Question

Write short notes on primitive streak.

Answer 1

Hint: The gastrulation is the period of the embryonic development. The epiblast and hypoblast layer plays an important role in the formation of primitive streaks. It is formed on the day fifteen of human development and initiates the process of gastrulation.

Complete answer:
-During the early embryonic development the structure formed in the blastula stage is called a primitive streak. It develops in the posterior side of the embryo. It is the transient structure which initiates the start of gastrulation. The process by which the inner cell mass is converted into the trilaminar embryonic disc is called gastrulation. It consists of three layers: ectoderm, mesoderm and endoderm.
-The epiblast is a single epithelial layer in the blastodisc. It acts as the source of all the embryonic material in amniotes and some of its cells will give rise to the primitive streak. It also responds to the signals from the marginal zone. If the signals are induced in the given region and streak formation starts, the remaining cells will no longer respond to these inductive signals and prevent the formation of another streak.
-This structure establishes bilateral symmetry, determines the site of gastrulation and initiates germ layer formation. The second embryonic axis is developed while the formation of primitive streak. It extends through the midline and creates the left- right and cranial –caudal body axis. It occurs by the ingression of the mesoderm. It also helps in the coordinated movement and re-arrangement of the cells in the epiblast.

Note: The implantation of the zygote in the uterine wall occurs after the blastocyst stage. It occurs in the endometrium wall of the uterus. The formation of a complex network of signaling pathways ensures the formation of primitive streaks. Hypoblast also plays an important role in the streak formation. In chick, hypoblast serves to inhibit formation of the primitive streak.