2nd WEEK OF DEVELOPMENT Formation of Bilaminar Disc Dr Rania Gabr

OBJECTIVES By the end of this lecture the student should be able to: Describe the formation of the Bilaminar disc Describe the formation of the yolk sac and amniotic cavity Explanation of formation of Extraembryonic mesoderm and its types

End of the First week of Development 11/19/2018 BLASTOCYST Inner cell mass or embryoblast- EMBRYO Outer cell mass or trophoblast

Inner cell mass develops into bilaminar embryonic disc Hypoblast – Yolk sac forms here Epiblast – amniotic cavity forms here Yolk sac – formed by an extension of hypoblast Digestive tube forms from yolk sac Tissues around yolk sac Gives rise to earliest blood cells and blood vessels Amniotic sac – formed by an extension of epiblast Outer membrane – forms the amnion Inner membrane – forms the amniotic sac cavity Filled with amniotic fluid

2- (bilaminar germ disc) The inner cell mass is differentiated into: a- hypoblast = endoderm (small cuboidal cells facing the blastocyst cavity and b- epiblast= ectoderm (tall columnar cells facing the amniotic cavity).

Trophoblast  Bilaminar Embryonic Disc

Formation of Amniotic Cavity As implantation of the blastocyst progresses, changes appear in the inner cell mass (embryoblast) A cavity, amniotic cavity appears separating embryoblast from the trophoblast, which soon becomes lined by amnioblasts derived from inner cell mass The cavity gradually increases in size and is filled with amniotic fluid

Formation of Primitive Yolk Sac The blastocyst cavity becomes lined with exocelomic membrane and is called exocelomic cavity The hypoblastic cells soon replace the exocelomic membrane and the cavity is then named as the primitive (primary) yolk sac

3- FORMATION OF THE EXTRA-EMBRYONIC MESODERM AND EXTRAEMBRYONIC COELOM - Cells appears between the inner surface of the cytotrophoblasts and the outer surface of the yolk sac cavity a. These cells are derived from the yolk sac cells and form the extra-embryonic (primary) mesoderm. b. They fill the space between the trophoblasts, the amnion and yolk sac cavities.

Large spaces develop in the extraembryonic mesoderm and coalesce to form extraembryonic coelom. These spaces rapidly fuse to form a large fluid filled, C-shaped cavity, the extraembyonic celome surrounding the amniotic cavity and the yolk sac

Formation of Connecting Stalk The region where no cavity has appeared, forms the connecting stalk, that connects the amniotic cavity, yolk sac and the embryonic disc to the outer wall The site of the connecting stalk determines the caudal pole of the embryonic disc

With the formation of extraembryonic ceolum: The extraembryonic mesoderm splits into two layers: an outer extraembryonic parietal (somatic) mesoderm an inner extraembryonic visceral (splanchnic) mesoderm

Clinical application: The syncytiotrophoblast secretes (human chorionic gonadotrophin hormone) HCG. It stimulates the production of progesterone which in turn is important in sustaining the placenta. By the end of the 2nd week the amount of this hormone HCG will be sufficient to be detected in the maternal blood and urine. This is the basis of pregnancy test.

EXTRAEMBRYONIC MESODERM 11/19/2018 Cells are derived from epiblast and hypoblast 2 layers: Somatopleuri c mesoderm- lining the amnion Splanchnople uric mesoderm- lining the yolk sac Free template from www.brainybetty.com

The primary yolk sac decreases in size and becomes the secondary (definitive) yolk sac Wall of the yolk sac, amnion & chorion are formed

The second week of development is the week of two`s, because of the following: The trophoblast differentiates into 2 layers, cytotrophoblast & syncytiotrophoblast The inner cell mass differentiates into 2 layers, epiblast & hypoblast. The primary mesoderm splits into somatopleuric primary mesoderm & splanchnopleuric primary mesoderm. Starting of formation of the amniotic and yolk sac cavities.

11/19/2018

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