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
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CHROMOSOME ABNORMALITIES IN OTHER SPECIES
From all the studies mentioned above it has become clear that from the moment of fertilization, there is a continuous reduction or ‘selection’ of conception products showing chromosome abnormalities. Starting from around 38% at conception and ending at 0.6% at birth [38], selection against aneuploid embryos most probably starts at the morula/blastocyst transition [7]. It is assumed that about 10% of the zygotes do not proceed to the cleavage stage embryos, which means that the development is stopped immediately after fertilization. Trisomy 1 is probably 100% lethal at the preimplantation stage, because it has only been observed following in vitro fertilization.
In contrast to humans, meiotic non disjunction is a rare occurrence in most other species [24]. In the yeast Saccharomyces cerevisiae the frequency is as low as 1/10,000. Also in Caenorhabditis elegans, the error rate is in the same order of magnitude. In Drosophila melanogaster females, estimates vary between 1/1,700 and 1/6,000. In the female mouse, the frequency is higher and exceeds 1% [24].
Reports of chromosomal abnormalities in mammalian embryos of non human species at later stages of development are also relatively rare. In 10-day-old pig embryos [60], in 13- to 14-day-old sheep blastocysts [61] and in bovine blastocysts obtained after IVF [62], polyploidy and in particular, mixoploidy, i.e., mosaicism of diploid and polyploid cells, has been reported. However, it is clear that no other species is comparable to the human with respect to the frequency of chromosome abnormalities resulting from non disjunction.
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