Why does the fertilization failure sometimes? At the early stage of fertilization, egg and sperm cells are fused, which in the narrow sense is the definition of fertilization. In unfertilized eggs, meiosis ceases at a species-specific stage. The early stage of fertilization breaks the pause in cell division and meiosis resumes. This is called “oocyte activation”. This activation allows meiosis to be completed, and then pronuclei of males and females are formed. The fusion of the male and female genomes (syngamy) completes fertilization in a broad sense. On the other hand, fertilization is not established if fertility of eggs or sperm cells deteriorates.
It is remarkable to note the improvement in fertilization rates through micro insemination (intracytoplasmic sperm injection, ICSI). However, fertilization is not achieved even after ICSI in many cases. In such severe cases of infertility, artificial activation of the oocyte is one of the possible solutions. Thus, the effectiveness of various ways to activate eggs from both aspects of cytological evaluation and biochemical evaluation.
Motility, capacitation, and the acrosome reaction of human sperm are important events in the in vivo fertilization process. However, when performing ICSI, they are not significant. Confirmation of the second polar body and pronuclei indicates successful fertilization. Even in the case of round spermatid cells and immature spermatogenic cells, fertilization was confirmed when egg activation was induced artificially.
The Reasons of Fertilization Failure
The reasons of fertilization failure can be divided into cases originating from the gamete itself and the environment surrounding the gamete in vivo. The first is the quantity and quality of gametes and the latter is a problem in the environment within the female reproductive tract. When we consider the reasons of fertilization failure in terms of sperm, spermatogenesis, spermiogenesis, conditions within the female reproductive organs, the chemical composition of the tubal fluid surrounding the cumulus cells, interaction with zone pellucida, membrane fusion, and egg activation are common problems.
Whereas with respect to egg cause infertility, egg reservations, oogenesis, egg nuclear and cytoplasmic maturation, the nature of the zona pellucida, membrane fusion, the ability to activate the egg itself, resuming the second meiosis, interacting with a penetrating sperm, and the process leading to nuclear fusion are common problems.
What happens on ICSI oocytes that do not form pronuclei? On the part of the sperm, this phenomenon implies the absence or insufficiency of the sperm factor to initiate the egg activation. From the egg side, this means that egg activation, depending on the intracellular signaling caused by sperm factors, does not occur.
In general, the main reason of fertilization failure in ICSI, as has been shown, is egg activation failure, indicating the involvement of sperm factors.