Changes of organelles associated with the differentiation of epidermal melanocytes in the mouse

Electron microscopic observations on normally differentiating and alpha-MSH (melanocyte-stimulating hormone)-treated epidermal melanocytes of newborn mouse skin were carried out. The process of melanocyte differentiation from premelanosome-containing melanoblasts was investigated in detail with respect to melanosomes as markers. Melanoblasts containing unmelanized premelanosomes gradually decreased in number after birth, while the number of melanocytes rapidly increased. The epidermis of alpha-MSH-treated 3-day-old mice and normal 6-day-old mice contained melanocytes with numerous fully melanized melanosomes, and with no or only a few melanoblasts. Changes in other organelles in differentiating melanocytes were also noticeable. Golgi apparatus and RER (rough endoplasmic reticulum) decreased in number during the normal or alpha-MSH-induced differentiation of the epidermal melanocytes, though the number of mitochondria showed no notable change. The number of SER (smooth endoplasmic reticulum) per cell did not change in the cells of newborn mice, while in alpha-MSH-treated cells the number increased significantly. These results led us to an assumption that Golgi apparatus or RER transforms into other forms of organelles including melanosomes and SER during the differentiation of melanocytes.