Answer
Millions of sperms are ejaculated into a vagina in an act of sexual intercourse. The object of sexual intercourse is for one sperm to reach a an antral ( Graafain ) follicle and fuse with the haploid nucleus of a secondary oocyte. There are many obstacles between the vagina and the fallopian tube where a few fortunate sperms may arrive after a tough obstacle course Even here there may be many sperms trying to get trough the follicles defences a penetrate and fuse with the oocyte. It is not desirable that more than one sperm fuse with an oocyte and so there are safeguards against this exercised by both male and female reproductive organs and structures
The following are some of the safeguards.
1 . Reduction of number of sperms
Most sperms that enter the vagina leak out
Large numbers are killed by vaginal acids
many are killed by phagocytes that live on the uterine wall
2. Capacitation. Freshly ejaculated sperms cannot penetrate the oocyte so to have a hope of getting to the oocyte sperms must be capacitated. This is a process by which the uterus secretes enzymes that remove covering of sperm acrosome to enable the sperm acrosomal reaction to take place
In the acrosomal reaction the sperm acrosome secretes enzymes that enable it to get through the cells of the corona radiata and reach the zona pellucida. As the corona cell separate under the digestive action of sperm hyaluronidase the fortunate sperm bind to glycoprotein receptors on the zona. Hundreds of sperms commit suicide (exocitosis) to produce the proteases that digest a hole in the zona pellucida.. By flagellar action the lucky sperm propels through the hole in the zona . The posterior sperm plasma membrane binds to the oocyte plasma membrane. The oocyte forms microvilli and surround the sperm . Then the cytoplasmic contents of the sperm enters the oocyte. Resu;l the contents of both cells-- sperm and secondary oocyte are combined in one cell .
Work Step by Step
Blocks to polyspermy:
1. The cortical reaction
When the sperm head enters the oocyte the oocyte releases enzymes that remove the sperm receptors on the zona so no more sperms can bind.
2. Slow block to polyspermy
Water released by enzyme action. Egg membrane swells then hardens . All sperms still attached to the egg membrane are shed
This ensures that usually only one sperm gets to fertilize an oocyte
