Definition - What does Blastocyst mean?
An embryo that has developed for five to six days post fertilization is referred to as a blastocyst. A blastocyst contains 3 distinct features, including a fluid-filled cavity and two distinct types of cells:
- Trophectoderm (T) cells - T cells consist of a single layer of cells around the circumference of the embryo that become the placenta and embryonic sac.
- Inner cell mass (ICM) - The ICM is a distinct clump of cells that form the actual baby
The term blastocyst is particularly relevant in in vitro fertilization (IVF) when blastocysts are developed in a laboratory setting and then transferred to a uterus, hopefully resulting in implantation and pregnancy.
A blastocyst may also be referred to as a Day 5 embryo or a Day 6 embryo.
FertilitySmarts explains Blastocyst
When a sperm and egg meet, fertilization occurs. At this point, a zygote is formed. Cells continue to divide and around 5 to 6 days after fertilization, the embryo becomes a blastocyst. The blastocyst differs from an embryo because of its advanced cell development and growth. This includes a fluid-filled cavity and two distinct types of cells: T cells, and the ICM. Around 5 or 6 days after fertilization, a healthy blastocyst will hatch from its protective outer shell known as the zona pellucida. It is around 24 hours after this hatching process that embryo implantation is ready to occur.
Having high-quality embryos to transfer to a uterus is the most important part of the IVF process. Blastocysts are graded by an embryologist to determine which have the greatest likelihood of resulting in pregnancy. The amount of fluid in the cavity, along with the appearance of the T cells and ICM are the features that determine the quality of a blastocyst.
For a couple or individual undergoing IVF, it may be required to decide between an embryo transfer and a blastocyst transfer. This is a decision that is informed by the reproductive endocrinologist and embryologist based on the development of the embryos. This decision comes down to the number of days that are optimal for the embryo/blastocyst to reside in the lab before being inserted into the uterus or the overall number of embryos that continue to develop.
With a blastocyst transfer, the timing is the most dominant factor. An embryo has been fertilized for just two to three days. With intercourse, the embryo would still be residing in the fallopian tubes, having not yet reached the uterus. With IVF, introducing the embryo to the uterus several days before it would biologically arrive is thought to be potentially disruptive to the process. Therefore, the blastocyst transfer may be considered a closer imitation of the naturally occurring biological process. Additionally, blastocyst transfer allows for a more selective choice of transfer specimens. While lab-developed embryos may not have enough time to fully demonstrate their overall viability, more advanced blastocysts can. Additionally, because a more informed decision can be made, fewer blastocysts are typically transferred which reduces the chances of multiple pregnancies.
Disadvantages of a blastocyst transfer include:
- Additional expense
- Prolonged exposure of the embryo to the conditions of a lab (versus the uterus)
- Requirement of high-level lab equipment and capabilities
- Potential of having no embryos to transfer