Pre-Implantation Genetic Diagnosis (PGD)
When there is a history of genetic or chromosomal transmissible family illnesses, it can be of great help to know if an illness is present in the precursor cells (eggs) or in the embryo before the woman becomes pregnant (before the embryo is transferred to the uterus). A technique called Pre-Implantation Genetic Diagnosis is used in cases when it is possible. PGD has to be carried out in conjunction with IVF because to attain the genetic material to be studied, the techniques of the Assisted Reproduction laboratory are needed.
The diagnosis can be made in two different ways:
Pre-Implantation Genetic Diagnosis with embryos
The embryos are studied before being transferred into the uterus. The procedure takes place in the laboratory after IVF and when the embryos are at the 4-8 cell stage. This technique allows for the selection and transfer into the uterus of only those embryos which have been diagnosed as normal. Having been informed and after having accepted to be included on an IVF programme, the patient undergoes a hormonal induction for follicular stimulation. All the collected mature eggs are inseminated to obtain embryos. After 24 hours a biopsy is performed on all the embryos which meet the necessary requirements, and one of their cells is taken with a micromanipulator. The embryos are identified and kept in an incubator while the study continues. The obtained cells are prepared under special conditions and the genetic material is examined. This examination allows the biologist to find out if the embryo they are taken from is affected or not by the illness. Once the condition of each embryo is known, those which are free of the illness are selected and transferred into the woman’s uterus. The affected embryos have to be destroyed.
Preconception Diagnosis with eggs
In the case of PGD with embryos, the couple have to be aware of the fact that in order to have an illness-free pregnancy, only healthy embryos are selected which necessarily implies that the affected ones will be disposed of.
Therefore, one step forward in diagnosing these genetic or chromosomal pathologies even before the embryo is formed is the Preconception Diagnosis using female gametes (eggs). In order to do so, a specific egg cell called second polar body is used. This technique cannot be carried out on a spermatozoon and its application is limited to the inherited pathology of maternal origin.
The couples who may need PGD are those where the woman is a carrier of a chromosomal reorganisation, mainly reciprocal and Robersonian translocations, and the male partner has a normal karyotype. In 94.1% of cases with this technique a polar body with a good chromosomal morphology can be obtained.
All the mature eggs are processed for their genetic study. A biopsy is performed on the polar body by means of a micromanipulator making a tiny incision in the zona pellucida surrounding it. The biopsied eggs are identified and kept in an incubator. To ensure the result, it is important to try the reagents called probes in the lymphocyte preparation of the woman (blood cells) before making a Preconception Diagnosis.
Once the normal eggs are identified they are inseminated via ICSI (intracytoplasmic sperm injection). Therefore, all the resulting embryos are suitable to be transferred and none of them has to be destroyed. The unused ones can be frozen and exploited in future possible attempts.