The quality of the embryos is evaluated according to various morphological parameters after fertilization. Apart from various other factors, the quality of the transferred embryo significantly determines the chances of pregnancy. It is important to keep in mind that the woman's age and some other factors are as important for pregnancy as embryo quality. Each embryo is examined under a microscope to determine embryo quality. Each embryo is evaluated according to three parameters:
Number of cells;
Whether the cells are of equal size or not;
Presence of “fragmentation” and its proportional percentage relative to embryo size.
The number of cells in each embryo is between 1-12. It is thought that the more cells there are in the embryo, the faster the embryo divides, and therefore the highest chance of pregnancy. On the 3rd day after egg collection, a cell count of 6 or more indicates a good quality embryo. Pregnancy can also be achieved with embryo transfers of 4 or fewer cells, albeit with lower chances. In a perfect quality embryo, the cells are of similar size and there is no fragmentation. The structure we call fragmentation is non-living cell residues. As fragmentation increases, the embryo quality (grade) decreases and the chance of pregnancy decreases.
It is also important to know that embryo quality is a very rough indicator of pregnancy success. A good quality embryo may not always provide the ideal genetic and biochemical content for a healthy baby. Embryo quality can change within hours. For this reason, it is recommended that people do not focus on embryo quality more than necessary. The point to keep in mind is that the most important indicator of embryo quality is to obtain a healthy baby.
Embryo selection criteria
Embryos are examined with special microscopes, classified and scored. The higher the embryo score, the higher its potential to attach and cause pregnancy. The embryo/embryos with the highest score on the day of transfer are preferred for transfer. Many parameters of the embryo are used in scoring.
First day evaluation
Core position and volume
Number, shape and distribution of nucleoli
Location of polar body
Evaluation of the division phase on day 2 and 3
Early division
The momentum of division
Blastomere volume
Fragmentation degree
Blastomere multinucleation (more than one nucleus)
Cytoplasmic image
Features of perivitelline area (extracytoplasmic area) and zona pellucida (outer shell)
Day 5 and 6 blastocyst stage evaluation
Blastocele (central cavity) size
Inner cell mass
Outer cell mass (Trophectoderm)
Day One Review
Core position and volume
16-18 hours after the microinjection application, 2 nuclei carrying chromosomes from the mother and father begin to form in the middle of the egg. The female pronucleus progresses until it gets very close to the male pronucleus and they merge. Observing 2 nuclei in the middle of the egg means that the oocyte is fertilized.
Research and studies have shown that for pronuclear oocytes to be considered morphologically normal, both PNs must be in a central position in the cytoplasm, close to each other and of the same size.
Number, shape and distribution of nucleoli
Nucleoli are tiny, numerous and scattered within the nuclei. Over time, they fuse with each other, decrease in number, grow larger and line up tightly next to each other. Studies show that the same number and order of nucleoli in both nuclei is an indication of higher retention potential.
According to the Tesarik and Greco scoring system, P0 has the highest holding potential and P5 has the lowest holding potential.
Location of polar body
The polar body is a structure located in the perivitelline space and formed during meiosis. The appearance of the polar body means that the egg is mature. The small angle between the nuclei and the polar body indicates that the embryo's retention potential is high.
The cytoplasmic halo is the region of low density seen around the egg. Its presence is an indication that the embryo has a high retention potential.
Day 1 fertilized egg: Two nuclei can be seen, one carrying chromosomes from the mother and the other from the father.
2nd and 3rd day division phase examination
In embryo scoring systems, the cleavage stage should also be evaluated to distinguish embryos with high implantation potential.
The presence of early cleavage at the cleavage stage, cleavage acceleration, blastomere volume, fragmentation rate, nuclear status of blastomeres, cytoplasmic image, perivitelline area and zona pellucida features are examined. Many different cleavage stage embryo scoring systems have been identified in the literature. After providing information on the parameters evaluated in these scoring systems, an example 'cleavage stage embryo scoring system' will be given.
Early Division
The fertilized egg begins to divide after an average of 20 hours and forms a 2-cell embryo. At the twenty-fifth hour, an average of 20% of fertilized eggs are in the 2-cell stage. Studies have shown that embryos that divide early (20-26 hours after microinjection ) have a higher pregnancy and implantation potential. Based on this information, the time of first cell division of fertilized eggs is examined as an important criterion used when choosing an embryo.
Division Speed
A good quality embryo should have 4-5 blastomeres on day 2 (hour 42-44) and at least 7 cells on day 3 (hour 66-68). Studies have shown that embryo development that is too slow or too fast negatively affects embryo implantation rates.
Blastomere (cell) size
Cell division that is not of the same size is considered a parameter that provides information about embryo quality and embryo development potential. Studies have shown that pregnancy and implantation rates are negatively affected by the transfer of embryos that are not divided equally.
Fragmentation degree
The structures within the embryo that are not cells are called fragments. The number of fragments without a nucleus and the ratio of their amount to the embryo volume should be examined when determining embryo quality. The cause of cell fragmentation and its effect on embryo development are not fully known and are specific to both the embryo and the patient.
Structural features of blastomeres
Another morphological parameter used for embryo scoring is the presence of more than one nucleus in a cell (multinucleation). This is an undesirable situation and embryos with this condition should not be transferred.
Cytoplasmic View
If the cytoplasm is light colored, transparent or slightly granulated (spongy structure), it is considered normal.
Perivitelline area and zona pellucida features
When scoring embryos, width, narrowness and inclusions in the perivitelline region (extra-cytoplasmic area) should be evaluated, and the structure and thickness of the zona pellucida (outer shell) are also examined.
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