Cytogenetic and morphological observations of single pronucleated human oocytes after in-vitro fertilization.

At 16-18 h after insemination, 5.5% of the inseminated oocytes displayed only one pronucleus. The incidence of single-pronucleated oocytes was not correlated with the age of the patient. Cytogenetic analysis of 41 embryos derived from single-pronucleated oocytes revealed a haploid chromosome complement in 12.2%, a diploid chromosome complement in 80.5% and a triploid set in 7.3% of the embryos. In one diploid metaphase, a Y chromosome could be clearly demonstrated. A total of 312 single-pronucleated oocytes were evaluated twice (at 16-18 h and at 20-24 h after insemination). In 25% of the single-pronucleated oocytes a second pronucleus was observed 4-6 h later, suggesting asynchronous or delayed pronuclear formation. The cleavage of these embryos was similar to the cleavage of embryos with two pronuclei at the first evaluation. Parthenogenetic activation and asynchronous pronuclei may both be mechanisms leading to the morphological observation of a single pronucleus. In clinical practice, the second repeat observation of single-pronucleated oocytes became part of the standard procedure.