Somites

The thickening of the mesoderm layer of cells on either sides of the developing embryo’s midline gives rise to the somitomeres (from the paraxial mesoderm).

The somites are visible on the surface of the developing embryo and and will go on to give rise to the axial musculoskeletal system and the body wall of the developing embryo.

Cells within the somites will differentiate to form the following structures:

1. Ventral sclerotomes

   • These will migrate to surround the notochord and eventually gives rise to the vertebrae, intervertebral discs, and ribs.

2. Dorsal dermomyotomes

   • These will differentiate to form:

     • Dermotomes

       • These will contribute to the formation of the dermis and subcutaneous tissue of the skin in the region of the neck and trunk

     • Myotomes

       • These will eventually give rise to myoblasts to form the skeletal muscles

Dermatomes

A dermatome is a region of skin that is innervated by the sensory component of one spinal nerve and can be mapped as seen above.

Clinical Notes:

1. Varicella zoster - this occurs as a result of the virus that causes chicken pox lying dormant in the dorsal root ganglia. The virus might follow the path of the spinal nerve as mapped in the dermatome map above and travel to the skin, causing shingles.