What is Spermatozoon?

A sperm cell attempts to penetrate an ovum coat to fertilize it.
Diagram of a human spermatozoon
	subject #258 1243
MeSH	Spermatozoa

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]

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.

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 110^-2, so it is known that the spermatozoa exhibits 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 cell is characterized by a minimum of cytoplasm. 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 ^[3].

In other organisms

The fruit fly^[1] 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 form entering the egg. The entire sperm, tail included, gets incorporated into the oocyte cytoplasm.^[4]

Sperm in other organisms

Animals

Fertilization relies on sperm cells for most, if not all, sexually reproductive animals.

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

The gametophytes of bryophytes, ferns and some gymnosperms produce motile sperm, contrary to pollens or other more complex arrangement of gametophytes, 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.

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 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.

References

External links

• The Handbook of Andrology
• Sperm hyperactivity
• Slower conception 'leads to boys'
• Photos of sperm under a microscope

•  • [ e] Male reproductive system
Scrotum	layers (skin, Dartos, External spermatic fascia, Cremaster, Internal spermatic fascia) • Perineal raphe • Spermatic cord
Testes	layers (Tunica vaginalis, Tunica albuginea) • Appendix • Mediastinum • Lobules • Septa • Leydig cell • Sertoli cell • Blood-testis barrier
Spermatogenesis	Spermatogonium • Spermatocytogenesis • Spermatocyte • Spermatidogenesis • Spermatid • Spermiogenesis • Spermatozoon
seminal tract	Seminiferous tubules (Tubuli seminiferi recti, Rete testis, Efferent ducts) • Epididymis  (Appendix) • Vas deferens • Ejaculatory duct  Seminal colliculus
urinary tract	Internal urethral orifice • Urethra (Prostatic, Intermediate, Spongy) • Urethral crest • Urethral gland • External urethral orifice
Penis	Corpus cavernosum • Corpus spongiosum • Navicular fossa of male urethra • Glans penis • Fundiform ligament • Suspensory ligament • Foreskin • Frenulum
accessory glands	Seminal vesicles  (Excretory duct of seminal gland) • Prostate  (Prostatic utricle, Prostatic sinus) • Bulbourethral glands