The nerve fibers reach the surface ectoderm during the 5th week of development.At this stage the related spinal nerves supply segmental bands of skin around the trunk and on the limb bud. The 5th cervical nerve supplies a preaxial strip of upper limb bud and the first thoracic nerve a postaxial strip. In between these strips, the intermediate nerves (ie, C4,5,6) supply parallel strips of skin. As the limb bud elongates, the distribution of intermediate nerves migrates along it so that they no longer reach the surface of the proximal part of the limb. Area of skin supplied by a single spinal nerve is called a dermatome*.
*lt is to be noted that in anatomy the term dermatome is used to convey two different meanings:
(1) In embryology, this term is used to denote one of the three subdivisions of a somite.
(2) In gross anatomy, this term is used to imply the area of skin supplied by a single spinal nerve.
The dermatomal sequence of cutaneous innervation descends along the preaxial border and ascends along the postaxial border of each limb: C4,5, 6, 7, 8, T1, 2 in the upper limb and Li, 2, 3,4,5 and 81,2, 3 in the lower limb. After rotation of the limbs, the limb dermatomes can be traced down the lateral side of the upper limb and up its medial side, whereas those of the lower limb can be traced down the ventral surface and up the dorsal surface of the limb. Dorsal and ventral axial lines can be defined on the surface of the developing limbs. These lines separate adjacent areas of skin, which are supplied by non-adjacent derma-tomes.
Adjacent dermatomes overlap in their cutaneous nerve supply, but there is no overlap across the axial lines.