515. An abrasion results in the total loss of epidermis over a large area of an arm, but one month later, the abrasion has healed, with regrowth of the epidermis. Which of the following mechanisms accounts for the restoration of the epidermis over the abraded area?

 A.  Growth of epidermis from hair follicles and sweat glands in the dermis
 B.  Migration of endothelial cells from newly grown capillaries
 C.  Transformation of dermal fibroblasts into epidermal cells
 D.  Transformation of macrophages into epidermal cells
 E.  Transformation of melanocytes into epidermal cells



The correct answer is A. The dermis contains skin appendages (e.g., hair  follicles), which contain
epithelial stem cells. In the process of healing a large area where the epidermis has been lost but the
dermis is intact, re-epithelialization occurs by growth of epidermal cells from the underlying skin
appendages, as well as from the intact epidermis along the wound edges. Physiologically, the dermis
lies beneath the epidermis. It has two major components, a superficial papillary layer and a deeper
reticular layer. The papillary layer contains the capillaries and the sensory neurons, which supply the
surface of the skin. The reticular layer consists of an interwoven meshwork of dense irregular
connective tissue.
None of the other cell types are known to directly contribute to the regeneration of epidermis over
abraded skin.