What are Chromosomes?
Chromosomes are string-like structures found in the center of every cell (the nucleus). Chromosomes contain genes that are made of DNA. Therefore, our inherited information is housed on the chromosomes. Normal human cells (embryo, fetus, baby or adult) contain 46 Chromosomes or 23 pairs. We receive 23 chromosomes from each parent. The first 22 pairs of are the same for men and women and labeled largest to smallest: 1 through 22. The 23 rd pair determines our sex. To test for a chromosome abnormality such as Down syndrome, the chromosomes are studied.
Chromosomal Aneuploidy
Spermatozoa or eggs that have extra or missing chromosomes will pass this problem on to the embryo after fertilization. This situation is known as aneuploidy. There can be extra (trisomy) or missing (monosomy) chromosomes. Both conditions are a problem. If the aneuploidy involves the larger chromosomes, the embryo may not attach to the wall of the uterus or may stop developing soon after and miscarry. In some cases, however, the aneuploidy may cause the fetus to be abnormal but carry to birth. Down syndrome is an example of this, but there are several other types. The features of the chromosome condition depend upon which chromosome is extra or missing, but can include physical abnormalities and mental retardation.
Avoiding Transfer of Chromosomally Abnormal Embryos
Aneuploid embryos are mostly indistinguishable morphologically and developmentally from chromosomally normal ones, thus, without genetically testing them, the embryologists doing your IVF procedure cannot recognize them and may transfer chromosomally abnormal embryos to you.
The PGD Procedure
A technique called Preimplantation Genetic Diagnostics (PGD) has been developed to test your embryos prior to the transfer of the embryos to the uterus. This technique consists of the removal (biopsy) of one cell of each embryo, followed by a very fast genetic analysis using a technique called fluorescence in situ hybridization (FISH), and the subsequent replacement of those embryos identified as normal. Normal embryos have a higher chance of implanting, resulting in pregnancy and not miscarrying, than abnormal embryos.
The cells to be analyzed are either polar bodies or blastomeres. The ripening egg produces two small cells called polar bodies that degenerate after fertilization. The chromosomal or genetic content of these cells allows us to infer the chromosomal content of the egg. If one is testing the polar body, an opening is made in the zona pellucida, the covering of the egg, and the polar body is removed via aspiration with a pipette, a very thin glass tube. The polar body is then analyzed while the egg is placed in an incubator. By analyzing polar bodies, we obtain information from only the mother.
Paternal or sperm genetic information or chromosome abnormalities that may occur after fertilization will not be detected. At least one third of abnormalities occur during or after fertilization and are not found in the egg. That is why we prefer to biopsy cells from the embryo. A blastomere is a cell from an embryo. To obtain the blastomere, an opening is made in the covering of the embryo during its third day of development when the embryo has 6 to 10 cells. A blastomere is removed by gentle suction. The embryo is placed in an incubator while the cell is analyzed. Chromosomal disorders are tested by direct study of the chromosomes.
Not all genes or chromosomes can be studied by PGD and one cannot test for both genes and chromosomes from the singles cell concurrently. Neither test is 100% accurate because we can only biopsy a single cell from the embryo, thus follow-up prenatal testing via chorionic villous sampling (CVS) or amniocentesis is highly recommended.
Risks of the PGD Procedure
While PGD is a relatively new procedure in IVF, the micromanipulation or biopsy techniques required to perform the procedure have been in use for many years. The risk of accidental damage to an embryo during the removal of the cell(s) is less than 1% in experienced fertility centers. Additionally, no part of the future fetus will be lacking because of the removal of a cell. There is no clear evidence that biopsy of one cell is a problem for the developing embryo later on.
The test may occasionally classify an abnormal embryo as normal. Very few of such pregnancies have occurred. The reverse may happen, too - a normal embryo that is tested may be classified as abnormal by mistake, though the chance of this is also small. Again, due to the small chance of misdiagnosis as well as the presence of conditions not tested for via PGD, prenatal testing is still recommended.
How Can You Have PGD?
PGD is usually preformed in a very specialized genetics center. Your IVF center will arrange for or perform the biopsy and process the cell and then send it to Reprogenetics. Reprogenetics scientists developed the first tests for PGD of aneuploidy, and therefore, are one of the PGD centers with the most experience in this technique. For more information regarding Reprogenetics, please check www.reprogenetics.com. If you are interested in PGD through PRS, please email us info@prsfertility.com.
What About Cost?
As you might expect, this technology doesn't come cheaply. This procedure may add close to $6,000 to the cost of IVF. Few insurance policies cover the expense.