Immunoelectron microscope analyses of rat germinal vesicle-stage oocyte nucleolus-like bodies

Germinal vesicle (GV) stage oocytes isolated from rat ovaries were investigated by immunoelectron microscopy for the presence of several nuclear proteins in the prominent 'compact nucleoli' (nucleolus-like bodies named here NLB). Specific spliceosomal components including the Sm antigen of the nucleoplasmic small nuclear ribonucleoproteins (snRNP) and the non-snRNP splicing factor SC-35 were clearly detected in the dense finely fibrillar mass of the NLB. Moreover, the presence of small nuclear RNA (snRNA) in the NLB was demonstrated by means of an antibody which recognized the m3G-capped snRNA. The level of immunolabelling for the nuclear proteins fibrillarin and p80-coilin was relatively lower in the NLB. p80-coilin was distinctly localized, in small, poorly morphologically defined structural constituents in the nucleoplasm. Aggregates of intranuclear granules having similar antigenic composition to the NLB were also detected. Our observations suggested that the 'compact nucleoli' of rat GV oocytes represented nuclear compartments containing significant amounts of non-nucleolar, spliceosomal components. These NLB have a molecular composition closer to the composition of certain nuclear bodies than to the functional nucleoli of somatic cells. The NLB may represent a compartment in the mammalian oocyte, akin to the sphere organelle of the amphibian oocyte (also reported to contain spliceosomal components and a p80-coilin-related protein). Both structures may serve as temporary storage organelles for different maternal macromolecules, which support early embryonic development, inter alia of that involved in the maturation of pre-mRNA to be transcribed from the embryonic genome.