The cell nucleus in early bovine and caprine preimplantation embryos: fine structural cytochemistry and...

Fine structural cytochemistry and immunocytochemistry were used to study nucleic acids and nuclear proteins in nuclear bodies (NB) of pronuclear and 2-cell bovine and caprine embryos on ultrathin sections of paraformaldehyde fixed and Lowicryl K4M or LR White embedded specimens. The most striking feature detected in some of these nuclear bodies (NBs) was the presence of non-nucleolar proteins known to be involved in pre-mRNA splicing. One category of such intranuclear bodies (showing a rather dense finely fibrillar composition and named here dense body-DB) contained the Sm-antigen (an antigen common to a major group of nucleoplasmic spliceosomal snRNPs). Another, more numerous category of NBs differed morphologically from the former one by a much looser composition of fibrillogranular elements (loose body-LB). Moreover, it showed the presence of the non-snRNP splicing factor SC-35, in addition to the Sm-antigen. Both categories of these nuclear bodies were distinguished clearly from the nucleolar precursor bodies (NPBs) by an absence of immunolabeling of NPB with antibodies against nuclear proteins involved in splicing. Moreover, the former NBs are not stained with silver, while NPBs already in pronuclei exhibit strong affinity to silver. In addition to the immunolabeling in prominent (approx. 0.2-2.0 microns) NBs, regularly occurring high concentration of snRNP was revealed in very small (approx. 0.05 micron), morphologically poorly defined areas (named here small snRNP-enriched areas-SSA), harboring moreover a set of nuclear proteins similar to that of the coiled body. Numerous observations of the presence of these small areas in nuclear bodies and in their close vicinity, in nucleoplasm, in proximity of the nuclear envelope and also in ooplasm suggested that they are possible carriers of certain nuclear proteins moving between nuclear bodies, nucleoplasm and cytoplasm. A functional relationship of all these embryonic subnuclear elements has not been elucidated so far but their mutual relation is suggested, since the NPBs and other nuclear bodies usually occur in a close association. Fine structural and immunoelectron microscopic observations further suggest a similarity of the nuclear bodies in the early ruminant embryo with specific intranuclear bodies ("snurposomes") known from Xenopus laevis oocytes. A new and striking feature emerging from these observations is a possible involvement of a group of nucleoplasmic proteins in a yet unknown way in the differentiation processes concomitant with early embryonic nucleologenesis.