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Freeze-fracture replication of melanocytes and melanosomes.

Ultrastructural studies spanning the past fifteen years have greatly extended knowledge of the functional morphology of the melanocyte, and its specific organelle, the melanosome (e.g. Birbeck & Barnicot, 1959; Moyer, 1963; Seiji, Shimao, Birbeck & Fitzpatrick, 1963; Breathnach & Wyllie, 1966; Seiji, 1967; Toda & Fitzpatrick, 1972; Lutzner & Lowrie, 1972). Considerable attention has been focused upon the origin, formative stages, and basic structure of the melanosome, but certain differences of opinion still remain concerning these matters. For example, some of the above authors believe that the internal structural components of the melanosome are arranged in the form of 'filaments' or 'fibrils' while others believe the arrangement to be primarily 'membranous'. The necessity of employing chemical fixatives and heavy-metal 'stains' for conventional transmission electron microscopy has almost certainly affected analysis of this and other features of the developing melanosome, and the progressive deposition of electron-dense melanin within it also tends to obscure detail. The freeze-fracture replication technique does not require fixation, dehydration, embedding or staining of tissues, and is proving to be a valuable supplement to thin section techniques in that it permits examination of potentially viable material, and can reveal particular features of membranes and organelles not apparent in conventionally processed specimens (Steere, 1957; Weinstein & Someda, 1967; Koehler, 1968; Bullivant, 1969; Moor, 1971; Branton, 1971; Breathnach, Goodman, Stolinski & Gross, 1973). Accordingly, it seemed worthwhile to apply this technique to the examination of the melanocyte and melanosome, with a reasonable expectation that observations of interest might emerge.