| 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.) | |
| Latin | somitus |
| Gray's | subject #9 52 |
| Carnegie stage | 9 |
| Days | 20[1] |
| Precursor | paraxial mesoderm |
| Gives rise to | dermatome, myotome, sclerotome |
| Code | TE E5.0.2.2.2.0.3 |
| MeSH | Somites |
A somite (or primitive segment in older texts) is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage; in arthropods it is a characteristic of a hypothetical ancestor.
Contents |
In the developing vertebrate embryo, somites 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).
Because the sclerotome differentiates before the other two structures, the term "dermomyotome" is sometimes used to describe the combined dermatome and myotome.[2]
The mesoderm forms at the same time as ectoderm and endoderm. The mesoderm that is lateral to (at the side of) the neural tube is called paraxial mesoderm. It is distinct from the mesoderm underneath the neural tube which is called the chordamesoderm and becomes the notochord. The paraxial mesoderm is initially called the “segmental plate” in the chick embryo or the “unsegmented mesoderm” in other vertebrates. As the primitive streak regresses and neural folds gather (to eventually become the neural tube), the paraxial mesoderm separates into blocks called somites. These somites have four compartments: the sclerotome forms the vertebrae and the rib cartilage; the myotome forms the musculature of the back, the ribs and the limbs; the dermatome forms the skin on the back; and the syndetome forms the tendons and some blood vessels. In addition the somites specify the migration paths of neural crest cells and spinal nerve axons.
The cells which will become somites are committed before mesoderm becomes capable of forming somites. The formation of somites is called "somitogenesis". Somitogenesis depends on a clock and wave mechanism. Ocsilating Notch and Wnt signaling pathways provide the clock. The wave is a gradient of the FGF protein that is rostral to caudal (nose to tail gradient). Somites form one after the other down the length of the embryo from the head to the tail, with each new somite forming on the caudal (tail) side of those already in existing somites.
The timing of the interval is not universal. Different species have different interval timing. In the chick embryo somites are formed every 90 minutes. In the mouse the interval is variable.
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.
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