Primitive streak

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Primitive streak

Surface view of embryo of a rabbit. arg. Embryonic disk. pr. Primitive streak.
Gray's	subject #6 47
Carnegie stage	6b
MeSH	Primitive+Streak
Dorlands/Elsevier	s_25/12761719

The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development. The formation of the primitive streak is one of the first signs of gastrulation. It is characterized as a furrow in the midline of the embryonic disk at the future caudal end of the embryo. This furrow is formed by the ingression of epiblast cells which will go on to form the definitive endoderm and mesoderm by replacing hypoblast cells. The epiblast cells then develop into the definitive ectoderm.

The primitive streak establishes a visible longitudinal axis of bilateral symmetry around which all embryonic structures will organize and align. The primitive streak consists of the following:

Primitive groove - shallow valley extending along the length of the streak.
Primitive node - a raised circle of cells at the rostral tip of the primitive streak. (Hensen's node)
Primitive pit - a depression in the center of the primitive node.

The primitive streak is an important concept in bioethics, where some experts have argued that experimentation with human embryos is permissible only until the primitive streak develops, generally around the fourteenth day of existence. The development of the primitive streak is taken, by such bioethicists, to signify the creation of a unique, potential human being. See The President's Council on Bioethics, Human Cloning and Human Dignity: An Ethical Inquiryat 143.