Quality of oocytes (eggs) is a factor, which limits women’s fertility. Of course, sperm plays an important role in the formation of the embryo (number of father’s chromosomes and some factors, which are necessary for fertilization and further epigenetic changes) but the embryo’s destiny depends mainly on egg.
Oocytes characteristics, which can influence on their quality, can be divided into 3 categories: morphological, cellular and molecular. Usually oocytes quality is determined morphologically by appearance and properties in the period after fertilization.
Visual methods are safe because their use does not affect the viability of eggs.
Evaluation of two other categories is not performed in medical practice today, since these methods are incompatible with the viability of oocytes but still are more precise and objective in quality evaluation.
Morphological oocyte abnormities can appear to 13% of unfertilized eggs after in-vitro fertilization and to 60-70% of oocytes, which underwent the denudation of cumulus cells and which are ready to intracytoplasmic insemination of sperm. Morphological characteristics include appearance of cumulus, maturity core, appearance of the polar body, presence of cleavage spindle, size, form, color, presence of inclusions in the oocyte and perivicular space, condition of the area of the pelulicide.
On the first stage of quality evaluation, the oocyte-cumulus complexes received after the retrieval are evaluated. It is considered that mature oocytes have to be surrounded by more than three lines of cumulus cells, which tightly wrap the egg. Oocyte maturity is important factor, which determines the success of ART. About 85% of oocytes are mature with characteristic delay of division on the stage of MII.
Oocyte maturity determines by the presence of the primary polar body. Immature eggs can ripen spontaneously in conditions of in-vitro cultivating, however the level of their fertilization is much lower and degree of embryos development also decreases. In the area of polar body detachment, the cleavage spindle is located, which secure the correct chromosomes difference during the fragmentation. The damage of the structure, changes of location and/or absence of spindle can lead to aneuploidy, cell death, decrease of fertilization frequency and blastocysts formation. It is shown that woman after age of 40 can often have damage of cleavage spindle. About 0,3% of received oocytes are bigger in size (almost in 30%). Although, such oocytes are fertilized and are able to grow to stage of blastocyst. However, such embryos are not recommended to transfer because of possible chromosome abnormities and fetal miscarriage. The presence of granulation in the cytoplasm (what means its immaturity) do not influence on the ability to fertilize, development and embryos quality, however, correlated with low pregnancy rate. Despite this, preimplantation diagnostics data claims that more than 50% of cells of such embryos are aneuploidy, what explains the high level of pregnancy interruptions. In contrast to granulation the formation of vacuoles and various inclusions in the cytoplasm of oocytes has the negative influence on the ability to fertilize and blastocysts formation.
Form and color of oocyte, size, thickness and color of the area of the pelulicide belong to extracitoplasmic characteristics and do not influence on the frequency of fertilization, the percentage of developing embryos and their ability to implantation. Significant changes in cytoplasm oocyte such as central granulation, the presence of vacuoles and inclusions, changes of cleavage spindle, and especially a combination of several defects lead to violations of the embryo development and its implantation.
The general approach in human embryology is at present the postponement of the problem: call available oocytes that meet the basic criteria of MII phase are subjected to fertilization. With the selection based only on the morphological characteristics after fertilization, seemingly appropriate but intrinsically handicapped embryos may be cultured and transferred resulting in compromised in vitro development, or low pregnancy rates, abortions and further negative consequences.
To guarantee high quality of the oocytes we can by ensuring appropriate donor selection, adequate stimulation protocol, precise and timely triggering, small number of retrieved oocytes, oocyte selection in compliance with the following criteria.
Oocyte selection criterias.
To guarantee high quality of the oocytes we can by ensuring appropriate donor selection, adequate stimulation protocol, precise and timely triggering, small number of retrieved oocytes, oocyte selection in compliance with the following criterias:
- maturity (М ІІ), М І and GV oocytes are exclusion criteria);
- The polar body size (considerable deviations are exclusion criteria).
- Oocyte size.
- The presence of vacuoles (considerable amount or their large size (over 20-25%) are exclusion criteria.
- ASER (aggregation of smooth endoplasmic reticulum) (considerable amount or their large size (over 20-25%) are exclusion criteria.
- Oocyte shape (considerable deviations are exclusion criteria).
- The zona pellucida shape, colour and thickness (considerable deviations are exclusion criteria).