Updated by: Arabinda Naik
Fertilization is the process of fusion of a haploid male gamete (sperm) with a haploid female gamete (ovum) to form a diploid zygote. This marks the beginning of embryonic development.
Site of Fertilization: In humans, fertilization is internal and occurs in the ampulla of the fallopian tube.
Process of Fertilization: Fertilization is a stepwise process that involves several stages:
During sexual intercourse (copulation), the male releases semen into the vagina of the female. This is called insemination. A single ejaculation contains about 200-400 million sperms. Only a few hundred sperms reach the ampulla of the fallopian tube.
Movement of Sperm in the Female Reproductive Tract. After ejaculation, sperm must navigate through various obstacles within the female reproductive system to reach and fertilize the ovum. The journey of sperm involves several key stages:
1. Entry into the Vagina
Ejaculation: Millions of sperm (about 200-300 million) are deposited into the vagina near the cervix during sexual intercourse.
Acidic Environment: The vagina has an acidic pH, which is hostile to sperm. Many sperm die due to this harsh environment.
2. Passage Through the Cervix
The cervical mucus plays a role in selecting sperm. During ovulation, the mucus becomes less viscous, making it easier for sperm to pass through. Some sperm get trapped in thick mucus and die.
Uterine contractions help propel sperm upwards toward the fallopian tubes. Immune Response: White blood cells in the uterus may destroy many sperm, reducing their number. Only a few thousand sperm survive and continue their journey toward the fallopian tubes.
Sperm that enter the wrong fallopian tube (without an ovum) will eventually die.
Chemotaxis: The ovum releases chemical signals that attract sperm toward the correct fallopian tube. Only about 100 sperm reach the ampulla of the fallopian tube, the site of fertilization. Arrival of Secondary Oocyte the Matured Graafian follicle in the ovary releases the secondary oocyte during ovulation. This occurs around the 14th day of a 28-day menstrual cycle. Reception by the Infundibulum The released secondary oocyte is caught by the fimbriae (finger-like projections) of the infundibulum, the funnel-shaped opening of the fallopian tube. Surrounding of Sperm, the secondary oocyte moves through the fallopian tube and is surrounded by numerous sperms if sperm are present in the female reproductive tract. Fertilization Process Out of all the sperm surrounding the oocyte, only one sperm succeeds in fertilizing it. Once fertilization occurs, the secondary oocyte completes meiosis-II, forming a haploid ovum and a second polar body. Formation of the Ovum After fertilization, the secondary oocyte is officially termed the ovum. The ovum's nucleus fuses with the sperm nucleus, leading to the formation of a diploid zygote, which will undergo further development.
Capacitation: Before fertilization, sperm undergo changes in the female reproductive tract, becoming more active and capable of penetrating the ovum. The sperm reach the ovum and bind to the zona pellucida (the egg's outer layer).
Acrosomal Reaction: The sperm release enzymes to dissolve the zona pellucida, allowing one sperm to penetrate.
Once one sperm enters, the ovum undergoes a cortical reaction, preventing other sperm from entering (polyspermy prevention).