THE CELL-NUCLEUS IN EARLY BOVINE AND CAPRINE PREIMPLANTATION EMBRYOS - FINE-STRUCTURAL CYTOCHEMISTRY AND IMMUNOELECTRON MICROSCOPY

Fine structural cytochemistry acid immunocytochemistry were used to st udy nucleic acids and nuclear proteins in nuclear bodies (NB) of pronu clear and 2-cell bovine and caprine embryos on ultrathin sections of p araformaldehyde fixed and Lowicryl K4M or LR White embedded specimens. The most striking feature detected in some of these nuclear bodies (N Bs) 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 nu cleoplasmic spliceosomal snRNPs). Another, more numerous category of N Bs differed morphologically from the former one by a much looser compo sition of fibrillogranular elements (loose body-LB). Moreover, it show ed the presence of the non-snRNP splicing factor SC-35, in addition to the Sm-antigen. Both categories of these nuclear bodies were distingu ished clearly from the nucleolar precursor bodies (NPBs) by an absence of immunolabeling of NPB with antibodies against nuclear proteins inv olved in splicing. Moreover, the former NBs are not stained with silve r, while NPBs already in pronuclei exhibit strong affinity to silver. In addition to the immunolabeling in prominent (approx. 0.2-2.0 mu m) NBs, regularly occurring high concentration of snRNP was revealed in v ery small (approx. 0.05 mu m), morphologically poorly defined areas (n amed here small snRNP-enriched areas - SSA), harboring moreover a set of nuclear proteins similar to that of the coiled body. Numerous obser vations of the presence of these small areas in nuclear bodies and in their close vicinity, in nucleoplasm, in proximity of the nuclear enve lope and also in ooplasm suggested that they are possible carriers of certain nuclear proteins moving between nuclear bodies, nucleoplasm an d cytoplasm. A functional relationship of all these embryonic subnucle ar elements has not been elucidated so far but their mutual relation i s suggested, since the NPBs and other nuclear bodies usually occur in a close association. Fine structural and immunoelectron microscopic ob servations further suggest a similarity of the nuclear bodies in the e arly ruminant embryo with specific intranuclear bodies (''snurposomes' ') known from Xenopus laevis oocytes. A new and striking feature emerg ing from these observations is a possible involvement of a group of nu cleoplasmic proteins in a yet unknown way in the differentiation proce sses concomitant with early embryonic nucleologenesis.