Question

Describe the events that take place from the time of entry of human sperm into the ovum till fertilization is complete.

Answer 1

1. Millions of sperm are deposited in the vagina during ejaculation; only a few reach the fallopian tube, where fertilization occurs.
2. Sperm undergo capacitation in the fallopian tube, which enhances their motility and removes the acrosomal membrane to expose enzymes.
3. The sperm releases hyaluronidase enzyme that helps remove follicle cells (corona radiata) and dissolve the zona pellucida near the ovum.
4. Compatibility between sperm and ovum is checked chemically through fertilizin (from ovum) and antifertilizin (from sperm).
5. Only one sperm enters the ovum (monospermy) as the zona pellucida undergoes zona reaction or cortical reaction, preventing entry of others.
6. Entry of sperm activates the ovum to complete its second meiotic division, extruding a second polar body and forming a mature ovum.
7. The nuclei of the sperm and ovum form male and female pronuclei, respectively.
8. Male and female pronuclei fuse to form a zygote with restored diploid chromosome number (46 chromosomes).
9. This marks the completion of fertilization and initiation of a new organism’s development.