The potential significance of binovular follicles and binucleate giant oocytes for the development of genetic abnormalities

Normal development of a fertilizable female gamete emanates from a follicle containing only one oocyte that becomes haploid after first meiotic division. Binovular follicles including two oocytes and binucleate giant oocytes that are diploid after first meiosis constitute notable exceptions from this rule. Data provided by programmes of human-assisted reproduction on the occurrence of both phenomena have been reviewed to evaluate possible implications for the formation of genetic abnormalities. To exclude confusion with oocytes aspirated from two adjacent individual follicles, true binovularity has been defined as inclusion of two oocytes within a common zona pellucida or their fusion in the zonal region. A total of 18 conjoined oocytes have been reported and one of the oocyte was normally fertilized in seven cases. Simultaneous fertilization of both female gametes occurred only once. No pregnancy was achieved after transfer of an embryo from a binovular follicle. Binucleate giant oocytes have been observed sporadically but a few reports suggest an incidence of up to 0.3% of all gametes retrieved. Extensive studies performed by two independent centres demonstrated that giant oocytes are diploid at metaphase II, can undergo fertilization in vitro with formation of two or three pronuclei and develop into triploid zygotes and triploid or triploid/mosaic embryos. In summary, giant binucleate oocytes may be responsible for the development of digynic triploidy whereas the currently available data do not support a role of conjoined oocytes in producing dizygotic twins, mosaicism, chimaeras or tetraploidy. However, more information on the maturity and fertilizability of oocytes from binovular follicles is needed. Future studies should also evaluate a possible impact of pharmaceutical and environmental oestrogens on the formation of multiovular follicles.