Cleavage, Blastocyst Formation and Implantation

Once fertilisation is complete, the zygote's first mitotic division produces two identical cells, called blastomeres; further rapid divisions give four cells, then eight, and so on, and by about 72 hours after fertilisation these divisions have produced a loose, berry-shaped cluster of 16 or more cells called the morula. Under the relaxing influence of progesterone, the smooth muscle of the fallopian tube eases, and over the next 4-5 days it gently carries the still-dividing embryo down into the uterine cavity. By the time it arrives, the embryo has become a blastocyst: a fluid-filled hollow ball of roughly 100 cells, made up of an outer layer of large, flattened trophoblast cells and an inner cluster of 20-30 smaller, rounded cells called the inner cell mass. The inner cell mass of the blastocyst goes on to become the embryo proper, and its embedding into the endometrium of the uterus — implantation — is the event that actually establishes pregnancy. If a fertilised ovum implants somewhere other than the uterus, the result is called an ectopic pregnancy; about 95% of these occur in the fallopian tube itself, and as the embryo grows there it can cause internal bleeding, infection, and in some cases even death from rupture of the tube. This section also introduces twins: monozygotic (identical) twins arise when a single fertilised egg splits into two during its first cleavage, producing two babies of the same sex …