MULTICOLOR FISH DETECTS FREQUENT CHROMOSOMAL MOSAICISM AND CHAOTIC DIVISION IN NORMAL PREIMPLANTATION EMBRYOS FROM FERTILE PATIENTS

We have used multicolour fluorescent in situ hybridisation (FISH) with DNA probes for chromosomes X, Y and 1 to analyse spare untransferred cleavage-stage embryos after preimplantation diagnosis to avoid X-link ed disease. In total, 93 morphologically normal embryos were available from seven patients (six of proven fertility) who had undergone fourt een in vitro fertilisation (IVF) cycles. The chromosome patterns obser ved were classified into four groups; normal, abnormal (non-mosaic), m osaic and chaotic (uncontrolled division). Approximately half of the e mbryos were normal for the chromosomes tested. Two embryos only were a neuploid (nonmosaic) throughout but, after excluding those showing cha otic division, 30% were considered to be chromosomal mosaics. Of these , a minority had arisen because of mitotic non-disjunction or chromoso me loss or gain, whereas the majority were ploidy mosaics, with haploi dy being the most common. The occurrence of chaotically dividing embry os was strongly patient-related, i.e. some patients had 'chaotic' embr yos in repeated cycles, whereas other patients were completely free of this type of anomaly. 'Chaotic' embryos are unlikely to progress beyo nd implantation. These findings have important implications both for r outine IVF and preimplantation genetic diagnosis.