The skin is the outermost layer covering the body surface. It is composed of two main layers, on is called the epidermis, which is the outermost covering of the skin. Deep to it is the dermis layer, which is a connective tissue that contains blood vessels, nerves and glands.
The epidermis does not have blood supply,therefor it does not bleed if injured. It is composed of cells that are arranged in several layers and is called stratified epithelium. The epithelium is also keratinized, namely it contains the protein keratin which protects skin from heat and infectious agents and from hazardous chemicals.
The epidermis contains several types of cells. These are: first, keratinocytes which constitute approximately 90% of the cell population in the epidermis. The second type of cells in the epidermis is the melanocytes. Melanocytes produce the skin pigment melanin which gives the skin its typical brown color.
The skin of black people differ from that of white people in the amount of melanin in the skin. They have much more melanin in their skin. after its production, melanin is transferred to keratinocytes and there it forms a shield around the nucleus and thus protect the keratinocytes from the damaging effect of ultra-violet radiation.
Melanin contributes to the brown color of the skin. In the case of jaundice another yellow pigment (bilirubin) accumulates in the skin and give it a yellowish color. Bilirubin is a product of hemoglobin degradation. Another type of cells in the epidermis is called langerhans cells. These cells are of immunologic origin and help in performing immune response. They originate in the bone marrow. They constitute a small amount of the total cells in the epidermis.
The other type of cells in the epidermis is called merkel cells. These are neurons or nerve cells that are specialized in the sensation of touching. Deep to the epidermis is the dermis layer. The color of the skin is determined primarily by the pigment melanin. However, other compounds impart color to skin as well. These are hemoglobin ( the oxygen carrier in the blood), and carotene ( the precursor of vitamin A which is important in the physiology of vision).
Melanin is synthesized by melanocytes from the amino acid tyrosine and the synthesis requires an enzyme that is called tyrozinase which catalyzes the conversion of tyrosine to melanin.
Two skin diseases that are concerned with pigment loss are albinism and vitiligo. In albinism, which is an inherited disease the patient is unable to produce melanin. This condition is typified by the inability of melanocytes to produce the enzyme tyrozinase which is essential for the production of melanin.
The other condition ( vitiligo) is characterized by white spots on the skin due to the absence of melanocytes at these white spots. This loss of melanocytes can be due to an autoimmune reaction in which the immune system produces antibodies which attacks melanocytes. Another skin disease which is not related to melanocytes or pigment is called psoriasis.
It is related to keratinocytes. It is characterized by fast division of keratinocytes and their move from the deeper layer of the skin where they form to the epidermis. In this condition keratinocytes produce keratin in excess which is manifest in the skin. The skin has many functions and one of them is a protecting function. It forms a physical barrier against pathogens and chemicals. Its stratified epithelium gives it the ability to form a barrier between the outer and inner environment of the body.
Another function of the skin is regulation of temperature. It do so by the mechanism of sweating and the evaporation of sweat from the skin. The other function of skin is to synthesize precursor of vitamin D or calcitriol which is important in bone metabolism.