. Describe the process of fertilization in human beings
- About 200 million sperm are introduced into the vagina, only 1% of the sperm reach the cervix of the uterus and about 200 reach the secondary oocyte.
- About 12 to 24 hours after ovulation, fertilization takes place in the fallopian tube.
- Sperm with the help of flagella swim down to the cervical canal through the vagina.
- From the vaginal canal, the contraction of the wall of organs of the rest of the uterus helps sperm to reach the fallopian tube.
- Until about seven hours the sperm reaches the oocyte and cannot be able to fertilize.
- During this period sperm undergoes the process of capacitation (the process in which sperms are getting ready for fertilization).
- Sperm that are capacitated are attracted by the surrounding cells of the oocyte and can react to chemical factors.
- Sperm first penetrate two layers-corona radiata and the zona pellucida.
- The acrosomal enzyme helps the sperm to penetrate corona radiata and the protein ZP3 molecules associated with the zona pellucida trigger acrosomal reaction in sperms.
- Only one out of many sperm first binds to the ZP3 molecule and penetrates zona pellucida and reaches the oocyte’s plasma membrane and fuses with the oocyte.
- When sperm enters the secondary oocyte, the oocyte first completes its meiosis II. It divides into a larger ovum and a second smaller polar body.
- The nucleus is present in the head of the sperm and the nucleus in the fertilized ovum fuse to produce a single nucleus by the process of syngamy.
- Now fertilized ovum is called a zygote.
In fertilization, sperm reaches the secondary oocyte, where the secondary oocyte first completes its meiosis II and produces ovum and polar nuclei,
where the ovum nucleus fuses by syngamy fuse with sperm to form a zygote.