Day 5 Embryo Grading Evaluates Two Types of Cells in the Embryo.
On day 5, embryos should be reaching the blastocyst stage of development. They are continuing to divide and the number of cells continues to increase, but the cells are also ifferentiating into specific cell types.
By this time embryos should have started to outgrow the space inside the zona pellucida (ZP or “shell”) that surrounds the embryo. They start to expand and thin the ZP in preparation for the stage of development when the blastocyst bursts through this membrane (“hatching”) to prepare for implantation into the uterine lining.
There are two cell types in the day 5 embryo or blastocyst.
One cell type forms the Inner Cell Mass (ICM). It is a ball of cells that will eventually grow into the fetus.
The other cell type, is the Trophectoderm Epithelium (TE). It is a sheet of cells that will go on to make tissues needed during pregnancy (like the placenta).
Together these cell types make a fluid filled sphere with the TE cells on the outside and the ICM inside. Think of a balloon. If you blow up a balloon and put a ping-pong ball inside, that is what a blastocyst looks like. The latex of the balloon is the TE and the ping-pong ball is the ICM. Both of these cell types are necessary to establish a healthy pregnancy. You cannot have a baby without a placenta and you cannot have a pregnancy without a fetus, so when we grade embryos at the blastocyst stage, we assign a letter grade to each of the cell types .
Examples of the expansion grades are:
Very Early Blastocyst in which the cavity is just beginning to form in the embryo and the cell types are not yet distinguishable.
Expanded Blastocyst in which the cavity is fully formed, the embryo contains 100 to 125 cells, but is still contained within the thinned ZP.
Hatched Blastocyst in which the embryo is outside of the ZP, and contains upwards of 150 cells.
Below are some examples of embryos that we typically see on day 5.
|Expanding Blastocyst (DC)||50-75 cells, Developing ICM, growing trophectoderm, clear 100-125 cells, compacted, good quality ICM, nicely populated trophectoderm, clear blastocoel|
|Expanded Blastocyst (CB)||100-125 cells with trophectoderm beginning to protrude outside of the zona pellucida, good quality ICM, nicely populated trophectoderm, clear blastocoel|
|Hatching Blastocyst (BB)||100-125 cells with trophectoderm beginning to protrude outside of the zona pellucida, good quality ICM, nicely populated trophectoderm, clear blastocoel|
|Hatching Blastocyst (AB)||100-125 cells with trophectoderm beginning to protrude outside of the zona pellucida,large, good quality ICM, nicely populated trophectoderm, clear blastocoel|
As you might notice from the examples above, blastocyst grading is complex and therefore there are no absolute grades. While an A is “better” than a D, an embryo with a D grade ICM, for example, may be still developing and when viewed later, the ICM may have compacted into a B or even an A. Many times a Very Early Blastocyst on Day 5 becomes an Expanded Blastocyst on Day 6 and may be frozen if the other indicators are also good. The determination of whether an embryo has good potential or not is made by taking all of the components of the embryo into account.