How Fertilization Happens | What to Expect
Conception: From Egg to Embryo. Take a journey with us from the moment a single sperm fertilizes an egg. From tiny egg to growing embryo, follow the. Something magical is about to happen! Watch as the ovulation process occurs, and then millions of sperm swim upstream on a quest to fertilize. Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube. The result of this union is the production of a zygote cell, or fertilized egg, plasma membrane, the sperm head disconnects from its flagellum and the egg travels down the Fallopian tube to reach the uterus.
Cortical granules filled with enzymes line the inside of the cell membrane, and will help make sure that only one sperm can fertilize the egg.
The setting Egg and sperm travel in opposite directions to meet in most often the fallopian tubes. During ovulation, ovaries release an egg into one of the fallopian tubes, and the egg proceeds down the tube toward the uterus, which is being prepared for possible implantation.
Part of this preparation involves elevated levels of estrogen and luteinizing hormone LH. LH triggers the ovaries to release the egg, while higher blood estrogen levels stimulate the vaginal membrane to secrete glycogen, which is then metabolized to lactate. This lowers vaginal pH to as low as 3. However, this environment can also be toxic to sperm, though the semen a basic fluid can buffer the vaginal acidity to preserve sperm cells.
Only about 1 in 1 million sperm that are ejaculated into the vagina will reach the site of fertilization. Estrogen also relaxes the cervix, causes cervical mucus to become watery and more alkaline, and stimulates uterine contractions — all of which help sperm penetrate and navigate the female reproductive system.
Relaxing the cervix allows sperm to pass from the vagina into the uterus and reduces a potential physical barrier. 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.
Human fertilization - Wikipedia
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.
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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. 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.The difficult journey of the sperm - Signs
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. The sperm's tail and mitochondria degenerate with the formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin.
Still, a considerable amount of RNA from the sperm is delivered to the resulting embryo and likely influences embryo development and the phenotype of the offspring. Their membranes dissolve, leaving no barriers between the male and female chromosomes.
How Pregnancy Happens
During this dissolution, a mitotic spindle forms between them. The spindle captures the chromosomes before they disperse in the egg cytoplasm. Upon subsequently undergoing mitosis which includes pulling of chromatids towards centrioles in anaphase the cell gathers genetic material from the male and female together.
Thus, the first mitosis of the union of sperm and oocyte is the actual fusion of their chromosomes. Thus, they are genetically identical. Fertilization age[ edit ] Fertilization is the event most commonly used to mark the zero point in descriptions of prenatal development of the embryo or fetus.