Sperm Meets Egg: Weeks 1 to 3 of Pregnancy | Parents
Something magical is about to happen! Watch as the ovulation process occurs, and then millions of sperm swim upstream on a quest to fertilize. nos animation. Image: Chris Patton, Stanford University. FERTILIZATION is a complex process. The cytoplasm of the sperm contains NOS. After intercourse, it takes one to 2 hours for sperm to reach the ends of the fallopian tubes. The sperm usually meet the egg in the outer one-third of the fallopian.
Relaxing the cervix allows sperm to pass from the vagina into the uterus and reduces a potential physical barrier.
Animation: fertilisation of egg by sperm
Cervical mucus may prevent sperm from passing into the uterus, but during ovulation when the egg is released from the ovaries, the mucus gets thinner and lower in pH. These changes make the mucus a great transport medium for the sperm, and help the sperm continue traveling.
As we can see, the progress of sperm is really influenced by where in the menstrual cycle the female is. The closer to ovulation, the easier it is for sperm to pass. The vagina and uterus are very susceptible to infection, so the body has to balance on a fine line between protecting these areas and allowing sperm to come through. Diagram showing the female reproductive tract. The path of the sperm is highlighted via a blue arrow. As the sperm approach the egg, they bind to the zona pellucida in a process known as sperm binding.
This triggers the acrosome reaction, in which the enzymes of the acrosome are freed.
Egg meets sperm (article) | Embryology | Khan Academy
When the sperm cell finally reaches the egg cell, the plasma membranes of the two cells fuse together and the sperm releases its genetic material into the egg. Fusion also triggers the cortical reaction. When the sperm and egg fuse it triggers a release of calcium ions, which cause the cortical granules inside the egg to fuse with the plasma membrane.
As they fuse, these granules release their contents outside of the cell, toward the remains of the zona pellucida. The enzymes of the cortical granules further digest the zona pellucida, making it unable to bind more sperm, while other molecules found in the granules create a new protective layer around the fertilized egg. By creating a new barrier and destroying the initial interface between sperm and egg, the cortical reaction prevents polyspermy, or the fertilization of a single egg by multiple sperm.
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Other sperm reaching the egg now are just shunted away. A diagram showing the steps sperm take to fertilize the egg.
Copper intrauterine devices, or IUDs take advantage of sperm cell properties to prevent fertilization. The copper released by these contraceptives is a natural spermicideand ovicide, though it more strongly affects sperm. Though the devices release less copper than what could be found in our diets, the copper build-up in the mucous lining of the cervix and uterine is enough to halt the movement of sperm. IUDs in general also trigger a mild inflammatory reaction that brings in immune cells that make it even harder for the sperm to complete their journey.
When Sperm Meets Egg - Scientific American
Thus activated, the egg would start dividing and growing into an embryo. But how the sperm triggered this calcium release had remained a mystery. David Epel and his colleagues from Stanford University may have found the answer, studying the gametes--eggs and sperm--of sea urchins.
Epel and the lead author of the study, graduate student Richard C. Kuo, report the findings in the August 10th issue of Nature. They have long been popular models in early development studies because, unlike humans, their fertilization takes place in the open sea where it is easy to observe.
To date, sea urchins have helped solve several important questions about human fertilization, including how an egg becomes activated. This time, the scientists found that both sperm and eggs contain an enzyme that can produce NO from precursor molecules: Sperm start producing NO, largely inside their bulky heads, within seconds of coming into contact with the egg coat, a jellylike layer on the outside of the egg that mainly consists of sugar-coated proteins.
Sperm Meets Egg: Weeks 1 to 3 of Pregnancy
Touching this layer also makes the sperm release its protein-destroying enzymes, which it stores in a vesicle called the acrosome near the front of its head. These enzymes pave the way for the sperm to penetrate the egg membrane and release its genetic material inside. Of interest, when Epel's group inhibited this acrosome reaction using chemicals, the sperm no longer made a lot of NO.
So this reaction seems crucial for NO production in the sperm. Eggs, however, only showed large increases in NO about 40 seconds after a sperm had fertilized them or after they had been artificially activated.The difficult journey of the sperm - Signs
So the idea is that NO from the sperm triggers the release of calcium in the egg, which in turn activates the NOS in the egg, producing more NO and further calcium release.
To answer it, the researchers injected one of two molecules into the eggs: