Even though the central portion of an epidermal wound may extend to the dermis, the edges of the wound usually involve only slight damage to superficial epidermal cells. Common types of epidermal wounds include abrasions, in which a portion of skin has been scraped away, and minor burns. In response to an epidermal injury, basal cells of the epidermis surrounding the wound break contact with the basement membrane. The cells then enlarge and migrate across the wound (Figure 5.6a). The cells appear to migrate as a sheet until advancing cells from op-posite sides of the wound meet. When epidermal cells encounter one another, they stop migrating due to a cellular response called contact
inhibition. Migration of the epidermal cells stops completely when each is finally in contact with other epidermal cells on all sides. As the basal epidermal cells migrate, a hormone called epider-mal growth factor stimulates basal stem cells to divide and re-place the ones that have moved into the wound. The relocated basal epidermal cells divide to build new strata, thus thickening the new epidermis (Figure 5.6b).