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Larry Sawchuk

Dark skin evolved pari passu with the loss of body hair and was the original state for the genus HomoThe largest and most massive of the organs of the body the skin of the average adult human exceeds 2 m2 yet is generally no thicker than 2 mmLacking adequate protection from hair human skin has undergone numerous adaptive structural changes that give it strength resilienceand sensitivityThe skin of humans like that of all tetrapods acts as a sun shield to protect the body from most solar UV radiation UVR and is the locus for the initiation of the important UVRdriven process of vitamin D production in the bodyEpidermis a thinner outer layer the epidermis and a thicker and more internally complex inner layer the dermisThe epidermis is a smooth abrasionresistant surface that is interrupted only by hair follicles and the pores of sweat glandsKeratinocytes are the principal cell type found in epidermis and are composed largely of filamentous proteins known as keratins which are imbedded in an amorphous matrixThe skins elasticity and resistance to physical and chemical attack can be attributed to the high elastic modulus and unique amino acid composition of the keratinized layer of the epidermisThe epidermis also contains populations of three types of immigrant dendritic cells melanocytes Langerhans cells and Merkel cellsMelanocytes produce the skins primary pigment melaninLangerhans cells are specialized cells of the immune system that present and respond to antigens coming in contact with the skinMerkel cells are associated with nerve terminals that together function as slowadapting mechanoreceptors for touch they are most common on the glabrous skin of the fingertipsThe epidermis is subdivided into four layers from deep to superficial the stratum basale the germinative layer of keratinocytes the stratum spinosum the stratum granulosum and the stratum corneumStratum corneum acts as a barrier to the unrestrained passage of water and solutes through the skin defends against invasion by microorganisms and the penetration of toxic substances and protects against most mechanical injury caused by friction abrasion pricks or arthropod bitesthe epidermis is in a constant state of turnover as the outermost cornified cells of the stratum corneum are shed as they are replaced from belowThe stratum corneum of darkly pigmented or heavily tanned people is more compact and consists of more cornified cell layers than that of lightly pigmented people these characteristics enhance the barrier protection functions of the skinConsiderable variation in epidermal thickness exists within human populations and is likely related to age and history of sun exposureIn all primates the epidermis of the volar surfaces of the hands and feet exhibit welldeveloped epidermal ridges or dermatoglyphics which impart greater resistanceagainst frictionDermatoglyphics are also found on the ventral surfaces of the tails of prehensiletailed New World monkeys and on the knuckle pads of chimpanzees and gorillas
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