Development of the Musculoskeletal System: Basic Mesoderm Development

• Paraxial mesoderm: the thickened clump of mesoderm cells next to the notochord and neural tube.
• Lateral plate mesoderm: the thinner strip of mesoderm lateral to the paraxial mesoderm.
• Extra-embryonic mesoderm: The mesoderm surrounding the yolk sac.

Right in the occipital region, caudal to where the brain will form, the paraxial mesoderm begins to divide into somites. Above the occipital somite, there is diffuse mesoderm, organized roughly into somitomeres. Below the occipital somite, 38 pairs of somites begin to form cranially to caudal. The somites further divide into three parts:

1. Dermatome: most lateral portion of the somite – it will migrate to the skin and become the dermis.
2. Sclerotome: The middle segment that will eventually form the vertebrae and IV discs. Even though this will eventually end up closest to the notochord, it does not start right next to it.
3. Myotome: The most medial segment, and will eventually become the muscles, tendons, and fascia. These tissues must pick up innervation from the neural tube before they migrate, so they are the closest to the middle.

Category: Anatomy Notes