Folding of the Embryo Figures or photographs used

Folding of the Embryo Figures or photographs used in this presentation are originally reproduced from Langman’s Medical Embryology by T. W. Sadler, 10 th Edition, Lippincott Williams & Wilkins, for teaching purpose only.

Folding Of Embryo Ø Flat trilaminar disc folds into a somewhat cylindrical embryo. Ø Results from rapid growth of the embryo. Ø Long axis increases rapidly than the sides. Ø Occurs simultaneously on both axis. Ø Constriction at the junction of embryo & yolk sac.

Folding Of Embryo - Folding means conversion of the flat trilaminar embryonic disc into a cylindrical embryo (Fig. 46). Time: Folding of the embryo begins by the end of the 3 rd week. It is completed by the 4 th week.

Ø Folding of the embryo is due to rapid growth of the embryo specially the nervous system and ectoderm. Ø The head folds first then the tail. At the same time, side to side folding occurs.

• 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 Cardiogenic plate. • 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. * Placenta will face the umblical cord.

Results of folding 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). 4 - Allantois& connecting stalk become dorsal then caudal then ventral.

Results of folding 5 - formation of umbilical cord. 6 - The oral membrane was cranially ventral. 7 - The cloacal membrane and allantois was caudal ventral.

After Tail Fold Ø The connecting stalk (primordium of umbilical cord) is attached to the ventral surface of the embryo. Ø Allantois (a diverticulum of yolk sac) is partially incorporated into the embryo as a part of hindgut.

Thank You Prof. (Dr) SHIVARAM BHAT