Structure and function of skin, hair and nails

The skin is the largest organ of the human body. It is a complex epithelial and mesenchymal tissue comprising a multilayered stratified epidermis, adnexal structures such as hair follicles, sweat glands and sebaceous glands, a dermis containing collagen and elastic fibres, and underlying subcutaneous fat. More than 1000 disease entities involving the skin have been described, and up to 20% of all patient referrals to general practitioners involve skin pathology. Infections, drug reactions and diseases such as psoriasis, eczema, urticaria and skin cancer impose a considerable burden on healthcare resources and significantly affect patients' quality of life. Knowledge of the structure and function of the skin and its appendages is paramount to understanding the biology of healthy skin and the pathophysiology of skin diseases.