Spermatozoa sometimes fail to fertilize even when they are artificially placed in close proximity of egg during conventional in vitro fertilization (IVF). Fertilization failure in IVF is particularly common where there are grossly abnormal semen parameters or when the number of spermatozoa is insufficient. Gamete micromanipulation is the only way to overcome this problem in most cases.

The different techniques developed in this regard focused initially on the obstacle to sperm penetration represented by the zona pellucida (ZP), by thinning it through exposure to enzymes or creating an opening through localized chemical digestion, mechanical breach, or even photoablation. The placing of the spermatozoon beneath the zona has yielded consistent results achieving a fertilization rate of ~20%. However, these techniques have been almost abandoned because of limiting factors such as the need for many functional spermatozoa with good progressive motility, and complications such as multiple sperm penetration.

The Intracytoplasmic sperm injection (ICSI) procedure entails the deposition of a single spermatozoon directly into the cytoplasm of the oocytes, thus bypassing the ZP and the oolemma. The ability of ICSI to achieve higher fertilization and pregnancy rates regardless of sperm characteristics makes it the most powerful micromanipulation procedure yet with which to treat male factor infertility. In fact, the therapeutic possibilities of ICSI go from case in which, after sperm selection, the spermatozoa show poor progressive motility, to its application to azoospermic men where spermatozoa are microsurgically retrieved from epididymis and the testis. Retrieval of a low number of oocytes represent a further indication for this procedure, because only after cumulus cell removal is it possible to identify the oocytes that have extruded the first polar body and then inseminate them accordingly.

ICSI is also suggested when oocytes are to be considered for preimplantation genetic diagnosis (PGD) is to be performed on oocytes, the removal of the polar body requires the stripping of cumulus corona cells, thus leaving ICSI the only option to avoid polyspermy. When embryos need to be analyzed for gene defects, the avoidance of contaminating spermatozoa on the zona pellucida reduces the chance of false positivity with PCR.

Reference

David K Gardner, DPhil. Textbook of assisted reproductive techniques: laboratory and clinical Perspectives. 1 st ed. United kingdom. Taylor & Francis. 2001.147-56