Chromosome studies in oocytes and zygotes from an IVF programme.

Chromosome studies have been carried out in 117 oocytes, 17 one-cell zygotes and four, two- to four-cell zygotes in our IVF programme. Three apparently unfertilized oocytes were, in fact, diploid zygotes. Two of 14 apparently polyspermic zygotes were also diploid. One zygote with four pronuclei was pentaploid. This indicates that pronuclei can either be confused with other cytoplasmic structures, like vacuoles, or be eliminated. Endoreduplication was observed in one tetraploid, apparently polyspermic zygote, and in one two-cell degenerated zygote. The incidence of aneuploidy in unfertilized oocytes, taken as twice the level of hyperhaploidy, was 15.4%. Five oocytes showed fragmented metaphase II chromosomes (4.3%). The incidence of unreduced oocytes, due to a lack of extrusion of the first polar body, was 6.8%. Thus the total number of potentially aneuploid, polyploid or non-viable zygotes due to chromosome aberrations in the oocyte was 26.5%.