Spermatozoon
| Spermatozoon | |
|---|---|
 | |
 Diagram of a human spermatozoon | |
| Identifiers | |
| MeSH | D013094 |
| Anatomical terminology | |
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
In Male Humans
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 Female Humans
A new scientific breakthrough may lead to women in future being able to produce sperm.
Scientists in England have turned stem cells from an embryo into sperm which are capable of producing offspring.
The breakthrough is likely to lead to new advances in treating male infertility and even the possibility that women could manufacture sperm.
The researchers at Newcastle University say that the advance, when developed further, could help men with certain types of infertility to become fertile and even one day could enable a lesbian couple to have children that genetically would be their own.
The experiment used embryo cells to produce seven baby mice, six of whom lived into adulthood, although the survivors suffered adverse events of the kind seen in cloning experiments.
The researchers isolated embryonic stem cells from an embryo only a few days old consisting of a cluster of cells. The cells were grown in a laboratory and screened to isolate the spermatogonial stem calls which were grown and then injected into female mouse eggs and grown in early stage embryos.
The research team says its project will aid the understanding of the biological process through which sperm is produced, which should help in the future treatment of infertility.
It is hoped that this new knowledge could be translated into treatments for men whose sperm is dysfunctional, although could be some years into the future.
The research was published in the journal Developmental Cell.
In other animals
The largest spermatozoa belongs to the fruit fly.[1]
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).
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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 sperm cell sinks into the egg.
Upon penetration the membrane of the egg cell undergoes a change and becomes impenetrable, preventing further fertilization of the ovum.
See also
References
- ^ "Timeline: Assisted reproduction and birth control". Retrieved 2006-04-06.