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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CYTOGENETIC TECHNIQUES
Cytogenetic studies can be carried out on metaphase chromosomes or on interphase cells. In the former, classical karyotypes are made, while in the latter, non dividing cells are studied by in situ hybridzation. In situ hybridization can be used to visualise specific DNA sequences in chromosomes of metaphase spreads. This method is also ideal for the study of chromosomes in interphase nuclei. Fluorescent in situ hybridization allows the detection of numerical as well as structural chromosome abnormalities in single cells.
The visualization of chromosomes at the single cell level with great accuracy and optimal efficiency is a formidable task but new approaches have been proposed for the study of oocytes and first polar bodies [4,5], second polar bodies [6], as well as for single blastomeres [7]. If chromosomes have been obtained for karyotyping, staining methods such as multicolor-FISH (M-FISH) and spectral karyotyping (SKY) simultaneously visualzse each human chromosome in a specific color. Both methods differ only in the way the colors are produced [8] and have been applied for the single cell diagnosis of aneuploidy [5,9].
Another promising approach is comparative genomic hybridization (CGH), which was originally described for cytogenetic analysis of solid tumors [10]. It has been successfully applied in pre implantation embryos [11,12] and in early spontaneous abortion [13]. The principle of this method is that test and control DNA are labeled with different fluorochromes. The two genomic DNA sequences are then simultaneously hybridized to normal metaphase chromosomes. The ratio of fluorochrome intensity along the length of a chromosome is proportional to the ratio of test and control DNA. On the basis of a shift in the ratio of the two colors, quantitative fluorescence microscopy reveals which chromosomes (in aneuploidy) or chromosomal regions (in unbalanced structural rearrangements) are lost or have become amplified.
In the future, molecular genetic techniques, such as fluorescent polymerase chain reaction of small and short tandem repeats [14,15] and microarrays for the measurement of DNA copy number [16,17] may replace cytogenetic methods.
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