Lecture 6 2nd Week Development Summary PDF

‫”قالوا سبحانك ال علم لنا إال ما‬ ‫الحكيم“‬ ‫علمتنا إنك أنت العليم سورة البقرة‬ ‫آية (‪)32‬‬  By: Dr. Elsayed Aly Metwally Professor of Anatomy & Embryology Master Degree in tropical medicine - Nutrition Diploma  2nd week development Different...

‫”قالوا سبحانك ال علم لنا إال ما‬ ‫الحكيم“‬ ‫علمتنا إنك أنت العليم سورة البقرة‬ ‫آية (‪)32‬‬  By: Dr. Elsayed Aly Metwally Professor of Anatomy & Embryology Master Degree in tropical medicine - Nutrition Diploma  2nd week development Differentiation of trophoblast Formation of bilaminar germ disc Formation of 2 cavities Formation of extra-embryonic mesoderm & coelom  The trophoblast differentiates into: 1- Cytotrophoblast: mononucleated inner layer 2- Syncytiotrophoblast: multinucleated outer cell layer without distinct cell boundaries.  Functions of the syncytiotrophoblast: 1- It forms finger like processes called villi. 2- It erodes the maternal tissues, so lacunae filled with maternal blood surround columns of syncytiotrophoblast. 3- the lacunae coalesce to form intervillous spaces. The inner cell mass is differentiated into: a- Epiblast (ectodermal) layer: tall columnar cells facing the amniotic cavity b- Hypoblast (endodermal) layer: small cuboidal cells facing the blastocyst cavity. Both layers will form the bilaminar germ disc. Formation of amniotic sac. Vacuoles develop within the epiblast and coalesce to form the amniotic cavity. The roof is formed by flattened cells called amnioblast. The floor is formed by the epiblast layer (ectoderm) Formation of yolk sac. Another cavity is formed ventrally called 1ry yolk sac (exocelomic cavity). The roof is formed by hypoblastic layer (endoderm) The floor is formed by exocelomic membrane (derived from endodermal layer)  The syncytiotrophoblast secretes human chorionic gonadotrophin(HCG) hormone which prevents the degeneration of the corpus luteum. It also 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 will be sufficient to be detected in the maternal blood and urine, this is the basis of pregnancy test. a. The endoderm of the yolk sac gives a new type of cells called the extra-embryonic (primary) mesoderm. b. They fill the space between the trophoblasts, the amniotic and yolk sac cavities. Large cavities appear in the 1ry mesoderm, that coalesce to form extraembryonic celom (chorionic cavity). The 1ry mesoderm will persist in 3 areas: 1. Somatopleuric 1ry mesoderm: surrounds the trophoblast and amnion. 2. Splanchnopleuric 1ry mesoderm: surrounds the yolk sac. 3. Body (connecting) stalk: between germ disc and trophoblast.  The trophoblast differentiates into 2 layers; cytotrophoblast & syncytiotrophoblast The inner cell mass differentiates into 2 layers; epiblast & hypoblast. Formation of 2 cavities; amniotic and yolk sac cavities. The primary mesoderm splits into 2; somatopleuric & splanchnopleuric primary mesoderm. Practical Questions  What is the pointed structure?  Answer : - hypoblast  What is the pointed structure?  Answer : - Exocelomic membrane  What is the pointed structure?  Answer : - Extraembryonic celom Interactive Questions