الفهرس 
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Abstract
Summary
Preimplantation genetic diagnosis(PGD ) is an early form of prenatal diagnosis, aimed at eliminating embryos carrying serious genetic diseases before implantation. It is currently being performed in over 20 centers around the world. The pioneer idea of Preimplantation diagnosis was elaborated by Edwards and Gardner in 1968. more than 160 children have already been born following PGD in over 1200 clinical cycles performed for single gene and chromosomal disorders.
During fetal life, human oocytes arrest at prophase I in the first meiotic division. In this state they contain 46 chromosomes. At ovulation, meiosis resumes as far as metaphase II and results in extrusion of first polar body, containing 23 chromosomes. At fertilization, the second meiotic division is completed resulting in extrusion of the second polar body containing 23 single chromatids, leaving a haploid number of single maternal chromosomes in the oocyte. After Fertilization, the oocyte undergoes a series of cleavage divisions within the zona pellucida, at each of which the blastomeres double in size. By day 3 after Fertilization the embryo is usually at the 8-cell stage. By day 5-6(32-64 cells) the first stages of differentiation occur with blastocyst formation.
There are two main groups of inherited genetic diseases: single gene defects which may affect the autosomes ( chromosome 1-22 ) or the sex chromosomes (X & Y) and inherited by autosomal recessive, autosomal dominant or X-linked (sex linked) modes and chromosomal abnormalities which may be structural or numerical abnormalities.
There are numerous indications for offering Preimplantation genetic diagnosis including advanced maternal age, previous child with chromosome abnormality, family history of a chromosome abnormality, family history of a single gene disorder and other high risk factors such as parental consanguinity, a poor obstetric history as repeated implantation failure and certain maternal illness.
There are two main approaches in PGD. The first and less informative is to remove a polar body with a micropipette, this polar body contains a copy of chromosomal material that is not needed for development by the egg and can be tested to infer the chromosomal makeup of the egg. The second and preferred method is to remove one or two cells ”blastomere”with a micropipette from the embryo on day 3 ; at this stage the embryo usually has 6 to 10 developing identical totipotent cells, each has a full set of chromosomal material.
Embryo biopsy can be performed at three stages, polar body, cleavage stage and blastocyst, but it is recommended to use biopsy on the morning on day 3 post-insemination. It is acceptable to use analysis of both polar bodies to determine recombination events between the first and second polar bodies. Although, in some cases cleavage stage biopsy may be required to confirm the polar body diagnosis.
For zona breaching, laser or mechanical zona breaching is acceptable for polar body biopsy, but acid Tyrode’s is not recommended as it may adversely affects the spindle. Acceptable methods for zona breaching during cleavage stage or blastocyst biopsy include acidified Tyrode’s solution, laser or mechanical methods. As regard cell removal, it is recommended to remove polar bodies and blastomeres by displacement and push methods whereas removal of trophectoderm cells during blastocyst biopsy by aspiration and stitch and pull .
The biopsy obtained from an embryo can be analyzed in a number of different ways. A diagnosis can be made directly from the DNA(single cell diagnosis) by karyotyping, identifying the presence or absence of specific genetic loci in metaphase or interphase chromosomes using fluorescent in situ hybridization (FISH); or by amplifying the minute amount of DNA from one or two cells many times over using PCR. Alternatively the genetic disease may be diagnosed at a post-transcriptional level by measuring the intracellular product of messenger RNA (m RNA) translation. This may be the abnormal protein or an enzyme causing the genetic illness.
The PGD depends mainly upon two analytical procedures. The first is polymerase chain reaction(PCR) which can be used for the diagnosis of single gene defects at the DNA level. The second is fluorescence in situ hybridization (FI``SH) which is used for chromosomal abnormalities diagnosis as well as sexing of embryos. Other methods used for diagnosis, has been developed such as comparative genomic hybridization (CGH); real time PCR and microarrays.
The FISH procedure can be used for chromosomal abnormalities and for aneuploidy screening. A probe set of at least five chromosome pairs from 13,14,15,16,18,21,22, X and Y is recommended. PCR can be used for the diagnosis of single gene defects at the DNA level. When sexing only is being performed for X-linked diseases, it is recommended that FISH is used, as FISH identifies chromosomal abnormalities of the sex chromosomes and is not influenced by contamination. As regard insemination, ICSI is recommended for all PCR cases to reduce the chance of paternal contamination from extraneous sperm attached to the zona pellucida or non-decondensed sperm within blastomeres. On the other hand, ICSI or conventional insemination is acceptable for FISH cases.
Currently, gender diagnosis can be performed by using PGD. The autosomal recessive single gene defects that can be diagnosed by PGD include b-thalassemia, cystic fibrosis, Sickle cell anemia, spinal muscular atrophy, Tay Sach’s disease, adrenogenital syndrome, and Hypophosphataemia. Autosomal dominant diseases that can be diagnosed by PGD include Marfan’s syndrome, familial adenomatous polyposis coil, Huntington’s disease, myotonic dystrophy, and osteogenesis imperfect. X-linked diseases that can be diagnosed by PGD include Duchenne muscular dystrophy, haemophilia, Lesh-Nyhan syndrome and Charcot Marie Tooth.
It was hypothesized that PGD of numerical chromosome abnormalities would be able to reduce the chances of delivering a trisomic baby, to increase pregnancy rate in women with advanced maternal age and to reduce embryo loss during pregnancy. Several PGD cycles have been undertaken for diagnosis of Robertsonian translocations, which are easier to diagnose using PGD, and reciprocal translocation, which is more difficult to be diagnosed by PGD.
The great advantage from the ethical point of view that PGD provides is that it avoids implantation of defective embryos, and this process of selection eliminates the need for future termination of pregnancies. Couples at high risk are offered the opportunity to
overcome the worrisome burden of a possible abortion, as defective embryos are detected in-vitro and only healthy unaffected embryos are implanted.
The conservative method of prenatal diagnosis is closely linked with the heated issue of selective abortion. The medical staff and the ethicists are divided into those who allow selective abortion and those who oppose it. Preimplantation diagnosis avoids such a debate in society and in individual cases.
The selection of embryos on genetic ground appears to be ethically acceptable. The islamic law may accept research on excess embryos resulting from IVF in order to increase knowledge and this may be possible in cases where it will be for the sake of the individual embryo. Research conducted on Pre–embryos should be limited to therapeutic research, and this includes genetic diagnosis of a portion of the embryo, one blastomere or its nucleus for a specific genetic defect. Research aimed at changing the inherited characteristics of Pre–embryos, including sex selection, is forbidden.