Ultrastructural changes in the cell cycle of cultured melanoma cells

The fine structure of the synchronized cultures of B16 melanoma cells at different days of incubation was studied. Our observations demonstrate that the endoplasmic reticulum first develops in the dividing cells as simple flattened smooth cisternae, which become paired or multilamellated during nuclear reconstruction and form annulate lamellae as well as typical endoplasmic reticulum with ribosome‐studded membranes in the postmitotic and interphase cells. The presence of both premelanosomes and melanosomes in dividing cells is unequivocal evidence that functionally mature melanocytes are capable of proliferation.