In our Austin fertility lab, embryos on day 3 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 from one cell to two, to four, to eight and so on.
However, actual embryos do not divide synchronously. We commonly see three, five, six cell 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 cells) 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 given a cell designation (number of cells) and a grade of one through 4 (including 2.5). The number of cells is just that, the number of cells present in the embryo when it is observed under the microscope. Typically, a good, well growing day 3 embryo will contain between 6 and 10 cells. From studies that we have done in our lab and from other studies published, embryos that contain these numbers of cells are more likely to grow 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. In the table below, each of the grades are described.
|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, an embryo that is graded a 3 may be simply dividing asynchronously and not have uneven cells due to poor development. In these cases, day 3 embryos have excellent developmental potential. Also, the number of cells in the day 3 embryo is a better indicator of potential than the grade. Therefore, an 8 cell Grade 3 embryo would have better potential than a 4 cell grade 2 embryo on day 3.