Question

List all the changes that occur in the ovum during fertilisation.

Answer 1

When a sperm enters the secondary oocyte, the following changes occur:

1. The oocyte is activated and completes meiosis II, forming the mature ovum and a second polar body.
2. Cortical granules release their contents, causing changes to the egg coverings. The zona pellucida and vitelline membrane harden or alter, forming a barrier that prevents entry of additional sperm (block to polyspermy).
3. The female and male pronuclei form and then fuse, restoring the diploid chromosome number.
4. The fertilised egg, or zygote, is formed and begins cleavage.