The skin is a complex tissue that serves as a barrier to fluid loss and to the entry of pathogenic organisms; it must be flexible but nevertheless withstand mechanical stress and abrasion. The outermost layer (epidermis) is a stratified squamous epithelium; daughter cells from the stem cells (basal cells) become progressively filled with keratin and are eventually shed as squames. Dysfunction in the keratinization process (cornification) can cause palmoplantar keratoderma, Naxos disease, and Vohwinkel's syndrome whereas simple mechanical irritation can cause the formation of calluses or corns. Neoplasia of basal cells, the major cell type, can produce basal cell carcinoma or Gorlin's syndrome. Other cells of the epidermis are melanocytes and cells involved in immune surveillance (dendritic cells, Langerhans cells, skin associated lymphoid tissue). Underlying the epidermis is the vascularized connective tissue of the dermis containing fibroblasts which are important in wound healing. Because they are visible, skin diseases are well recognized. Some have an inflammatory basis (e.g. atopic dermatitis; cheiropompholyx; eczema; psoriasis); in others the defects are in cellular components such as desmosomes that are important for mechanical resistance (see ectodermal dysplasia; epidermolysis bullosa; Hailey–Hailey disease; pemphigus). See also acanthoma; dermatofibrosarcoma protuberans; dermatographia; dermatomyositis; dermatophytes; familial primary localized cutaneous amyloidosis; hyperkeratosis; ichthyosiform erythroderma; ichthyosis; keratoacanthoma; lichen planus; Merkel cell carcinoma; Molluscum contagiosum virus; mycosis; pigmentation; ringworm; Rothmund–Thomson syndrome; tuberous sclerosis. The synthesis of vitamin D in the skin is important.
Subjects: Medicine and Health.