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As we had discussed in the last lecture of somitogenesis and changes in intraembryonic mesoderm, that the intraembryonic mesoderm is not a single, homogenous undifferentiated layer. It is, instead, made up of different parts viz., Notochord or axial mesoderm, paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm. This process called gastrulation, during which there is the formation of different types of mesoderm occurs, is not a random, haphazard process but rather a nicely coordinated process. As we had seen in the lecture of gastrulation, the cells from the epiblast layer ingress downwards through the primitive streak and primitive node to first replace the hypoblast layer and then form different components of intraembryonic mesoderm between the two layers. The primitive streak, along with the primitive node [Hensen’s node], is the real guiding star during gastrulation. The fate of these ingressing cells is determined by the location or part of primitive streak and node from which they ingress downward. Let me explain how. The cells that ingress downward from the primitive or Hansen’s node will form the notochord and prechordal plate. Cells that ingress down from the cranial most part and caudo-lateral part of primitive node will form the paraxial mesoderm. Cells that ingress from the midstreak will form the intermediate mesoderm Cells that ingress from the caudal part of strak will form the lateral plate mesoderm. Finally, the cells that ingress from the caudal most end of the streak will contribute slightly to the extraembryonic mesoderm. Remember that most of the extraembryonic mesoderm is thought to have been originated, during the second week, from the hypoblast cells. However, some contribution also done by these ingressing cells during the gastrulation in third week of development. What is extraembryonic mesoderm by the way. As you must be knowing the ectoderm continues with amnion and endoderm layer continues with the yolk sac. So, baby is provided with these two fluids filled balloons. Outside these two balloons and the embryo proper, whole this structure is plastered outside with a layer of extraembryonic mesoderm. Website: Email: draizaz@
