Human embryos can develop very differently from one another. Even in a single cycle, embryos within a group rarely have uniform characteristics. The speed of division is one parameter by which early embryos can be distinguished from one another. All other parameters fall under the category of “morphology” or the shape or appearance of cells in the embryo. Embryo grading systems have been developed in which embryo division rate and morphology is expressed on a number or letter scale (for example, 1-4 or A-D). In our laboratory, embryos are examined multiple times, from fertilization until embryo transfer or freezing. We follow generally accepted models of embryo grading but we have also established our own guidelines.
Fertilization and all other stages of growth are assessed under a specialized microscope using monitors so that embryologists can discuss their findings with colleagues in case of a special or conflicting observation. Embryos are graded on a scale of 1-5, 1 representing embryos with the best morphology. Embryos are then selected for transfer or cryopreservation based on their quality as represented by this grade. A large number of studies conducted over the past three decades of IVF have established a clear relationship between embryo morphology and speed of development and the clinical outcome of IVF cycles. However, it is important to remember that these parameters are not absolute, that is, future growth is not guaranteed even if an embryo is given the best possible grade.
The following are some of the criteria used in our laboratory for grading embryos:
Number of Pronuclei and Polar Bodies
- The normally fertilized egg (zygote) should have two pronuclei, one derived from the sperm cell and the other from the egg. As a by-product of maturation and fertilization, the fertilized egg will also have two tiny cells called polar bodies adjacent to the main body of the egg. The zygote is considered to be abnormal if the numbers of pronuclei and polar bodies are below two or exceed two.
- Early cleavage, that is division of the zygote into two cells by about 27 hours after insemination, is considered to be a positive indicator of embryo quality.
- The first cells (or blastomeres) of the embryo are only loosely connected and these blastomeres may be associated with small remains of cellular material called fragments. The process of fragmentation during division is very common and is considered to be normal in most embryos. However, in some embryos, fragmentation may be extensive and may inhibit further growth, thus fragmentation may be another indicator of embryo quality and potential for development.
- A normal cell should contain a single nucleus but one or more blastomeres in human embryos may show more than one nucleus. Multi-nucleation is often (but not always) abnormal and embryos affected with multi-nucleation may have a reduced development potential.
- By the fifth day of development in the laboratory, the embryo is expected to form a blastocyst. This is marked by the formation of a fluid-filled cavity, a blastocoel, surrounded by a single layer of cells called the trophectoderm (or trophoblast) and a cluster of cells on the inside, called the inner cell mass. The inner cell mass or ICM later develops into the fetus while the trophectoderm forms the placenta. The blastocyst is graded according to its developmental stage; the trophectoderm and inner cell mass components of the blastocyst are graded separately based on their general appearance, number of cells, and their structure.