Tyrosinase activity was studied in dermal nevus cells using a modification of the ultrastructural DOPA technique. Tyrosinase was localized to melanosomes and premelanosomes in nevus cells of the superficial dermis. Nevus cells in the middermis and deep dermis, in addition to having melanosomal tyrosinase activity, also demonstrated linear accumulations of DOPA-melanin in the smooth endoplasmic reticulum. Nevus cells having a neuroid appearance also contained this linear reaction product after incubation in DOPA, suggesting a pigment cell origin similar to other dermal nevus cells. The absence of tyrosinase activity in the smooth endoplasmic reticulum of the superficial dermal nevus cells may be related to auto-phagic breakdown of melanin with the information of melanosome complexes. The findings of this study support the hypothesis that premelanosomes can be formed directly from the smooth endoplasmic reticulum without prior processing by the Golgi apparatus.
Thorne EG, Mottaz JH, Zelickson AS. Tyrosinase Activity in Dermal Nevus Cells. Arch Dermatol. 1971;104(6):619-624. doi:10.1001/archderm.1971.04000240043005