Nucleoskeleton of early bovine embryos and differentiated somatic cells: an ultrastructural and immunocytochemical comparison.

The nucleoskeleton is a complex structure involved in structural and functional organisation of the genome in eukaryotic cells. As little information on the nucleoskeleton is available from early embryonic development stages, we describe here the morphology and composition of the nucleoskeleton in cleaving bovine embryos (stages 1-16 cells). Ultrastructural observations were performed using thick resinless sections after chromatin removal by nucleases. The localisation of nucleoskeleton-related lamins A and C, NuMA, SRm160 and hnRNP H was tested by immunofluorescence. The characteristic structures of the nucleoskeleton (nuclear lamina, core filaments and the 'diffuse' nucleoskeleton) were present throughout all embryonic stages studied, although less discernable during the 1-cell stage. Lamins A and C as well as the NuMA protein were observed in embryo nuclei from the 1-cell stage; a diffuse hnRNP H and speckled SRm160 immunofluorescence appeared from the 4-cell stage. During the 8- to 16-cell stages (major transcriptional activation), the immunofluorescence patterns were identical with those of differentiated cells (fibroblasts). The temporal pattern of immunolabelling confirmed that transcription and splicing compartmentalisation was established progressively during cleavage, and that some of the proteins tested can be used as markers in studies on nuclei reprogramming after transfer into enucleated oocytes.