Four types of cellular fine structure associated with human amelanotic melanoma.

Fine-structural studies of amelanotic foci within primary cutaneous melanomas and in amelanotic metastatic deposits have shown four distinctive kinds of melanoma cells, possibly suggesting different cellular mechanisms which may be associated with cessation of pigment synthesis in neoplastic systems. Type I. The cytoplasm shows numerous premelanosomes which approach the normal structure, showing both fibrillary and striated forms. Evidence for melanin deposition on the melanofilaments of the melanosomes is lacking. Type II. The cytoplasm shows the largest number of organelles of any of the kinds of cells. The cytoplasm is crowded with spherical organelles which contain only one or two single melanofilaments without evidence for pigment deposition. Type III. The cytoplasm shows a moderate number of spherical membrane-delimited organelles having either a granular or a lamellar internal structure. The granular form is the more prominent of the two. Type IV. The cytoplasm is a classical dedifferentiated cytoplasm seen in a neoplastic cell. The cytoplasm is crowded with mitochondria, polyribosomes, and free ribosomes, but only a rare melanosome is noted. The observations suggest that cessation of pigment synthesis may be associated with a lack of tyrosinase activity in melanosomes which approach normal form (Type I); it may be associated with faulty melanosomal formation (Type II); or it may be associated with the formation of an entirely abnormal organelle (Type III); and lastly, it is clearly associated with the dedifferentiated cell which does not form any or forms few melanosomes (Type IV). The most common of these types of cells associated with cessation of pigment synthesis is the second type of cell, that is, the one which shows an abnormal melanosomal form.