A skin condition, also known as a cutaneous condition, is any medical condition affecting the integumentary system—the organ system enclosing the body and including skin, nails, and related muscles and glands. The primary function of this system is as a barrier against the external environment.

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 Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many non-pathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin condition descriptions. Classification of these conditions often presents many nosological challenges since underlying causes and pathogenetic are often unknown. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), and (skin conditions resulting from physical factors).

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 Clinically, the diagnosis of any particular skin condition begins by gathering pertinent information about the presenting skin lesion(s), including location (arms, head, legs); symptoms (pruritus, pain); duration (acute or chronic); arrangement (solitary, generalized, annular, linear); morphology (macules, papules, vesicles); and color (red). Some diagnoses may also require a skin biopsy yielding histologic information correlate with the clinical presentation and laboratory data. In addition, the introduction of cutaneous ultrasound has allowed the detection of cutaneous tumors, inflammatory processes, and skin diseases.

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A layer of skin involved

 The skin weighs an average of 4 kg (8.8 lb) and covers an area of 2 m2 (22 sq ft), defined by three distinct layers: the epidermis, dermis, and subcutaneous tissue. The two main types of human skin are glabrous skin, the nonhairy skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, hairs in structures called pilosebaceous units have a hair follicle, sebaceous gland, and associated arrector pili muscle. The epidermis, hair, and glands in the embryo are from the ectoderm, chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.

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Epidermis

Main article: Epidermis (skin)

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. Nourishment is provided to these layers via diffusion from the dermis since the epidermis lacks a direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Keratinocytes are the primary component of these, constituting roughly 95% of the epidermis. This stratified squamous epithelium is present by cell division within the stratum basale, slowly differentiating cells displaced outwards through the stratum spinosum to the stratum corneum, where cells continually shed from the surface. In normal skin, the production rate equals the loss rate; a cell needs about two weeks 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.

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Dermis

Main article: Dermis

 The dermis is the layer of skin between the epidermis and subcutaneous tissue and comprises two sections, the papillary dermis and the reticular dermis. The superficial papillary dermis interdigitates with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. Structural components of the dermis are collagen, elastic fibers, and ground substance, also called extra fibrillar matrix. Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. In addition, the dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which connect by vertical communicating vessels. The function of blood vessels within the dermis is fourfold: to supply nutrition, regulate temperature, modulate inflammation, and participate in wound healing.

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Subcutaneous tissue

Main article: Subcutaneous tissue

 The subcutaneous tissue is a layer of fat between the dermis and underlying fascia. This tissue may divide into two components, the actual fatty layer, or panniculus adiposis, and a deeper vestigial layer of muscle, the Panniculus Carnosus. The main cellular component of this tissue is the adipocyte or fat cell. The structure of this tissue is composed of septal (i.e., linear strands) and lobular compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.

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Diseases of the skin

 For a list, see List of skin conditions.

Diseases of the skin include skin infections and skin neoplasms (including skin cancer).

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History

See also: History of dermatology.

In 1572, Geronimo Mercuriali of Forlì, Italy, completed De Morbis Cutaneous ('On the diseases of the skin'). It is considered the first scientific work dedicated to dermatology.

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Diagnoses

The physical examination of the skin and its appendages, as well as the mucous membranes, forms the cornerstone of an accurate diagnosis of cutaneous conditions. Most of these conditions present cutaneous surface changes, term "lesions," which have more or less distinct characteristics. Often proper examination will lead the physician to obtain appropriate historical information and laboratory tests that can confirm the diagnosis. Upon examination, the important clinical observations are the (1) morphology, (2) configuration, and (3) distribution of the lesion(s). Regarding morphology, the initial lesion characterizes a condition known as the "primary lesion," and identifying such lesions is the essential aspect of the cutaneous examination. Over time, these primary lesions may continue to develop or be modified by regression or trauma, producing "secondary lesions" However, the lack of standardization of basic dermatologic terminology has been one of the principal barriers to successful communication among physicians in describing cutaneous findings. Nevertheless, some commonly accepted terms describe the macroscopic morphology, configuration, and distribution of skin lesions, which are listed below.