Folding of the EmbryoFolding of the Embryo

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

Folding Of EmbryoFolding Of Embryo Flat trilaminar disc folds into a somewhat Flat trilaminar disc folds into a somewhat

cylindrical embryo.cylindrical embryo. Results from rapid growth of the embryo.Results from rapid growth of the embryo. Long axis increases rapidly than the sides.Long axis increases rapidly than the sides. Occurs simultaneously on both axis.Occurs simultaneously on both axis. Constriction at the junction of embryo & Constriction at the junction of embryo &

yolk sac.yolk sac.

Folding Of EmbryoFolding 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 3rd week. It is completed by the 4th week.

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

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

Results of foldingResults of 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 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 foldingResults of folding1- Embryo1- Embryo change into change into

cylinderical embryo.cylinderical embryo.2-Transposition between 2-Transposition between

septum transversum and septum transversum and cardiogenic plate( S.T lies cardiogenic plate( S.T lies cranial then ventral and lastly cranial then ventral and lastly caudal).caudal).

3- Yolk sac reduced in size &divided into:3- Yolk sac reduced in size &divided into: a- intraembryonic ( gut).a- intraembryonic ( gut). b- extraembryonic ( atrophies).b- extraembryonic ( atrophies). c- yolk stalk (degenerates).c- yolk stalk (degenerates).4- Allantois& connecting stalk become dorsal then caudal then 4- Allantois& connecting stalk become dorsal then caudal then ventral.ventral.

Results of foldingResults of folding5- formation of 5- formation of

umbilical cord.umbilical cord.

6- The oral 6- The oral membrane was membrane was craniallycranially ventral.ventral.

7- The cloacal 7- The cloacal membrane and membrane and allantois was allantois was caudalcaudal ventral. ventral.

After Tail FoldAfter Tail Fold

The connecting stalk (primordium of The connecting stalk (primordium of umbilical cord) is attached to the ventral umbilical cord) is attached to the ventral surface of the embryo.surface of the embryo.

Allantois (a diverticulum of yolk sac) is Allantois (a diverticulum of yolk sac) is partially incorporated into the embryo as a partially incorporated into the embryo as a part of hindgut.part of hindgut.

Thank YouThank You

Prof. Dr. wafaa abdel rahmanProf. Dr. wafaa abdel rahman