Preimplantation Genetic Screening (PGS)

Preimplantation genetic screening (PGS) is a procedure carried out on in vitro fertilization (IVF) embryos to detect any chromosomal aneuploidy (Acog.org, 2015). The procedure was first done by Munné et al., (1995) and was once called preimplantation genetic diagnosis (PGD) for aneuploidy screening (Harper and SenGupta, 2012). Rather than detecting gene disorder, PGS was introduced to help selecting the best embryos for IVF thus increasing the rate of success of the procedure. This essay will explain the rationale for PGS and the procedures with their technical challenges. It will also discuss the reason methods were changed for second generation of PGS, the advantages and disadvantages of these new methods and whether PGS is worthwhile. …show more content…
The blastomere is removed from zona pellucida on the third day of development by using acid Tyrodes (Thornhill et al., 2004). However in recent years, the usage of acid Tyrodes has been replaced with laser. Ca2+Mg2+ free biopsy media is also used to make the biopsy easier by reducing the junctions between blastomeres (Harper and SenGupta, 2012). Blastocyst biopsy has become more preferable in recent years. The cell is obtained in two ways. A hole is drilled on the third day and is left to allow herniation of trophectoderm cells before being biopsied two days later. This could be challenging as there is possibility that inner cell mass may herniate instead of trophectoderm. The second way is to drill the hole on the morning of day 5 and after a few hours, the cells are cut from the embryo using laser (Harper and SenGupta, 2012).
The old method of analysis uses FISH. The biopsied embryo is transferred into a spreading solution of HCl/Tween. The cells are then mixed with several DNA probes tagged with different fluorochromes that will bind to complementary sequence (Harper et al., 1994). Up to 15 probes have been allowed to hybridize to maximise the number of chromosomes to be screened (Baart et al., 2007). The result can be visualised under a fluorescent microscope (Harper and SenGupta, 2012). The embryos are considered aneuploid if the chromosome abnormality is above 90% (Baart et al., …show more content…
In SNP array, the amplified test and control DNA are labelled with red and green fluorescent molecules respectively (Figure 1). The assessment is done by doing a comparison between the alleles detected and those of the parents to reveal the parental chromosomes inherited by the embryo. Monosomies are detected through homozygosity for all loci in the affected chromosome while trisomies are detected through the presence of three distinct chromosomal haplotypes (Wells et al., 2008). However, just like aCGH, the DNA amplification process could be

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