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A transient, multipotent, migratory cell population that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage, bone, smooth muscle, peripheral and enteric neurons, and glia.
In the development of the human embryo the intraembryonic coelom (or somatic coelom) is a portion of the conceptus that forms in the mesoderm. During the second week of development the lateral mesoderm splits into a dorsal somatic mesoderm (somatopleure) and a ventral splanchnic mesoderm (splanchnopleure).
By the third week of development, this process gives rise to a cavity between the somatopleure and splanchnopleure referred to as the intraembryonic celom. This space later gives rise to both the thoracic and abdominal cavities.
In the developing vertebrate embryo, somites are masses of mesoderm that can be found distributed along the two sides of the neural tube. They will eventually become dermis (dermatome), skeletal muscle (myotome), vertebrae (sclerotome), and tendons and cartilage (syndetome).
The mesoderm found lateral to the neural tube is called the paraxial mesoderm. It is separate from the chordamesoderm underneath the neural tube. The paraxial mesoderm is initially called the unsegmented
mesoderm in vertebrates, but is called the segmented mesoderm in chick embryos.
As the primitive streak regresses and the neural folds gather preceding the formation of the neural tube, the paraxial mesoderm divides into blocks called somites. Somites play a critical role in early development by participating in the specification of the migration paths of neural crestcells and spinal nerveaxons.
Later in development, somites separate into four compartments:
The dermatome is the
dorsal portion of the paraxial mesoderm somite. In the human embryo it arises
in the third week of embryogenesis.
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 the lateral plate mesoderm.
The myotome is that part
of a somite that forms the muscles. Each myotome divides into an epaxial part (epimere),
at the back, and a hypaxial part (hypomere) at the front.
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.
The sclerotome forms the vertebrae
and the rib cartilage and part of the occipital bone. It forms the musculature
of the back, the ribs, and the limbs.
The syndetome forms the tendons and some blood vessels.