A spermatozoon or spermatozoan (pl. spermatozoa), from the ancient Greek σπέρμα (seed) and ζῷον (living being) and more commonly known as a sperm cell, is the haploid cell that is the male gamete. It joins an ovum to form a zygote. A zygote is a single cell, with a complete set of chromosomes, that normally develops into an embryo.
| Spermatozoon | |
|---|---|
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 Diagram of a human spermatozoon | |
| Identifiers | |
| MeSH | D013094 |
| Anatomical terminology | |
Sperm cells contribute half of the genetic information to the diploid offspring. In mammals, the sex of the offspring is determined by the sperm cells: a spermatozoon bearing a Y chromosome will lead to a male (XY) offspring, while one bearing an X chromosome will lead to a female (XX) offspring (the ovum always provides an X chromosome). Sperm cells were first observed by a student of Antoni van Leeuwenhoek in 1677.[1]
Spermatozoan structure and size
Humans
The human sperm cell is the reproductive cell in males. Sperm cells come in two types; "male" and "female." The females differ in that they carry an X chromosome, while the male sperm carry a Y chromosome. The female sperm cell also differs phenotypically in that it has a larger head in comparison to the male sperm cell.
Males
In male humans, sperm cells consists of a head 5 µm by 3 µm and a tail 50 µm long. The Reynolds number associated with spermatozoa is in the order of 1×10−2, so it is known that the spermatozoa exhibits laminar flow. Spermatozoan stream lines are straight and parallel. The tail flagellates, which propels the sperm cell (at about 1-3 mm/minute in humans) by whipping in an elliptical cone. [2]. The cell is characterized by a minimum of cytoplasm. During fertilization, the sperm's mitochondria are destroyed by the egg cell, and this means only the mother is able to provide the baby's mitochondria and mitochondrial DNA, which have an important application in tracing maternal ancestry. However it has been recently discovered that mitochondrial DNA can be recombinant [3].
In other organisms
The fruit fly[3] has the largest known spermatazoon relative to its size.
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Sperm in other organisms
Animals
Fertilization relies on sperm cells for most, if not all, sexually reproductive animals.
Sea urchins such as Arbacia punctulata—are the workhorses of sperm research, because they spawn large numbers of sperm into the sea, making them well-suited as model organisms for research experiments.
Plants
The gametophytes of bryophytes, ferns and some gymnosperms produce motile sperm, contrary to pollens or other more complex arrangement of gametophytes, employed in most gymnosperms and all angiosperms. This renders sexual reproduction in the absence of water impossible, since water is a necessary medium for sperm and egg to meet.
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Genetic odds
It is clear that the odds of the sexes can be manipulated in differing species. For example, temperature affects the sex of offspring in some reptiles, such as crocodiles and turtles. Cool incubation temperature produces male offspring while warm temperature produces female offspring. However, sex determination in most mammals, as in humans, is chromosomal.
Sperm cell production
Spermatozoa are produced in the seminiferous tubules of the testes in a process called spermatogenesis. Round cells called spermatogonia divide and differentiate eventually to become spermatozoa. During copulation the cloaca or vagina gets inseminated, and then the spermatozoa move through chemotaxis to the ovum inside a Fallopian tube or the uterus.
The acrosome reaction
Mammalian sperm cells become even more active when they approach an egg cell. They swim faster and their tail movements become more forceful and erratic. This behaviour is called "hyperactivation."
A recent discovery links hyperactivation to a sudden influx of calcium ion into the tails. The whip-like tail (flagellum) of the sperm is studded with ion channels formed by proteins called CatSper. These channels are selective, allowing only calcium ion to pass. The opening of CatSper channels is responsible for the influx of calcium. The sudden rise in calcium levels causes the flagellum to form deeper bends, propelling the sperm more forcefully through the viscous environment. Sperm hyperactivity is necessary for breaking through two physical barriers that protect the egg from fertilization.
The first barrier to sperm is made up of so-called cumulus cells embedded in a gel-like substance made primarily of hyaluronic acid. The cumulus cells develop in the ovary with the egg and support it as it grows.
The second barrier coating the oocyte is a thick shell formed by glycoproteins called the zona pellucida. One of the proteins that make up the zona pellucida binds to a partner molecule on the sperm. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of different species from fusing. There is some evidence that this binding is what triggers the acrosome to release the enzymes that allow the sperm to fuse with the egg.
When a sperm cell reaches the egg the acrosome releases its enzymes. These enzymes weaken the shell, allowing the sperm cell to penetrate it and reach the plasma membrane of the egg. Part of the sperm's cell membrane then fuses with the egg cell's membrane, and the contents of the head sink into the egg.
Upon penetration, the oocyte is said to have become activated. It undergoes its secondary meiotic division, and the two haploid nuclei (paternal and maternal) fuse to form a zygote. In order to prevent polyspermy and minimise the possibility of producing a triploid zygote, several changes to the egg's cell membranes renders them impenetrable shortly after the first sperm enters the egg.
References
- ^ "Timeline: Assisted reproduction and birth control". Retrieved 2006-04-06.
- ^ Sumio Ishijima, Shigeru Oshio, Hideo Mohri, "Flagellar movement of human spermatozoa", Gamete research, 1986, vol. 13, no3, pp. 185-197 (27 ref.) [1]
- ^ Marilena D'Aurelio et al., "'Heterologous mitochondrial DNA recombination in human cells'", Human Molecular Genetics 2004 13(24):3171-3179; doi:10.1093/hmg/ddh326 [2]