Primitive streak

Image Caption : Human Embryo 18 Day Old with Primitive Streak : Computer Generated Image from Micro-MRI, actual size in length = 1.5 mm - This image presents a side-view of an embryo during its third week of development. The age is calculated from the day of fertilization. The embryo is attached to the uterine wall and attains a pear-shaped structure. The white line seen on the embryo is the primitive streak, which establishes the longitudinal axis of the embryo and signals the development of the right and left sides of the body. The primitive streak also indicates where the division of the brain will occur.

Primitive Streak

A linear band of rapidly proliferating cells that begins near the posterior end of an embryo and grows cranially. Primitive streak is formed during GASTRULATION by the convergent migration of primary ectodermal cells (EPIBLAST). The knot at the tip of the streak is called HENSEN NODE.

National Center for Biotechnology Information, U.S. National Library of Medicine

The primitive streak is a structure that forms in the blastula during the early stages of avian, reptilian and mammalian embryonic development. It forms on the dorsal (back) face of the developing embryo, toward the caudal or posterior end.

The presence of the primitive streak will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the second embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation. The primitive streak extends through this midline and creates the left–right and cranial–caudal body axes, and marks the beginning of gastrulation. This process involves the ingression of mesoderm progenitors and their migration to their ultimate position, where they will differentiate into the mesoderm germ layer that, together with endoderm and ectoderm germ layers, will give rise to all the tissues of the adult organism.

The material on this site is for informational purposes only and is not intended as medical advice. It should not be used to diagnose or treat any medical condition. Consult a licensed medical professional for the diagnosis and treatment of all medical conditions and before starting a new diet or exercise program. If you have a medical emergency, call 911 immediately.