The behaviour of mitotic nuclei after transplantation to early meiotic ooplasts or mitotic cytoplasts

Abstract

SummaryThis study evaluates the ability of the cytoplasm to determine the nature of the division cycle (meiotic or mitotic) in nuclei obtained from mitotically dividing cells. Using mouse oocytes in different stages of development two types of cytoplasm were prepared: firstly, early meiotic ooplasts were obtained by enucleation of non-matured, prophase-stage oocytes; secondly, mitotic cytoplasts were prepared by enucleation and activation of metaphase II (Mll)-stage oocytes. These two types of cytoplasts were then used in fusion experiments, in which mouse primitive type A spermatogonia (prospermatogonia) or mouse fibroblasts were used as a source of donor nuclei. While the fusion of prospermatogonia with mitotic cytoplasts resulted, as expected, in normal premature chromosome condensation (PCC) and subsequent pronuclear formation (58.1%), the majority of hybrids obtained by fusion of prospermatogonia with early meiotic ooplasts (40.3%) displayed unique morphology consisting of two sets of chromosomes organised in two spindle centres connected by microtubules. Each set of chromosomes contained the haploid (1n) number of chromosomes as revealed by chromosome analyses. The same morphology was observed also in 44.2% of hybrids in which the differentiated nuclei of fibroblasts were used as a source of donor mitotic nuclei. In both cases the hybrids were blocked at this stage with high activity of maturation promoting factor (MPF), resistant to any kind of activation and not able to undergo further development. These results suggest that the early meiotic ooplasm was able to induce the initiation of a meiosis-like reducing division in mitotic nuclei originating both from the germline cells and from more differentiated somatic cells.