Three cases of preimplantation genetic diagnosis (PGD) (two for sexing and one for aneuploidy screening) are presented. Embryo biopsy was performed at day 3 and diagnosis was established with fluorescent in situ hybridization (FISH). Embryos not used for replacement were cultured in sequential media for blastocyst development. Blastocyst rate was 39.3 per cent. Confirmations of diagnosis were established with FISH in blastocysts and arrested embryos. Mosaicism was observed in 7/8 blastocysts (mean number of cells analysed: 55.5) and 5/8 arrested embryos. The percentage of abnormal cells was 17.1 per cent for blastocysts and 54 per cent for arrested embryos. Polyploid cells were observed in 4/8 blastocysts. Confirmation of diagnosis at the blastocyst stage is a useful tool in PGD. Copyright © 1999 John Wiley & Sons, Ltd.

Summary A new methodology for blastocyst biopsy that uses a 1.48μm in diode laser is described. Trophectoderm cells are biopsied after laster zona drilling and culture, fixed and processed for fluorescent in situ hybridisation (FISH) analysis. Preliminary results on the efficiency of the procedure and blastocyst recovery rate are promising. Blastocyst laser biopsy is a useful tool in preimplantation genetic diagnosis (PGD) as it allows a more reliable diagnosis and widens the diagnostic possibilities on account of the higher number of cells obtained in the biopsy.

Although karyotyping is the ideal method for diagnosing chromosome anomalies in the human embryo, the detection of chromosome anomalies in preimplantation human embryos by classical cytogenetics is extremely difficult. Nowadays, fluorescent in situ hybridization (FISH) in interphase nuclei is the method of choice. Both karyotype studies and FISH analysis of human preimplantation embryos obtained after in vitro fertilization (IVF) treatment revealed a high rate (25-51%) of chromosomally abnormal embryos. FISH analysis showed that chromosome mosaicism (22-24%) and chaotic embryos (7-26%) were the most frequent chromosome anomalies. Therefore, it seems that natural selection may be the reason for the low implantation rates of human preimplantation embryos in IVF programs. Although it is not possible to carry out such studies on embryos from natural conceptions, frequent occurrence of chromosome mosaicism and chaotically dividing embryos may apply to in vivo conception and explain the low fecundity rates in humans.