AbstractThe perineal pigment spot of the C57 Black mouse was examined macro‐ and microscopically in animals ranging in age from 14 days post‐fertilization to 28 days post‐parturition. Dendritic pigment‐containing cells first become evident in the dermis of the area on the sixteenth day post‐fertilization. Throughout later stages of development, these pigmented cells continue to be restricted to the dermis of this area. Since the dopa treatment reveals the presence of a melanin synthesizing system within these cells, they are true melanocytes which have become restricted to this layer of the integument.Histological examination reveals a population of melanocytes in the epidermis and/or hair follicles also. These are rarely found over the center of the area but are seen in increasing numbers laterally from the ventral midline. In contrast, dermal melanocytes are heavily concentrated near the ventral midline but decrease in numbers laterally. There are, therefore, two opposing but complementary gradients of melanocytes in the integument of the pigment spot. This suggests the possibility that a precocious differentiation of the basement membrane in this area during embryonic development inhibits the passage of melanoblasts into the epidermis thereby causing them to be restricted to t
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