> NEXT: 06. Completion of the Second Meiotic Division
++++++++++
Fertilization typically occurs in the outer third of the fallopian tube when one sperm successfully penetrates the egg's outer layer.
++++++++++++++++
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.
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.
This process represents one of the most studied yet still fascinating areas of reproductive biology, with ongoing research continuing to reveal new details about the molecular mechanisms involved