Пациенти на клиники Бахчеджи тема 2

After fertilization:
classifying cleavage-stage embryos22 to 25 hours after fertilization, the zygote (cell produced by the union of the two gametes, before cleavage) undergoes its first division (cleavage) and becomes a two-cell embryo. A normally growing embryo is then expected to have 3-4 cells on the 2nd day (42-44 hours), 6-8 cells on the 3rd day (66-68 hours) and more than 10 cells on the 4th day.

The number of cells is the first parameter that will determine the grade of the embryo. Thus, if the number of blastomeres (any cell produced during cleavage) is unexpectedly low considering its developmental day, the embryo is evaluated as slow-growing. Additionally, cleavage-stage embryos are checked for the following:  

  Size and shape of blastomeres,    Degree of fragmentation between blastomeres (uneven, unclean division of the cells of the embryo, resulting in cytoplasmic debris),    Nucleus number in each blastomere,    Cytoplasmic appearance,    
Signs of compaction (please see section below “Going Beyond Day 3”).

Grade 1: Embryos with equal-sized blastomeres, including 0-10% fragmentation and with no granular structure in the cytoplasm are ranked as top quality;

Grade 2: Embryos with slightly unequal-sized blastomeres or including 0-10% fragmentation;

Grade 3: Embryos with either slightly unequal-sized blastomeres including 10-20% fragmentation or with significantly unequal-sized blastomeres including 20-25% fragmentation;

Grade 4: Blastomeres cannot be counted or are distinctly different from each other with a fragmentation more than >25%.

Going beyond day3: assessing late-stage embryos

Compaction starts on the 4th day past fertilization, after approximately 96 hours, when the number of cells in the embryo reaches 16 to 20. Intercellular tight junctions (desmosomes and gap junctions) are formed between cells, creating a compact mass called a morula. Cavitation occurs next where the outer layer of cells (trophectoderm) secrete fluid into the morula. Thus, a central, fluid-filled cavity (blastocoele) is formed (please see the pictures below).  

The quality of 4th day embryos is assessed as follows:

Grade 1: Early blastocyst, cavitated embryo or morula without any anomaly (such as fragmentation or vacuolization);

Grade 2: Morula or compacted embryo with one anomaly (such as fragmentation or vacuolization);

Grade 3: Morula or compacted embryo with 2 or 3 anomalies or embryo with no signs of compaction (10 or more blastomeres);
Grade 4: Embryo with no compaction (10 or fewer blastomeres).

Blastocyst culture

Blastocyst formation occurs at day 5 or day 6 after insemination. One of the greatest advantages of blastocyst culture is that it can identify slow growing or growth-arrested embryos which ultimately give a low chance of pregnancy. In our center, to select the best embryo with the best survival capacity, zygotes are cultured until blastocyst stage and the transfer is usually done on day 5, resulting in a pregnancy rate of 65-70%

Another advantage of blastocyst transfer is the reduction in the number of multiple pregnancies. The main reason for this complication is the practice of transferring more than one embryo, in order to increase pregnancy chances after inadequate embryo selection on day 3. In the past 7 years, our laboratory has successfully established blastocyst culture, allowing the selection of the embryo with the best morphology and the best chance of implanting in the uterus and producing a healthy pregnancy.

Classifying blastocysts

The blastocyst is composed of two differentiated cell groups: the inner cell mass that will later develop into the fetus and the outer cell mass, or trophectoderm, which will produce the placenta and embryonic sac. To transfer the embryo with the best morphology, each blastocyst is assessed regarding:    

the development (from cavitation to hatching –through a series of expansion-contraction cycles the embryo bulges out and emerges from the rigid envelop, the zona pellucida–),    the inner cell mass    the trophectoderm

Score    Blastocyst development

1    Starting cavitation; volume of blastocoele <50% of the whole embryo
2    Blastocoele volume > 50% of the embryo
3    Blastocoele reaches the volume of the whole embryo
4    Blastocoele exceeds the embryo in size, thinning the zona pellucida
5    Hatching starts
6    Complete hatching, the embryo leaves the zona pellucida
Score    Trophectoderm quality
A    Epithelial structure with many closely attached cells
B    Epithelial structure with some loose cells
C    Epithelial structure with many loose cells
Score    Inner cell mass quality
A    Many cells packed together;
  B    Many cells but loosely packed
C    Few cells