Gastrulation and Formation of Tri-Laminar Disc Part 1 PDF

03/11/23 Gastrulation and formation of tri-laminar disc: part 1 Learning objective: de ne and describe the process of gastrulation and formation of tri-laminar embryonic disc ( three germ layers ). Learning objective: describe the development of the notochord. Learning objective: appreciate the impo...

03/11/23 Gastrulation and formation of tri-laminar disc: part 1 Learning objective: de ne and describe the process of gastrulation and formation of tri-laminar embryonic disc ( three germ layers ). Learning objective: describe the development of the notochord. Learning objective: appreciate the importance of cellular positioning and migration during mesoderm formation. Week 3: Gastrulation happens during the third week of development. This is the process which converts the bi-laminar disc into the tri-laminar disc containing three germ layers: ectoderm, mesoderm and endoderm. These three layers will give rise to all of the tissues and organs. Gastrulation is the beginning of morphogenesis ( development of the form and structure of organs and parts of the body ). The process of Gastrulation: Gastrulation begins with the formation of a transient structure called the primitive streak. This occurs on day 15. P.S is a thickening of epiblast cells that rst appears at the caudal end of the embryo and elongates into the primitive groove, as cells proliferate and migrate towards the cranial end- formation of the P.S establishes the cranial/caudal axis of the embryo ( top and bottom ). At the cranial end of the P.S epiblast cells ingress at a greater rate forming a circular cavity, primitive pit, and cells of the P.S proliferate and become enlarged to form the primitive nodeprimary tissue organiser. Epithelial cells in the lateral edge of the epiblast undergo EMT ( epithelial to mesenchymal transition ). They become ask-shaped, detach and migrate down/ into P.S ( invagination or ingression ). The rst set of cells to move down P.S integrate into the hypoblast layer and transform into endoderm. The second set of cells to detach and ingress ll the space bet ween endoderm and epiblast and forms the mesoderm. The remaining epiblast cells transform into the ectoderm. Good video of Gastrulation of overview: https://www.youtube.com/watch?v=ADlYn0ImTNg Gastrulation abnormalities: Caudal dysgenesis: Insuf cient mesoderm is formed in the caudal most region of the embryo and as a result, abnormalities in the lower limbs and Urogenital system occur. Sacrococcygeal teratomas: Tumours that occur when ruminants of the primitive streak persist in the sacrococcygeal region. Development of the notochord: Day 18-21: Progenitor mesoderm ( mesenchymal ) cells from the primitive node and pit migrate to form a median cellular cord - the notochordal process. This soon acquires a lumen which is called the notochordal canal. The primitive streak moves down as the notochordal process advances cranially bet ween the ectoderm and the endoderm until it reaches the prechordal plate. Some mesenchymal cells from the p.s migrate cranially and meet to form the cardiogenic mesoderm in the cardiogenic region. This is where the premordium of the heart begins to develop at the end of week 3. More development: As the hypoblast is replaced to form the endoderm, notochordal process cells proliferate and detach from the endoderm forming notochordal plate that will ultimately form a solid cylinder of cells, the de nitive notochord. FIG The notochord is a transient structure that provides support and de ne the axial axis of the embryo ( top and bottom ). Transient means short lived. Fused layers of the ectoderm and endoderm form the oropharyngeal membrane located at the future site of the oral cavity and the cloacal membrane at the future site of the anus. I Notochord is a developmental patterning structure. Serves as the basis for the development of the axial skeleton and also induces the overlying embryonic ectoderm to thicken and form the neural plate. This is the primordial of the CNS. Indicates the future sites of the vertebral bodies. Fate map established during Gastrulation: Speci c regions of the epiblast migrate through different parts of the node and P.S to form mesoderm: Cells migrating at the central and most cranial part of the node will form the notochord. Cells migrating more posteriorly will form paraxial mesoderm. The next region will form intermediate mesoderm. Cells migrating through the more caudal part of the streak will form lateral plate mesoderm. Cells migrating through the most caudal part will contribute to extraembryonic mesoderm.