Skin: Tissue Function

Physical Protection

The most important function of the skin is protection against foreign, and possibly dangerous, intrusions into the body. Skin provides a barrier against pathogens, air, water, toxins, and other substances.

The natural pH of the skin is acidic and ranges from 4.0 to 7.0, creating what is referred to as an “acid mantle.” This acidic environment is not only optimal for the survival of endogenous flora that live on the skin, but it is inhospitable to many pathogens. The resident flora are symbiotic by preventing colonization by potentially harmful foreign bacteria. The acidity of the skin is attributable to many mechanisms, such as the presence of lactic acid, free fatty acids in sebum, and active proton pumps.

Another protective function is served by melanin, produced by melanocytes in the lowest layers of the epidermis. The dark-brown granules act as a shield for the exposed DNA in the dividing cells of the stratum basale.

Physical protection is conferred by the many squamous layers of keratinocytes in the epidermis, mainly in the stratum corneum. The constant sloughing of dead cells and the tight junctions within underlying layers resist minor degrees of penetration from trauma or pathogens. The hard keratin of nails provides additional protection to soft fingertips and toes that are usually subject to a high degree of daily wear and tear.

Immune Protection

The epidermis contains keratinocytes that release agents such as tumor necrosis factor (TNF) and antimicrobial peptides in response to trauma. Epidermal Langerhans cells are modified dendritic cells that take up pathogens and present antigen to naïve T lymphocytes in secondary lymphoid tissue.

[Page 1025]The dermis also contains components of the innate and adaptive immune system, such as macrophages and mast cells that release cytokines upon stimulation by pathogens, as well as dendritic cells that present antigens. Locally released cytokines cause inflammation and recruitment of more immune cells to the site of pathogenic invasion or trauma, thus facilitating healing.

Metabolism

The skin has metabolic functions that are essential to human life, namely the production of vitamin D. The keratinocytes in the stratum basale and stratrum spinosum layers of the epidermis convert 7-dehydrocholesterol to cholecalciferol, or vitamin D3, upon exposure to UV radiation. Melanin, which has the ability to absorb UV radiation, thus may diminish the ability of the epidermis to produce vitamin D3.

Vitamin D3 is subsequently converted to its active form calcitriol via two steps of enzymatic hydroxylation in the liver and the kidneys. With regard to the effect of vitamin D on the skin, keratinocytes express the Vitamin D receptor (VDR) and utilize the hormone for proliferation and differentiation.

Sensation

The skin allows us to consciously perceive the external environment through its sensory receptors. Receptors exist either as encapsulated or free nerve endings in the epidermis. Mechanoreceptors sense pressure and differ in number all over the body. They tend to exist at a higher density in the fingers and lips, where humans are accordingly the most sensitive to touch. The type of stimulation present will cause different mechanoreceptors to fire. Merkel cells and Ruffini corpuscles have slowly adapting fibers that respond to continuous stimulation, whereas Meissner and Pacinian corpuscles have rapidly adapting fibers that respond to changes in stimulation.

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