| 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 termprimitive segments is used to identify the somites.) | |
| Details | |
| Carnegie stage | 9 |
| Days | 20[1] |
| Precursor | Paraxial mesoderm |
| Gives rise to | Dermatome,myotome,sclerotome,syndetome |
| Identifiers | |
| Latin | somitus |
| MeSH | D019170 |
| TE | E5.0.2.2.2.0.3 |
| FMA | 85522 |
| Anatomical terminology | |
Thesomites (outdated term:primitive segments) are a set of bilaterally paired blocks ofparaxial mesoderm that form in theembryonic stage ofsomitogenesis, along the head-to-tail axis insegmented animals. Invertebrates, somites subdivide into thedermatomes,myotomes,sclerotomes andsyndetomes that give rise to thevertebrae of thevertebral column,rib cage, part of theoccipital bone,skeletal muscle,cartilage,tendons, andskin (of the back).[2]
The wordsomite is sometimes also used in place of the wordmetamere. In this definition, the somite is ahomologously-paired structure in an animalbody plan, such as is visible inannelids andarthropods.[3]
Themesoderm forms at the same time as the other twogerm layers, theectoderm andendoderm. The mesoderm at either side of the neural tube is calledparaxial mesoderm. It is distinct from the mesoderm underneath the neural tube, which is called thechordamesoderm that becomes the notochord. The paraxial mesoderm is initially called the "segmental plate" in the chick embryo or the "unsegmented mesoderm" in other vertebrates. As theprimitive streak regresses and neural folds gather (to eventually become theneural tube), the paraxial mesoderm separates into blocks called somites.[4]
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 ofsomitogenesis.[4]
The development of the somites depends on a clock mechanism as described by theclock and wavefront model. In one description of the model, oscillatingNotch andWnt signals provide the clock. The wave is a gradient of thefibroblast growth factor protein that isrostral 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 the previous one.[5][6]
The timing of the interval is not universal. Different species have different interval timing. In thechick embryo, somites are formed every 90 minutes. In themouse the interval is 2 hours.[7]
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 byNoggin-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.[4][8]
As cells within the paraxial mesoderm begin to come together, they are termedsomitomeres, indicating a lack of complete separation between segments. The outer cells undergo amesenchymal–epithelial transition to form anepithelium around each somite. The inner cells remain asmesenchyme.
The Notch system, as part of the clock and wavefront model, forms the boundaries of the somites.DLL1 andDLL3 are Notchligands, mutations of which cause various defects. Notch regulatesHES1, which sets up the caudal half of the somite. Notch activation turns onLFNG which in turn inhibits the Notch receptor. Notch activation also turns on the HES1 gene which inactivates LFNG, re-enabling the Notch receptor, and thus accounting for the oscillating clock model.MESP2 induces theEPHA4 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 andN-cadherin are key to the mesenchymal–epithelial transition process 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 thecytoskeleton.
TheHox 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.[4]
In the developing vertebrateembryo, somites split to form dermatomes, skeletal muscle (myotomes),tendons and cartilage (syndetomes)[9] andbone (sclerotomes).
Because the sclerotome differentiates before the dermatome and the myotome, the termdermomyotome refers to the combined dermatome and myotome before they separate out.[10]
Thedermatome is the dorsal portion of the paraxial mesoderm somite which gives rise to the skin (dermis). In the human embryo, it arises in the third week ofembryogenesis.[2] It is formed when a dermomyotome (the remaining part of the somite left when the sclerotome migrates), splits to form the dermatome and the myotome.[2] The dermatomes contribute to the skin,fat andconnective tissue of theneck and of the trunk, though most of the skin is derived fromlateral plate mesoderm.[2]
Themyotome is that part of a somite that forms the muscles of the animal.[2] Each myotome divides into anepaxial part (epimere), at the back, and a hypaxial part (hypomere) at the front.[2] Themyoblasts from the hypaxial division form the muscles of the thoracic and anterior abdominal walls. The epaxial muscle mass loses its segmental character to form theextensor muscles of the neck and trunk of mammals.
In fishes, salamanders, caecilians, and reptiles, the body musculature remains segmented as in the embryo, though it often becomes folded and overlapping, with epaxial and hypaxial masses divided into several distinct muscle groups.[citation needed]
Thesclerotome (orcutis plate) forms thevertebrae and the rib cartilage and part of the occipital bone; the myotome forms themusculature of the back, the ribs and the limbs; the syndetome forms the tendons and the dermatome forms theskin on the back. In addition, the somites specify the migration paths ofneural crest cells and theaxons ofspinal nerves. From their initial location within the somite, the sclerotome cells migrate medially towards thenotochord. These cells meet the sclerotome cells from the other side to form the vertebral body. The lower half of one sclerotome fuses with the upper half of the adjacent one to form each vertebral body.[11] From this vertebral body, sclerotome cells move dorsally and surround the developingspinal cord, forming the vertebral arch. Other cells move distally to the costal processes ofthoracic vertebrae to form the ribs.[11]
Incrustacean development, a somite is a segment of the hypothetical primitive crustacean body plan. In current crustaceans, several of those somites may be fused.[12][13]
