EFFECTS OF EXOGENOUS MSH ON THE TRANSFORMATION FROM PHAEOMELANOGENESIS TO EUMELANOGENESIS WITHIN C57BL/6J- AY/A HAIRBULB MELANOCYTES

The extent to which alpha-melanocyte-stimulating hormone (MSH) is a true in vivo regulator of melanogenesis in mice is unknown. The objective of this study was to determine if MSH-induced eumelanogenesis in hairbulb melanocytes of yellow (A(y)/a) mice mimics the natural program of eumelanogenesis occurring in genetically black (a/a) hairbulb melanocytes. We conducted quantitative transmission electron microscopy on melanosome differentiation within MSH-treated regenerating 9-d hairbulbs of A(y)/a and a/a mice. Results of exogenous alpha-MSH injections (5 d at 0.15 mM MSH) showed that the striking visual darkening of hair was accompanied by an incomplete transformation of phaeo- to eumelanogenesis. Ontogenetic data on developmental stages I-IV of 3678 melanosomes based on geometric considerations (length, width, shape, and area) showed that MSH did not induce a complete transformation from spherical phaeomelanosomes to elliptical eumelanosomes. Also, observations on the number of vesiculoglobular bodies and matrix organization revealed that MSH-treated A(y)/a melanosomes retained distinct features of phaeomelanogenesis even after 5 d of MSH treatment. Thus, MSH induced a partial but incomplete pattern of eumelanogenesis in regenerating hairbulb melanocytes of A(y)/a mice. The continued investigation of the dynamics of melanin synthesis in MSH-induced A(y)/a mice melanocytes possessing "mosaic" melanosomes could be productive in understanding fundamental relationships between tyrosinase activity, matrix function, matrix structure, and regulation of melanin (phaeo- and/or eumelanin) synthesis.