This technique which was accidentally invented towards the end of 1993 is used to achieve egg fertilisation mechanically with cases of impaired sperm motility, count and morphology in normally ejaculated spermatozoa and during microsurgical sperm aspiration or testicular sperm extraction. Other indications include repeated fertilisation failures using conventional egg insemination and for couples where a fertility presentation could not be elucidated at the time (idiopathic infertility).

Most IVF clinics worldwide use ICSI as the preferred method of choice for managing egg fertilisation regardless of the indication. In essence, the technique which necessitates the use of delicate micromanipulation systems entails the selection and activation of a single morphologically normal motile spermatozoon and its subsequent release into the inner part of the egg (cytoplasm) by direct injection. Fertilisation rates after ICSI vary between 70%-90% and these are dependent on the skills of the embryologist performing the procedure and the quality of the gametes themselves.

Since the cascade of mechanisms involved during the normal fertilisation procedure is circumvented with ICSI, the egg does not recognize the presence of the sperm in its cytoplasm unless this is immobilized prior to its injection. Immobilization causes a disruption to the sperm tail triggering the gradual disruption of other parts of the sperm membrane facilitating the entry of the sperm nucleus decondensing factor of the egg to induce the swelling of the head. It is because of this swelling that the sperm head plasma membrane breaks releasing sperm associated oocyte activating factors into the cytoplasm of the egg necessary for its activation.

A few hours following these molecular mechanisms, the sperm and egg chromosomes become indistinguishable from each other and further decondensation causes the development of the male and female pronuclei. During the injection procedure aspiration of the egg, the cytoplasm is necessary to ensure that the oocyte’s membrane is broken therefore the incorporation of the sperm is successfully implemented. The proper orientation of the egg at the time of injection and the needle position is essential parameters towards successful fertilisation minimizing thereby the risks of damage to the egg.