ROLE OF CHROMOSOMES IN EMBRYO DEVELOPMENT
Geraedts JPM*
*Corresponding Author: Professor Dr. Joep P.M. Geraedts, Department of Genetics and Cell Biology, University of Maastricht, P.O. Box 1475, 6201 BL Maastricht, The Netherlands; Tel.: +31-43-3875840; Fax: +31-43-3877877; E-mail: joep.geraedts@gen.unimaas.nl
page: 3

CHROMOSOME ABNORMALITIES IN ZYGOTES AND PREIMPLANTATION EM¬BRYOS

In zygotes studied for the presence of pronuclei, about 20 hours after insemination, the main abnormalities de­tected were parthenogenetic activation (one pronucleus) and triploidy (three pronuclei) [37]. The reported rates of abnormal fertilization observed during IVF vary from laboratory to laboratory [29]. Tripronuclear zygotes, that result in most cases from dispermy [38], are not transferred to the uterus.

The first cytogenetic studies of early embryos were carried out using classical techniques [38]. To obtain suffi­cient metaphases for study, cleaving embryos were re­quired. Only a minority of the embryos studied this way could be analyzed. Furthermore, the proportion of cases in which results were obtained in all cells was even less [39-41]. The rate of reported abnormalities using this tech­nique varied from 23 to 90%.

Study of preimplantation embryos arising from tripro­nuclear zygotes has made it clear that they display a vari­ety of chromosomal abnormalities that include: 1) com­plete triploidy in all cells after regular division, 2) gross abnormalities in all cells due to chaotic chromosome movement after multipolar spindle division, 3) cell sub­populations with either a haploid or a diploid chromo­somal content because of extrusion of a haploid nucleus during the first cleavage division, and 4) cell subpopula­tions with a diploid or a triploid chromosomal content as a result of extrusion of a haploid nucleus during the first cleavage division and subsequent incorporation in one of the two nuclei [37].

The development of FISH technology enabled the study of arrested human embryos with high efficiency, and made it possible to draw conclusions about the presence of normal and abnormal chromosome copy numbers. The FISH studies on nuclei of embryos that resulted from ab­normal fertilization revealed mostly mosaic chromosome complements [42,43]. In human preimplantation embryos, especially those with abnormal morphology, chromosomal mosaicism was a normal feature [40,41,44]. Chromosomal abnormality in preimplantation embryos, at day 2/3 of development, ranged between 30 and 70% in embryos [45-47]. The differences appear to depend on the number of probes applied simultaneously, the type of probes used, embryo morphology, embryo development and the pres­ence of multinucleated blastomeres.


At least 29% of morphologically normal embryos are chromosomally abnormal [22] at the cleavage stages. In human blastocysts chromosomal mosaicism was reported in 29% [48]. During preimplantation development, the percentage of embryos showing chromosomal mosaicism increased to almost 100% at the blastocyst stage [49,50]. Anaphase lagging appeared to be the major mechanism through which human embryos acquire a mosaic chromo­some pattern during preimplantation development to the blastocyst stage. The percentage of abnormal cells per embryo was 16%. The comparative genome hybridization technique showed that three out of 12 preimplantation embryos consist of only normal cells in each of two studies [11,51]. Even though there is a strong selection against chromosomally abnormal embryos, extended culture to day 5 or 6 cannot be used as a reliable tool to select against clinically relevant chromosome abnormalities such as trisomies [52].




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