Ultrastructure of melanocytes and premelanosomes in the human epidermis during melanogenesis.

Two types of melanocytes differing in their ultrastructural organization--active and "spent," or reserve--are present in the adult human epidermis. In active melanocytes, intensive melanogenesis takes place, and six stages of melanosomes development can be distinguished in them. The "spent" melanocytes do not contain premelanosomes, and the presence of glycogen inside and around them is evidently indicative of a phase preceding restoration of their function.