Melanocytes in Human Embryonic and Fetal Skin: A Review and New Findings

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Abstract
Melanocytes account for approximately 5–10% percent of the cells in adult epidermis. Unlike the ectodermally derived keratinocytes, they originate in the neural crest and migrate into the epidermis early in development. There has been an interest in melanocytes in developing human skin since the late 1800s, when concentrated pigmented cells were identified in the sacro‐coccygeal skin of Japanese fetuses. This observation led to speculation and subsequent investigation about the racial nature of the melanocytes in this site (the Mongolian spot), the presence of melanocytes in fetuses of other races, the timing of appearance of these cells in both the dermis and epidermis, and their origin. The early investigators relied primarily on histochemical methods that stained either the premelanosome or the pigmented melanosome, or relied upon the activity of tyrosinase within the melanosome to effect the DOPA reaction. Studies by electron microscopy added further documentation to the presence of melanocytes in the skin by resolving the structure of the melanosome regardless of its state of pigmentation. All of these methods recognized, however, only differentiated melanocytes. The thorough investigations of melanocytes in the skin from a large number of black embryos and fetuses by Zimmerman and colleagues between 1948 and 1955 provided insight into the time of appearance of melanocytes in the dermis (10–11 weeks' menstrual age) and the epidermis (11–12 weeks) and revealed the density of these cells in both zones of the skin of several regions of the body. The precise localization of the melanocytes in the developing hair follicles was contributed by the studies of Mishima and Widlan (J Invest Dermatol 1966; 46:263–277). More recently, monoclonal antibodies have been developed that recognize common oncofetal or oncodifferentiation antigens on the surface or in the cytoplasm of melanoma cells and developing melanocytes (but not normal adult melanocytes). These antibodies recognize the cells irrespective of the presence or absence of melanosomes or their activity in the synthesis of pigment and therefore are valuable tools for re‐examining the presence, density, and distribution patterns of melanocytes in developing human skin. Using one of these antibodies (HMB‐45), it was found that dendritic melanocytes are present in the epidermis between 40 and 50 days estimated gestational age in a density comparable with that of newborn epidermis and are distributed in relatively non‐random patterns. A number of questions about the influx of cells into the epidermis, potential reservoirs of melanoblasts retained within the dermis, division of epidermal melanocytes, and the interaction of melanocytes and keratinocytes during development remain unresolved. The tools now appear to be available, however, to begin to explore many of these questions.