Bilaminar Embryonic Disc - Embryology Lecture 4 PDF

Bilaminar Embryonic Disc Anatomy Unit201 Anatomy Unit  Objectives 1. Describe the formation of the Bilaminar disc 2. Describe the formation of the yolk sac and amniotic cavity 3. Explanation of formation of Extraembryonic mesoderm and its types ...

Bilaminar Embryonic Disc Anatomy Unit201 Anatomy Unit  Objectives 1. Describe the formation of the Bilaminar disc 2. Describe the formation of the yolk sac and amniotic cavity 3. Explanation of formation of Extraembryonic mesoderm and its types  Case scenario Q. At the end of week 2 of development (day 14), what is the composition of the embryonic disk? (A) Epiblast only (B) Epiblast and hypoblast (C) Ectoderm and endoderm (D) Ectoderm, mesoderm, and endoderm (E) Epiblast, mesoderm, and hypoblast End of the First week of Development BLASTOCYST Inner cell mass or embryoblast-EMBRYO Outer cell mass or trophoblast  Changes in the shape of the bilaminar embryonic plate: The embryonic plate changes from circular to oval to pear shaped structure  Changes in the Trophoblast The trophoblast undergoes repeated mitotic division and differentiated into 2 layers: ❑ Cytotrophoblast: inner mono- nucleated layer of cells. ❑ Sycytiotrophoblast: outer multinucleated mass.  Changes in the Trophoblast Isolated cavities (lacunae) appear in The adjacent Syncytiotrophoblast the Syncytiotrophoblast which become lacunae fused to form lacunar filled with maternal blood. networks (will form the intervillous spaces of the placenta).  Changes in the Inner cell mass Inner cell mass develops into bilaminar embryonic disc ✓ Epiblast (1ry ectoderm)– upper high columnar cells ✓ Hypoblast (1ry endoderm) – small cubical cells  Yolk Sac ▪ formed by an extension of hypoblast Boundaries: - ▪ Roof: hypoblast ▪ Floor and sides: Hauser's membrane.  Amniotic Sac ▪ formed by an extension of epiblast ▪ appears on the 7th or 8th day, Boundaries: ▪ Roof and sides: amnioblast. ▪ Floor: epiblast.  Formation of the extra-embryonic mesoderm Origin: hypoblastic & Cytotrophoblast. Development: The extra embryonic mesoderm appears between the trophoblast externally and the primary yolk sac and amniotic cavity internally.  Formation of extraembryonic coelom Isolated spaces appear within the extra embryonic mesoderm. These spaces rapidly fuse to form a large cavity, which is the extra embryonic coelom.  Layers of extraembryonic mesoderm The extra-embryonic mesoderm divided into two layers: 1. Somatic mesoderm: lining the trophoblast 2. Splanchnic mesoderm: covering the primary yolk sac.  Extraembryonic mesoderm layers ▪ The two layers connected at the caudal end of the embryo and are called the connecting stalk (the future umbilical cord). ▪ The trophoblast together with somatic layer of the extra – embryonic mesoderm form the chorion  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.  Case scenario  Q. At the end of week 2 of development (day 14), what is the composition of the embryonic disk? (A) Epiblast only (B) Epiblast and hypoblast (C) Ectoderm and endoderm (D) Ectoderm, mesoderm, and endoderm (E) Epiblast, mesoderm, and hypoblast More (Prader-Willi syndrome) (Langman's Medical Embryology, p66)