Primitive streak
Surface view of embryo of a rabbit. arg. Embryonic disk. pr. Primitive streak.
Gray's	subject #6 47
Carnegie stage	6b
Days	15
MeSH	Primitive+Streak

The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.

Contents 1 Formation 2 Components 3 Ethical implications 4 References 5 External links 6 Additional images

[edit] Formation

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.

A group of cancers, germ cell tumors, may be the result of an error during this stage of development.

[edit] Components

The primitive streak consists of the following:^[1]

• Primitive groove - shallow valley extending along the length of the streak.
• Primitive knot - 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.

[edit] Ethical implications

The primitive streak is an important concept in bioethics, where some experts have argued that experimentation with human embryos is permissible only before 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.^[2]

[edit] References

1. ^ "The trilaminar germ disk (3rd week)". http://www.embryology.ch/anglais/hdisqueembry/triderm01.html. Retrieved 2007-10-19. 
2. ^ The President's Council on Bioethics, Human Cloning and Human Dignity: An Ethical Inquiryat 143.

[edit] External links

• bdyfm-013 — Embryology at UNC

[edit] Additional images

Human embryo—length, 2 mm. Dorsal view, with the amnion laid open. X 30.

Section through the embryo.

v • d • e Developmental biology > Human embryogenesis (development of embryo) and development of fetus First three weeks Week 1 Fertilization · Egg activation · Zygote · Cleavage · Morula · Blastula (Blastomere) · Blastocyst · Inner cell mass Week 2 (Bilaminar) Hypoblast · Epiblast Week 3 (Trilaminar) Germ layers Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) · Gastrula/Gastrulation · Regional specification Ectoderm Surface ectoderm · Neuroectoderm · Somatopleure · Neurulation · Neural crest Endoderm Splanchnopleure Mesoderm Chorda- · Paraxial (Somite/Somitomere/Sclerotome/Myotome/Dermatome) · Intermediate · Lateral plate (Intraembryonic coelom, Splanchnopleure/Somatopleure) Extraembryonic/ uterus Trophoblast (Cytotrophoblast, Syncytiotrophoblast) Blastocoele · Yolk sack/exocoelomic cavity · Heuser's membrane · Extraembryonic coelom · Vitelline duct Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) · Allantois Placenta · Decidua (Decidual cells) · Chorionic villi/Intervillous space · Gestational sac (Amnion/Amniotic sac/Amniotic cavity, Chorion) Histogenesis Programmed cell death (Apoptosis) · Stem cells · Germ line development Organogenesis Limb development: Limb bud · Apical ectodermal ridge/AER other structures: Eye development · Cutaneous structure development · Heart development · Development of the urinary and reproductive organs (Some dates are approximate—see Carnegie stages and a timeline.)

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