Definition - What does Cleavage Embryo mean?
A cleavage embryo refers to an embryo at the stage of its development in which it is undergoing a series of cell divisions or cleavages. This stage begins about 24 hours after the egg is fertilized and lasts until the morula is formed around 4 days after fertilization.
FertilitySmarts explains Cleavage Embryo
About 24 hours after an egg has been fertilized, its first division (cleavage) occurs. The result of this division is a 2-cell organism. These cells further divide or cleave, to progress through 4-cell, 8-cell, and 16-cell stages. Each of the cells in these stages is known as a blastomere and some will later go on to form a blastocyst.
Although the cells within the embryo are increasing, the embryo itself remains about the same size throughout this process. As the cells are dividing, small portions of the cell may break off and form a bleb. This is called fragmentation and can impact the quality of the embryo.
In Vitro Fertilization (IVF)
During in vitro fertilization (IVF), cleavage stage embryos are examined on the third day after fertilization and graded on their quality. The cleaving process is primarily driven by the maternal genome and often occurs on autopilot. Many eggs fertilize and cleave but some embryos arrest when the maternal and paternal genome fails to work together after day three.
That is why some cleaving embryos fail to develop to the blastocyst stage. An embryologist can never visually identify if failure to develop from the maternal or paternal genome but recent advances in PGT-A testing can identify the source of the abnormality.
Also, early egg or cleavage-stage evaluations may reveal indicators that a reproductive endocrinologist may use to determine pregnancy success for future cycles. These indicators may be egg granularity, a high degree of early-stage fragmentation, etc. These indicators are often processed retrospectively and are not 100% predictive of success. The embryos with the best grades have been shown more likely to result in a successful pregnancy.
Cleavage embryos are graded based on the number of cells they contain and their appearance under a microscope. On day 3 after fertilization, a good quality embryo usually contains 6-10 cells at this stage, have cells that are arranged in a regular pattern, have cells that are roughly the same size and cells that contain little to no fragmentation. Embryos with the best grades will be used for transfer in hopes that they will produce a successful pregnancy.
Day 3 Transfer
An embryo can be transferred at the cleavage stage in what is known as a Day 3 transfer. Alternatively, the embryo will hopefully continue to grow in the lab setting until s after fertilization. This allows it to develop into a blastocyst before being transferred or frozen. Health care providers have differing opinions on which type of transfer is more likely to produce a successful pregnancy.
- Cleavage and Blastocyst Formation. (n.d.).
- Embryonic Development. (2020).