

The coelom is a body cavity completely surrounded by mesoderm.Mesoderm & coelom form

simultaneously.

In enterocoely, the coelom forms as outpocketing of the gut.

Deuterostome Development

Typical deuterostomes have coeloms that develop by enterocoely.Vertebrates use a modified version of

schizocoely.

Protostome DevelopmentProtostomes include flatworms,

annelids and molluscs.Spiral cleavage

Protostome DevelopmentMosaic

development – cell fate is determined by the components of the cytoplasm found in each blastomere.Morphogenetic

determinants.An isolated

blastomere can’t develop.

Protostome DevelopmentProtostome means first mouth.

Blastopore becomes the mouth.

The second opening will become the anus.

Protostome DevelopmentIn protostomes, a mesodermal band of tissue

forms before the coelom is formed.

The mesoderm splits to form a coelom.Schizocoely

Not all protostomes have a true coelom.Pseudocoelomates have a body cavity between

mesoderm and endoderm.Acoelomates have no body cavity at all other

than the gut.

Two Clades of Protostomes

Lophotrochozoan protostomes include annelid worms, molluscs, & some small phyla.Lophophore – horseshoe shaped feeding

structure.Trochophore larva

Feature all four protostome characteristics.

Two Clades of Protostomes

The ecdysozoan protostomes include arthropods, roundworms, and other taxa that molt their exoskeletons.Ecdysis – shedding of the cuticle.Many do not show spiral cleavage.

Building a Body PlanAn organism’s development is

determined by the genome of the zygote and also by differences that arise between early embryonic cells.Different genes will be expressed in

different cells.

Building a Body PlanUneven distribution of

substances in the egg called cytoplasmic determinants results in some of these differences.

Position of cells in the early embryo result in differences as well.Induction

Restriction of Cellular Potency

In many species that have cytoplasmic determinants only the zygote is totipotent, capable of developing into all the cell types found in the adult.

Restriction of Cellular Potency

Unevenly distributed cytoplasmic determinants in the egg cell:Are important in establishing the body axes. Set up differences in blastomeres resulting

from cleavage.

Restriction of Cellular Potency

As embryonic development proceeds, the potency of cells becomes progressively more limited in all species.

Cell Fate Determination and Pattern Formation by Inductive

SignalsOnce embryonic cell division creates cells

that differ from each other,The cells begin to influence each

other’s fates by induction.

InductionInduction is the

capacity of some cells to cause other cells to develop in a certain way.

Dorsal lip of the blastopore induces neural development.Primary

organizer

Spemann-Mangold Experiment

Transplanting a piece of dorsal blastopore lip from a salamander gastrula to a ventral or lateral position in another gastrula developed into a notochord & somites and it induced the host ectoderm to form a neural tube.

Building a Body PlanCell differentiation – the specialization

of cells in their structure and function.

Morphogenesis – the process by which an animal takes shape and differentiated cells end up in their appropriate locations.

Building a Body PlanThe sequence includes

Cell movementChanges in adhesionCell proliferation

There is no “hard-wired” master control panel directing development.Sequence of local patterns in which one step

in development is a subunit of another.Each step in the developmental hierarchy is a

necessary preliminary for the next.

Hox GenesHox genes control

the subdivision of embryos into regions of different developmental fates along the anteroposterior axis.Homologous in

diverse organisms.

These are master genes that control expression of subordinate genes.

Formation of the Vertebrate Limb

Inductive signals play a major role in pattern formation – the development of an animal’s spatial organization.

Formation of the Vertebrate Limb

The molecular cues that control pattern formation, called positional information: Tell a cell where it is with respect to the

animal’s body axes.Determine how the cell and its descendents

respond to future molecular signals.

Formation of the Vertebrate Limb

The wings and legs of chicks, like all vertebrate limbs begin as bumps of tissue called limb buds.

The embryonic cells within a limb bud respond to positional information indicating location along three axes.

Formation of the Vertebrate Limb

One limb-bud organizer region is the apical ectodermal ridge (AER).A thickened area of ectoderm at the tip of the

bud.

The second major limb-bud organizer region is the zone of polarizing activity (ZPA).A block of mesodermal tissue located

underneath the ectoderm where the posterior side of the bud is attached to the body.

MorphogenesisMorphogenesis is a major aspect of

development in both plants and animals but only in animals does it involve the movement of cells.

The Cytoskeleton, Cell Motility, and Convergent Extension

Changes in the shape of a cell usually involve reorganization of the cytoskeleton.

Changes in Cell Shape

The formation of the neural tube is affected by microtubules and microfilaments.

Cell MigrationThe cytoskeleton also drives cell

migration, or cell crawling.The active movement of cells from one place

to another.

In gastrulation, tissue invagination is caused by changes in both cell shape and cell migration.

Evo-DevoEvolutionary developmental biology

- evolution is a process in which organisms become different as a result of changes in the genetic control of development.Genes that control development are similar in

diverse groups of animals.Hox genes

Evo-DevoInstead of evolution proceeding by the

gradual accumulation of numerous small mutations, could it proceed by relatively few mutations in a few developmental genes?The induction of legs or eyes by a mutation in

one gene suggests that these and other organs can develop as modules.

The Common Vertebrate Heritage

Vertebrates share a common ancestry and a common pattern of early development.Vertebrate

hallmarks all present briefly.Dorsal neural

tubeNotochordPharyngeal gill

pouchesPostanal tail

AmniotesThe embryos of birds, reptiles, and

mammals develop within a fluid-filled sac that is contained within a shell or the uterus.Organisms with these adaptations form a

monophyletic group called amniotes.Allows for embryo to develop away from

water.

AmniotesIn these three types of organisms, the

three germ layers also give rise to the four extraembryonic membranes that surround the developing embryo.

AmniotesAmnion – fluid

filled membranous sac that encloses the embryo. Protects embryo from shock.

Yolk sac – stores yolk and pre-dates the amniotes by millions of years.

AmniotesAllantois - storage of metabolic wastes

during development.

Chorion - lies beneath the eggshell and encloses the embryo and other extraembryonic membrane.As embryo grows, the need for oxygen increases.

Allantois and chorion fuse to form a respiratory surface, the chorioallantoic membrane.

Evolution of the shelled amniotic egg made internal fertilization a requirement.

The Mammalian Placenta and Early Mammalian Development Most mammalian embryos do not

develop within an egg shell.Develop within the mother’s body. Most retained in the mother’s body.

Monotremes Primitive mammals that lay eggs.Large yolky eggs resembling bird eggs.Duck-billed platypus and spiny anteater.

The Mammalian Placenta and Early Mammalian DevelopmentMarsupials

Embryos born at an early stage of development and continue development in abdominal pouch of mother.

Placental Mammals Represent 94% of the class Mammalia.Evolution of the placenta required:

Reconstruction of extraembryonic membranes.

Modification of oviduct - expanded region formed a uterus.

Mammalian DevelopmentThe eggs of placental mammals:

Are small and store few nutrients.Exhibit holoblastic cleavage.Show no obvious polarity.

Mammalian DevelopmentGastrulation and organogenesis resemble

the processes in birds and other reptiles.

Mammalian Development

Early embryonic development in a human proceeds through four stages:Blastocyst reaches

uterus.Blastocyst implants.Extraembryonic

membranes start to form and gastrulation begins.

Gastrulation has produced a 3-layered embryo.

Mammalian DevelopmentThe extraembryonic membranes in

mammals are homologous to those of birds and other reptiles and have similar functions.

Mammalian DevelopmentAmnion Surrounds embryo Secretes fluid in

which embryo floatsYolk sac Contains no yolk Source of stem cells that give rise to blood and lymphoid cellsStem cells migrate to into the developing embryo Allantois Not needed to store wastesContributes to the formation of the umbilical cordChorion Forms most of the placenta

OrganogenesisVarious regions of the three embryonic

germ layers develop into the rudiments of organs during the process of organogenesis.

OrganogenesisMany

different structures are derived from the three embryonic germ layers during organogenesis.

Derivatives of Ectoderm: Nervous System and Nerve Growth

Just above the notochord (mesoderm), the ectoderm thickens to form a neural plate.Edges of the neural

plate fold up to create an elongated, hollow neural tube.Anterior end of neural

tube enlarges to form the brain and cranial nerves.

Posterior end forms the spinal cord and spinal motor nerves.

Derivatives of Ectoderm: Nervous System and Nerve Growth

Neural crest cells pinch off from the neural tube.Give rise to

Portions of cranial nervesPigment cellsCartilageBoneGanglia of the autonomic systemMedulla of the adrenal glandParts of other endocrine glands

Neural crest cells are unique to vertebrates.Important in evolution of the vertebrate head

and jaws.

Derivatives of Endoderm: Digestive Tube and Survival of

Gill ArchesDuring gastrulation,

the archenteron forms as the primitive gut.

This endodermal cavity eventually produces:Digestive tractLining of pharynx and

lungsMost of the liver and

pancreasThyroid, parathyroid

glands and thymus

Derivatives of Endoderm: Digestive Tube and Survival of

Gill ArchesPharyngeal pouches are derivatives of

the digestive tract.Arise in early embryonic development of all

vertebrates.During development, endodermally-lined

pharyngeal pouches interact with overlying ectoderm to form gill arches.

In fish, gill arches develop into gills.In terrestrial vertebrates:

No respiratory function1st arch and endoderm-lined pouch form

upper and lower jaws, and inner ear.2nd, 3rd, and 4th gill pouches form tonsils,

parathyroid gland and thymus.

Derivatives of Mesoderm: Support, Movement and the

Beating HeartMost muscles

arise from mesoderm along each side of the neural tube.

The mesoderm divides into a linear series of somites (38 in humans).

Derivatives of Mesoderm: Support, Movement and the

Beating HeartThe splitting, fusion and

migration of somites produce the:Axial skeletonDermis of dorsal skinMuscles of the back, body wall,

and limbsHeart

Lateral to the somites the mesoderm splits to form the coelom.