BBC Bitesize - KS3 Biology - Human reproduction - Revision 3
Once the sperm cells and egg cell meet, the egg binds the sperm using receptors , and enzymes allow the cells to By Melissa Mayer; Updated May 04, Sperm may be guided to the oocyte by a combination of thermotaxis and Because sperm are terminally differentiated cells, deprived of an As it is, of the millions of sperm inseminated at coitus in humans, only a few thousand reach normal abilities for binding and fusion to the egg plasma membrane. 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, its flagellum and the egg travels down the Fallopian tube to reach the uterus. It may be regarded as a mechanism of kin selection.
A sperm cell with all 23 chromosomes the same shape is said to have an X chromosome.
Egg meets sperm: the female side of the story
A child resulting from fertilization of an egg by a sperm with an X chromosome will be female, and a child resulting from fertilization of an egg by a sperm with a Y chromosome will be male.
There are sperm-sorting processes available to increase the likelihood of a particular sex for couples using intrauterine insemination to achieve pregnancy. Some in vitro fertilization clinics offer couples the option of sex testing before choosing which embryos to transfer. For couples seeking pregnancy the traditional way, however, there is no way to control which type of sperm fertilizes the egg.
Eggs[ edit ] Egg cells are released from the ovaries, usually one or two per menstrual cycle. Every egg has an X chromosome. During fetal development, a female fetus will have developed between 6 and 7 million eggs by the fourth or fifth month of pregnancy. Most of the eggs gradually waste away; by puberty, the average female haseggs remaining. During menopause, the body stops releasing egg cells.
Sperm transport in the female reproductive tract | Human Reproduction Update | Oxford Academic
Fertilization[ edit ] A sperm meeting an egg. Fertilization, also called conception, happens when a sperm cell combines with an egg cell, usually in the uterus.
They are created by the cellular division process known as meiosis, which creates 4 sperm from a single germ cell. Sperm are ejaculated in semen, a basic fluid with a pH of about 7. Since it is so much bigger than sperm, the egg is the source of cytosol and organelles,particularly mitochondria, for the future zygote.
This means that the egg is haploid but with sister chromatids still attached to each other.
Also unlike sperm, the meiotic division to create eggs, oogenesis, only makes one viable egg. The egg is covered in a thick outer coating known as the zona pellucida, a layer of carbohydrate-covered proteins that surrounds the plasma membrane.
The zona pellucida helps protect the egg and is responsible for mediating the initial meeting of sperm and egg. 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.
- Female Reproductive System
- Fertility basics
- Egg meets sperm
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. 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.