The chromosomal constitution of embryos developing from abnormally fertilized oocytes after intracytoplasmic sperm injection and conventional in-vitro fertilization

The aim of this study was to analyse the chromosomal constitution of embryos developing from mono- (1PN) and tripronuclear (3PN) oocytes, after in-vitro fertilization (IVF) and after intracytoplasmic sperm injection (ICSI) into oocytes, by means of the fluorescent in-situ hybridization (FISH) technique with specific probes for the chromosomes X, Y and 18. FISH analysis was carried out on embryos from 3PN oocytes: 106 after ICSI and 71 after conventional IVF. In the 3PN embryos after ICSI, equal ratios of XXX and XXY were observed and no XYY embryos were present. This shows the digynic origin of such 3PN embryos. On the other hand, after conventional IVF, the XYY status indicative of dispermic fertilization was observed in some embryos. After IVF, only 12.7% of the 3PN oocytes developed into embryos with uniformly triploid blastomeres, compared with 55.7% after ICSI (P < 0.001). On the other hand, after ICSI only 16.0% of the embryos developing from 3PN oocytes were mosaic, compared with 42.3% after conventional IVF (P < 0.001). FISH was also carried out on embryos from 1PN oocytes: 61 after ICSI and 115 after conventional IVF. In 35.6% of IVF embryos developing from 1PN oocytes Y-specific hybridization signals were observed. This indicates that in 70-75% of such cases a spermatozoon had penetrated the oocyte and that only 25-30% of them were parthenogenetic. A significantly higher proportion (P < 0.001) of embryos developing from 1PN oocytes were diploid after IVF (48.7%) than after ICSI (27.9%); equal ratios of XX and XY embryos were observed in the two groups. Formation of a single pronucleus in an embryo subsequently shown to be diploid indicates that normal fertilization was followed by asynchronous formation of pronuclei. A significantly (P < 0.001) higher proportion of 1PN oocytes developed into haploid embryos after ICSI (31.2%) than after conventional IVF (13.1%). In both groups most of the haploid embryos were X-bearing (IVF, 93.3%; ICSI, 84.2%) and only a few were Y-bearing (IVF, 6.7%; ICSI, 15.8%). A contribution of normal fertilization and androgenetic activation thus led to 1PN oocytes. Gynogenetic and/or parthenogenetic activation, both leading to indistinguishable chromosomal distributions, also contributed to the formation of 1PN oocytes after ICSI and IVF.