THE BEHAVIOR OF MITOTIC NUCLEI AFTER TRANSPLANTATION TO EARLY MEIOTICOOPLASTS OR MITOTIC CYTOPLASTS

This study evaluates the ability of the cytoplasm to determine the nat ure of the division cycle (meiotic or mitotic) in nuclei obtained from mitotically dividing cells. Using mouse oocytes in different stages o f development two types of cytoplasm were prepared: firstly, early mei otic ooplasts were obtained by enucleation of non-matured, prophase-st age oocytes; secondly, mitotic cytoplasts were prepared by enucleation and activation of metaphase II (MII)-stage oocytes. These two types o f cytoplasts were then used in fusion experiments, in which mouse prim itive type A spermatogonia (prospermatogonia) or mouse fibroblasts wer e used as a source of donor nuclei. While the fusion of prospermatogon ia 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 wi th early meiotic ooplasts (40.3%) displayed unique morphology consisti ng of two sets of chromosomes organised in two spindle centres connect ed by microtubules. Each set of chromosomes contained the haploid (1n) number of chromosomes as revealed by chromosome analyses. The same mo rphology was observed also in 44.2% of hybrids in which the differenti ated nuclei of fibroblasts were used as a source of donor mitotic nucl ei. In both cases the hybrids were blocked at this stage with high act ivity of maturation promoting factor (MPF), resistant to any kind of a ctivation and not able to undergo further development. These results s uggest that the early meiotic ooplasm was able to induce the initiatio n of a meiosis-like reducing division in mitotic nuclei originating bo th from the germline cells and from more differentiated somatic cells.