PRESENCE OF CHROMOSOMAL MOSAICISM IN ABNORMAL PREIMPLANTATION EMBRYOSDETECTED BY FLUORESCENCE IN-SITU HYBRIDIZATION

The extent of chromosomal mosaicism in human preimplantation embryos w as examined using an improved procedure for the preparation and spread ing of interphase nuclei for use in fluorescence in situ hybridisation , allowing the analysis of every nucleus within an embryo. One cell sh owed no hybridisation signals in only three of the 38 embryos that wer e included in this study, i.e. the hybridisation efficiency per succes sfully spread nucleus was 99% (197/200). Double-target in situ hybridi sation analyses with X- and Y-chromosome-specific probes was performed to analyse nine embryos resulting from normal fertilisation, 22 polyp ronucleate embryos and seven cleavage-stage embryos where no (apronucl eate) or only one pronucleus (monopronucleate) was observed. We also a nalysed autosomes 1 and 7 by double-target in situ hybridisation in th e nuclei of two apronucleate, one monopronucleate and four polypronucl eate embryos. All nine embryos that resulted from normal fertilisation were uniformly XY or XX. None of the apronucleate or monopronucleate embryos was haploid: three were diploid, one was triploid and three we re mosaic. Fertilisation was detected by the presence of a Y-specific signal in four of these embryos. Of the polypronucleate embryos, two w ere diploid, two were triploid and 18 were mosaic for the sex chromoso mes and/or autosomes 1 and 7. These results demonstrate that fertilisa tion sometimes occurs in monopronucleate embryos and that chromosomal mosaicism can be detected with high efficiency in apronucleate, monopr onucleate and polypronucleate human embryos using fluorescence in situ hybridisation.