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In a PGD cycle, embryos created by IVF are cultured in the laboratory for 3 days, at which time they contain approximately 8 cells (blastomeres). The genetic material of the embryos (which is derived from both parents) is not altered in any way during a PGD cycle, and early embryological development is similar to natural conception, except that it occurs in the laboratory. Embryos that show normal development on day 3 are biopsied with micromanipulation techniques involving the use of very fine glass needles and tools under microscopic observation and control to remove 1or 2 cells for analysis. At this early stage of embryological development, the blastomeres have not yet become committed to form cells of a specific organ or tissue and are capable of becoming any type of cell (totipotent). Removal of a small number of cells from the early embryo does not decrease the ability of that embryo to become a complete, normal fetus and child. Data from many years of PGD in animals and approximately 1200 live births in humans indicates that PGD does not lead to an increase in birth defects or chromosomal disorders. The physical characteristics of the embryo (morphology) are currently used to select embryos with the highest potential for successful pregnancy. Embryologists give a "score" to the embryo based on the uniform size of the different cells, the degree of fragmentation present, as well as other criteria reflecting the physical appearance of the embryo. In our experience, embryos that do not have at least 5 cells on day three or are of poor quality rarely go on to successfully implant. While embryo morphology is helpful in picking the best embryos for transfer, it is known that many embryos with significant abnormalities of the chromosomes can have normal morphology. Even among embryos that successfully reach an advanced developmental stage (blastocyst), 25% are still chromosomally abnormal. Chromosome screening therefore is an improved method to select the best embryos for transfer in an IVF cycle. Back to Top^ PGD Screening at Genetics & IVF PGD for aneuploidy Standard karyotyping for chromosome analysis takes about 5-7 days. In PGD, chromosome information must be available within 48 hours of biopsy, therefore, fluorescence in-situ hybridization (FISH) is utilized. The biopsied blastomere is first fixed to a microscope slide and then the cellular material is digested away leaving the nucleus, which contains the DNA, in a spread out and decondensed form. The cell being studied is mixed with a fluorescent DNA probe, which will only attach to its exact DNA match on a particular chromosome. Excess probe is washed off, and the cell is examined under the fluorescent microscope. Genetics & IVF Institute's PGD laboratory uses FISH probes for the chromosomes most commonly found in abnormalities: chromosomes 13, 15, 16, 18, 21, 22, X and Y. The laboratory report of the PGD chromosome screening will have a table (shown below) that lists the embryo number and the number of signals observed for each chromosome. After the list of observed signals for each chromosome, the individual result for the cell and, finally, the embryo will be listed to aid in decision making.
Back to Top^ Normal embryos show two signals for the numbered chromosomes and either 2 X chromosomes (female) or 1 X and 1 Y chromosome (male). Abnormal embryos show a numerical abnormality of a single chromosome i.e. trisomy (three copies), monosomy (1 copy) or nullisomy (no copies). The result for Embryo #3 above shows trisomy for chromosome 16. Embryos are said to have complex abnormalities if more than a single chromosome is abnormal, (as for chromosomes 13, 15 and 16 in Embryo #4 in the sample report). These embryos are also called chaotic, and are the most frequently seen abnormalities, comprising half of all abnormal embryos. This type of complex abnormality arises when an already abnormal embryo encounters additional abnormal cell divisions resulting in an almost random distribution of chromosomes in subsequent cells. Embryos with this type of chromosome abnormality will not produce ongoing pregnancies.
When a PGD biopsy is done, 1-2 cells are removed from the embryo. In order to obtain results of the biopsy, the cell removed must contain a nucleus, as the nucleus contains the genetic information necessary for testing. If the cell removed has no nucleus or if the nucleus ruptures during the extraction or fixation process, that cell cannot be scored for chromosome problems. As embryos are actively dividing, sometimes the biopsied cell contains two nuclei (as the cell was preparing to divide). Most often, embryos which contain such “binucleate” cells are abnormal. Both of these situations may not allow a PGD result to be interpretable for that particular cell. This is one of the reasons we try and take two separate cells to analyze whenever possible. Those embryos considered to be unaffected on the basis of the PGD testing will then be available to be transferred into the woman's uterus or cryopreserved for future use. In about 30% of couples, PGD testing will show that there are no normal embryos available for transfer. There are many reasons why this can occur. In some women, poor response to ovarian stimulation results in a low number of eggs and embryos. In others, despite production of an adequate number of embryos, the chromosome results are uniformly abnormal. Many problems are related to egg quality, while other factors such as sperm quality may also play a role in the number of abnormal embryos produced during an IVF cycle. In general, couples who do not produce normal embryos after two or three IVF/PGD cycles have a low chance of success in future cycles. Back to Top^
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