Week 2 Embryonic Period Spring 2024 PDF

Weeks 2 Embryonic period Bilaminar germ disc Spring Semester 2024 Dr. Ismail Memon MBBS, PhD Saba University School of Medicine  Learning outcomes By the end of the lecture student will be able to: 1. Describe the differentiation of the trophoblast and formation...

Weeks 2 Embryonic period Bilaminar germ disc Spring Semester 2024 Dr. Ismail Memon MBBS, PhD Saba University School of Medicine  Learning outcomes By the end of the lecture student will be able to: 1. Describe the differentiation of the trophoblast and formation of cytotrophoblast and syncytiotrophoblast. 2. Describe the differentiation of the embryoblast and formation of epiblast and hypoblast. 3. Describe both of the amniotic and the yolk sac cavities. 4. Describe formation of extraembryonic mesoderm, somatic mesoderm and visceral mesoderm. 5. Describe connecting stalk and allantois. 6. Identify the importance of hCG. 7. Describe some trophoblast diseases and oncofetal antigens. 8. Briefly describe the different sites of ectopic pregnancy. Dr. Ismail Memon  LO-1 At the end of 1 week st At the end of 1st week embryo has inner cell mass and outer cell mass. Inner cell mass, the Embryoblast forms the embryo proper. Outer cell mass, the Trophoblast will contribute to formation of placenta. Dr. Ismail 3 Memon  LO-1 Week-2 of human development Further development of the outer cell mass (trophoblast) Further development of the inner cell mass (embryoblast) Development of extraembryonic mesoderm 4 Dr. Ismail Memon Differentiation of the trophoblast LO-1 What is the trophoblast? It is the outer flat squamous epithelial layer of cells of the blastocyst. Importance of the trophoblast: blastocyst. 1- When the blastocyst hatches from the zona pellucida, the trophoblast layer play an important role in the adhesion of the blastocyst to the endometrium. endometrium. 2- After attachment of the blastocyst to the endometrium, the trophoblast will start the process of implantation by penetrating the lining of the uterine wall. Trophoblast of the blastocyst 3- As implantation continues, the trophoblast layer proliferates and differentiates into two different layers LO- Dr. Ismail Memon  LO-1 Further development of the When the trophoblast blastocyst is partially embedded in the endometrium, the cells of the trophoblast layer become differentiated into two distinct layers: 1. Cytotrophoblast: Inner layer of mononucleated cells. 2. Syncytiotrophoblast: Outer multinucleated zone which consists of a large pool of cytoplasm without distinct cell boundaries between the cells. This layer is called syncytium (syn=together, cyte=cell) LO-1 Dr. Ismail  LO-1 What is the source of the syncytiotrophoblast? Mitotic figures are found in the cytotrophoblast but not in the syncytiotrophoblast. Thus, cells in the cytotrophoblast divide and migrate into the syncytiotrophoblast, where they fuse and lose their individual cell membranes. LO- Dr. Ismail  Further development of LO-2 embryoblast The inner cells, the embryoblast differentiates in: Dorsal epiblast layer (columnar cells) and ventral hypoblast layer (cuboidal cells). Within the epiblast, clefts begin to develop and eventually coalesce to form amniotic cavity. The amniotic cavity will eventually become filled with amniotic fluid. 8 Dr. Ismail  Further development of LO-3 embryoblast Epiblast cells adjacent to the cytotrophoblast are called amnioblasts. Hypoblast cells migrate, line the blastocyst cavity and form exocoelomic membrane. Blastocyst cavity is now called exocoelomic cavity or primary yolk sac. Epiblast and hypoblast together form a flat disk known as bilaminar disc. 9 Dr. Ismail  LO-4 Development of extraembryonic mesoderm  Yolk sac cells form a fine, loose connective tissue called extraembryonic mesoderm.  Extraembryonic mesoderm fill all space between cytotrophoblast externally and amnioblast and exocoelomic membrane internally.  Spaces appear in extraembryonic mesoderm, coalesce to form extraembryonic coelom/chorionic cavity.  Chorionic cavity surrounds whole part of embryo except where the embryo is attached to the cytotrophoblast by connecting stalk. 10 Dr. Ismail  Differentiation of extraembryonic mesoderm LO-4  Extraembryonic coelom divides extraembryonic mesoderm into somatic and visceral part.  Extraembryonic somatic mesoderm lines cytotrophoblast and Extraembryonic visceral mesoderm covers yolk sac.  Extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast together form chorion.  Exocoelomic cavity (primary yolk sac) covered by visceral mesoderm forms definitive yolk sac.  Cytotrophoblast proliferates into the syncytiotrophoblast and form primary chorionic villi. Dr. Ismail  Differentiation of extraembryonic mesoderm LO-4 The chorionic cavity divides the extraembryonic mesoderm into two layers, the splanchnic (visceral) and somatic (parietal) layers This chorionic cavity surrounds both yolk sac and amniotic cavity, except where the bilaminar germ disc is connected to the trophoblast by the connecting stalk. LO- Dr. Ismail Dr. Ismail Dr. Ismail  Connecting LO-5 Stalk Formed by extraembryonic mesoderm. The extraembryonic coelom expands to form a large cavity (chorionic cavity), within which the embryo and the attached amniotic cavity and yolk sac are suspended by the body stalk (connecting stalk) from which the umbilical cord forms. Dr. Ismail  LO-5 Allantois The posterior wall of the yolk sac forms a small diverticulum which is called allantois and extends into the connecting stalk. Allantois Dr. Ismail  Summary of Main Events of Week-2 Embryoblast forms 2 layers  Epiblast  Hypoblast Trophoblast differentiates into 2 layers  Cytotrophoblast  Syncytiotrophoblast Extraembryonic mesoderm splits into 2 layers  Somatic  Visceral 2 cavities are formed  Amniotic cavity  Chorionic cavity Appearance of primary chorionic villi Anterior visceral endoderm  Establishment of cranio-caudal axis of Week 2 Summary Dr. Ismail  LO-6 Clinical Considerations Human chorionic gonadotropin (hCG)  Glycoprotein produced by syncytiotrophoblast  Stimulates production of progesterone by corpus luteum  Assayed in maternal blood at day 8 or maternal urine at day 10  Basis of pregnancy testing and detectable throughout pregnancy  Low hCG may predict a spontaneous abortion or may indicate an ectopic pregnancy  High hCG may indicate a multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia 20 Dr. Ismail  LO-7 Gestational trophoblastic disease Hydatidiform mole / Molar pregnancy – Marked benign enlargement of chorionic villi (trophoblast). – Characterized by grapelike vesicles in uterus and absence of embryo, high hCG level. Gestational trophoblastic neoplasia - choriocarcinoma – Malignant tumor of trophoblast that may occur after normal or ectopic pregnancy, abortion, or hydatiform mole. – Sometimes they metastasize to liver 21 Dr. Ismail  LO-7 Oncofetal antigens Cell surface antigens (proteins), normally present during fetal development For unknown reasons re-express themselves in human malignant cells Used as tumor markers for diagnosis & treatment prognosis – e.g., alpha-fetoprotein, which is produced by hepatocellular carcinoma and some germ cell tumors – Carcinoembryonic antigen, which is elevated in people with colon cancer – Beta 2 microglobulin in multiple myeloma  LO-6 Ectopic pregnancy LO-8 Occasionally, implantation of the blastocyst takes place outside the uterus, resulting in ectopic pregnancy. 95% of ectopic pregnancies occur in the uterine tube. The blastocyst may also attach itself to the peritoneal covering of the intestinal tract. When the blastocyst develops in the ovary proper, the case is called primary ovarian pregnancy. In most ectopic pregnancies, the embryo Dr. Ismail