Amniote egg amd human fetus have the same features
yolk sac
chorin
all antois
Amnion
(Extra embryonic membrane)
Avian Reproduction - Eggs
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
Cleavage
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
Therefore equatorial and vertical division:
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
Formation of Chick Blastoderm
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
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
•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
Migration through Primitive Streak
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
Hensen’s Node
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
Migration through Hensen’s node
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
Formation of Presumptive Head and Notochord
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 and Hensen’s node
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
Chick Gastrulation
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
The Chick “Organizer"
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
Epiboly of Ectoderm
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
Axis formation
Dorso-vental axis (back-belly): created by pH gradient (axis can be reversed by manipulating pH)
--> shifts lighter particles towards one side of blastoderm --> posterior end
--> Opposite is anterior end
Right-Left axis
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 Axis
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 Axis
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
Author
ChipzThatLeo
ID
266783
Card Set
Chapter 9: EARLY DEVELOPMENT OF VERTEBRATES: BIRDS AND MAMMALS