Somite

From Wikipedia, the free encyclopedia

Somite

Transverse section of half of a chick embryo of forty-five hours' incubation. The dorsal (back) surface of the embryo is towards the top of this page, while the ventral (front) surface is towards the bottom.

Dorsum of human embryo, 2.11 mm. in length. (The older term 'primitive segments' is used to identify the somites.)
Gray's	subject #9 52
Carnegie stage	9
Precursor	paraxial mesoderm
Gives rise to	dermatome, myotome, sclerotome
MeSH	Somites
Dorlands/Elsevier	s_15/12743985

In the developing vertebrate embryo, somites (or primitive segments in older texts) are masses of mesoderm distributed along the two sides of the neural tube and that will eventually become dermis (dermatome), skeletal muscle (myotome), and vertebrae (sclerotome). They originate from paraxial mesoderm which, towards the end of the third gestational week, becomes organized into loose masses of cells called somitomeres. Driven by changes in the expression of adhesion molecules, somitomeres compact and bud off to form the somites. Approximately 44 somites form and give rise to the bones of the face, vertebral column, associated muscles, and overlying dermis.

In crustacean biology, a somite is a segment of the hypothetical primitive crustacean body plan. In current crustaceans, several of those somites may be fused.