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Quantitative Study of Melanosome and Mitochondrial Populations in Pigmented and Amelanotic S-91 Mouse Melanomas¹

Department of Pathology, New York University Medical Center, New York, New York 10016

The relative numbers of mitochondria and melanosomes were enumerated in pigmented and amelanotic S-91 mouse melanomas. There were 6 times more mitochondria per sq µm of cytoplasm in amelanotic S-91 tumors than in melanotic S-91 tumors. This relative paucity of mitochondria in pigmented S-91 melanomas is significant in view of the identical in vivo growth rates of the two tumors and also because the two tumor types are so closely related in terms of common origin and ease of producing one from the other by serial selective transplantation. The respiratory rate of melanotic S-91 melanomas is only about 30% less than that of nonpigmented S-91 tumors. The 6-fold difference in mitochondrial numbers and the ultrastructural similarities of the mitochondria in the two tumor types suggest that the respiratory capacity of the melanotic S-91 melanocytes is attributable in part to melanosomes. These organelles contain high levels of tyrosinase activity, an enzyme that is coupled directly to molecular oxygen. This quantitative study of mitochondria and melanosome populations in these two closely related melanomas is harmonious with the concept that tyrosinase and melanosomes play a vital respiratory role in the relatively mitochondria-poor pigmented S-91 melanomas.

¹ This work was supported by National Cancer Institute Grant Ca 10807, USPHS.

Received 1/13/71. Accepted 10/ 6/72.