Embryo development begins with fertilization. The mechanisms underlying it are still not fully understood and represent a combination of the interaction of germ cells long before the very moment of their unification into a new organism. A huge number of molecules and factors affect the communication between the reproductive system cells. To overcome the cumulus cells and the oocyte zona pellucida, at least 50000-100000 spermatozoa are needed. Moving among the cells of the corona radiata surrounding the oocyte, spermatozoa are activated and acquire a number of changes aimed to the possibility of interaction with the oocyte covering. This phenomenon is called capacitation. The next stage of gamete interaction is a contact specific recognition of protein receptors. All these processes have undergone changes in the course of evolution and represent a unique combination of selective and regulatory cascades.

One of the fertilization methods in the ART laboratory (conventional IVF) is as close as possible to natural conditions: oocyte-cumulus complexes and a sufficient number of spermatozoa are mixed in the container with culture medium, and then we rely on the independent interaction of gametes, similar to their behavior in the body. However, it is not always possible to obtain the necessary amount of fertile spermatozoa for the IVF method, and in a complex cascade of molecular interaction of germ cells, various problems may arise that do not always depend on sperm factors, especially with an existing history of infertility.

In such cases, the embryologist may resort to micromanipulation techniques for selecting and injecting a single sperm directly into the cytoplasm of the oocyte: ICSI. When performing ICSI under the control of special equipment containing micromanipulators and an inverted microscope, the embryologist manually selects the most promising sperm for fertilization. All manipulations are carried out on a special anti-vibration table in order to minimize the external impact on the fertilized cells. The oocyte is retained in a certain position by a holding micropipette, and the sperm is immobilized and injected into the cytoplasm with a very thin glass needle. Under the control of a microscope, destroying the sperm tail membrane, the embryologist activates the same biochemical processes of its competence activation that occur with the natural contact interaction of gametes during fertilization. For an adequate selection of the most promising sperm, a large magnification of the microscope is used – about 600x, and the specialist performing the procedure must have not only honed micromanipulation skills, but also extensive experience allowing the procedure to be carried out in non-standard situations.

Contrary to popular belief, an embryologist is not a wizard and cannot improve what was obtained before him. There is no magic wand or magic in the hands of an embryologist to help him to achieve the desired result. In other words, the entire result of the laboratory’s work will depend primarily on the initial features of patients and the conducted stimulation protocol, and the main task of the embryologist is to be sure that all manipulations are really necessary and rational, and the protocols used are safe and have proven effectiveness. For example, it is impossible to obtain fertilization of oocytes that have not reached the appropriate stage of development (MI, GV), such cells need additional protocols for their maturation in vitro, and the IVF program itself needs to be heavily modified. Therefore, only oocytes that are at the stage of the block of the second meiotic division – MII – are used for fertilization. Each cell has its own internal “clock” of molecular cascades, and by conducting stimulation and assigning a trigger, we mean the smooth operation of this complex intracellular regulation. In the event that the oocyte is not able to respond adequately to it, we cannot count on getting a full-fledged embryo from it. Since our goal is an effective care, and not the maximum number of oocytes or zygotes produced, GV and MI cells are culled before fertilization. In situations where the entire protocol of patient management is aimed in advance to obtain immature cells, it is possible to mature the oocytes using an additional stage of their maturation in special media – artificial maturation of oocytes in vitro – IVM.

After fertilization, the obtained zygotes are evaluated after 18-24 hours in order to determine the successful completion of the gamete interaction process. We have collected information on further evaluation of embryo development in the corresponding section.