Characterization of abnormal one pronuclear human oocytes by morphology, cytogenetics and in-situ hybridization

The morphology, chromosomal constitution and developmental capability of abnormal human oocytes (94/3500 oocytes; 2.7%) which after insemination exhibited only one pronucleus were examined. The majority of one pronuclear oocytes exhibited two or more distinct polar bodies. Dividing oocytes showed irregular chromosome distribution from haploid to diploid. Embryos resulting from abnormal oocytes displayed limited developmental potential. Many of them underwent fragmentation or were arrested at the 2- or 8-cell stage of development, and only some reached the morula or blastocyst state (11/35 oocytes). In approximately 45% (15/33) of examined oocytes, decondensed sperm heads or tiny nucleus-like structures were found in addition to a single nucleus. Chromosome Y was also detected in chromosomal preparations in approximately 10% of the oocytes by in-situ hybridization utilizing the human Y chromosome-specific DNA probe (DYZ3). These observations provided strong evidence that many of these oocytes originated from fertilized oocytes. The origin of the other one pronuclear oocytes could not be determined. Parthenogenetic activation of some oocytes cannot be excluded and other explanations concerning the origin of abnormal oocytes are discussed.