Amniotes - Chick

Amniotes

Vertebrates with embryos fully enclosed in a fluid filled sac

• common in reptiles, birds, and mammals

• originated in anphibian ancestor

Parts of the Amniote Egg

8 main components

• Yolk Sac

• Embryo

• Shell 

• Shell membranes

• Air sac

• Chalazae 

• Amnion

• Allantois

Embryo

Forms from the blastodisc that sits on top of the egg

Yolk Sac

Provides nutrients to the developing embryo

Amnion

Fluid filled cushion and water source for the embryo

Allantois

Stores embryonic wastes and the location of gas exchange

Chorion

Inner shell membrane that interacts with the environment

• becomes the placenta in mammals

Telolecithal Egg

Egg with central yolky mass that does not get cleaved

Meroblastic and Discoidal Cleavage

Discoidal in that cleavage is limited to the blastodic, meroblastic in that yolk is not cleaved and forms sub germinal space

Egg Rotations

Rotations and gravity is important in eggs for even distribution of components

• 10-12 rotations per hour 

• lightest components rise and will develop into the posterior site of gastrulation

Blastoderm Formation

Meroblastic cleavage forms the subgerminal space which eventually forms the blastocoel which is bounded by the epiblast on top and the hypoblast on the bottom

Area Pellucida

Area Opaca

Gastrulation

Cells ingress into blastocoel from the epiblast along the midline

• form Hensen’s node and the primitive streak

Scatter Factor

Down regulates cadherin in order to allow for migration

• essential for the cell movements that occur during gastrulation

• converts epitheliial cells to mesenchymal cells

Gastrulation: First Wave

Migrates to the bottom of the blastocoel to displace the hypoblast

• will become endoderm 

Gastrulations: Second Wave

Aided by scatter factor disperses into blastocoel

• becomes mesoderm

Hensen’s Node

Site where gastrulation and neural tube fromation begins moving anterior to posteriorly 

• induces mesoderm differentiation 

Primitive Streak

A posterior-anterior structure that arises from the edge of the zona pellucida

• extended through anterior convergent extension anteriorly

• forms a depressiom 

• anterior end is thickened to form Hensen’s node

Chick at 24 Hrs

Differentiation will be ahead anteriorly

Posterior Signaling

blastoderm is a watery cytoplasm

• rotations shift components and create posterior marginal zone 

• defines origin of primitive streak and the A/P axis 

Neural Tube Formation

Follows that seen in frogs

• MHP and DLHP present

• the transition from neural tube to CNS occurs over 24 hrs 

BPA And Neural Tube Formation

BPA had disruptive effects on chick development 

• induced abnormal embryogenesis that were mainly head/brain & neural tub defects 

• did not significantly impact somite number

Chick at 33 hrs

• Regions of brain and heart differentiating 

• Gastrulation still occurring at Hensen’s node 

Mesoderm

Spread laterally over the yolk

• Hypomeric on outside forms body wall/gut wall 

• Mesomeric forms kidneys and gonads 

• Epimeric closest to notochord forms somites

Somites

Derived from epimeric mesoderm

• form paired structures that give rise to vertebrae, ribs, back muscle etc 

Chick at 48 hrs

• Gastrulation is complete

• Neuralation almost complete 

• Circulation largely extraembryonic 

• Blood islands form primitive blood vessels 

• Germinal crescent positions germ cells 

Chick at 4 days

Cephalic and cervical flexure

Extraembryonic membranes

Chorion, allantois, and amnion

Chick at 96 hrs

Wing buds, allantois, and amnion

Gastrulation: Epiblast

Forms ectoderm and neural ectoderm

Subgerminal Space

Space between yolk and blastodisc

Blastocoel

The space inside of the layer or epiblast and hypoblast

Hypoblast

Formed in two part

• cells that have migrated individually from area pellucida to form primary hypoblast islands

• sheets of cells derived from area opaca push the islands together