As is so clearly obvious, the coloration of
human skin is highly variable. This variation is primarily hereditary,
but environmental factors certainly have an effect. Genes found
in the human genome determine the amount of blood vessels in the
skin, thickness of the stratum corneum, and sensitivity to light.
Genes also determine the production of three pigments which influence
|Melanocytes are specialized cells deep within the epidermis which produce the pigment melanin. This pigment, like all pigments, absorb light, but melanin absorbs many of the lights harmful wave lenghts. This pigment is produced in great quantity and packaged into granules for extracellular transport by the numerous Golgi Bodies found in the cell. This is significant because melanocytes will actually pass onto the regular epithelial cells some of this pigment through a process called cytocrine secretion. Melanocytes have long cytoplasmic extensions which reach amongst the epithelium, delivering to those cells the pigmentation. In reality, some epithelial cells actually end up with more melanin than the melanocytes!|
Hemoglobin is the pigment found in the blood that is responsible for transporting oxygen throughout the body. Oxygenated blood is red, and the amount of blood flowing through the skin determines the 'pinkness of the skin, something which is easliy seen in individuals of European heritage. Actions which may cause an increase of blood flow will likewise increase the intensity of the red coloration of the skin, such as overheating (blood flow increase for heat loss) or rapid emotional change (such as an embarressing moment). Birthmarks are congenital disorders of the capillaries in the dermis.
Carotene is a pigment related to vitamin A. This yellowish-orange pigment is found in cells of the epidermis and within adipose cells of the hypodermis. The coloration typical of individuals of Asian heritage is frequently a result of this pigment, although it is in conjunction with melanin.
Melanin is the brownish-black pigment produced by melanocytes found in the epidermis. These cells have branches which reach into the epidermis, delivering melanin to the keratinocytes, providing them with melanin which protects those cells from the suns rays. These cells are not always distributed uniformly, and dense clusters of these cells can produce freckles and moles.
Some people lack a gene needed to produce an
enzyme necessary for the production of melanin. Albinos
have a normal number of melanocytes, but because of the inability
to produce the proper enzyme, they lack the pigment.
Environmental Factors and Skin Color
Environmental factors also influence skin color. Tanning as a response to sun exposure is a frequent example of how the environment effects skin color. After exposure to significant amounts of UV light (the harmful rays of the sun), a hormone, Melanocyte Stimulating Hormone (MSH) is released from the pituitary gland. This hormone travels through the blood, stimulating the melanocytes to increase their activity level and thus produce increased amounts of melanin. Thus the skin becomes darker.
Tanning is the bodies attempt to prevent damage to the skin, protecting the ever dividing epithelial cells which forms our bodies number one protective device from the suns rays. UV can damage the DNA of these cells, leading to serious problems, such as cancer.
A yellowing of the skin, called Jaundice, can be caused by liver infection or damage. Normally the liver removes the yellow pigment Bilirubin from the blood. This ppigment is produced from the breakdown of hemoglobin from dead red blood cells. If this accumulates in the blood, the skin becomes yellowed instead of it's customary pink hue.