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27.4C: Somite Development

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    8279
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    Somites develop from the paraxial mesoderm and participate in the facilitation of multiple developmental processes.

     

    LEARNING OBJECTIVES

     

    Describe the functions of somites

     

    KEY TAKEAWAYS

    Key Points

     

    • The paraxial mesoderm is distinct from the mesoderm found more internally in the embryo.
    • Alongside the neural tube, the mesoderm develops distinct paired structures called somites that develop into dermis, skeletal muscle, and vertebrae.
    • Each somite has four compartments: the sclerotome, myotome, dermatome, and the syndetome. Each becomes a specific tissue during development.

     

    Key Terms

     

    • neural crest cells: 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.
    • conceptus: The fetus or embryo, including all the surrounding tissues protecting and nourishing it during pregnancy.
    • somite: One of the paired masses of mesoderm, distributed along the sides of the neural tube, that will eventually become dermis, skeletal muscle, or vertebrae.

     

    Intraembryonic Coelom Development

     

    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.

     

    Somite Development

     

    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.

    This is drawing of a dorsal view of a human embryo. The repetitive somites are marked with the older term primitive segments.

    Somites: A dorsal view of a human embryo. The repetitive somites are marked with the older term primitive segments.

    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 crest cells and spinal nerve axons.

    Later in development, somites separate into four compartments:

    Dermatome

    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.

    Myotome

    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.

    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.

    Sclerotome

    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.

    Syndetome

    The syndetome forms the tendons and some blood vessels.