Ultrastructural effects of thymidine analogs in melanosomes and virus activation in cloned hamster melanoma cells in culture.

Melanosomes without the characteristic structure of normal melanocytes have been found in experimental melanomas and human nodular melanomas (1-3). They are associated with a unique type of premelanosome, the granular body (1, 2, 4, 5). Melanosomes and premelanosomes in melanomas are tyrosinase positive, but it has been shown that they also contain lysosomal enzymes (6-8). Novikoff et al. (6) proposed that melanosomes are modified lysosomes which tend to break down melanin in "melanosome-complexes" (5, 9) and become large autophagosomes, supporting an earlier view based on Drochman's observations (10), but no physiochemical evidence of melanin degradation has been reported. In order to investigate the unusual pigmentation system, we examined the ultrastructure of four cell lines established from a transplantable hamster melanoma associated with a R-type virus (1, 11). These clones differ in degree of melanogenesis and virus formation. One clone (MB line) closely resembled the most predominant cell found in the tumor in vivo (1). Ultrastructurally, melanosomes filled the cytoplasm and a few granular bodies were found (Figs. la, b). As colonies aged, large autophagosomes appeared, containing "melanosome complexes" and lamellated structures. Slight to moderate numbers of R-type viruses were observed in the cisternae of rough endoplasmic reticulum (RER) which intertwined with mitochondria, forming a characteristic relationship. Another clone (TD line) appeared similarly in the electron microscope except for an unpredictable tendency to produce large amounts of virus. The other two clones were distinguished by a block in the melanization process, which appeared most dramatically in the KF line (Fig. 2a, b). These cells contained large golgi with many membrane-lined, smooth-surfaced vesicles (SSV) and granular bodies (premelanosomes), but virtually no melanosomes, and melanosome complexes were never seen, although large autophagosomes containing unidentifiable structures occurred in aged cultures. These cells were not completely amelanotic but Dopaand Fontana-positive cells did not exceed more than 10%