Naturally, the fertilisation process involves the penetration of a single sperm from the ejaculate into the egg (called oocyte in scientific terms) released during the menstrual cycle. Said sperm must penetrate the shell of the oocyte (called zona pellucida) to immediately fuse with the female gamete.
Sometimes, either due to physiological deficiencies that may lead to a drastic reduction or absence in the production of gametes or due to poor gamete quality or defects in the interaction, the process of fertilisation can be diminished or even prevented.
From the Assisted Reproduction Techniques (ART) perspective there are two main approaches
1. Conventional IVF
The procedure Conventional IVF is based on the interaction between the oocytes and the sperm. After the egg retrieval, each oocyte and the set of cells that surround it (granulose cells) will be put in a drop of culture media containing a suspension of sperm at a given concentration. From this interaction a single sperm should penetrate the egg to fertilise it.
Limitations of the technique: -
Although the technique mimics the natural interaction between the gametes that occur in the fallopian tubes. It requires a certain semen concentration due to which not all samples are capable of being used for conventional IVF.
Likewise, the technique cannot solve the possible problems of interaction between female and male gametes so, the uncertainty about the success of the procedure is generally higher.
2. Intra-Cytoplasmic Sperm Injection (ICSI)
ICSI is based on the insertion of a selected sperm into the oocyte, overcoming the limitations that conventional IVF faces with poor semen samples and deficiencies in the interaction between the gametes. Because of this, ICSI has become the most commonly used IVF technique. With the help of an advanced micromanipulation station (including microscope, microinjectors and micropipettes) the sperm has to be individually selected and immobilised. During the microinjection the oocyte must be held while the sperm is inserted and released inside the inner part of the oocyte. The day after the microinjection, signs of proper fertilisation will be checked in each oocyte.
The indications of conventional IVF and ICSI are tubal factor, uterine factor, premature ovarian failure, ovulatory dysfunction, endometriosis, diminished ovarian reserve and unexplained infertility. ICSI is specially recommended for samples presenting alterations in the semen analysis (mostly in cases of low concentration or motility), situations in which there are a limited number of oocytes available or in cases of previous fertilisation failures with conventional IVF. This leads conventional IVF to be usually indicated in good prognosis patients where the quality of both gametes is not an apparent limitation.