Folding of embryo

Folding of embryoDefinition: Conversion of the flat trilaminar disc into a cylindrical embryo Time: End of 3rd week and completed by the fourth week Types:1.

Cephalocaudal folding: due to rapid longtudinal growth of C.N.S. Lateral folding: Produced by rapidly growing somites.

2.

Folding of the embryo

Folding of the embryo is due to rapid growth of the embryo specially the nervous system.

The head folds first then the tail . At the same time, side to side folding

The amniotic cavity enlarged. The Yolk sac smaller & divided into (intraembryonic Y.S, Yolk stalk& extra embryonic Y.S). Allantois& connecting stalk shifted caudally. S.T Shifted anterior to

The amniotic cavity more enlarged. Allantois& connecting stalk shifted ventrally and form the umbilical cord which contains the extra embryonic Y.S and stalk. S.T Shifted caudal to Cardiogenic plate. pericardial coelom lies ventral to the

Lateral foldingThe ventrolateral body wall forms fold towards the median plane to form cylindrical embryo. As the abdominal walls form, part of the endoderm is incorporated in the embryo as the midgut. The initial wide connection between the midgut and yolk sac is reduced to the yolk stalk. The umbilical cord is formed from the connecting stalk. The amniotic cavity expands and obliterated most of the extraembryonic coelom and forms the epithelial covering of the umbilical cord.

1- Embryo change into cylinderical embryo. 2-Transposition between septum transversum and cardiogenic plate( S.T lies cranial then ventral and lastly caudal). 3- Yolk sac reduced in size &divided into: a- intraembryonic ( gut). b- extraembryonic (atrophies). c- yolk stalk (degenerates).

Results of folding

Results of folding

4. Allantois& connecting stalk become dorsal then caudal then ventral.

5- formation of umbilical cord. 6- The oral membrane was cranially ventral. 7- The cloacal membrane was

THANK YOU