Day 3 embryos are referred to as “cleavage stage” embryos.
The reason for this designation is that the cells in the embryo are dividing (or cleaving) but the embryo itself is not growing in size. Think of a pizza. When you slice it, you create more pieces of pizza, but you do not increase the size of the pizza itself. This is what a cleavage stage embryo is like. The genetic material replicates and the cells divide, but the volume of the embryo does not differ from the volume of the unfertilized egg.
Conceptually, you would think that embryos would divide in a very specific sequence; one cell becomes two; two cells become four; four cells become eight and so on. However, actual embryos do not divide synchronously. We commonly see three, five, six cell, etc. embryos. This is not an indication of a poor embryo, but of one that is growing normally. Also, as embryos divide, sometimes a small portion of cytoplasm (the inside of the cell) breaks off and forms a bleb that we call a fragment. Fragments do not contain nuclei and are not considered cells. The causes of fragmentation are poorly understood, but embryos containing a lot of fragmentation are developmentally disadvantaged simply because the cells lose too much cytoplasm and thus, cellular machinery, to the fragments.
Cleavage stage embryos are graded using 2 criteria: the number of cells in the embryo and their appearance under a high-power microscope. While the cell number is objective, the score for appearance is subjective using a score of 1-4. Typically, a good, normally growing day 3 embryos will contain between 6 and 10 cells. From studies that we have done in our lab and from other published studies, we know that embryos that contain these numbers of cells are more likely to develop into viable blastocysts than embryos with fewer cells.
The embryo grade refers to how the cells in the embryos look. A grade one embryo, for example, is one in which all of the cells are the same size and there is no fragmentation in the embryo. The system we use to grade embryo appearance is presented in the following table.
|Grade 1||Cells are of equal size; no fragmentation seen|
|Grade 2||Cells are of equal size; minor fragmentation only|
|Grade 2.5||Cells are mostly of equal size; moderate fragmentation|
|Grade 3||Cells are of unequal size; no fragmentation to moderate fragmentation|
|Grade 4||Cells are of equal or unequal size; fragmentation is moderate to heavy|
Grade 1 through 2.5 embryos seem to have the greatest potential for developing to the blastocyst stage. However, a grade 3 embryo may also be of good quality if it’s appearance can be explained by asynchronous cell division rather than by poor development. We have published data showing that the number of cells in the day 3 embryo is a better indicator of potential than the grade of the embryo. Therefore, an 8 cell Grade 3 embryo would have better potential than a 4 cell grade 2 embryo on day 3.