For patients having their embryos transferred at the blastocyst stage, the grading procedure used to assess the embryos can seem complicated. However, we simply look to see that the embryos are developing normally, are not slowing down, and are preparing for implantation in the uterus.

In the 2 days following fertilization, embryos go through 3 rounds of cell division. The fertilized oocyte divides in 2, these cells each divide again to give 4, and then these divide to give 8. In the resulting 8-cell embryo, each cell should be 1/8 the size of the original oocyte since there is no growth in size, and each cell should be intact and symmetrical. When we assess embryos at this stage, we first count the number of cells and we then assign a grade based on how good the embryo looks. Embryos that have disintegrating or asymmetrical cells are assigned a lower grade.

At this early stage, the individual cells stay together because they are contained within a shell called the zona pellucida. However, as the embryo progresses past the 8-cell stage, dividing to 16 and then 32, the cells attach to each other and cooperate to form a tight ball called a morula. At the morula stage, the cells are pressed so tightly together that individual cells cannot easily be identified or counted. Once the attachments between cells are formed, the cells begin to pump fluid into the center of the ball, giving rise to a tiny fluid filled cavity or cyst. As long as the junctions between cells hold, no fluid can escape from the cyst, and the cyst grows larger as more fluid is pumped in.

These are critical days for the embryo. In addition to forming the central cyst, the embryo is also busy organizing its cells into two distinct populations. As the embryo moves beyond the 8-cell stage, some cells stay on the outside of the ball and some are pushed to the inside. In the typical 16-cell embryo, there are 12 outer and 4 inner cells. At the 32-cell stage, 22 of the cells are outer cells and 10 are inner cells. Creating more outer cells is deliberate, because these cells are needed to maintain the integrity of the cavity as it becomes larger. More importantly however, these cells will become the placenta, and having enough cells to establish the placenta is critical to successful implantation in the uterus. Once the placenta is established, it can feed the inner cells which become the developing fetus.

The appearance of the cyst at the center of the morula marks the next embryo stage, the blastocyst. In assessing the blastocyst, we look at the size of the cyst and the integrity of the outer and inner cells. Depending on the size of the cyst, the blastocyst is referred to as early, expanding or fully expanded. If the cyst has become large enough to cause the embryo to burst through its shell, we call it a hatching blastocyst. Occasionally, we even see fully hatched blastocysts. Hatching is a natural process that frees the embryo from its shell to allow implantation to occur. The more expanded the cyst has become, the more we favor the embryo for transfer.

In addition to looking at cyst expansion, the grade of the blastocyst is further determined by the integrity of the inner and outer cells. Embryos with more cells are better, and the best blastocysts are well expanded with distinct inner and outer cell populations. In poor quality blastocysts, there can be few cells in one or both populations, and/or the cavity can be small. And sometimes, even in embryos with beautiful outer cells, we cannot see any inner cells at all. These embryos are destined to fail since a full blastocyst with 32 cells is incapable of making inner cells if they do not already exist.

The embryos that are most difficult to assess are those where the cavity has just begun to open up, but has not expanded sufficiently to allow us to see inside. These early blastocysts are usually assigned lesser grades as we are unable to determine whether any inner cells are present. We often look at these embryos again several hours later to see if further expansion has revealed the presence of those critical inner cells. We would then re-grade the embryo, if appropriate.

All of this development, from fertilization to blastocyst expansion and hatching, normally follows a tight timeline that is independent of cell number. The embryo attempts to hatch from its shell approximately 5 or 6 days post fertilization, regardless of the number of cells it contains. If development is slow, and cell number is consequently low, the outer cells stretch to enclose the cyst and expansion continues. This is important, as the uterus waits only a few days for the embryo to implant. If the embryo takes too long to make the “right” number of cells for expansion and hatching, it may miss the implantation window. The practical result of this is that we still get high implantation rates even if only early blastocysts are available for transfer.

The above phenomenon is relevant to frozen embryo transfer cycles too, because many embryos lose one or more cells as a result of freezing and thawing. Such embryos still try to form blastocysts according to their original timeline, even though they may have less than the ideal number of cells. The consequences of arriving with plenty of cells but too late for the uterus are worse than having a chance to implant even with fewer cells. As a result, frozen-thawed embryos that have lost a cell or two are not assigned a lower grade since we still consider them to have high implantation potential.  

Tags: Embryo Freezing, IVF - In Vitro Fertilization, Lab, Oocyte, Treatment Options

This entry was posted on Wednesday, July 2nd, 2008 at 12:37 pm and is filed under Science Pulse. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.