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BACKGROUND: The aim of this study was to investigate haploidization of somatic cell nuclei in non-enucleated mature oocytes regarding spindle formation, chromosomes and developmental potential. METHODS: Mouse cumulus cells were injected into metaphase II oocytes. Some injected oocytes were examined for morphological changes of chromosomes and the spindle immediately, and at 30 min, 1 h or 2 h after the injections. The remaining oocytes were activated by Sr 2+ after various incubation periods and observed for formation of a second polar body and pseudo-polar body. Cytogenetic analysis was performed for some of the resulting zygotes. The progress to blastocysts in vitro and the possibility of conception in vivo were assessed. RESULTS: Immediately after injection, the cumulus cell nucleus was still in interphase without spindle formation. The occurrence of premature chromosome condensation (PCC) and spindle formation increased as the incubation time increased. The percentages of activated oocytes increased with the incubation time after nuclear transfer, but the difference was not significant between 1 (58%) and 2 h (62%). The incidence of chromosomal aberrations was high for the derived embryos. Development in vitro was poor, and no procreation of pups occurred after transfer of the 324 embryos. CONCLUSIONS: The PCC and spindle formation induced by cumulus cell nuclei in mature oocytes was time dependent, as was the chance for successful activation. The chromosomal abnormalities from segregation errors presented one obvious cause, apart from the potential epigenetic defects, of developmental failure of the semi-cloned embryos.