Question

Write a short note on primitive streak.

Answer 1

Hint:
Primitive streak works together with signaling pathways. It has three layers ectoderm, mesoderm and endoderm. It is very useful during the formation of layers during embryo development. Hypoblast plays an important role in the formation of streaks. The process of mesenchymal transition is very important.

Complete answer:
The primitive streak first appears at the caudal end of embryo and grows cranially. It appears as long thickened band. Cell proliferates to form the primitive knot (primitive node). It is a transient structure whose formation, on day 15 of human development. It makes the start of gastrulation. By the process of gastrulation the inner cell mass is converted into the trilaminar embryonic disc. It mainly consists of three layers ectoderm, mesoderm and endoderm. Primitive node is also known as a Hensen's node. It
contains a depression called the primitive pit. Cells migrate from the epiblast into the interior of the embryo. This process is termed as ingression. It involves mesenchymal transition (EMT). Hypoblast cells become definitive endoderm, which ultimately produces the future gut derivatives and gut linings. Mesoderm layer is formed between the epiblast and the definitive endoderm. After the formation of mesoderm, remaining epiblast cells cease to ingress and form the ectoderm. The prechordal plate also formed during gastrulation. Polonaise movement is essential for the formation of primitive streak.

Note:
The primitive streak regresses and addition of cells to caudal end and form notochord. The true cranial growth of the notochord is minimal. The absence of hypoblast in a chick results in multiple streaks. Presence of hypoblast is important for regulating the formation of a single primitive streak. This type of structure is also known as Anterior Visceral Endoderm (AVE).