Week 2 Bilaminar Germ Disk PDF

WE MAKE DOCTORS WEEK 2: BILAMINAR GERM DISK DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Describe the formation of the blastocyst and how it becomes bilaminar germ disc Describe the establishment of the amniotic cavity Describe the formation of the yolk sac and chorionic cavity Identify...

WE MAKE DOCTORS WEEK 2: BILAMINAR GERM DISK DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Describe the formation of the blastocyst and how it becomes bilaminar germ disc Describe the establishment of the amniotic cavity Describe the formation of the yolk sac and chorionic cavity Identify the clinical correlation of the previous concepts. DAY 8 AT THE EIGHTH DAY OF DEVELOPMENT, THE BLASTOCYST IS PARTIALLY EMBEDDED IN THE ENDOMETRIAL STROMA. In the area over the embryoblast, the trophoblast has differentiated into two layers: (1) an inner layer of mononucleated cells, the CYTOTROPHOBLAST (2) an outer multinucleated zone without distinct cell boundaries, the SYNCYTIOTROPHOBLAST Cells of the inner cell mass or embryoblast also differentiate into two layers: (1) a layer of small cuboidal cells adjacent to the blastocyst cavity, the HYPOBLAST LAYER (2) a layer of high columnar cells adjacent to the amniotic cavity, the EPIBLAST LAYER TOGETHER, THE LAYERS FORM A FLAT DISC.  At the same time a small cavity appears within the epiblast: AMNIOTIC CAVITY Epiblast cells adjacent to the cytotrophoblast: AMNIOBLAST Together with the rest of the epiblast, they line the amniotic cavity. The endometrial stroma adjacent to the implantation site is edematous and highly vascular. The large, tortuous glands secrete abundant glycogen and mucus. DAY 9 THE BLASTOCYST IS MORE DEEPLY EMBEDDED IN THE ENDOMETRIUM, AND THE PENETRATION DEFECT IN THE SURFACE EPITHELIUM IS CLOSE BY A FIBRINE COAGULUM. The trophoblast shows progress in development, particularly at the embryonic pole, where vacuoles appear in the syncytium. When these vacuoles fuse, they form LARGE LACUNAE. This phase of trophoblast development is known as: THE LACUNAR STAGE Trophoblastic Enlarged blood cells lacunae Syncitiotrophoblast Cytotrophoblast Fibrin coagulum LACUNAR STAGE At the abembryonic pole, there are cells originating from the hypoblast form a thin membrane, the exocoelomic (Heuser) membrane that lines the inner surface of the cytotrophoblast. This membrane, together with the hypoblast, forms the lining of the: ALSO CALLED PRIMATIVE YOLK SAC ”EXOCOELOMIC CAVITY”  Trophoblastic lacunae EMBRYONIC Syncitiotrophoblast POLE Cytotrophoblast Amniotic cavity Epiblast AMBEMBRYONIC POLE Hypoblast Exocoelomic (Heuser) Exocoelomic cavity membrane (primite yolk sac) DAYS 11 & 12  UTEROPLACENTAL CIRCULATION: The blastocyst is completely embedded in the endometrial stroma and the surface epithelium almost entirely covers the original defect in the uterine wall. Cells of the syncytiotrophoblast penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries (sinusoids). The syncytial lacunae become continuous with the sinusoids, and maternal blood enters the lacunar system establishing the uteroplacental circulation. surface epithelium A new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the primitive yolk sac forming the EXTRAEMBYONIC MESODERM Large cavities in the extraembryonic mesoderm become confluent and form a new space: the extraembryonic cavity or CHORIONIC CAVITY or “CHORIONIC CAVITY” EXTRAEMBRYONIC MESODERM Growth of the bilaminar disc is relatively slow compared with that of the trophoblast. Consequently, the disc remains very small (0.1 to 0.2 mm). Cells of the endometrium, meanwhile: Become loaded with glycogen and lipids Intercellular spaces are filled with extravasate The tissue is edematous. These changes, known as the DECIDUA REACTION, at first, are confined to the area immediately surrounding the implantation site but soon occur throughout the endometrium. DAY 13 The surface defect in the endometrium has usually healed, however, bleeding occurs at the implantation site as a result of increased blood flow into the lacunar spaces (28th day of menstrual cycle) Cells of the cytotrophoblast proliferate locally and penetrate the syncytiotrophoblast, forming cellular columns surrounded by syncytium: known as PRIMARY VILLI The hypoblast produces additional cells that migrate along the inside of the exocoelomic membrane, proliferates and gradually form a new cavity within the exocoelomic cavity: SECONDARY YOLK SAC or DEFINITIVE YOLK SAC. During its formation, large portions of the exocoelomic cavity are pinched off. These portions are represented by EXOCOELOMIC CYST, which are often found in the extraembryonic coelom or chorionic cavity. The extraembryonic mesoderm lining the inside of the cytotrophoblast is then known as the CHORIONIC PLATE. The only place where extraembryonic mesoderm traverses the chorionic cavity is in the CONNECTING STALK. With development of blood vessels, the stalk becomes the UMBILICAL CORD. https://youtu.be/VoaxRbve42o?si=v-l2nHalNabjcVdw  Abnormal Implantation References: Chapter 4. Second Week of Development: Bilaminar Germ Disc Bevis, R. (1992). Langman’s Medical Embryology, 6th edition T W Sadler Langman’s Medical Embryology, 6th Edition Williams & Wilkins 412pp £20.50 0-683- 07473-8. Nursing Children and Young People, 4(3), 14. https://doi.org/10.7748/paed.4.3.14.s14 Thank You!