INTRA CYTOPLASMIC SPERM INJECTION (ICSI)

From a historical point of view, PGD can be seen as na extension of prenatal diagnosis. The gamete and embryo genetic diagnosis before implantation into the uterus was only possible after recent advancesin the field of in vitro fertilization (IVF). In the same way that IVF was introduced in the world, PGD application generated some controversy, when referred for the first time. In spite of that, after the report of the first PGD generated pregnancy, the number of PGD cases has risen constantly. Since 1997 over 100 healthy babies came to the world thanks to this new technology.

PGD is a technically challenging procedure, because it demands a good understanding of embryology and molecular biology. During IVF, oocytes and pre-embryos in the firt cleavage stages are easily accessible, permitting one biopsy or a series of biopsies in the cells. However, the amount of nuclear material secure enough to perform the tests, is still limited to confirm diagnoses. The current transference protocols demand a synchronization between embryonic stage and uterine receptiveness. There is just a small “opportunity window” to complete this preimplantation genetic diagnosis task (the “implantation window” time). In spite of all these barriers, PGD is quickly becoming part of many IVF programs. Until now, there are about 30 centers in the world that can offer PGD.

Up to now, the three most important indications for PGD are:

1. The pre-embryonic sex can be securely determined by FISH technique (flourescent in vitro hybridization) using specific probes for the X and Y chromosomes, or by DNA sequence analyses using the PCR (Polymerase Chain Reaction) technique. In this way, sex linked diseases can be determined and avoided.

2. The choromosomal composition can be obtained through FISH allowing to determine the exact pre-embryo ploidy and thus avoiding aneuploidy. In older women this reduces or eliminates the risk of trissomies like the trissomy of pair 21 (Down Syndrome). FISH can also be used to detect structural chromossomal abnormalities, like balanced translocations.

3. Single gene diseases, like Cystic Fibrosis, Tay Sachs, Sickle Cell Anemia, and other common genetic alterations that can be detected by PCR.

POLAR BODY BIOPSY

The first critical aspect about PGD is to obtain genetic material sufficiently informative without damaging the pre-embryo development potential. These efforts may include the first polar body biopsy (or, to be more precise, pre-conceptional genetic diagnosis), the second polar body biopsy, cleavage stage blastomere biopsy or trophoectoderm cell biopsy. Theoretically, trophoectoderm cell biopsy represents the procedure with better potential to retrieve enough cell quantity for analysis, compared to other methods. However, first polar body biopsy and blastomere biopsy continue being the preferred methods in most clinics. Specific technical details for biopsy tend to be developed according to the embryologist’s personal preferences and experience. Although we can directly biopsy a blastomere, the most common and reliable method to obtain it is to use Tyrode’s Acid (TA) to drill a hole in the zona pellucida. Due care must be taken to correctly position the embryo and identify one of its blastomere nucleus.

ADVANCE