Spermatozoon

Image:Fertilisation.jpg

A sperm cell, spermatozooa (pl. spermatozoa) (in Greek: sperma = seed and zoon = alive), or spermatozoan, is the haploidcellthat is the male gamete. It is carried in fluid called semen, and is capable of fertilisingan egg cellto form a zygote. A zygote can grow into a new organism, such as a human being.
Sperm cells contain half of the genetic informationavailable to the diploidoffspring. In mammals, the sexof the offspring is determined by the sperm cells, through the chromosomalpair "XX" (for a female) or "XY" (for a male). Sperm cells were first observed by Antoni van Leeuwenhoekin 1679.

Spermatozoan structure and size

Individual spermatozoa are highly differentiated cells, composed normally of a head, basal body (or midpiece), and tail. The head contains some cytoplasmand the nuclearmaterial for fertilization. It also contains some enzymes which give it the ability to break through the egg to commence fertilisation. The basal bodycontains a large concentration of mitochondriathat provide the energy for sperm mobility through the production of ATP. The spermatozoan tail is typically a flagellumused for propulsion.

In humans, sperm cells consists of a head 5 µmby 3 µm and a tail 50 µm long. The Reynolds numberassociated with spermatazoa is in the order of 1E-2, so it is known that the spermatazoa exhibits laminar flow. Spermatazoan stream lines are straight and parallel. The tail flagellates, which we now know propels the sperm cell (at about 1-3 mm/minute in humans) by rotating like a propeller, not side to side like a whip. The cell is characterized by a minimum of cytoplasm. During fertilization, the sperm's mitochondriagets destroyed by the egg cell, and this means only the mother is able to provide the baby's mitochondria and mitochondrial DNA, which has an important application in tracing maternal ancestry. However it has been recently discovered that mitochondrial DNA can be recombinant.

The largest spermatozoa belongs to the fruit fly.

Sperm cell production

Main article: Spermatogenesis

Spermatozoa are produced in the seminiferous tubulesof the testesin a process called spermatogenesis. Round cells called spermatogoniadivide and differentiate eventually to become spermatozoa. During copulationthe cloacaor vaginagets inseminated, and then the spermatozoa move through chemotaxisto the ovum inside a Fallopian tubeor the uterus.

The acrosome reaction

The female ovumis coated in a thick protective membrane. When a sperm cell reaches the egg the acrosome releases its enzymes. These enzymes break down the cell membrane, allowing the sperm cell passage into the egg where the sperm fuseswith the egg cells membrane, and empties its genetic content into the egg. Upon penetration the membrane of the egg cell undergoes a change and becomes impenetratable, preventing further fertilizationof the ovum.

See also

• sperm competition
• sperm heteromorphism

External links

• The Handbook of Andrologyaf:Semen

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This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Spermatozoon Wikipedia article Spermatozoon.