The day after bringing the sperm and eggs together, the embryologists check to see how many eggs got fertilised. They can determine this by checking for the presence of two “pronuclei” in the egg; one pronucleus comes from the sperm and the other from the egg and is proof that the sperm penetrated and fertilised the egg. Occasionally abnormal fertilisation occurs and 3 pronuclei are seen. Typically, around 70-80% of eggs become fertilised. The pronuclear stage is the earliest embryonic stage, also known as the zygote.
Embryos are then maintained in the lab within specialised media droplets under strictly controlled temperature and gassing conditions. Embryo development is monitored by the embryologists using microscopes. This very early embryo is referred to as the preimplantation embryo. Preimplantation development involves “cleavage” into smaller cells; as cell numbers increase, the size of individual embryonic cells decreases. By Days 4-5 of development, the embryo has typically developed into a morula, and by Days 5-6, it reaches the blastocyst stage.
The blastocyst has a small fluid-filled cavity (the “blastocoele”) and at this stage, the embryo now has two distinct cell-types, the outer “shell” or trophectoderm and an inner cell mass. The trophectoderm will form the placenta and the inner cell mass will form the fetus.
During natural pregnancy, eggs become fertilised at the outer end of the Fallopian tube and subsequent steps of embryo development occur as the embryo makes its way along the tube towards the cavity of the womb. The embryo is usually at the blastocyst stage by the time it reaches the womb cavity and begins implantation. It is for this reason that blastocyst stage embryos often produce higher success rates during IVF than earlier cleavage-stage embryos. Professor Homer wrote the chapter on Fertilisation in one of the world’s most popular textbooks of Gynaecology (Gynaecology 4th Edition; Chapter 19).
