Blastocyst, a distinctive stage of a mammalian embryo. It is a form of blastula that develops from a berrylike cluster of cells, the morula. A cavity appears in the morula between the cells of the inner cell mass and the enveloping layer. This cavity becomes filled with fluid. The blastocyst differs from the blastula in that it is composed of two already differentiated cell types, the inner cell mass and the enveloping layer. Further differentiation produces a thin layer of cells, called the hypoblast, between the inner cell mass and the cavity. These cells contribute to the formation of the embryonic endoderm, from which derive the respiratory and digestive tracts. The enveloping layer is now referred to as the trophoblast. It does not contribute directly to the formation of the embryo but rather serves to establish a connection with the maternal uterus.

In the IVF laboratory, after the egg retrieval procedure, fertilization is carried out by IVF (In-vitro fertilization) or by ICSI (intra-cytoplasmic sperm injection) and the resulting embryos are placed in an incubator for further development. The stages of development are as follows:

  • The day of egg retrieval is considered as Day 0 (IVF or ICSI is done after egg retrieval)
  • The pronuclear stage on day 1 (embryologist checks as to how many eggs have successfully fertilised)
  • The two to four cell stage on Day 2
  • The eight cell stage on day 3
  • The morula stage on day 4
  • The blastocyst stage on day 5 or day 6

The blastocyst contains two different cell types- the inner cell mass which develops into the fetus, and an outer layer of cells called trophectoderm which develops into the placenta.

Historically, embryo transfer following IVF or ICSI has been carried out on Day 2 or Day 3 after egg collection. However, advances in the IVF laboratory advances allow us to now culture embryos for five or six days. This gives the embryos more opportunity to prove their developmental potential. Some embryos stop developing (arrest) prior to day 5/6. This ‘natural selection’ enables the embryologist to identify and choose the best embryo/s for embryo transfer.