Fertilisation is not a single event. It is a multi-stage process that takes several hours. It is important to understand this to give careful consideration to the question of when life begins.
A little bit of sperm and egg anatomy may help.
Fertilisation is the process whereby a single sperm fuses with an egg to form a zygote. The diploid human zygote has the potential to become an embryo, then after about 14 days, a foetus, then after 40 weeks, a baby.
About 300 million sperm are deposited at the woman’s cervix during normal intercourse, and they begin their arduous journey through her uterus and Fallopian tubes. If an egg cell or ovum is present in the section of the tube nearest to the ovary, fertilisation may occur. About 100,000 healthy sperm usually cluster around the ovum before one is able to fertilise.
During this time, the sperm undergo a structural change in which the membrane over the acrosome ruptures and the enzymes escape. This is called the acrosome reaction and it helps to digest some of the layers around the ovum, thus assisting the passage of the sperm through the cumulus cells.
Events of fertilisation
- The sperm makes a neat slit in the zona pellucida and slides through until the whole sperm cell, including the tail is incorporated into the perivitelline space.
- The cell surface membrane on the head of the sperm begins to fuse with the egg surface membrane. The cortical granules merge with the egg surface membrane and release their contents. It is thought that this plays a part in preventing the entry of any other sperm.
- The sperm nucleus is incorporated into the egg cytoplasm. The egg resumes its second meiotic division producing a second polar body.
- The genetic material in both the sperm head and the egg cytoplasm form structures called the male and female pronuclei.
- The zygote is formed. Note that the two pronuclei do not merge. The zygote splits into two and four cells before the genetic material unites. This stage is called syngamy.
At what stage do you think a new life is formed?
Do you think the gametes should be treated any differently from any other human cells or tissues? Give your reasons.
The early embryo