The effect of actinomycin D on the nucleolus and on pigment synthesis in pigment cells of Xenopus laevis: an ultrastructural study.

The effect of AMD on the nucleolus and on melanogenesis in differentiating pigment cells of Xenopus laevis was investigated in cultured neural crest cells. Cultures were treated with either 2 or 10 microgram/ml AMD for 41/2 hours. Following treatment the antibiotic was removed. Observations of the cells were made with both scanning and transmission electron microscopes. Actinomycin D almost entirely stopped pigment formation in neural crest cultures during treatment. The morphological sequence in the formation of melanin granules in the untreated pigment cells appears to be as follows: The earliest identifiable premelanosome is membrane-bound and contains very thin laminae and/or small vesciles. The premelanosomes become until they are approximately the size of mature melanin granules and there is thickening of the laminae, which appear to have a periodic substructure. The cells eventually become packed with electron-dense melanin granules. Compared with controls, the cytoplasm of treated cells showed a greater abundance of smooth- than of fuzzy-membraned vesicles, less rough endoplasmic reticulum, dilatation of the Golgi cisternae, and a smaller number of premelanosomes. The necleolus showed segregation and blebbing of its components, decrease in size and even disappearance; sometimes confluence of the components occurred. The most consistent morphological effect of AMD on the nucleolus was the separation of the fibrillar and granular areas. The granular component appeared to undergo marked changes in size and arrangement and is thought to be the source of ribosomal RNA precursors. The alteration in size of the outer component of the nucleolus went hand in hand with disappearance of free ribosomes from the cytoplasm of treated cells and inhibition of pigment synthesis.