Embryo selection in IVF consists on selecting an embryo for transfer with the maximum implantation potential.
The evaluation of embryos depends on dynamic and morphological criteria. The cell division rythm and symmetry, the formation of small cell fragments, the number of nuclei present in each cell, the compacting cells on day 4 and blastocyst formation on day 5 (and their characteristics) among others, are the best factors to predict embryo implantation.
Embryo development and quality depend on the sperm and the oocyte. During the first 3 days the embryo evolution depends more on the oocyte machinery, whereas from day 4, the embryonic genome is activated, which appears to reflect the sperm effect on embryo quality. To achieve high quality embryos, it is essential to improve the care of oocytes, sperm and embryos. Keeping in mind that these cells are not normally outside the human body, its removal and maintenance in the laboratory must be performed with the maximum care.
The goal of the laboratory is to obtain conditions “in vitro” as close as possible to the “in vivo” environment of fallopian tubes and uterus, in order to cause the minimum stress to gametes and embryos. It is necessary to monitor the stability of the environmental temperature (23° C) and the internal temperature of the incubator (37°). The incubators also maintain a relative humidity of 95% and a mixed gas of 7% CO2 and 5% oxygen, which are the most similar “in vivo” conditions and allow to stabilize the embryo culture pH among 7,20-7.34. To ensure that the purity of the air in contact with gametes and embryos is the best possible (without particles or volatile alcohols, perfumes, etc.), multiple filters (HEPA and active carbon) are used: external filter units for the air entering the laboratory; 2 internal laboratory and surgical units to re-filter the air; and filters in the gas entrance to the incubators to ensure that the air comes free of toxic particles. Gases used for the incubators (CO2 and Nitrogen) are of the highest purity available in the market (99.995%). We also work with a dim light to decrease potential free radicals in the environment (not normally present in the human body). Any variation from the optimal values triggers the alarm bells. Materials used for the handling of gametes and embryos enter the laboratory and operating room after stringent external and internal quality and embryotoxicity controls.
It is noteworthy that culture media have substantially improved over the years. Incorporating sequential media, having different concentrations and different nutrients depending on the degree of development of the embryo, have enabled us to keep embryos in culture for 5 days, allowing us to study their evolution.
With all these tools in our hands, and the team’s experience, we are able to obtain a high percentage of embryos of excellent quality and select the best embryo for transfer, and thus give patients…..their best opportunity of pregnancy.