Sperm

From New World Encyclopedia
Sperm
A sperm cell attempts to penetrate an ovum coat to fertilize it.
Gray6.png
Human spermatozoön. Diagrammatic. A. Surface view. B.Profile view. In C the head, neck, and connecting piece are more highly magnified.
Gray's subject #258 1243
MeSH Spermatozoa

A spermatozoon or spermatozoan (pl. spermatozoa), from the ancient Greek σπερμα (seed) and ζων (alive), is more commonly known as a sperm cell. It is the haploid cell that is the male gamete, or reproductive cell. It fertilizes an ovum, which serves as the female gamete, to form a zygote, or fertilized egg. A zygote can then grow and develop into a new organism.

Sperm cells contribute half of the genetic information to the diploid offspring, which contain two sets of chromosomes. In mammals, the sex, or gender, of the offspring is determined by the sperm cell since the ovum always provides and X chromosome. A spermatozoon bearing a Y chromosome will lead to a male (XY) offspring, while a spermatozoon bearing an X chromosome will lead to a female (XX) offspring.

Sperm cells were first observed by a student of Anton van Leeuwenhoek in 1677.[1]

Sperm cell structure

In humans, a sperm cell consists of a head, which is 5 µm by 3 µm, and a 50 µm long tail. Spermatazoan stream lines are straight and parallel. The tail flagellates and propels the sperm cell at about 1 to 3 mm/minute in humans by rotating like a propeller, not side to side like a whip. The cell contains a minimum amount of cytoplasm. During fertilization, the sperm's mitochondria are destroyed by the egg cell, meaning only the mother is able to provide the offspring's mitochondrial DNA. This plays an important fact in tracing maternal ancestry. However, it has been recently discovered that mitochondrial DNA may be recombinant, or a combination of genes not found together in either parents.

In marine invertebrates, the sperm cell is a flagellate cell consisting of a flagella, acrosome, and perforatorium. Such organisms practice external fertilization (Baccetti 1986).

The largest spermatozoa belongs to the fruit fly.

The Reynolds number associated with spermatazoa is in the order of 1E-2, so it is known that the spermatazoa exhibits laminar flow.

Sperm cell production

Spermatozoa are produced in the male testes, which are the male gonads, or the organs responsible for the production of gametes. Gametes are the reproductive cells that unite to form a new individual. In the testes of a newborn boy, immature spermatogonia are present. These cells are germ cells, or the embryonic gonadal cells that produce gametes. Some of these spermatogonia continually duplicate themselves through the process of mitosis. Other spermatogonia undergo meiosis and eventually develop into sperm.

During the process of spermatogenesis, germ cells become sperm cells. Spermatogonia initially undergo meiosis to become primary spermatocytes. Then, in the first meiotic division, each primary spermatocyte divides into two secondary spermatocytes.

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

acrosome reaction on a Sea Urchin cell

Sperm cells become even more excited when they approach an egg cell. They swim faster and their tail movements become more forceful and erratic. This behaviour is called "hyperactivity."

A recent discovery links hyperactivity 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 a protein called CatSper1. These channels are selective, allowing only calcium ion to pass. The opening of CatSper1 channels is responsible for the influx of calcium. The sudden rise in calcium levels causes the flagellum to spin faster, 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 that encase the egg like chain mail. The cumulus cells develop with the egg and support it as it grows, and then provide a physical barrier to fertilization.

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. 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.

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 impenetrable, preventing further fertilization of the ovum.

See also

  • sperm competition
  • sperm heteromorphism

References
ISBN links support NWE through referral fees

External links

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.