Fertilization
Fertilization is when a sperm from a male fuses with an egg (ovum) from a female to form a zygote, marking the beginning of a new organism.
Location and Timing
Fertilization occurs in the ampulla of the fallopian tube. Once released, a human egg is viable for approximately 12–24 hours, while sperm can live within the female reproductive tract for up to 5 days.
The Journey of Sperm
When sperm are deposited in the vagina during sexual intercourse, they travel through the cervix and uterus into the fallopian tubes.
Of the hundreds of millions of sperm released during ejaculation, only about 200 reach the site of fertilization in the oviduct.
During their journey, sperm undergo a crucial process called capacitation, which takes about 5-6 hours in humans. This process improves sperm motility and depletes cholesterol molecules in the membrane, helping facilitate the release of enzymes needed to penetrate the egg's exterior.
Sperm-Egg Binding
Recent research has identified specific proteins involved in sperm-egg binding. When the sperm enters the space around the egg, a sperm-specific protein called Izumo on the sperm head binds to Juno receptors on the egg membrane. This binding triggers mechanisms to prevent multiple sperm from fertilizing the same egg (polyspermy), which would be lethal in humans.
-
Penetration of the corona radiata: Sperm release enzymes and use their flagella to pass through the outer layer of cells surrounding the egg
-
Penetration of the zona pellucida: Sperm bind to receptors on this protective coating and digest through it
-
Membrane fusion: The sperm and egg membranes fuse, allowing the sperm to enter
-
Nuclear fusion: The egg completes its second meiotic division, and the sperm nucleus fuses with the egg nucleus, enabling fusion of their genetic material
Once a sperm enters the egg, the egg immediately changes its surface to prevent other sperm from entering. The genetic material from the sperm (23 chromosomes) combines with the genetic material from the egg (23 chromosomes) to create a complete set of 46 chromosomes. This newly formed zygote contains all the genetic information needed to develop into a human being.
Formation of the Zygote
The fusion creates pronuclei containing genetic material from each parent. These pronuclei migrate toward each other, decompress, expand, replicate their DNA, and then intermingle when their nuclear envelopes disintegrate. This completes fertilization, creating a diploid zygote with 46 chromosomes—23 from each parent.
After Fertilization
The zygote begins dividing as it travels down the fallopian tube toward the uterus over the next several days. By the time it reaches the uterus (usually 5-7 days later), it has become a ball of cells called a blastocyst, which then implants into the uterine lining. This implantation is when pregnancy is officially established.
About six days after fertilization, the developing embryo (now a blastocyst) implants into the uterine lining, marking the true beginning of pregnancy and triggering hormone production that maintains the pregnancy.