Fertilization is the fusion of the sperm nucleus with the egg nucleus to form a diploid cell known as the zygote.
After ejaculation, sperms undergoes capacitation to be capable of fertilization which occurs in the ampulla of the uterine tube.
Enzymes-hyaluronidase released by the acrosomes of several sperms digest a path through the material that holds the granulosa (follicle) cells together.
The sperm move by lashing their tails and reach the outer surface of the zona pellucida-a thick layer surrounding the secondary oocyte. The zona pellucida has special receptors to which the sperm heads can bind.
Another acrosomal enzyme digest a path through the zona pellucida and the sperm moves through to the surface of the secondary oocyte. Here, the head of a sperm will fuse with the microvilli surrounding the secondary oocyte and penetrates its cytoplasm.
Immediately the sperm has penetrated, the lysosomes in the outer region of the secondary oocyte-cortical granules, release their enzymes which causes the zona pellucida to thicken and harden, forming the fertilization membrane. This is called the cortical reaction.
It prevents the entry of further sperm and therefore the possibility of more than one sperm fertilizing the same egg. The enzymes also destroy the sperm receptor sites, so sperm can no longer bind to the zona pellucida
The entry of a sperm acts as the stimulus for completion of the second meiotic division of the secondary oocyte which produces the ovum and the second polar body.
The second polar body immediately degenerates and the tail of the sperm is lost within the cytoplasm of the ovum.
The nucleus of the sperm swells as its chromatin becomes less tightly coiled. At this stage the nuclei of the sperm and secondary oocyte are called pronuclei.
The male pronucleus fuses with the female pronucleus. This is the actual act of fertilization. The new nucleus formed has two sets of chromosomes, one from the egg and one from the sperm. The cell is now diploid and is called the zygote. The new nucleus divides immediately by mitosis. The zygote then undergoes cytokinesis or cell division and produces two diploid cells.