Keratosis: Any horny growth such as a wart or callus.Keratosis, Actinic: White or pink lesions on the arms, hands, face, or scalp that arise from sun-induced DNA DAMAGE to KERATINOCYTES in exposed areas. They are considered precursor lesions to superficial SQUAMOUS CELL CARCINOMA.Keratosis, Seborrheic: Benign eccrine poromas that present as multiple oval, brown-to-black plaques, located mostly on the chest and back. The age of onset is usually in the fourth or fifth decade.Darier Disease: An autosomal dominantly inherited skin disorder characterized by warty malodorous papules that coalesce into plaques. It is caused by mutations in the ATP2A2 gene encoding SERCA2 protein, one of the SARCOPLASMIC RETICULUM CALCIUM-TRANSPORTING ATPASES. The condition is similar, clinically and histologically, to BENIGN FAMILIAL PEMPHIGUS, another autosomal dominant skin disorder. Both diseases have defective calcium pumps (CALCIUM-TRANSPORTING ATPASES) and unstable desmosomal adhesion junctions (DESMOSOMES) between KERATINOCYTES.Facial DermatosesBowen's Disease: A persistent progressive non-elevated red scaly or crusted plaque which is due to an intradermal carcinoma and is potentially malignant. Atypical squamous cells proliferate through the whole thickness of the epidermis. The lesions may occur anywhere on the skin surface or on mucosal surfaces. The cause most frequently found is trivalent arsenic compounds. Freezing, cauterization or diathermy coagulation is often effective. (From Rook et al., Textbook of Dermatology, 4th ed, pp2428-9)Keratoderma, Palmoplantar: Group of mostly hereditary disorders characterized by thickening of the palms and soles as a result of excessive keratin formation leading to hypertrophy of the stratum corneum (hyperkeratosis).Keratoacanthoma: A benign, non-neoplastic, usually self-limiting epithelial lesion closely resembling squamous cell carcinoma clinically and histopathologically. It occurs in solitary, multiple, and eruptive forms. The solitary and multiple forms occur on sunlight exposed areas and are identical histologically; they affect primarily white males. The eruptive form usually involves both sexes and appears as a generalized papular eruption.Photosensitivity Disorders: Abnormal responses to sunlight or artificial light due to extreme reactivity of light-absorbing molecules in tissues. It refers almost exclusively to skin photosensitivity, including sunburn, reactions due to repeated prolonged exposure in the absence of photosensitizing factors, and reactions requiring photosensitizing factors such as photosensitizing agents and certain diseases. With restricted reference to skin tissue, it does not include photosensitivity of the eye to light, as in photophobia or photosensitive epilepsy.Eyebrows: Curved rows of HAIR located on the upper edges of the eye sockets.Skin Neoplasms: Tumors or cancer of the SKIN.Carcinoma, Basal Cell: A malignant skin neoplasm that seldom metastasizes but has potentialities for local invasion and destruction. Clinically it is divided into types: nodular, cicatricial, morphaic, and erythematoid (pagetoid). They develop on hair-bearing skin, most commonly on sun-exposed areas. Approximately 85% are found on the head and neck area and the remaining 15% on the trunk and limbs. (From DeVita Jr et al., Cancer: Principles & Practice of Oncology, 3d ed, p1471)Lichenoid Eruptions: Conditions in which there is histological damage to the lower epidermis along with a grouped chronic inflammatory infiltrate in the papillary dermis disturbing the interface between the epidermis and dermis. LICHEN PLANUS is the prototype of all lichenoid eruptions. (From Rook et al., Textbook of Dermatology, 4th ed, p398)Lichen Planus: An inflammatory, pruritic disease of the skin and mucous membranes, which can be either generalized or localized. It is characterized by distinctive purplish, flat-topped papules having a predilection for the trunk and flexor surfaces. The lesions may be discrete or coalesce to form plaques. Histologically, there is a "saw-tooth" pattern of epidermal hyperplasia and vacuolar alteration of the basal layer of the epidermis along with an intense upper dermal inflammatory infiltrate composed predominantly of T-cells. Etiology is unknown.Scalp DermatosesOintment Bases: Various mixtures of fats, waxes, animal and plant oils and solid and liquid hydrocarbons; vehicles for medicinal substances intended for external application; there are four classes: hydrocarbon base, absorption base, water-removable base and water-soluble base; several are also emollients.Parakeratosis: Persistence of the nuclei of the keratinocytes into the stratum corneum of the skin. This is a normal state only in the epithelium of true mucous membranes in the mouth and vagina. (Dorland, 27th ed)Leukoplakia: A white patch lesion found on a MUCOUS MEMBRANE that cannot be scraped off. Leukoplakia is generally considered a precancerous condition, however its appearance may also result from a variety of HEREDITARY DISEASES.Carcinoma, Squamous Cell: A carcinoma derived from stratified SQUAMOUS EPITHELIAL CELLS. It may also occur in sites where glandular or columnar epithelium is normally present. (From Stedman, 25th ed)Keratolytic Agents: Agents that soften, separate, and cause desquamation of the cornified epithelium or horny layer of skin. They are used to expose mycelia of infecting fungi or to treat corns, warts, and certain other skin diseases.Skin DiseasesSunlight: Irradiation directly from the sun.Petrolatum: A colloidal system of semisolid hydrocarbons obtained from PETROLEUM. It is used as an ointment base, topical protectant, and lubricant.Leukoplakia, Oral: A white patch seen on the oral mucosa. It is considered a premalignant condition and is often tobacco-induced. When evidence of Epstein-Barr virus is present, the condition is called hairy leukoplakia (LEUKOPLAKIA, HAIRY).Arsenic Poisoning: Disorders associated with acute or chronic exposure to compounds containing ARSENIC (ARSENICALS) which may be fatal. Acute oral ingestion is associated with gastrointestinal symptoms and an encephalopathy which may manifest as SEIZURES, mental status changes, and COMA. Chronic exposure is associated with mucosal irritation, desquamating rash, myalgias, peripheral neuropathy, and white transverse (Mees) lines in the fingernails. (Adams et al., Principles of Neurology, 6th ed, p1212)Conjunctival DiseasesAcitretin: An oral retinoid effective in the treatment of psoriasis. It is the major metabolite of ETRETINATE with the advantage of a much shorter half-life when compared with etretinate.Skin Diseases, Genetic: Diseases of the skin with a genetic component, usually the result of various inborn errors of metabolism.Ichthyosis Vulgaris: Most common form of ICHTHYOSIS characterized by prominent scaling especially on the exterior surfaces of the extremities. It is inherited as an autosomal dominant trait.Hyperpigmentation: Excessive pigmentation of the skin, usually as a result of increased epidermal or dermal melanin pigmentation, hypermelanosis. Hyperpigmentation can be localized or generalized. The condition may arise from exposure to light, chemicals or other substances, or from a primary metabolic imbalance.Aminolevulinic Acid: A compound produced from succinyl-CoA and GLYCINE as an intermediate in heme synthesis. It is used as a PHOTOCHEMOTHERAPY for actinic KERATOSIS.Precancerous Conditions: Pathological processes that tend eventually to become malignant. (From Dorland, 27th ed)Dermatologic Agents: Drugs used to treat or prevent skin disorders or for the routine care of skin.Skin: The outer covering of the body that protects it from the environment. It is composed of the DERMIS and the EPIDERMIS.Mouth Mucosa: Lining of the ORAL CAVITY, including mucosa on the GUMS; the PALATE; the LIP; the CHEEK; floor of the mouth; and other structures. The mucosa is generally a nonkeratinized stratified squamous EPITHELIUM covering muscle, bone, or glands but can show varying degree of keratinization at specific locations.Aminoquinolines: Quinolines substituted in any position by one or more amino groups.Photochemotherapy: Therapy using oral or topical photosensitizing agents with subsequent exposure to light.Ointments: Semisolid preparations used topically for protective emollient effects or as a vehicle for local administration of medications. Ointment bases are various mixtures of fats, waxes, animal and plant oils and solid and liquid hydrocarbons.Warts: Benign epidermal proliferations or tumors; some are viral in origin.Microscopy, Acoustic: A scientific tool based on ULTRASONOGRAPHY and used not only for the observation of microstructure in metalwork but also in living tissue. In biomedical application, the acoustic propagation speed in normal and abnormal tissues can be quantified to distinguish their tissue elasticity and other properties.Skin Pigmentation: Coloration of the skin.Photosensitizing Agents: Drugs that are pharmacologically inactive but when exposed to ultraviolet radiation or sunlight are converted to their active metabolite to produce a beneficial reaction affecting the diseased tissue. These compounds can be administered topically or systemically and have been used therapeutically to treat psoriasis and various types of neoplasms.Epidermis: The external, nonvascular layer of the skin. It is made up, from within outward, of five layers of EPITHELIUM: (1) basal layer (stratum basale epidermidis); (2) spinous layer (stratum spinosum epidermidis); (3) granular layer (stratum granulosum epidermidis); (4) clear layer (stratum lucidum epidermidis); and (5) horny layer (stratum corneum epidermidis).Keratinocytes: Epidermal cells which synthesize keratin and undergo characteristic changes as they move upward from the basal layers of the epidermis to the cornified (horny) layer of the skin. Successive stages of differentiation of the keratinocytes forming the epidermal layers are basal cell, spinous or prickle cell, and the granular cell.Arsenic: A shiny gray element with atomic symbol As, atomic number 33, and atomic weight 75. It occurs throughout the universe, mostly in the form of metallic arsenides. Most forms are toxic. According to the Fourth Annual Report on Carcinogens (NTP 85-002, 1985), arsenic and certain arsenic compounds have been listed as known carcinogens. (From Merck Index, 11th ed)Administration, Topical: The application of drug preparations to the surfaces of the body, especially the skin (ADMINISTRATION, CUTANEOUS) or mucous membranes. This method of treatment is used to avoid systemic side effects when high doses are required at a localized area or as an alternative systemic administration route, to avoid hepatic processing for example.Ultraviolet Rays: That portion of the electromagnetic spectrum immediately below the visible range and extending into the x-ray frequencies. The longer wavelengths (near-UV or biotic or vital rays) are necessary for the endogenous synthesis of vitamin D and are also called antirachitic rays; the shorter, ionizing wavelengths (far-UV or abiotic or extravital rays) are viricidal, bactericidal, mutagenic, and carcinogenic and are used as disinfectants.Biopsy: Removal and pathologic examination of specimens in the form of small pieces of tissue from the living body.