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.
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Only one sperm binds to specific receptors on the egg's outer layer, the zona pellucida.
- The sperm membrane fuses with the oocyte membrane, enabling sperm entry and fertilization.
Completion of the Second Meiotic Division
The immediate result of sperm entry is to activate the oocyte to complete the second meiotic division and extrude the second polar body.
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Block to Polyspermy:
- Once a sperm successfully fuses with the egg, the egg undergoes changes to its membrane to prevent other sperm from entering.
- Zygote Formation:
- At fertilization, the diploid number of chromosomes is restored, the chromosomal sex of the new individual is determined and the ovum is activated to proceed to the first cleavage division.
- The sperm's nucleus and the egg's nucleus combine to form a single cell with a diploid nucleus called a zygote..
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- The zygote begins to divide and develop as it travels toward the uterus, where it may implant and continue growing into an embryo.
Thereafter, Embryogenesis occurs.
Video
Significance of Fertilization
Reference
1. http://gizmodo.com/what-happens-to-sperm-once-theyre-inside-a-woman-1716992251
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Key Molecular Interactions
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.
The Four Main Stages
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Penetration of the corona radiata: Sperm release enzymes and use their flagella to pass through the outer layer of cells surrounding the egg
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Penetration of the zona pellucida: Sperm bind to receptors on this protective coating and digest through it
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Membrane fusion: The sperm and egg membranes fuse, allowing the sperm to enter
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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.
What Happens Next
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.
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