Immunolocalization of DNA in preimplantation mouse embryos

Distribution of embryonic DNA in mouse preimplantation embryos was studied by the immunoelectron microscopic method with colloidal gold. The embryos (one-cell, early or late two-cell, four-cell) were sampled separately, and immunogold labelling has been performed on thawed ultrathin cryosections. Condensed chromatin blocks were labelled very intensively. DNA labelling was predominantly visible over peripheral chromatin blocks and over small patches of chromatin localized throughout the nucleus of one-cell embryo. DNA distribution appeared to be more extended in the whole nucleus of early two-cell embryo. A few labelled chromatin clumps were observed to be associated with prenucleolar body (nucleolar precursor body--NPB) in this stage. Convex-shaped electron-light areas (fibrillar centres?) were found at the periphery of NPB in the late two-cell stage. These areas were labelled by scattered individual gold particles. In reticulated nucleolar regions (detected in four-cell embryos) immunolabelling appeared as individual gold particles present over the periphery of areas with loosely arranged material (fibrillar centres) surrounded by a denser component (dense fibrillar component). These consecutive changes are correlated with nucleologenesis and with the onset of transcriptional activity. We conclude that at the onset of nucleolar transcriptional activity condensed chromatin does not penetrate the NPB of mouse two-cell embryo.