IMMUNOLOCALIZATION OF DNA IN PREIMPLANTATION MOUSE EMBRYOS

Distribution of embryonic DNA in mouse preimplantation embryos was stu died by the immunoelectron microscopic method with colloidal gold. The embryos (one-cell, early or late two-cell, four-cell) were sampled se parately, and immunogold labelling has been performed on thawed ultrat hin cryosections. Condensed chromatin blocks were labelled very intens ively. DNA labelling was predominantly visible over peripheral chromat in blocks and over small patches of chromatin localized throughout the nucleus of one-cell embryo. DNA distribution appeared to be more exte nded in the whole nucleus of early two-cell embryo. A few labelled chr omatin clumps were observed to be associated with prenucleolar body (n ucleolar precursor bod - NPB) in this stage. Convex-shaped electron-li ght areas (fibrillar centres?) were found at the periphery of NPB in t he late two-cell stage. These areas were labelled by scattered individ ual gold particles. In reticulated nucleolar regions (detected in four -cell embryos) immunolabelling appeared as individual gold particles p resent over the periphery of areas with loosely arranged material (fib rillar centres) surrounded by a denser component (dense fibrillar comp onent). These consecutive changes are correlated with nucleologenesis and with the onset of transcriptional activity. We conclude that at th e onset of nucleolar transcriptional activity condensed chromatin does not penetrate the NPB of mouse two-cell embryo.