|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|
|Gives rise to||dermatome, myotome, sclerotome|
A somite is a division of the body of an animal.
In vertebrates, somites are bilaterally paired blocks of mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somites give rise to skeletal muscle, cartilage, tendons, endothelial cells, and dermis.
The word "somite" is also used in place of the word "metamere." In this definition, the somite is a homologously paired structure in an animal body plan, such as is visible in annelids and arthropods.
Origin in mesoderm 
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—the somites.
Because the sclerotome differentiates before the other the dermatome and the myotome, the term "dermomyotome" is sometimes used to describe the combined dermatome and myotome.
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. In addition, the somites specify the migration paths of neural crest cells and spinal nerve axons.
Formation of somites 
The pre-somitic mesoderm commits to the somitic fate before mesoderm becomes capable of forming somites. The cells within each somite are specified based on their location within the somite. Additionally, they retain the ability to become any kind of somite-derived structure until relatively late in the process of somitogenesis.
Somitogenesis depends on a clock mechanism as described by the clock and wavefront model. In one description of the model, oscillating Notch and Wnt signals 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.
For some species, the number of somites may be used to determine the stage of embryonic development more reliably than the number of hours post-fertilization because rate of development can be affected by temperature or other environmental factors. The somites appear on both sides of the neural tube simultaneously. Experimental manipulation of the developing somites will not alter the rostral/caudal orientation of the somites, as the cell fates have been determined prior to somitogenesis. Somite formation can be induced by Noggin-secreting cells. The number of somites is species dependent and independent of embryo size (for example, if modified via surgery or genetic engineering). Chicken embryos have 50 somites; mice have 65, while snakes have 500.
As cells within the paraxial mesoderm begin to come together, they are sometimes called somitomeres, indicating a lack of complete separation between segments. The outer cells undergo a mesenchymal–epithelial transition to form an epithelium around each somite. The inner cells remain as mesenchyme.
Notch signalling 
The Notch system, as part of the clock and wavefront model, forms the boundaries of the somites. DLL1 and DLL3 are Notch ligands, mutations of which cause various defects. Notch regulates Hairy1, which sets up the caudal half of the somite. Notch activation turns on Lunatic Fringe which in turn inhibits the Notch receptor. Notch activation also turns on the HES gene which inactivates Lunatic Fringe, re-enabling the Notch receptor, and thus accounting for the oscillating clock model. MESP2 induces the EPHA4 gene, which causes repulsive interaction that separates somites by causing segmentation. EPHA4 is restricted to the boundaries of somites. EPHB2 is also important for boundaries.
Fibronectin and N-cadherin are key to epithelialization in the developing embryo. The process is probably regulated by paraxis and MESP2. In turn, MESP2 is regulated by Notch signaling. Paraxis is regulated by processes involving the cytoskeleton.
The Hox genes specify somites as a whole based on their position along the anterior-posterior axis through specifying the pre-somitic mesoderm before somitogenesis occurs. After somites are made, their identity as a whole has already been determined, as is shown by the fact that transplantation of somites from one region to a completely different region results in the formation of structures usually observed in the original region. In contrast, the cells within each somite retain plasticity (the ability to form any kind of structure) until relatively late in somitic development.
In arthropods 
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.
In older texts 
In some older texts, somites are referred to as "primitive segments."
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