Inhibition of melanosome transformation in embryonic chick pigment retina cultured in vitro.

When the retinal pigment epithelial cells of the chick embryo are transferred to monolayer cultures, they lose their phenotypic trait-- melanin granules-- after a few days. Within the first 24 hours almost all of the melanosomes and premelanosomes are transformed into the degradative structures of the dense bodies or the melanosome complexes. Then, within a few days, these structures disappear completely from the cytoplasm. Actinomycin D, added to the culture medium during the first four hours, almost completely prevents the transformation of melanosomes and premelanosomes. The inhibition of cell proliferation, caused by the addition of colcemid, does not prevent the transformation, though the time of initiation of transformation is delayed considerably. The mechanisms of the transformation of pigment granules are discussed.