![]() Vitamin D3 regulates epidermal differentiation and proliferation. In the liver, vitamin D3 is hydroxylated to 25-hydroxy D3 and in kidneys to the active form 1,25-vitamin D3. Sunlight acts on the epidermis to convert provitamin D3 (7-dehydrocholesterol) to vitamin D3 (cholecalciferol). The epidermis is also an important site of vitamin D regulation. Hair and melanin pigment deliver the bulk of photoprotection as well as provide thermoregulation. Constant immunosurveillance is provided by the skin immune system that consists of orchestrated interactions between Langerhans cells, keratinocytes, lymphocytes, and dermal dendritic cells. The SC also contains antimicrobial peptides (e.g., β-defensins, cathelicidins) and a variety of lipids that also aid in bacterial and chemical defense. The constant shedding (desquamation) and renewal aids in the removal of surface pathogens. This innocuous appearing layer of terminally differentiated (dead) keratin squames maintains hydration of the body by restricting water loss. The protective barrier of the skin is complex, and much of this function is provided by the stratum corneum (SC), the outermost layer of the epidermis. In farm animals, skin diseases produce severe economic losses because of damage to wool, hides, and meat or from decreased milk production or growth rates. In addition to causing undo suffering, severe skin disease may interfere with the human-animal bond. In companion animals, a healthy pelage (haircoat) stands as the interface between the human owner and pet. Skin diseases range from minor, esthetic problems to life-threatening conditions. Unlike diseases in internal viscera, lesions in the skin are readily apparent, and thus skin disease is one of the most common reasons for veterinary consultation. The vast surface area of the skin puts it in constant contact with environmental irritants, pollutants, and pathogens. ![]() The integument serves as the anatomic boundary between the body and the ambient environment. ![]()
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