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Gastrulation is the process in which the three germ layers (ectoderm, mesoderm and endoderm) are formed by successive waves of epiblast cells migrating through the primitive streak. Formation of the primitive streak, on day 15, marks the first event of gastrulation. Cells from the epiblast migrate into the interior of the embryo, via the primitive streak, in a process termed ingression, which involves a cellular epithelial-to-mesenchymal transition (EMT). The initial wave of migrating cells (day 16) streams through the primitive streak, displacing the hypoblast cells to become definitive endoderm, which ultimately produces the future gut derivatives and gut linings.
The second wave of migrating cells (also on day 16) populate a layer between the epiblast and the definitive endoderm, thereby forming the mesoderm layer. The intraembryonic mesoderm cells later give rise to five subpopulations of cells: paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm, cardiogenic mesoderm and a population that forms a midline tube called the notochordal process. The notochordal process originates in the primitive node and is the precursor of the flat-shaped notochordal plate, which, after detaching from the endoderm, fuses its free rims together to form a rod that is known as the notochord.
Once the mesoderm has formed, the remaining epiblast cells cease to ingress and form the ectoderm. The ectoderm gives rise to two distinct lineages, namely, the surface ectoderm and the neural ectoderm.
The mesoderm produces and contributes to the blood, endothelium, heart, kidney, reproductive system, bones, skeletal, smooth muscle and connective tissues. Mesoderm also contributes to tendons, ligaments, dermis and cartilage.