• At the end of week 3, the intra-embryonic mesenchyme differentiates into three loose aggregate pairs of mesenchyme on each side of the neural tube
• Medially, the paraxial mesoderm differentiates into the future dermatome (dorsal surface), myotome (middle layer), and sclerotome (ventral layer), forming dermis, muscle, and connective tissue respectively.
  Moving laterally, the second aggregate pair, called the intermediate mesoderm, will form the future urogenital system. Most laterally, the lateral plate mesoderm will develop into future body cavities (intraembryonic coelom) and parts of the body wall.

• The paraxial mesoderm will develop into paired cuboidal bodies, or somites (Gr. soma, body). These will eventually develop into the bones (sclerotome), muscles (myotome), and dermis (dermatome) of and surrounding the axial skeleton. Somites appear as bumps on the dorsal surface of the embryo.
• At the end of week 3, 4-12 somites are present (visible on the dorsal surface of the embryo). By the end of week 5, 42-44 can be counted. However, most appear between days 20-30, giving this period the title of the somite period of development.
• Somites appear cranially to caudally, beginning at the occipital end. They can be counted and are used to roughly estimate the age of the embryo.

• Eventually, they play a major role in segmentation of the embryo and the adult. Since several somites will disappear, the final number is 31 pairs of somites.
• Law of Original Innervation: The myoblasts (future muscle cells) form concurrently with the spinal nerves and they migrate out from the notochord together. This results in the formation of 31 spinal nerves with associated skin, muscle, and connective tissue.

*Definition to remember for Unit 5 and beyond...*dermatome- an area of skin receiving mesenchyme from a specific somite that is supplied by a single spinal nerve and its ganglion

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