Extraembryonic structures in reptiles and birds

1. Developmental Biology
Topic: Extraembryonic Structures in Reptiles and
Birds
Presented by: Bilawal Ghafoor

2. Blastodisc
 In reptiles and birds, like fishes, yolk is eventually enclosed in yolk sac but the way
it is done differs from fishes. The periphery of blastodisc is differenciated as area
opaca and external edge of this spreads over the surface of yolk and eventually
covers it.

4. AreaPellucida
Ectoderm
Mesoderm
Endoderm

5. Area Vasculosa and Area Vitellina
 Inner part of the area opaca
 Outer part of area opaca

6. Blood islands
 The development of blood vessels in
area vasculosa is intimately
connected with first blood cells.
 Group of densely packed
mesodermal cells called blood
islands

8. Differentiation of Blood Island Cells
 Blood island cells differentiate into two kinds:
 First; cells of the periphery join to form epithelial layer, the endothelial of future
blood vessels
 The central cells; become separated from one another and become blood
corpuscles
Thus from beginning the blood corpuscles lie inside the blood vessels.

9. Networking of vessels
 Meanwhile, the adjacent blood
vessels are transformed into very
irregular network.
 They are joined together at periphery
by the terminal sinus
 The vessels penetrate into area
pellucida and establishes connection
with embryo proper

10. Connection with blood system of embryo
At two points
Venous system
Arterial system

11. Venous system
Right & left vitelline veins
Ductus
Venosus
Sinus Venosus

12. Arterial System
Dorsal Aorta
Right vitelline artery Left vitelline artery

13. Middle of second day
 Heart of embryo starts to beat
 Between 38 to 40 hours
vascularization of yolk sac
 Vessels increase food absorption

14. Body folds
 Although Body folds eventually
surround the embryo.
 Head fold
 Tail fold

16. Umbilical cord
 The body folds gradually contract
underneath the embryo and
eventually a narrow stalk, the
umbilical cord, connects the embryo
with yolk sac and other
extraembryonic parts.

17. Continues…
The Cord includes
The outer
ectodermal lining
The double sheath
of mesoderm
Endodermal canal
connecting the
cavity of gut with
cavity of yolk sac

18. EXTRAEMBRYONIC MEMBRANES
 Beside the yolk sac the, the reptiles and birds develop three other extraembryonic
organs, the embryonic membranes.
amnion chorion allantois

19. The amnion and Chorion
 The amnion and chorion are developed together as upwardly projecting folds, the
amniotic fold, appearing on the area pellucida just outside the body folds and
eventually closing over the dorsal surface of the embryo
 The rudiment of amnion and chorion first appear as transverse fold anterior to the
head of the embryo, the fold bends backward over the anterior end of the head
and cover it, as with a hood.

20. Continues..
 The lateral ends of the fold prolong backward along both sides of the embryo.
They approximate one another over the body of the embryo and fuse from front
end backward, so that the more and more embryo becomes covered by the folds.
 Eventually a fold also develops behind the embryo, and free edges of the fold fuse
thus completely enclosing the body of the embryo in a cavity– the amniotic cavity.

21. Continues..
The amniotic cavity is at first the narrow slit between the embryo and inner wall of
amniotic fold.
But soon after a fluid is released into the cavity, which distends it so that the embryo
floats freely in cavity, connected to the extraembryonic parts through umbilical cord.

22. Amniotic fold
Some embryologist prefer to call it serosa instead of chorion
Two surfaces
Inner:
The
Amnion
Outer:
The
Chorion

24. Advantages to embryo
 In the presence of amnion, amniotic fluid and the amniotic cavity
 Embryo is immersed in a container filled with fluid although “on dry land”
 So the primary function of amnion is to prevent the embryo from dessication

25. Amnion
Prevent
from
Dssicatio
n
Act as
Shock
absorber
Isolate
embryo
from shell

26. Allantois
 An outgrowth of endodermal hind gut
 Act as urinary bladder
 Waste disposal is in the form of uric acid crystals
 By the time of incubation till hatching it grows larger and larger
 By the half time of the incubation allantois covers all surface underneath of choroin

27. Continues…
 Allantois supply embryo with oxygen
 the allantois fuses with chorion to form “chorioallantoic” membrane
 Blood vessels within it are responsible for gas exchange.