What is the significance of oocyte grading? Higher score oocytes give rise to morphologically normal embryos, which in turn have higher implantation potential. So, this just means that it is a good idea to track the embryo's journey right from the oocyte stage so that at the end on day five, if you see two similar looking blastocysts, you can just backtrack and see which had a good oocyte quality so that that particular embryo might be having a better implantation potential, and you can choose that for transfer. The oocyte scoring might be helpful in identifying embryos that are most likely to succeed in pregnancy, while all embryos appear to be otherwise of a good quality. So like I said, when all embryos are looking the same, you can backtrack with the oocyte quality.
Why Oocyte Scoring Matters in Embryo Selection?
Now how are oocytes graded? So first and foremost, they are graded on the basis of their maturity. If an oocyte is not mature, it is not going to fertilize and give rise to an embryo. So, second is the zona pellucida. Third is the peri wet lens space.
So that is the, space that exists between the zona and the oolemma, then the polar body morphology, and, finally, the characteristics of the ooplasm. So, we will address them one by one now. But before that, what is embryo grading? So, embryo grading like oocyte, it is the method for evaluating the embryos for transfer. Now it is critical to evaluate the egg maturity before fertilization because a mature egg has a greatest chance of successfully getting fertilized.
Parameters for Oocyte Grading
On the day of, pickup, we perform the IVF and the fertilization check is performed on the next day. That is day one. Embryos are typically graded on day three. And post this, we leave it for progress till day five depending on the situation. On day four, the embryos compact and they become modular, and no real grading is available for a modular as of now. So, we directly grade them on day five or when they have reached the blastocyst stage. So, this is the developmental stage that occurs prior to implantation of the embryo in the uterus. Now blastocyst implantation after day five is beneficial because it leads to more viable embryos that have already overcome the possible developmental hurdles. This just means that, when you culture embryos up to the blastocyst stage, this does not improve the quality of the embryo as such, but it is a method of natural selection. So not all good looking, day three embryos, they reach up to day five. Only the genetically normal and viable and embryos with better implantation potential reach up to the day five stage. So, this is a method of natural selection of the embryos. On day five, embryo grading then becomes blastocyst grading, and the goal becomes evaluating each embryo for optimal blastocyst development for a successful IVF outcome. If you have to perform a PGT screening, that is also typically performed on day five embryos or blastocyst. We earlier used to perform on day three embryos as well, but that has led to a high level of mosaicism. Okay? And on day five, it is possible to extract six to seven cells. So, as opposed to on day three when we could just extract two to, one to two cells for genetic analysis. So, this gives you a better report.
Embryo Assessment from Day 2 to Day 5
Now several consensuses have taken place, for the grading of the oocyte and the embryos and the blastocyst. So, first is the oocyte grading. Oocyte scoring is based on oocyte cytoplasmic dysmorphisms. That means if there is any dysmorphism or something that is not looking morphologically normal in the oocyte cytoplasm, then extra cytoplasmic dysmorphisms. Okay? Fragmentation in the PVS, inclusions in the PVS, or the zona itself, then the OCC complex or oocyte corona cumulus complex. That is the eggs as we see once they are retrieved from the follicles. OCC stage is the one that we see first and foremost. Okay? Second is the zygote scoring. Zygote is the stage of the embryo on day one when fertilization has just taken place, and you can see the pronuclei or p n.
Fertilization check is what you do on day one. So, fertilization and syngamy check. Syngamy means how the both the pronuclei have come together. Then on day two, we do an embryo assessment, day three, day four, and day five.
On day two, we expect to see four to six blastomeres. On day three, we expect to see about six to eight to 10 blastomeres. So, if this is the seed that you are seeing on day two that is with a very fast-growing embryo, no. You should this is what is expected to be seen on day three.
And, actually, for a day three, it is a very beautiful embryo because you can see all the blastomeres are of the same size, and this is kind of a picture-perfect embryo. Then the symmetry of the, blastomeres. If one blastomere is very huge and another is very small, then, you know, it is not a very symmetrical, embryo. Okay? So, symmetry also matters when you are creating. Finally, multinucleation. Okay? Multinucleation is you are supposed to see one nucleus inside one blastomere. So, if there is multinucleation, there is something very wrong with that cell.
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