In in vitro fertilization (IVF) treatments, there are two techniques for attempting to fertilize oocytes:
- Conventional IVF, in which previously capacitated sperm contact the oocytes so that one of the sperm penetrates the oocyte.
- Sperm microinjection (ICSI), in which a spermatozoid is chosen to microinject it into the oocyte previously isolated from the surrounding cells.
In both cases it is necessary to allow a series of hours (+/- 18 h) to pass in order to be able to confirm that the correct fertilization of the oocyte has taken place. With fertilization, the meiosis of the oocyte is completed (reducing division that allows the reduction of chromosomes by half), this is evidenced by the appearance of a small satellite structure called the 2nd polar corpuscle. In addition, two intracellular structures are formed, the pronuclei, which contain genetic information of each of the parents. The appearance of the pronuclei allows us to determine whether or not there has been fertilization and if this has been anomalous in which case the embryo would not be selected. These structures are visible for a few hours, so classically, it was necessary to organize the activity of the IVF laboratories to be able to evaluate the possible fertilization in a few very specific hours. If the assessment was not made at the right time, it could lead to diagnostic errors. The use of Time-Lapse systems, incubators with video systems that allow the evolution of embryos to be recorded, has allowed these practices to be modified. In such a way that currently, with morphokinetic incubators (GERI; Embryoscope...), embryologists review the images and can evaluate whether fertilisation has taken place, has been correct and at what point in time it has taken place. In this way an ideal evaluation of the embryo is achieved.
The fertilization rates of a center depend not only on the quality of the laboratory but also on the quality of the gametes. Poor quality oocytes can lead to fertilization difficulties that are not always ignored with ICSI. In a similar way. A minimum of seminal quality is necessary in order to be able to offer women classical IVF fertilization. In cases of serious alterations, a single spermatozoid per oocyte will be chosen for ICSI. The selection of cases for one technique or another is essential to ensure the success of the technique. Although it is true that some cases of failure of fertilization with classic IVF are resolved with ICSI, ICSI, which is carried out in more than 70% of cases in our country, is not always a guarantee of fertilization or the technique of choice.
Several lines of research are being carried out to combat fertilization failures with substances such as ionophores that activate the calcium channels of the oocyte.