Mechanisms of aneuploidies in human oocytes.

Objective: The aim of this study was the analysis of maternal aneuploidy mechanisms. Three origins have been described: (i) non-disjunction of bivalent chromosomes (NDJ) which could be age related: (ii) unbalanced predivision of sister chromatids at meiosis I (22 + 1/2 or 23 + 1/2); and (iii) balanced predivision (22 + 1/2 + 1/2), which could be an artefact caused by prolonged in-vitro culture. Materials/Methods: Metaphase II oocytes which failed to fertilize after IVF or intracytoplasmic sperm injection were fixed at days 1-4 after follicular puncture and stored at -20°C. Chromosomes 13, 16, 18, 21 and 22 were hybridized by fluorescence in-situ hybridization (FISH, Vysis kit and protocol, with modification). Aneuploidy was scored with strict criteria: if it was complemented in the polar body, if an additional chromosome or chromatid was found and the polar body was absent, and if sister chromatids were separated by more than two signal diameters. Results: From 81 patients 258 oocytes were fixed. Fixation efficiency was 52% (135/258). A total of 93 of 135 oocytes have been analysed by FISH, Hybridization efficiency was 90% (84/93). In 32% (27/84) the polar body could be analysed simultaneously. (i) NDJ was found in 26% {22/84}. (ii) Unbalanced predivision was found in 13% (11/84) and both were found in 6%, (5/84). (iii) Balanced predivision was found in 48% (40/84): in 35% (12/34) of day 1 oocytes, in 54% (22/41) of day 2 oocytes, and in 67% (6/9) of day 3 and 4 oocytes combined (P < 0.05). There was no significant difference for NDJ and unbalanced predivision between day 1 (24% and 18%), day 2 (29% and 13%) and days 3 and 4 combined (22% and 0%), Aneuploidies were found in all five chromosomes with a range of 6% (chromosome 21) to 26% (chromosome 13). Conclusions: Every second unfertilized oocyte can be successfully checked for chromosomal abnormalities by FISH. Every third oocyte was aneuploid for NDJ and unbalanced predivision. These aneuploidies are not related to in-vitro ageing, Balanced predivisions are linked to in-vitro ageing. Chromosome 13 is the one most frequently abnormal. [ABSTRACT FROM AUTHOR]