Induction of precocious melanogenesis of pigment cells in cultures of neuroretinal cells of chick embryo by amphotericin B.

When dissociated neuroretinal cells of the 9-day-old chick embryo were cultured, the cells formed monolayer sheets of somewhat flattened epithelial cells within 15 days after inoculation. During 15 to 30 days, numerous foci of non-pigmented epithelial cells were formed. During 30 to 50 days, melanin appeared in the cells of these foci. When amphotericin B (1 microgram/ml) was added to the culture medium on day 25 of culture, brown pigments appeared precociously, i.e. within the first two days, in the cells. The brown pigments were identified as melanins by histochemical and electron-microscopic methods. Induction of melanogenesis required continuous treatment with amphotericin B. With the precocious appearance of melanins, tyrosinase activity increased rapidly. This rapid increase in tyrosinase activity was inhibited by the addition of phenylthiourea or diethyl-dithiocarbamate. It was not enhanced by iodoacetamide, but was blocked by a low concentration of cycloheximide or actinomycin D. These findings indicate that amphotericin B induces de novo synthesis of tyrosinase rather than activation of pre-existing tyrosinase.