An approach to the study of cytodifferentiation in melanophores

Abstract

A method is described whereby the melanoblasts on the yolk sac of a fish ( Fundulus heteroclitus) can be identified and staged throughout the period of their early differentiation. The method is based on the nuclear volume increase observed in these cells. Using this method, several cytological and cytochemical changes were followed during differentiation: 1. 1. The cell volume, as estimated from area determinations, was found to parallel approximately the increase in nuclear volume except for a pause in the middle of growth. 2. 2. The DNA content of the nuclei, determined by Feulgen microphotometry, doubles during melanoblast growth. All early melanophores have the tetraploid (4C) DNA amount. 3. 3. The rate of RNA synthesis, determined by H 3 cytidine autography, and the amount of RNA per cell were found to reach a maximum preceding later growth stages and melanogenesis. 4. 4. C 14-tyrosine was incorporated by these cells from the beginning of their differentiation at a higher rate than in surrounding tissues, and it may be detected in small granules prior to the appearance of melanin. 5. 5. The Golgi apparatus makes its appearance in an active form early in differentiation. At about the same time small neutral redstaining granules make their appearance in the melanoblast cytoplasm. 6. 6. Melanoblast differentiation is broken down into early stages of migration and growth, a period of specialization and periods of preparation for and the onset of melanogenesis. The significance of the neutral red granules and the tyrosine containing granules that precede the appearance of melanin granules is discussed, and it is concluded that they may represent stages of increasingly specialized products, possibly of the Golgi apparatus.