Many conditions affect the human integumentary system—the organ system covering the entire surface of the body and composed of skin, hair, nails, and related muscle and glands.[1] The major function of this system is as a barrier against the external environment.[2] The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue.[1] The two main types of human skin are: glabrous skin, the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin.[3] Within the latter type, the hairs occur in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle.[4]In the embryo, the epidermis, hair, and glands form from the ectoderm, which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.[5][6][7]
The epidermis is the most superficial layer of skin, a squamous epithelium with several strata: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale.[8] Nourishment is provided to these layers by diffusion from the dermis, since the epidermis is without direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Of these, keratinocytes are the major component, constituting roughly 95 percent of the epidermis.[3] This stratified squamous epithelium is maintained by cell division within the stratum basale, in which differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where cells are continually shed from the surface.[3] In normal skin, the rate of production equals the rate of loss; about two weeks are needed for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional two weeks to cross the stratum corneum.[9]
The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary dermis and the reticular dermis.[10] The superficial papillary dermis interdigitates with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone.[10] Structural components of the dermis are collagen, elastic fibers, and ground substance.[10] Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands.[8] The dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which are connected by vertical communicating vessels.[8][11] The function of blood vessels within the dermis is fourfold: to supply nutrition, to regulate temperature, to modulate inflammation, and to participate in wound healing.[12][13]
The subcutaneous tissue is a layer of fat between the dermis and underlying fascia.[14] This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus.[3] The main cellular component of this tissue is the adipocyte, or fat cell.[14] The structure of this tissue is composed of septal (i.e. linear strands) and lobular compartments, which differ in microscopic appearance.[8] Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.[14]
Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails).[15][16] While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described.[14] Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known.[17][18] Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), etiology (skin conditions resulting from physical factors), and so on.[19][20] Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow).[21] Diagnosis of many conditions often also requires a skin biopsy which yields histologic information[22][23] that can be correlated with the clinical presentation and any laboratory data.[24][25][26]
Cutaneous congenital anomalies are a diverse group of disorders that result from faulty morphogenesis, the biological process that forms the shape of a human body.[35][40][41]
Dermal and subcutaneous growths result from (1) reactive or neoplastic proliferation of cellular components of the dermis or subcutaneous tissue, or (2) neoplasms invading or aberrantly present in the dermis.[1][35]
Disturbances of human pigmentation, either loss or reduction, may be related to loss of melanocytes or the inability of melanocytes to produce melanin or transport melanosomes correctly.[54][55][56]
Eosinophilic cutaneous conditions encompass a wide variety of diseases that are characterized histologically by the presence of eosinophils in the inflammatory infiltrate, or evidence of eosinophil degranulation.[62][63]
Merkel cell carcinoma (cutaneous apudoma, primary neuroendocrine carcinoma of the skin, primary small cell carcinoma of the skin, trabecular carcinoma of the skin)
Multiple keratoacanthomas (Ferguson–Smith syndrome, Ferguson-Smith type of multiple self-healing keratoacanthomas, multiple keratoacanthomas of the Ferguson–Smith type)
Linear and whorled nevoid hypermelanosis (linear nevoid hyperpigmentation, progressive cribriform and zosteriform hyperpigmentation, reticulate and zosteriform hyperpigmentation, reticulate hyperpigmentation of Iijima and Naito and Uyeno, zebra-like hyperpigmentation in whorls and streaks, zebra-line hyperpigmentation)
Scleroatrophic syndrome of Huriez (Huriez syndrome, palmoplantar keratoderma with scleroatrophy, palmoplantar keratoderma with sclerodactyly, scleroatrophic and keratotic dermatosis of the limbs, sclerotylosis)
Bacterium-related cutaneous conditions often have distinct morphologic characteristics that may be an indication of a generalized systemic process or simply an isolated superficial infection.[66][67]
Mycosis-related cutaneous conditions are caused by fungi or yeasts, and may present as either a superficial or deep infection of the skin, hair, or nails.[66]
Intravascular large B-cell lymphoma (angiotropic large cell lymphoma, intralymphatic lymphomatosis, intravascular lymphomatosis, malignant angioendotheliomatosis)
Jessner lymphocytic infiltrate of the skin (benign lymphocytic infiltration of the skin, Jessner lymphocytic infiltration of the skin, Jessner–Kanof lymphocytic infiltration of the skin, lymphocytic infiltrate of Jessner)
Melanocytic nevi and neoplasms are caused by either a proliferation of (1) melanocytes, or (2) nevus cells, a form of melanocyte that lack dendritic processes.[75][76]
Acral nevus (melanocytic nevus of acral skin, melanocytic nevus with intraepidermal ascent of cells)
Cutaneous lupus mucinosis (papular and nodular mucinosis in lupus erythematosus, papular and nodular mucinosis of Gold, papulonodular mucinosis in lupus erythematosus)
Confluent and reticulated papillomatosis (confluent and reticulated papillomatosis of Gougerot and Carteaud, familial cutaneous papillomatosis, familial occurrence of confluent and reticulated papillomatosis)
Keratosis punctata of the palmar creases (hyperkeratosis penetrans, hyperkeratosis punctata, keratodermia punctata, keratosis punctata, keratotic pits of the palmar creases, lenticular atrophia of the palmar creases, punctate keratosis of the palmar creases)
Carvajal syndrome (striate palmoplantar keratoderma with woolly hair and cardiomyopathy, striate palmoplantar keratoderma with woolly hair and left ventricular dilated cardiomyopathy)
Erythrokeratodermia variabilis (erythrokeratodermia figurata variabilis, keratosis extremitatum progrediens, keratosis palmoplantaris transgrediens et progrediens, Mendes da Costa syndrome, Mendes da Costa type erythrokeratodermia, progressive symmetric erythrokeratoderma)
Focal acral hyperkeratosis (acrokeratoelastoidosis lichenoides, degenerative collagenous plaques of the hand)
Haim–Munk syndrome (palmoplantar keratoderma with periodontitis and arachnodactyly and acro-osteolysis)
Hidrotic ectodermal dysplasia (alopecia congenita with keratosis palmoplantaris, Clouston syndrome, Clouston's hidrotic ectodermal dysplasia, Fischer–Jacobsen–Clouston syndrome, keratosis palmaris with drumstick fingers, palmoplantar keratoderma and clubbing)
Howel–Evans syndrome (familial keratoderma with carcinoma of the esophagus, focal non-epidermolytic palmoplantar keratoderma with carcinoma of the esophagus, palmoplantar ectodermal dysplasia type III, palmoplantar keratoderma associated with esophageal cancer, tylosis, tylosis–esophageal carcinoma)
Keratosis punctata palmaris et plantaris (Buschke–Fischer–Brauer disease, Davis Colley disease, keratoderma disseminatum palmaris et plantaris, keratosis papulosa, keratoderma punctatum, keratodermia punctata, keratoma hereditarium dissipatum palmare et plantare, palmar and plantar seed dermatoses, palmar keratoses, papulotranslucent acrokeratoderma, punctate keratoderma, punctate keratoses of the palms and soles, maculosa disseminata)
Keratitis–ichthyosis–deafness syndrome (erythrokeratodermia progressiva Burns, ichthyosiform erythroderma with corneal involvement and deafness, KID syndrome)
Mal de Meleda (acral keratoderma, Gamborg–Nielsen keratoderma, mutilating palmoplantar keratoderma of the Gamborg–Nielsen type, palmoplantar ectodermal dysplasia type VIII, palmoplantar keratoderma of the Norrbotten type)
Naxos syndrome (diffuse non-epidermolytic palmoplantar keratoderma with woolly hair and cardiomyopathy, diffuse palmoplantar keratoderma with woolly hair and arrythmogenic right ventricular cardiomyopathy of Naxos, Naxos disease)
Olmsted syndrome (mutilating palmoplantar keratoderma with periorificial keratotic plaques, mutilating palmoplantar keratoderma with periorificial plaques, polykeratosis of Touraine)
Scleroatrophic syndrome of Huriez (Huriez syndrome, palmoplantar keratoderma with scleroatrophy, palmoplantar keratoderma with sclerodactyly, scleroatrophic and keratotic dermatosis of the limbs, sclerotylosis)
Prurigo gestationis (Besnier prurigo, early-onset prurigo of pregnancy, linear IgM dermatosis of pregnancy, papular dermatitis of pregnancy, prurigo of pregnancy, Spangler's papular dermatitis of pregnancy)
Palmoplantar pustulosis (persistent palmoplantar pustulosis, pustular psoriasis of the Barber type, pustular psoriasis of the extremities, pustulosis of palms and soles, pustulosis palmaris et plantaris)
Skin conditions resulting from errors in metabolism are caused by enzymatic defects that lead to an accumulation or deficiency of various cellular components, including, but not limited to, amino acids, carbohydrates, and lipids.[16]
Skin conditions resulting from physical factors occur from a number of causes, including, but not limited to, hot and cold temperatures, friction, and moisture.[34][99][100]
Urticaria is a vascular reaction of the skin characterized by the appearance of wheals, which are firm, elevated swelling of the skin.[102]Angioedema, which can occur alone or with
urticaria, is characterized by a well-defined, edematous swelling that involves subcutaneous tissues, abdominal organs, or upper airway.[103]
^Any given cutaneous condition is only included once within this list.
^Parentheticals are used to indicate other names by which a condition is known. If there are multiple alternative names for a condition, they are separated by commas within the parenthetical.
^Citations for any given condition name and/or alternative name(s) may be found within the condition's respective article.
^This list uses American English; therefore, the symbols æ and œ, which are common to British English, are not used, but, rather, simplified to a single e. For example, the spelling of nevus is favored over nævus, edema over œdema, and so forth. For more information, see American and British English differences.
^Within this list, the terms human immunodeficiency virus and acquired immunodeficiency syndrome are abbreviated to HIV and AIDS, respectively.
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