What is the primitive streak and when does it form?

The primitive streak is a linear midline thickening of ectodermal cells at the caudal end of the embryonic disc. It appears on day 15 of development and is crucial for establishing the body plan.

What are the main functions of the primitive streak?

The primitive streak serves as a source of intraembryonic mesoderm, initiates notochord formation, and defines the craniocaudal axis while establishing left-right symmetry in the embryo.

What happens to the primitive streak after it forms?

The primitive streak remains active until the end of the third week, then regresses craniocaudally. It typically disappears by the end of the fourth week, leaving a small remnant in the sacrococcygeal region.

What is the clinical significance of the primitive streak?

If the primitive streak fails to regress fully, its pluripotent cells can form a sacrococcygeal teratoma. This is the most common tumor in newborns and can become malignant if not removed early.

What structures are associated with the primitive streak?

At its cranial end, the primitive streak forms the primitive node, which surrounds a small primitive pit. The streak itself appears as a groove flanked by bulges on the dorsal surface.

Why is the primitive streak important for embryonic development?

The primitive streak is essential because it provides pluripotent cells that differentiate into various cell types, helping to form structures like the mesoderm and notochord. It also sets up the basic body axes and symmetry.

Primitive Streak – Axis Formation & Mesoderm