Embryonic development of melanocytes in human hair and epidermis. Their cellular differentiation and melanogenic activity.

The biological and enzymic behaViDr of embryonic melanocytes and their cellular interaction in differentiating follicular and epidermal structures after leaving their dermal environment through the penetration of the dermalepidermal junction has not been significantly studied, although their neural crest origin (1) and developmental process within the dermis including their epidermal entry (2) is well described. Moreover, the melanocyte systems of adult human hair follicles excluding the infundibulum have been found to be different from that of the epidermis in their melanin synthesizing activity. The melanocyte in the peripheral layer of the outer root sheath below the melanogenic level has been described as being an "amelanotic melanocyte (3)" based on its negative reaction to dopa and Masson's premelanin technic. Furthermore, the hair bulb melanocyte has been shown to differ from the epidermal melanocyte in being not only dopa but also tyrosinase reaction positive in the anagen stage of the follicle (4). The origin and developmental genesis of these distinctive hair and epidermal melanocyte systems, however, have not been studied in relation to embryonic hair and epidermal differentiation despite its biological importance. Previous embryonic studies were mostly morphological characterizations which did not present information on the melanogenic activity of these developing melanocytes. In the present study the cellular differentiation, tyrosinase synthesis, and the formation of premelanin positive melanosomes and their melanization have been investigated in the melanocytes of developing embryonic human hair follicles and epidermis.