Baby Making

Fertilisation

When a sperm reaches the egg, it hits the zona pellucida, and undergoes the acrosome reaction, so it can digest a path through. When it reaches the egg’s plasma membrane, it fuses with it, passing into the cytoplasm. This sets off a series of reactions – ‘the block to polyspermia’ – stopping more than one sperm entering the egg:

• hardening of the zona pellucida
• inactivation of sperm binding zone in the zona pellucida
• ovum membrane sperm binding inactivation

The fertilised egg (a zygote) completes the second stage of meiosis, and the polar body is ejected. Both sets of chromosomes move to the centre of the cell, and mitosis begins.

Early stuff

The egg slowly moves down the fallopian tube over 4 days(ish). It remains there until progesterone levels are high enough to stop contractions squeezing closed the exit. Sometimes the egg can embed in the tube, or even in the abdominal cavity. This is an ectopic pregnancy (see below).

Eventually the muscles relax, and the zygote passes down, undergoing cleavage, where 16-32 cells end up, but taking up the same space – each of these cells is totipotent, meaning each one has the potential to become an entire individual. Once the zygote has reached the uterus, it floats in the fluid, which provides nutrients, for a few days, continually dividing, until it becomes a blastomere – no longer totipotent. The blastomere has two main bits:

• trophoblast
• inner cell mass

Clinical Conditions

Ectopic pregnancy

When the zygote embeds in the uterine tube, or in the abdominal cavity, the baby begins to grow in a bad place. The pregnancy cannot continue successfully, and must either spontaneously abort or be removed by surgery at risk of haemorrhage and death in the mother. It hurts a LOT too. Read more on the ectopic pregnancy page.