Localization of Nucleus Organizing Regions (NORs) in One-Cell Mouse Embryos and Oocytes by Fluorescence in situ Hybridization

In mouse zygotes, ribosomal genes (rDNA) are transcriptionally silent and so-called “nucleolar precursor bodies” are present instead of typical nucleoli. However, the functional significance of these structures remains obscure. Specifically, it remains unknown whether structural association between the nucleolar precursor bodies and rDNA are maintained when rDNA synthesis is switched off. Here, we described the rDNA topology in one-cell mouse embryos and MII oocytes using fluorescence in situ hybridization and mouse rDNA probes. Our data suggest that in the pronuclei of one-cell embryos, rDNAs form rather compact clusters, whose number does not exceed that of nucleolus organizing chromosomes characteristic for the haploid set of mouse chromosomes. In zygotic pronuclei, not all nucleolar precursor bodies are associated with rDNA and not all rDNA repeats are attached to the nucleolar precursor bodies. Altogether, these data favor the idea that spatial interactions of nucleolus organizing chromosomes and nucleolar precursor bodies are not obligatory. We assume that associations between nucleolar precursor bodies and nucleolus organizing chromosomal regions are mediated by centromeric heterochromatin. The total numbers of silver stained nucleolus organizing chromosomes in CBA and C57BL mice are different. rDNA genes are unequally distributed among nucleolus organizing chromosomes and nucleolus organizing regions of sister chromatids.