Embryology LC 2: Bilaminar Germ Disc PDF

UNIVERSITY OF NORTHERN PHILIPPINES COLLEGE OF MEDICINE, BATCH 2028 ○ 2 cavities formed: Amniotic cavity and Yolk COURSE OUTLINE Sac cavities I. BILAMINAR GERM DISC II....

UNIVERSITY OF NORTHERN PHILIPPINES COLLEGE OF MEDICINE, BATCH 2028 ○ 2 cavities formed: Amniotic cavity and Yolk COURSE OUTLINE Sac cavities I. BILAMINAR GERM DISC II. OVERVIEW II. OVERVIEW III. DAY 8 OF DEVELOPMENT ○ Trophoblast Differentiation DAY 8 OF DEVELOPMENT ○ Embryoblast Differentiation ○ Trophoblast Differentiation ○ Amniotic Cavity ○ Bilaminar germ disc IV. DAY 9 OF DEVELOPMENT ○ Amniotic cavity ○ Fibrin Coagulum Formation ○ Lacunar Stage DAY 9 OF DEVELOPMENT ○ Exocoelomic Membrane ○ Fibrin Formation ○ Lacunar stage V. DAY 11 & 12 OF DEVELOPMENT ○ Exocoelomic Membrane ○ Establishment of the Uteroplacental Circulation DAY 11 & 12 OF DEVELOPMENT ○ Lacunae – Sinusoids ○ Uteroplacental Circulation ○ Layers of the Extraembryonic ○ Exocoelomic Cavity Mesoderm ○ Lacunae Sinusoids ○ Decidua Reaction VI. DAY 13 OF DEVELOPMENT DAY 13 OF DEVELOPMENT ○ Primary Chorionic Villi ○ Secondary Yolk Sac / Definitive Yolk Sac ○ Secondary Yolk Sac / Definitive ○ Primary Villi Yolk Sac ○ Chorionic cavity VII. CLINICAL CORRELATIONS ○ Connecting Stalk ○ Pregnancy Test ○ Immunity ○ Abnormal Implantation III. DAY 8 OF DEVELOPMENT ○ Abnormal Blastocysts ○ Preimplantation & Postimplantation Reproductive Failure I. BILAMINAR GERM DISC This chapter gives a day-by-day account of the major events of the second week of development; however, embryos of the same fertilization age do not necessarily develop at the same rate. Indeed, considerable differences in rate of growth have Figure 2. Bilaminar Germ Disc (Day 8) been found even at these early stages of development. BLASTOCYST (58 CELLS) ○ 5 Inner cells ○ 53 Outer cells WEEK 2 ○ Blastocyst undergoes further differentiation. TROPHOBLAST DIFFERENTIATION ○ Trophoblast differentiates into Cytotrophoblast (lighter green) and Syncytiotrophoblast (darker green). ○ Cytotrophoblast (inner layer) Composed of primitive mononucleated Figure 1. Bilaminar Germ Disc (Day 8) cells that divide and contain syncytin. Syncytin: aids in cell fusion with the 2ND WEEK OF DEVELOPMENT IS KNOWN AS expanding outer layer of THE WEEK OF 2’S syncytiotrophoblast. ○ Trophoblasts differentiates into 2: helps embryo implantation in the uterus. Cytotrophoblast and Syncytiotrophoblast Mitotic figures are found in the ○ Embryoblasts forms 2 layers: Epiblast and cytotrophoblast but not in Hypoblast syncytiotrophoblast. ○ Extraembryonic mesoderm splits into 2 Cells in the Cytotrophoblast divide and layers: Somatic and Splanchnic layer migrate into the syncytiotrophoblast. BATCH 2028 1E 1 EMBRYOLOGY LC 2: BILAMINAR GERM DISC Dr. LICNACHAN M.M 09/16/2024 ○ Syncytiotrophoblast (outer layer) thereby forming the lining of the exocoelomic This is where cells will fuse and lose their cavity or the primitive yolk sac individual cell membranes after migration. Multinucleated syncytial layer that will provide transport functions of the placenta in the future. It has a single cytoplasm without cell borders that allows it to expand and branch. The cells in direct contact with the maternal blood that reaches the placental surface. Facilitates the exchange of nutrients, wastes, and gases between the maternal and fetal systems. EMBRYOBLAST DIFFERENTIATION ○ Inner cell mass or embryoblast differentiates into 2 layers: Hypoblast and Epiblast. Together, the layers form a flat disc called the Bilaminar Embryonic Disc. ○ Hypoblast: a layer of small cuboidal cells Figure 3. Bilaminar Germ Disc (Day 9) A 9 day human adjacent to the blastocyst cavity. blastocyst. The syncytiotrophoblast shows a large number ○ Epiblast: a layer of high columnar cells of lacunae. Flat cells form the exocoelomic membrane. adjacent to the amniotic cavity. The bilaminar disc consists of a layer of columnar epiblast ○ Within the epiblast, where amniotic cavity cells and a layer of cuboidal hypoblast cells. The original develops, while there is no urine, most amniotic surface defect is closed by a fibrin coagulum. fluid is from urine of embryo/fetus. The contents are formed within the epiblast; there V. DAY 11 & 12 OF DEVELOPMENT are cells called amnioblasts which are located between the epiblast and cytotrophoblast; it lines the amniotic cavity. AMNIOTIC CAVITY ○ A small cavity appears from within the epiblast and enlarges to become the amniotic cavity. ○ Amnioblasts: epiblast cells that are adjacent to cytotrophoblast. Amnioblast and the rest of the epiblast line the amniotic cavity. ○ The endometrial stroma adjacent to the implantation site is edematous(swollen) and highly vascularized. The large, tortuous glands secrete abundant glycogen and mucus. IV. DAY 9 OF DEVELOPMENT FIBRIN COAGULUM FORMATION ○ After the blastocyst is embedded to the Figure 4. Bilaminar Germ Disc (Day 11 & 12) Human endometrium, the formation of fibrin coagulum blastocyst of approximately 12 days. The trophoblastic will occur to plug the penetration defect at the lacunae at the embryonic pole are in open connection with entry site of the surface epithelium maternal sinusoids in the endometrial stroma. Extraembryonic mesoderm proliferates and fills the space LACUNAR STAGE between the exocoelomic membrane and the inner aspect ○ The trophoblast, especially at the embryonic of the trophoblast. pole, will further develop leading to the appearance of vacuoles in the syncytium At this stage, the blastocyst is now fully embedded ○ The vacuoles will fuse to form irregularly in the endometrial stroma, and the surface shaped large cavities called called epithelium almost entirely covers the defect in the trophoblastic lacunae uterine wall EXOCOELOMIC MEMBRANE FORMATION ESTABLISHMENT OF THE UTEROPLACENTAL ○ Flattened cells at the abembryonic pole that CIRCULATION probably originated from the hypoblast will form ○ The lacunar spaces in the syncytium will form a thin membrane called the exocoelomic an intercommunicating network called the (Heuser) membrane lacunae network ○ This membrane together with the hypoblast, ○ As the syncytiotrophoblast continue to will line the inner surface of the cytotrophoblast penetrate deeper into the stroma, it will erode the endothelial lining of the maternal capillaries BATCH 2028 1E 2 EMBRYOLOGY LC 2: BILAMINAR GERM DISC Dr. LICNACHAN M.M 09/16/2024 ○ Sinusoids – edges of the maternal capillaries ○ The lacunae network will connect with the VI. DAY 13 OF DEVELOPMENT sinusoids then the maternal blood will flow through the trophoblastic system, filling up the lacunae network with maternal blood, thereby establishing the uteroplacental circulation LACUNAE – SINUSOIDS ○ Lacunae forms from the vacuoles in the syncytium ○ The lacunae inside the syncytiotrophoblast connects to the sinusoids in order for blood supply to flow ○ Syncytiotrophoblast – secretes beta hCG ○ Positive pregnancy test result – when the syncytiotrophoblast and cytotrophoblast appears ○ As early as day 8, trophoblast differentiates and serum beta-hCG is detected in the blood ○ During day 11 and 12, when the blood enters the lacunar network and goes to the urinary system, the beta hCG is secreted Figure 5. A 13-day human blastocyst. Trophoblastic ○ Urine Pregnancy test kits – qualitative lacunae are present at the embryonic as well as the ○ Serum Pregnancy test – quantitative abembryonic pole, and the uteroplacental circulation has begun. Note the primary villi and the extraembryonic LAYERS OF THE EXTRAEMBRYONIC coelom or chorionic cavity. The secondary yolk sac is entirely lined with endoderm. MESODERM ○ The extraembryonic mesoderm appears, ○ Surface defects in the endometrium usually population of cells in the inner surface of the healed. cytotrophoblast. ○ Fibrin Coagulum is absent because it is ○ It will fill all the space between the trophoblast already closed and inside the endometrium. externally and the amnion and exocoelomic ○ Bleeding occurs at the implantation site as a membrane internally, it will eventually become result of increased blood flow into the lacunar the chorionic cavity. spaces, and occurs near the 28th day of the ○ It will separate into two layers: menstrual cycle. Ovulation occurs on the 14th Outer layer: extraembryonic somatic day. So if implantation occurs, it would be either mesoderm on the 12th or 13th day from the day of Inner layer: extraembryonic splanchnic ovulation. Meaning, it occurs on Day 26 or 27, mesoderm near the 28th day. This information can be used ○ It will remain small for about 0.1 to 0.2 mm, as a basis for counting the last menstrual period and the bilaminar germ disk grows slowly while of implantation. the trophoblast grows actively ○ Spotting – common complaints of patients. If there is bleeding at those times, it is more likely DECIDUA REACTION implantation bleeding, and not abortion or ○ The cell of endometrium become polyhedral menstruation. and loaded with glycogen and lipids ○ Intercellular spaces are filled with extravasate PRIMARY CHORIONIC VILLI ○ The tissue is edematous subchorionic hemorrhage is the implantation bleeding that shows in the ultrasound and is caused by the lacunar network Treatment: progesterone is given to supplement the corpus luteum ○ Small bleeding is normal and acceptable due to the establishment of uteroplacental circulation, unless the patient suffers from threatened abortion, the cervix opens, or the mother has immunodeficiency. Figure 6. Cellular columns of cytotrophoblast with syncytial covering arise from the buds of cytotrophoblast that protrude from the primitive syncytium Cells of the cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast, forming cellular columns surrounded by syncytium. Only cytotrophoblast has the capability to divide. BATCH 2028 1E 3 EMBRYOLOGY LC 2: BILAMINAR GERM DISC Dr. LICNACHAN M.M 09/16/2024 SECONDARY YOLK SAC OR DEFINITIVE YOLK ○ ECTOPIC PREGNANCY: SAC Diagnosis is based on the location or Hypoblast produces additional cells, which is implantation the secondary yolk sac or definitive yolk Ovarian Pregnancy – the egg did not enter sac. the fallopian tube and it only stayed in the Migrate along the inside of the exocoelomic ovary. The sperm got out of the fimbria then membrane into the ovary. Thus, fertilization and Forms a new cavity within the exocoelomic implantation occurred in the ovary. cavity Abdominal Pregnancy – eventually Secondary yolk sac or definitive yolk sac is pre-term. In this condition, there is no much smaller than the original exocoelomic endometrium – the implantation takes place cavity, or primitive yolk sac. anywhere in the abdomen. During its formation, large portions of the In abdominal pregnancy the delivery will be exocoelomic cavity will pinched off then the via the cesarean section, leaving the remnant will become the exocoelomic cyst placenta in the mother and it will only be which is redundant or doesn’t have a function. removed through chemotherapy. Extraembryonic coelom expands forming a In the abdominal cavity, the blastocyst most chorionic cavity. frequently attaches itself to the peritoneal The extraembryonic mesoderm lining the inside lining of the rectouterine cavity, or pouch of of the cytotrophoblast is then known as the Douglas and it may also attach itself to the chorionic plate. peritoneal covering of the intestinal tract or Connecting stalk with development of blood to the omentum. Sometimes, the blastocyst vessels will be the umbilical cord. develops in the ovary proper, causing a During this time, the extraembryonic membrane primary ovarian pregnancy. is continuous, lining the cytotrophoblast, it only uses the extraembryonic cavity (yolk sac) area ○ PLACENTA PREVIA: going to your blastocyst. The one that closes Due to the abnormal implantation of the connecting stalk is the future umbilical cord. blastocyst below or at the lower part of the uterus, which is the Cervix. VII. CLINICAL CORRELATIONS Implications: 1. Abnormal Labor 1. PREGNANCY TEST (URINE) ○ The patient cannot undergo normal labor ○ by the end of 2nd week because the placenta supports the fetus ○ 29th day of cycle ○ Abnormal placement of the blastocyst obstructs the birth canal 2. IMMUNITY 2. Painless Vaginal Bleeding ○ increased T-Helper Cells 2 ○ bright red vaginal bleeding in the second or ○ humoral immunity – becomes more severe third trimester (typically after 20 weeks) Ex: SLE (systemic lupus erythematosus) – worsen during pregnancy * Can Placenta Previa be resolved? ○ cell-mediated immunity – gets better during ○ It can be resolved through trophotropism the pregnancy ○ Placental trophotropism – refers to a ○ a shift from cell-mediated immunity to humoral phenomenon where there is a dynamic immunity occurs and this shift protects the migration of the placenta at its insertion embryo from rejection through gestation. The placenta tends to grow in areas of good blood supply and 3. ABNORMAL IMPLANTATION nutrition and atrophies in areas with poor The syncytiotrophoblast is responsible for hormone blood supply and poor nutrition. production, including human chorionic gonadotropin (hCG). By the end of the second week, quantities of * Myometrium this hormone are sufficient to be detected by ○ middle part of the uterus made of smooth radioimmunoassays, which serve as the basis for muscles that contract during labor to help the pregnancy testing. fetus go out. ○ allows for expansion/stretching of the uterus as the fetus grows. HOWEVER: In Placenta Previa, the maturation happens in the cervix which doesn’t have a muscle that would enable the expansion or stretching of the uterus. Result: Painless bleeding Figure 7. Abnormal implantation of the Blastocyst BATCH 2028 1E 4 EMBRYOLOGY LC 2: BILAMINAR GERM DISC Dr. LICNACHAN M.M 09/16/2024 Diagnosis: ○ Transabdominal or Transvaginal Ultrasound can confirm the location of the placenta relative to the cervix. ○ After 32 Weeks: Placenta previa is usually diagnosed after 32 weeks of gestation. Treatment: Cesarean Delivery (C-section) ○ The only safe method of delivering a baby when placenta previa is present. ○ Attempting vaginal delivery can lead to significant maternal and fetal risks due to hemorrhage. ABNORMAL BLASTOCYST ○ a fertilized egg that has failed to develop properly, leading to no formation of a fetus. ○ may appear as positive in a pregnancy test, but this is due to the development of the Trophoblast which formed the placental membranes. ○ little to no embryonic tissue was developed. Hydatidiform Mole or H-Mole A condition where moles secrete high levels of hCG (human chorionic gonadotropin) may produce benign or malignant tumors such as invasive mole or choriocarcinoma too much trophoblast = high levels of hCG What happens during H-Mole: 1. Sperm fertilizes an empty egg. 2. An empty egg cannot grow an embryo. 3. Placental tissue grows but is abnormal, as no fetus is formed and only contains fluid-filled cysts (or tumors). 4. Due to the presence of high hCG, the pregnancy test becomes positive. Types: ○ Complete H-Mole NO fetal tissue present Only abnormal trophoblastic tissue with cysts or tumors. ○ Partial H-Mole Presence of FEW or RESIDUAL fetal parts/tissue (e.g. leg, arm, etc.) NO fetus formed Reference(s): Sadler, T. W. (2011). Langman’s Medical Embryology. LWW. BATCH 2028 1E 5