How is a human egg fertilized?

Fertilization of a human egg is a complex process that occurs when a sperm cell successfully penetrates an egg cell, resulting in the formation of a zygote. Here's a step-by-step overview of how a human egg is fertilized:

1. Ovulation: During the menstrual cycle, typically around day 14, a mature egg (ovum) is released from one of the ovaries into the fallopian tube.

2. Sperm Transport: During intercourse or artificial insemination, semen is deposited into the vagina. The semen contains millions of sperm cells, which swim through the cervix, uterus, and into the fallopian tubes.

3. Capacitation: As sperm cells travel through the female reproductive tract, they undergo capacitation, a process that involves changes in their structure and biochemistry. Capacitated sperm cells become capable of penetrating the egg.

4. Sperm-Egg Contact: The capacitated sperm cells encounter the egg within the fallopian tube. Several sperm cells may come into contact with the egg, but usually, only one sperm cell successfully penetrates it.

5. Acrosomal Reaction: Upon contact with the egg's outer layer, the acrosome, a structure at the head of the sperm cell, undergoes an acrosomal reaction. Enzymes released from the acrosome help the sperm cell penetrate the egg's protective layers.

6. Penetration: The sperm cell's head, which contains the genetic material (DNA), enters the egg's cytoplasm. The tail of the sperm is left outside.

7. Zygote Formation: Once inside the egg, the sperm cell's nucleus fuses with the egg's nucleus, forming a single-celled zygote. The zygote contains a complete set of chromosomes, half from the sperm and half from the egg.

8. Cortical Reaction: The egg undergoes a cortical reaction in response to sperm entry. This prevents other sperm cells from penetrating the egg, ensuring fertilization by a single sperm.

9. Cleavage: The zygote undergoes a series of rapid cell divisions called cleavage. This process transforms the single-celled zygote into a multicellular embryo as it travels through the fallopian tube toward the uterus.

10. Implantation: Several days after fertilization, the developing embryo reaches the uterus and undergoes implantation. It attaches to the uterine wall, where it continues to grow and develop into a fetus over the course of pregnancy.

It's worth noting that fertilization is a very efficient process, but not all fertilized eggs successfully implant in the uterus. Some may fail to develop properly or may be reabsorbed into the uterine lining before they can be detected as a pregnancy.