WHOLE MOUNT ELECTRON MICROSCOPY OF THE NUCLEOLUS IN SALIVARY GLAND CELLS OF DROSOPHILA MELANOGASTER

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Abstract
The nucleolus of D. melanogaster salivary gland cells, examined by whole mount EM, consists of a fibrillar core region and a peripheral region containing both fibers and granules. These regions appear to correspond to the fibrillar and granular components, respectively, seen in thin sections. Most of the nucleoli were attached to the chromocenter region of the polytene chromosomes, containing the nucleolar organizer. Bundles of relatively straight chromatin fibers, 13 nm in diameter, extended from the chromocenter into the core region of the nucleolus; it was not possible to trace the path of these chromatin fibers through the nucleolus since they were obscured within the mass of nucleolar fibers. The nucleolar fibers in both the core and peripheral regions were irregular and knobby, with a diameter of about 15 nm. In the core region, the fibers appeared to be of considerable length and were characteristically clustered together to form small interconnected masses. The fibers in the peripheral region were relatively short and some appeared to blend with amorphous, poorly-defined pools of material. Electron dense granules 15-20 nm in diameter were also associated with this amorphous substance. The formation and subsequent packaging of the 28s rRNA may be represented by a morphological transition of the peripheral fibers, via an amorphous pool-like intermediate stage, into the nucleolar granules. Whole mount EM may be a useful alternative to thin sectioning in high resolution studies of the nucleolus.