Spermatozoon

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Template:Infobox Anatomy A spermatozoon or spermatozoan (pl. spermatozoa), from the ancient Greek σπερμα (seed) and ζων (alive) 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 can grow into a new organism, such as a human being.
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.Template:Ref

1 Spermatozoan structure and size
2 Sperm cell production
3 The acrosome reaction
4 See also
5 References
6 External links

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Spermatozoan structure and size

Image:Complete diagram of a human spermatozoa.svg In humans, sperm cells consists of a head 5 µm by 3 µm and a tail 50 µm long. The Reynolds number associated 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 mitochondria gets 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.

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Sperm cell production

Main article: Spermatogenesis

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. You can fit 100,000 sperm cells on the end of a pin.

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The acrosome reaction

Image:Acrosome reaction diagram.svg Recent study has revealed that sperms become even more excited when they zero in on an egg. They swim faster and their tail movements become more forceful and erratic. This behaviour is called "hyperactivity," to selective protein channels in the sperm's tail. Hyperactivity was associated with sudden influxes of calcium into the tails. The current study links two key findings made in recent years about sperm hyperactivity. The first was that a sperm's whip-like tail is studded with selective channels made from a protein called CatSper1 that allows only calcium ions through. Opening of CatSper1 is responsible for the influx of calcium into the sperm. This causes the sperm's tail to spin faster and more unevenly and the sperm is propelled through its viscous environment more forcefully.

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 that encase the egg like chain mail.

The second barrier is a membrane 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.

The female ovum is 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 fuses with 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 fertilization of the ovum.

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