Tuesday, February 17, 2009
Melanin, sometimes referred to as a chemical, is formed as part of the process of metabolizing an amino acid called tyrosine. In the skin, melanin is formed by cells called melanocytes. Certain medical conditions, such as albinism, are associated with the lack of melanin. Albinism is a condition marked by the abesnce of a normal amount of pigment in the body. Animals, humans, and even plants can have albinism.
Albinism exists in a number of variations. Depending on the type of albinism, the skin, hair, and eyes may all be affected. In fact, ocular albinism affects not only the color of the eyes, hair, and skin, but also results in poor vision. Additionally, some types of melanin deficiency are associated with increased mortality rates.
Melanin provides many benefits to human beings. One of the most recognized benefits involves ultraviolet rays of the sun. Melanin provides a natural protection against the harmful effects of these rays. However, it does not provide complete protection from the sun, and individuals with darker skin tones are still at risk from the sun's damaging rays.
Generally, those with darker skin tones and more melanin are able to tolerate exposure to the sun for hours without getting sunburn. By contrast, a person with lighter skin may get sunburn after spending only minutes in the midday sun. Skin cancer is directly related to exposure to the sun and the presence of less than optimal amounts of pigment. Sun exposure has even been linked with cataracts.
Melanin is the natural substance that gives color (pigment) to hair, skin, and the iris.
In humans, melanin is the primary determinant of human skin color and also found in hair, the pigmented tissue underlying the iris, the medulla and zona reticularis of the adrenal gland, the stria vascularis of the inner ear, and in pigment-bearing neurons within areas of the brain stem, such as the locus ceruleus and the substantia nigra.
Dermal melanin is produced by melanocytes, which are found in the stratum basale of the epidermis. Although human beings generally possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups more frequently or less frequently express the melanin-producing genes, thereby conferring a greater or lesser concentration of skin melanin. Some individual animals and humans have very little or no melanin in their bodies, a condition known as albinism.
Because melanin is an aggregate of smaller component molecules, there are a number of different types of melanin with differing proportions and bonding patterns of these component molecules. Both pheomelanin and eumelanin are found in human skin and hair, but eumelanin is the most abundant melanin in humans, as well as the form most likely to be deficient in albinism.
Eumelanin polymers have long been thought to comprise numerous cross-linked 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) polymers; recent research into the electrical properties of eumelanin, however, has indicated that it may consist of more basic oligomers adhering to one another by some other mechanism. Thus, the precise nature of eumelanin's molecular structure is once again the object of study. Eumelanin is found in hair and skin, and colors hair grey, black, yellow, and brown. In humans, it is more abundant in peoples with dark skin. There are two different types of eumelanin, which are distinguished from each other by their pattern of polymer bonds. The two types are black eumelanin and brown eumelanin, with black melanin being darker than brown. Black eumelanin is in mostly non-Europeans and aged Europeans, while brown eumelanin is in mostly young Europeans. A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of brown eumelanin in the absence of other pigments causes yellow (blond) color hair.
Pheomelanin is also found in hair and skin and is both in lighter skinned humans and darker skinned humans. In general women have more pheomelanin than men, and thus women's skin is generally redder than men's. Pheomelanin imparts a pink to red hue and, thus, is found in particularly large quantities in red hair. Pheomelanin is particularly concentrated in the lips, nipples, glans of the penis, and vagina. Pheomelanin also may become carcinogenic when exposed to the ultraviolet rays of the sun. Chemically, pheomelanin differs from eumelanin in that its oligomer structure incorporates benzothiazine units which are produced instead of DHI and DHICA when the amino acid L-cysteine is present.
Neuromelanin is the dark pigment present in pigment bearing neurons of four deep brain nuclei: the substantia nigra (in Latin, literally "black substance") - Pars Compacta part, the locus ceruleus ("blue spot"), the dorsal motor nucleus of the vagus nerve (cranial nerve X), and the median raphe nucleus of the pons. Both the substantia nigra and locus ceruleus can be easily identified grossly at the time of autopsy due to their dark pigmentation. In humans, these nuclei are not pigmented at the time of birth, but develop pigmentation during maturation to adulthood. Although the functional nature of neuromelanin is unknown in the brain, it may be a byproduct of the synthesis of monoamine neurotransmitters for which the pigmented neurons are the only source. The loss of pigmented neurons from specific nuclei is seen in a variety of neurodegenerative diseases. In Parkinson's disease there is massive loss of dopamine producing pigmented neurons in the substantia nigra. A common finding in advanced Alzheimer's disease is almost complete loss of the norepinephrine producing pigmented neurons of the locus ceruleus. Neuromelanin has been detected in primates and in carnivores such as cats and dogs.
Melanin is also a mechanism for absorbing heat from the sun. This purposes is of particular importance to cold-blooded animals. Snakes, lizards, certain types of fish, and a wide range of other animals depend on their surroundings, including the rays of the sun, to establish and maintain their body temperatures.
Also important for sharpness of vision, melanin serves to minimalize the number of light beams that enter the eye. It also provides for the absorption of scattered light within the eye. In this way, pigmentation allows for more keen sight.