Weeks 2 & 3 Gastrulation & Neurulation PDF

Weeks 2 and 3 Gastrulation and Neurulation Embryology Objectives Describe the development of the epiblast and hypoblast Describe the process of implantation, chorionic villus formation, and parts of decidua. Discuss clinical considerations regarding abnormal uterine implantation (placenta previa). D...

Weeks 2 and 3 Gastrulation and Neurulation Embryology Objectives Describe the development of the epiblast and hypoblast Describe the process of implantation, chorionic villus formation, and parts of decidua. Discuss clinical considerations regarding abnormal uterine implantation (placenta previa). Describe the process of gastrulation, parts of intraembryonic mesoderm Identify the derivatives of the ectoderm, mesoderm and endoderm. Understand formation of the neural tube and neural crest cells Describe the fate of neural crest cells Relation of vertebral column to the spinal cord during development Clinical correlates: NTDs, testing and diagnosis of NTDs, relevance of folic acid in preventing NTDs Recap & the road ahead 1st week – fertilization, morula, blastocyst, Implantation of blastocyst around day 6, hCG secretion 2nd week – Bilaminar germ disc (week 2 has 2 layers) epiblast and hypoblast 3rd week – Trilaminar germ disc (week 3 has 3 layers) ectoderm, intraembryonic mesoderm and endoderm. The process is known as gastrulation. Formation of the neural tube (neurulation) follows shortly after gastrulation. Embryonic cell mass –> bilaminar disc Embryonic pole reorganizes into two layers Epiblast- taller cells, facing the amniotic cavity Hypoblast – thinner layer of cells adjacent to exocelomic cavity (future umbilical vesicle) Chorionic villus formation Syncytiotrophoblast will erode the endometrium and with it maternal blood vessels and glands Cavities will develop in the Syncytiotrophoblast = lacunae Lacunae fill with blood from maternal endometrial capillaries and endometrial gland secretions This is the beginning of uteroplacental circulation End of the 2nd week Top View Cytotrophoblast begins growing into the syncytiotrophoblast These cells columns are the primary chorionic villi These are the first stage of the chorionic villi of the placenta Primary Villus Secondary Villus Immature Villus Tertiary Villus Matured Villus Essential elements of placenta are chorionic villi. Villi are surrounded by maternal blood. 1 3 2 4 4 cavities: 1. Amniotic cavity 2. Yolk sac 3. Chorionic cavity 4. Uterine cavity. 1 4 Amniotic cavity increases Amnion fuses with chorion k/as chorioamniotic membrane Chornionic cavity is obliterated Yolk sac becomes very small (not shown) chorioamniotic membrane 1 Amniotic cavity increases further Uterine cavity is obliterated Chorioamniotic membrane and decidua capsularis forms membrane. Membrane and all decidua are expelled after birth Fetoplacental circulation Umbilical Cord amnion left umbilical vein Remnants of vitellointestinal duct Remnants of allantois umbilical arteries Wharton Jelly Placental Barrier - Separates maternal and fetal blood Endothelial lining of fetal blood vessels Connective tissue in villus core Cytotrophoblast Syncytiotrophoblast IgG antibodies can pass through placental barrier. The term membrane is applied to those structures derived from the blastocyst which do not contribute to the embryo. The amnion, the chorion, the yolk sac and the allantois make up the fetal membranes Type 1 Type 2 Type 3 Type 4 Placenta previa is a placental implantation that overlies or is within 2 cm of the internal cervical os. Placenta is implanted on LUS (Lower Uterine segment). Painless, vaginal bleeding in the late second or third trimester, often after sexual intercourse. Investigation: Transvaginal ultrasonography Bleeding at time of delivery. Dizygotic twins or Fraternal twins Dizygotic; each embryo has its own amnion, chorion, and placenta - dichorionic/diamniotic May not be of same sex Separate genetic constitution Monozygotic or maternal; have same genetic constitution A: Chorion is separate, amnion separate – dichorionic/diamniotic days 0-4 B: Chorion is common, amnion separate - monochorionic/diamniotic days 4-8 C: Chorion is common, amnion common - monochorionic/monoamniotic days 8-12 D. Conjoint twin (shared body) - Chorion common, amnion common monochorionic/monoamniotic days more than 13 Thoracopagus is the most common form of conjoined twins, 1st week – fertilization, morula, blastocyst, Implantation of blastocyst around day 6, hCG secretion 2nd week – Bilaminar germ disc (week 2 has 2 layers) epiblast and hypoblast 3rd week – Trilaminar germ disc (week 3 has 3 layers) ectoderm, intraembryonic mesoderm and endoderm. The process is known as gastrulation Connecting stalk Cranial end Cells of epiblast near tail in central axis forms thickened band of cells known as primitive streak. Primitive streak is narrow groove with slight bulging on sides. Prechordal plate Cephalic end Wall of Yolk sac Cut edge of Amnion Epiblast Hypoblast Primitive Node Connecting stalk Caudal end Primitive Streak Gastrulation is process of formation of all 3 germ layers (ecto, meso and endoderm) Invagination - cells of epiblast migrate and pass-through primitive streak to reach between epiblast and hypoblast. These cells form intraembryonic mesoderm Prechordal plate – cranial end – will form oropharyngeal membrane (future mouth) and Cloacal membrane – caudal end – future anus Section Epiblast displace hypoblasts to form endoderm Epiblasts form ectoderm, mesoderm, endoderm Figure 6-4 (pg 53) Cells in primitive node multiply and migrate in midline up to prochordal plate, these cells form notochordal process Notochord Does not give rise to vertebral column Extent: primitive streak to prechordal plate Forms central axis for developing embryo, Induces formation of neural tube and provides central column around which vertebral bodies will form. Fate Nucleus pulposus of intervertebral disc Mesoderm Organization Neural plate forming neural groove Notochord induces overlying ectoderm to form neural plate Cells of plate will become the neuroectoderm Induction represent starting of neurulation Extends from primitive knot to prochordal plate Paraxial mesoderm differentiates into urogenital structures such kidney and gonads and their duct system. Neural tube Notochord Neural plate as it folds, passes deep to ectoderm It forms a tube deep to ectoderm Ectoderm fuses to form one layer On either sides of neural tube are paraxial mesoderm Notochord lies between neural tube and endoderm Neural Crest Cells Neural fold elevate and fuse, at that time the cells at lateral edge begin to dissociate and form neural crest cells Neural crest cells are derived from neuroectoderm Neural crest cells give rise to multiple structures including ganglia and adrenal medulla Neural Crest cells- Derivatives Melanocytes Tracheal cartilages Enterochromaffin cells Laryngeal cartilages Parafollicular cells Adrenal medulla Schwann cells Spiral septum Ganglia Neural fold fuse in midline Anterior end which is not fused = cranial neuropore (closes on day 25) Posterior end which is not fused = caudal neuropore (closes on day 28) Neural tube communicates with amniotic cavity through neuropores Caudal portion of neural tube is narrow, forms spinal cord Cranial portion is broad, forms brain 3rd month IUL 5th month IUL At birth Spinal cord occupies entire vertebral column at 3rd month of IUL. Then the vertebral column elongates. At birth spinal cord extends till lower border of L3. In adults spinal cord ends at the lower border of L1 or upper border of L2. Neural Tube Defects (NTDs) Neural tube closure fails to occur Anencephaly, spina bifida Spina Bifida Occulta – probably due to defective neurulation Prevention – 400μg of folic acid daily, 3 month prior to pregnancy to birth Valproate (VPA) (valproic acid) - used to treat epilepsy. It causes NTD if given during pregnancy. Anencephaly is a NTD in which a baby is born without parts of the brain and skull Neural tube defects (NTDs) Spina bifida occulta: bony defect is covered by skin; the spinal cord is intact. A patch of dark hair that grows may over the area. Meningoceles only a fluid-filled sac of meninges protrudes from the defect. Meningomyelocele include neural tissue in the sac. (Spina bifida cystica refers to spina bifida with meningocele or meningomyelocele) Rachischisis refers to NTDs in which the neural tube fails to close, exposure of neural tissue that often becomes necrotic. Spina Bifida Occulta Spina Bifida Occulta sometimes signs are visible on the skin above the defect, including an abnormal tuft of hair, or a small dimple or birthmark. Underdevelopment of the primary ossification center in the vertebral arch. NTD Test Triple screen test (blood test) – 16th and 18th weeks of pregnancy. 1. alpha-fetoprotein, - produced by fetus Elevated AFP levels might be indicative of a NTD 2. unconjugated estriol - is an estrogen produced by both the fetus and the placenta 3. human chorionic gonadotropin – produced within placenta Events occurring at 3rd week after fertilization a. Gastrulation – process of establishing all three germ layers (ectoderm, mesoderm, endoderm) b. Formation of primitive streak, primitive node, primitive pit, migration of epiblast to primitive streak and its invagination c. Formation of notochord, neurenteric canal d. Oropharyngeal and cloacal membrane formation e. Establishment of body axis f. Development of primary, secondary and tertiary villus in trophoblast