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The primitive streak is a transient structure whose formation, on day 15 of human development, marks the start of gastrulation, the process in which the inner cell mass in converted into the trilaminar embryonic disc, which is comprised of the three germ layers (ectoderm, mesoderm and endoderm). The primitive streak originates from the anterior epiblast, and appears as an elongating groove (primitive groove) on the dorsal midsagittal surface of the epiblast, along the anterior-posterior axis of the embryo. The rostro-caudal and medial-lateral axes of the embryo are defined by the primitive streak. The rounded primitive node, or Hensen's node, is situated at the cranial tip of the primitive streak, and contains a depression called the primitive pit. The primitive pit is continuous with the primitive groove.
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 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 populates 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.
The prechordal plate it also formed during gastrulation. It arises from the rostral end of the primitive streak and enters the endoderm and later contributes to the patterning of the rostral neural tube and to the oropharyngeal membrane, which forms the mouth. Prechordal plate cells also undergo EMT and contribute to head mesenchyme tissue; therefore, the prechordal plate is considered a rare mesendodermal structure.