Human fertilization occurs when the male reproductive cell, sperm, meets the female reproductive cell, an egg. The sperm of a male are produced inside the testes, and stored in the epidydymis. Ejaculation of sperm occurs through the head of the penis during during sexual intercourse, and millions of microscopic sperm are released into the uterus of a woman by way of her vaginal opening.
Ovulation is the release of a single female egg is released once a month by one of two ovaries. The egg, known as a oocyte, waits in one of the fallopian tubes of the female. Female hormones send signals to direct the sperm toward the egg cell. Fertilization occurs inside the fallopian tube when a single sperm cell penetrates the egg wall. The fertilized egg then falls down into the uterus and attaches to the uterine wall, where the growth of the newly fertilized egg occurs.
The female’s sexual hormone progesterone sends signals to the sperm swimming through the uterus and up the fallopian tube. To ensure fertilization, the oocyte is surrounded by cumulus cells that release the progesterone hormone. Progesterone serves as a chemical attractant to the male sperm. A calcium ion channel release is the active control of progesterone in women, and a sperm-specific identifier called the CatSper controls the swimming behavior of the sperm cells.
The female egg is in complete control of the direction in which the sperm travel. Progesterone not only attracts the sperm, but creates a hyperactivation of the sperm tails directing them to the exact location of the unfertilized egg. The tails of the sperm are directly affected by these calcium ion channels. The sperm alters its swimming effect based upon the signals released by the egg. Progesterone controls the entire pathway which the sperm take through alteration of the beating patterns of sperm tails.
More specifically, progesterone acts directly on what are known as the CatSper calcium channels. These are ion channels are found only in the tail of a sperm cell. The ion channels form pore proteins in the cell membrane, and acts as a “floodgate” for the release of ions. The female’s progesterone opens its sperm-specific CatSper channel allow calcium ions to flow into the tails of sperm redirecting their swimming pattern directly to the egg. The build-up of the calcium in sperm results in the egg having complete control.
A simplified way of examining this phenomenon is to think of the female egg as sending a “broadcast transmission” of calcium to the sperm. When the sperm receive the transmission, their tails become filled with calcium. The sperm then follow that broadcast transmission that is known as a a CatSper channel all the way to the egg cell that is awaiting fertilization. One male sperm will penetrate the female egg resulting in fertilization.
Sources:
The Scientist Magazine – http://www.the-scientist.com/news/display/58053/
Max-Planck-Gesselscfast – http://www.mpg.de/print/1225277
Wikipedia – http://en.wikipedia.org/wiki/Progesterone
