Further studies of the nucleolar material in salivary gland nuclei of Sciara coprophila

The nucleolar-like bodies or micronucleoli of Sciara coprophila salivary gland nuclei have been studied with phase contrast, the Nomarski optics, Azure B staining, and electron microscopy. In the late fourth instar the main nucleolus formed at the X-chromosome may become extensively fragmented and may appear as a large aggregate of micronucleoli. At about the same time large numbers of micronucleoli in a more peripheral location are also found. Studies in partially squashed and stained nuclei, as well as in unfixed glands have shown that, at a time when the nucleolar material is abundant, the X-NOR is highly ramified with its branches permeating much of the nuclear space. These observations make it appear probable that most or all of the nucleolar material, even the more peripherally located, is actually in contact with the main nucleolar organizer or its branches. On the other hand, many chromosomal bands are also in close association with micronucleoli. At the level of electron microscopy some of the associations between chromo somal bands and micronucleoli are very intimate with the nucleolar material often found deep within the band. In other instances there seems to be physical continuity between extensions of band chromatin and certain areas of the fibrillar component. The bands in question could be the sites of secondary nucleolar organizers. In the electron microscope a large aggregate of micronucleoli, interspersed with portions of chromatin can often be seen in an approximately central location. This is interpreted as the main nucleolus with portions of its NOR. Both the main nucleolus and the more peripheral micronucleoli are indistinguishable in their fine structure and show the components typically found in nucleoli, i.e., fibrils and granules. On the other hand the fine structure of both RNA and DNA puffs is strikingly different.Supported by funds from Public Service Grants GM 12191 from the National Institute of General Medical Sciences and 5 RO 1 AM 10016-06 from the National Institute of Arthritis and Metabolic Diseases (to Dr. A. M. Garcia).