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Gray19 with color.png
Latin somitus
Carnegie stage 9
Days 20[1]
Precursor paraxial mesoderm
Gives rise to dermatome, myotome, sclerotome
Gray's p.52
MeSH A16.254.425.660.750
Code TE E5.
Anatomical terminology

A somite is a division of the body of an animal or embryo.

Somites are bilaterally paired blocks of paraxial mesoderm that form along the head to tail axis of the developing embryo in segmented animals. In vertebrates, somites give rise to the vertebrae of the spine, rib cage, (and part of the occipital bone); skeletal muscle, cartilage, tendons, and skin.


Origin in mesoderm[edit]


The mesoderm forms at the same time as the other two germ layers the ectoderm and endoderm. The mesoderm at either side of the neural tube is called paraxial mesoderm. It is distinct from the mesoderm underneath the neural tube which is called the chordamesoderm 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 the primitive streak regresses and neural folds gather (to eventually become the neural tube), the paraxial mesoderm separates into blocks called somites.[3]


In the developing vertebrate embryo, somites split to form dermatomes, skeletal muscle (myotomes), tendons and cartilage (syndetomes)[4] and bone (sclerotomes).



The dermatome 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 of embryogenesis.[6] It is formed when a dermamyotome (the remaining part of the somite left when the sclerotome migrates), splits to form the dermatome and the myotome.[7] The dermatomes contribute to the skin, fat and connective tissue of the neck and of the trunk, though most of the skin is derived from lateral plate mesoderm.[7]


The myotome is that part of a somite that forms the muscles of the animal. Each myotome divides into an epaxial part at the back, and a hypaxial part at the front. The myoblasts from the hypaxial division form the muscles of the thoracic and anterior abdominal walls. The epaxial muscle mass loses its segmental character to form the extensor 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]


The sclerotome forms the vertebrae and the rib cartilage and part of the occipital bone; the myotome forms the musculature of the back, the ribs and the limbs; the syndetome forms the tendons and the dermatome forms the skin on the back. In addition, the somites specify the migration paths of neural crest cells and the axons of spinal nerves. From their initial location within the somite, the sclerotome cells migrate medially towards the notochord. 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.[8] From this vertebral body, sclerotome cells move dorsally and surround the developing spinal cord, forming the vertebral arch. Other cells move distally to the costal processes of thoracic vertebrae to form the ribs.[8]

Formation of somites[edit]

Transverse section of a human embryo of the third week to show the differentiation of the primitive segment. ao. Aorta. m.p. Muscle-plate. n.c. Neural canal. sc. Sclerotome. s.p. Dermatome






Notch signalling[edit]





Scheme showing how each vertebral centrum is developed from portions of two adjacent segments. (Myotome labelled in upper left.)


In arthropods[edit]


In older texts[edit]


See also[edit]


  6. ^ Larsen, WJ. Human embryology.Third edition. Publisher Date 2001. Page 54.
  7. ^ a b Page 504

See also[edit]

Developmental biology

External links[edit]