Spermatozoon

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.

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]

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Mammalian spermatozoan structure and size

Humans

The human sperm cell is the reproductive cell in males. Sperm cells come in two types; "male" and "female." Sperm cells that give rise to female (XX) offspring after fertilization differ in that they carry an X chromosome, while sperm cells that give rise to male (XY) offspring 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.

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 exhibit 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 spermatozoon is characterized by a minimum of cytoplasm and the most densely packed DNA known in eukaryotes. Compared to mitotic chromosomes in somatic cells, sperm DNA is at least sixfold more highly condensed.^[3]

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 ^[4].

Avoidance of immune system response

Glycoprotein molecules on the surface of sperm cells are recognised by all human immune systems, and interpreted as a signal that the cell should not be rejected. The male immune system might otherwise attack sperm whilst in the testes, and the female immune system would attack sperm in the reproductive tract. The specific glycoproteins coating sperm cells are also utilised by some cancerous and bacterial cells, some parasitic worms, and HIV-infected white blood cells, in order to avoid an immune response from the host organism.^[5]

Spermatozoa in other organisms

See also: Sperm

Animals

Fertilization relies on spermatozoa for most sexually reproductive animals.

The fruit fly^[3] has the largest known spermatozoon relative to its size. Drosophila melanogaster produces sperm that can be up to 1.8 mm in size, which is longer than the adult fly. The incredibly long tail is thought to block other sperm from entering the egg. The entire sperm, tail included, gets incorporated into the oocyte cytoplasm.^[6]

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, algae and fungi

The gametophytes of bryophytes, ferns and some gymnosperms produce motile sperm cells, contrary to pollen grains 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. Algae and lower plant sperm cells are often multi-flagellated (see image) and thus morphologically different from animal spermatozoa.

Some algae and fungi produce non-motile sperm cells, called spermatia. In higher plants and some algae and fungi, fertilization involves the migration of the sperm nucleus through a fertilization tube (e.g. pollen tube in higher plants) to reach the egg cell.

Spermatozoa production in mammals

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

Main article: 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.

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