sperm

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.^[1]

Spermatozoan structure and size

Humans

The human sperm is the main reproductive cell in males. The sperms differ in that each carry a set of chromosomes dividing each into either a male, or female sperm. The females differ in that they carry an X chromosome, while the male sperm carry only a Y chromosome. The female sperm also differ phenotypically in that they have a larger head in comparison to the male sperms. This contributes to the male sperm being lighter, and therefore faster and stronger swimmers than their female counterparts (although statistically there is still a 50% chance of an either XY or XX embryo forming).

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 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.

In other animals

The fruit fly.^[1] has the largest spermatazoa relative to its size.

The working horse for sperm researchers are sea urchins such as Arbacia punctulata which spawn their sperm into the sea at high numbers making them a perfect study tool for experiments.

Fertilization relies on sperm cells for most (all?) sexually reproductive animals.

Even some plants, such as bracken fern, use sperm for sexual reproduction (see papers by Brokaw).

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

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.

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 sperm cell sinks into the egg (at which point the sperm tail falls off).

Upon penetration the membrane of the egg cell undergoes a change and becomes impenetrable, preventing further fertilization of the ovum.

References

1. ^ Timeline: Assisted reproduction and birth control. Retrieved on 2006-04-06.

External links

• The Handbook of Andrology
• Sperm hyperactivity
• Women could make sperm
• Slower conception 'leads to boys'