Chapter 9: EARLY DEVELOPMENT OF VERTEBRATES: BIRDS AND MAMMALS

• yolk sac
• chorin
• all antois
• Amnion
• (Extra embryonic membrane)

Favorite organism for embryological studies

Internal fertilization in Oviduct, then albumin and shell are secreted

• Eggs are Telolecithal with a small blastodisc (actual embryo)
• Cleavage: Formation of Epiblast and Hypoblast in Blastoderm

Gastrulation: Primitive stalk




Meroblastic-Incomplete. ex: Teloecithal and Centrolecithal eggs

• Discoidal meroblastic cleavage – occurs only in blastodisc
• Further divisions result in a single-layered balstoderm –   basal region is in contact with yolk
• No cleavage divisions in the yolk

• 5 to 6 layered
• Blastoderms held together by tight junctions (adhesion by E-cadherin).
• Subgerminal space: between the outer layer and the yolk. Several layers of cells

Epiblast – top single layer of blastoderm --> forms embryo

Cells delaminate and migrate individually into subgerminal cavity to form poly-invagination island (primary hypoblast)






A sheet of posterior margin of blastoderm (Koller’s sickle) migrate anteriorly, join poly-invagination islands forming secondary hypoblast --> extra-embryonic endoderm

• Primitive streak is the principle structure in avian and mammalian gastrulation – structure through which all endodermal and mesodermal cells must pass
• 

• A.3-4 hrs after fertilization
• B.7-8 hrs after fertilization

• 1. Thickening of epiblast at the posterior region (just anterior to Koller’s sickle)
•   – caused by ingression of endodermal precursors from epiblast into blastocoel and
•   -  migration of cells from the lateral region of posterior epiblast

Further migration elongates primitive streak towards future head region

• •Primitive streak defines anterio-posterior axis
• •Migrating cells enter dorsal side and move towards its ventral side
• •Separates left from right side of embryo
• •Further convergence results in depression called primitive groove

• 1. Cells involute over edge of groove and pit and ingress into blastocoel
•   2. Once in blastocoel, cells flatten and migrate
•   3. In chick, the cells migrate individually after undergoing epithelial-to-mesenchymal cell transformation
•   - Scatter factor (190 kd protein), secreted by the   cells as they enter the streak – possibly down   regulate expression of E-cadherin (adhesion   protein) à loose mesenchymal cells

• C. 15-16 hrs
• D. 16-19 hrs

• Anterior end of primitive streak thickens to form primitive knot (Hensen’s node)
• Center of the node has a funnel-shaped depression – primitive pit – functional equivalent of dorsal lip of blastopore

• 1. First batch of cells to enter Hensen’s node --> migrate anterior--> pharyngial endoderm of foregut
•   2. Second batch of cells --> migrate anterior (not too far) ventrally, remain between endoderm and epiblast --> head mesenchme/ prechordal plate mesoderm
• These early-ingressing cells form Head process anterior to Hensen’s node

• 3. Next cells migrating through Hensen’s node --> form chordamesoderm (notochord) cells --> extend up to presumptive midbrain and meet precordal plate
•   4. Cells migrate inwardly through lateral part of primitive streak --> separate into two layers
•   - deeper layer --> endoderm   
•   - upper layer --> mesoderm
• By about 22 hrs, presumptive endodermal cells are inside and mesodermal cells are migrating to their respective positions

• Regression of primitive streak pushes the Hensen’s node downwards (posterior)
• Formation of notochord:
•   - anterior portion of notochord from the ingressed cells through Hensen’s node
•   - posterior portion of notochord (downwards from somite 17) from mesodermal tissue ingressed through primitive streak

Hensen’s node regresses to its most posterior portion --> ANUS

Head start: While posterior portion is undergoing gastrulation, cells at  anterior end begin to form organs. Anterior end is more advanced than posterior end for the next several days

• Koller’s sickle cells consists of:
•   - Hensen’s node (Dark green cells)
•   – Primitive streak (Red cells)
• Thus equivalent to amphibian ‘organizer”
• Hensen’s node is equivalent to amphibian dorsal lip of blastopore

• Ectodermal precursor cells proliferate --> migrate to surround yolk by epiboly through underside of vitelline envelope
•   - cells in the outer edge of growing ectodermal cells have  long filopodia
•   -takes about 4 days to complete

• Dorso-vental axis (back-belly): created by pH gradient (axis can be reversed by manipulating pH)
•   - albumin, pH 9.5; relatively –ve membrane potential --> DORSAL SIDE
•   - submarginal space, pH 6.5; relatively  +ve membrane potential --> VENTAL SIDE

• Anterio-posterior axis: aided by gravity
•   - developing ovum spins 10-20 revolutions per hr
•   --> shifts lighter particles towards one side of blastoderm --> posterior end
•   --> Opposite is anterior end
• 

• Left side - Heart and spleen
• Right side – Liver
• Regulated by Asymmetric gene expression
• Cilia in Hansen’s node direct the flow of fluid carrying expression of specific genes
• Eg – Shh; FGF8 and TGFb family
• 

• LEFT SIDE of Hensen’s node:
• – expresses sonic hedgehog (Shh) and Lefty-1 genes

-Lefty-1  blocks FGF8 expression

-Shh activates expression of Cerebus gene --> blocks BMP --> no block of Nodal and Lefty1 – repression of Snail and activation of Pitx2 -->  activate organ primordia --> left side structures

• RIGHT SIDE of Hensen’s node:
• – expression of activin and its receptor --> activates FGF8 protein
• -FGF8 blocks Caronte (car) gene
• Normal activation BMP --> repression of Nodal and Lefty 2
• --> expression of Snail and repression of Pitx2
• --> activation of morphogenic factors of right side