Question

Where is the ovum fertilised under normal conditions?

Answer 1

Hint: Human fertilisation involves the fusion of the sperm cell with the egg cell to form the zygote. It is regulated so that only one sperm fuses with each egg cell.

Complete Answer:
Once the oocyte is released from the mature follicle, it moves towards the fallopian tubes. They connect each ovary with the uterus on either side. The process of fertilization which is the fusion of the sperm nucleus with the oocyte nucleus to form the diploid zygote usually occurs at the distal end of the tube, or the ampulla. For this to occur, several other factors are involved.

Sperm which enter the uterus in the millions have to contend with the phagocytic cells in the female reproductive tract, only a few thousand enter the uterine or fallopian tubes. Here, they undergo capacitation where they become primed to encounter the oocyte. At the same time, the corona radiata which surrounds the oocyte releases chemicals that attract the sperm cells.

Sperms that reach the oocyte need to push or burrow through this outer, jelly-like layer, till they reach the second protective layer which is the Zona Pellucida. On the sperm head binding with receptors of the Zona pellucida, an acrosomal reaction takes place, allowing the sperm to go through and reach the oocyte membrane. Once one sperm cell manages to reach the oocyte, the membrane undergoes a change blocking the entry of any more sperm cells.

The oocyte’s sperm receptors are then blocked, and the fertilisation membrane forms.
Only once the sperm nucleus is released into the oocyte does the female nucleus complete the second meiotic division, converting it into an ovum. The male and female nuclei form the two pronuclei which combine to give the diploid zygote. And cell division to form the embryo can begin.

Note: There are times when fertilisation occurs outside of the Fallopian tubes and implantation is outside the uterus. These are known as ectopic pregnancies. These are extremely life threatening for the woman, and surgical intervention is required.