Sperm cells are the male gametes, or cells that function in sexual reproduction, before fertilization. Gametes merge with another gamete, in this case the female egg cell, to form a zygote. A zygote is the ovum, or egg, after fertilization. Sperm is short for spermatozoon, and spermatozoa in the plural form.
In mammals, sperm cells are produced in the male testes. The testes, or testicles, produce over 4 million new sperm every hour. Sperm cells are produced by a cell division called meiosis, which results in each cell having 23 chromosomes, which is half of the chromosomes found in every other cell in the human body except for the female egg, which also contains only 23. When joined, however, they make up the requisite 46 chromosomes, a pair made up of one chromosome from the male and one from the female, necessary for healthy development.
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There are three parts to a healthy sperm cell. There is a head capsule which contains a nucleus, the middle part is called the mitochondria and the long tail is called the flagellum. The nucleus contains the genetic material from the 23 chromosomes. Mitochondria in the body of the cell provides the energy for the swimming activity necessary for the sperm to reach the egg. The side to side movement of the flagellum is made possible by the alternate contraction of the protein fibers that make up the tail, providing the impetus to reach the female egg once the sperm have entered the vagina. There are between 200-500 million sperm cells in a single ejaculation.
Not all of the cells released into the vagina will survive the journey up the cervix to the fallopian tubes. Only the living sperm that are healthy and straight will have any chance of reaching the egg, a feat which requires the microscopic sperm to swim for up to an hour. If there is no egg to be fertilized, the cells can stay alive for up to five days or more in the female reproductive tract.
Male fertility depends on how healthy the sperm cells are and the health of the cells depend to a large extent on lifestyle. Men who are overweight, do not get enough exercise or follow diets lacking in fruit and vegetables may find that their sperm is not as fertile as normal. Stress and pollution also play a major role in decreasing sperm health.