Cytogenetic analysis of oocytes and embryos.

Abnormal embryo development represents the major cause of implantation failures and accounts for the low rate of human infertility in vivo or in vitro. Chromosome abnormalities are widely involved in this process. Indeed, 28.4% of oocytes carry a chromosome aberration, i.e. 25.6% aneuploidy and 2.8% structural anomalies. Fertilisation abnormalities (possibly increased by in vitro procedures) were recorded: 7 to 28% of oocytes from fertilisation failure showed a sperm premature chromosome condensation probably resulting from ooplasmic immaturity. Moreover, 1.6% and 3.8% of inseminated oocytes had either a single or 3 pronuclei demonstrating parthenogenesis or triploidy, respectively. In vitro developmental capacities of embryos depends on the degree of ploidy. Parthenogenetic embryos display a fairly normal development until implantation. Triploid zygotes show an original way of division: half of them divide first into 3 cells and then into 6 cells (via a tripolar spindle) whereas diploid zygotes divide into 2 and then 4 cells. As a consequence of either meiotic or mitotic non disjunctions or fertilisation anomalies, 25 to 71% preimplantation embryos carry a chromosome disorder. As an outgrowth of in vitro fertilisation and embryo transfer, detection of genetic and metabolic defects prior to implantation might be possible in the future. So far, 6 girls have been born in couples at risk of transmitting X-linked disease. This technic will increase the efficiency of IVF and avoid the trauma of repeated abortions.