Integumentary system.

Skin diseases.

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. There are two main types of human skin: 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, there are hairs in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle. 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.

Epidermis.

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 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. 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. In normal skin, the rate of production equals the rate of loss; it takes about two weeks 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.

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 extrafibrillar matrix (previously called ground substance). Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. 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.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.

Diseases of the skin.

Morphology.

Primary lesions.

• Macule – A macule is a change in surface color, without elevation or depression and, therefore, nonpalpable, well or ill-defined, variously sized, but generally considered less than either 5 or 10 mm in diameter at the widest point.
• Patch – A patch is a large macule equal to or greater than either 5 or 10 mm, across depending on one's definition of a macule. Patches may have some subtle surface change, such as a fine scale or wrinkling, but although the consistency of the surface is changed, the lesion itself is not palpable.
• Papule – A papule is a circumscribed, solid elevation of skin with no visible fluid, varying in size from a pinhead to less than either 5 or 10 mm in diameter at the widest point.
• Plaque – A plaque has been described as a broad papule, or confluence of papules equal to or greater than 1 cm, or alternatively as an elevated, plateau-like lesion that is greater in its diameter than in its depth.
• Nodule – A nodule is morphologically similar to a papule, but is greater than either 5 or 10 mm in both width and depth, and most frequently centered in the dermis or subcutaneous fat. The depth of involvement is what differentiates a nodule from a papule.
• Vesicle – A vesicle is a circumscribed, fluid-containing, epidermal elevation generally considered less than either 5 or 10 mm in diameter at the widest point.
• Bulla – A bulla is a large vesicle described as a rounded or irregularly shaped blister containing serous or seropurulent fluid, equal to or greater than either 5 or 10 mm, depending on one's definition of a vesicle.
• Pustule – A pustule is a small elevation of the skin containing cloudy or purulent material usually consisting of necrotic inflammatory cells.These can be either white or red.
• Cyst – A cyst is an epithelial-lined cavity containing liquid, semi-solid, or solid material.
• Erosion – An erosion is a discontinuity of the skin exhibiting incomplete loss of the epidermis, a lesion that is moist, circumscribed, and usually depressed.
• Ulcer – An ulcer is a discontinuity of the skin exhibiting complete loss of the epidermis and often portions of the dermis and even subcutaneous fat.
• Fissure – A fissure is a crack in the skin that is usually narrow but deep.
• Wheal – A wheal is a rounded or flat-topped, pale red papule or plaque that is characteristically evanescent, disappearing within 24 to 48 hours.
• Telangiectasia – A telangiectasia represents an enlargement of superficial blood vessels to the point of being visible.
• Burrow – A burrow appears as a slightly elevated, grayish, tortuous line in the skin, and is caused by burrowing organisms.

Secondary lesions.

• Scale – dry or greasy laminated masses of keratin that represent thickened stratum corneum.
• Crust – dried serum, pus, or blood usually mixed with epithelial and sometimes bacterial debris.
• Lichenification – epidermal thickening characterized by visible and palpable thickening of the skin with accentuated skin markings.
• Excoriation – a punctate or linear abrasion produced by mechanical means (often scratching), usually involving only the epidermis but not uncommonly reaching the papillary dermis.
• Induration – dermal thickening causing the cutaneous surface to feel thicker and firmer.
• Atrophy – refers to a loss of tissue, and can be epidermal, dermal, or subcutaneous. With epidermal atrophy, the skin appears thin, translucent, and wrinkled. Dermal or subcutaneous atrophy is represented by depression of the skin.
• Maceration – softening and turning white of the skin due to being consistently wet.

Burns.

Burns to the skin can occur in many ways.  Burns can be caused by dry heat such as fire, moist heat such as steam or hot liquids, Radiation, friction, heated objects, the sun, electricity, or  chemicals.

Thermal burns are the most common type. Thermal burns occur when hot metals, scalding liquids, steam, or flames come in contact with the skin. This type of burn result frequently from fires, automobile accidents, playing with matches, improperly stored gasoline, space heaters, and electrical malfunctions. Other causes include unsafe handling of fireworks and/or kitchen accidents (such as a child climbing on top of a stove, pulling down a hot pot, or grabbing a hot iron).

Inhaling smoke, steam, or superheated air can cause burns to your airways, or toxic fumes, often in a poorly ventilated space.

There are three levels of burns:

• First-degree burns affect only the outer layer of the skin. They cause pain, redness, and swelling.
• Second-degree burns affect both the outer and underlying layer of skin. They cause pain, redness, swelling, and blistering.
• Third-degree burns extend into deeper tissues. They cause white or blackened, charred skin that may be numb.

Ulcers.

An ulcer is a discontinuity or break in a bodily membrane that impedes the organ of which that membrane is a part from continuing its normal functions. Common forms of ulcers recognized in medicine include:

• Ulcer (dermatology), a discontinuity of the skin or a break in the skin
  • Pressure ulcers, also known as bedsores
  • Genital ulcer, an ulcer located on the genital area
  • Ulcerative dermatitis, a skin disorder associated with bacterial growth often initiated by self-trauma due to a possible allergic response
• Corneal ulcer, an inflammatory or infective condition of the cornea
• Mouth ulcer, an open sore inside the mouth
  • Aphthous ulcer, a specific type of oral ulcer also known as a canker sore
• Peptic ulcer, a discontinuity of the gastrointestinal mucosa (stomach ulcer)
• Venous ulcer, a wound thought to occur due to improper functioning of valves in the veins
• Ulcerative sarcoidosis, a cutaneous condition affecting people with sarcoidosis
• Ulcerative lichen planus, a rare variant of lichen planus
• Ulcerative colitis, a form of inflammatory bowel disease,

Allergies

An allergy is a reaction of your immune system to something that does not bother most other people. People who have allergies often are sensitive to more than one thing. Substances that often cause reactions are

• Pollen
• Dust mites
• Mold spores
• Pet dander
• Food
• Insect stings
• Medicines

How do you get allergies? Scientists think both genes and the environment have something to do with it. Normally, your immune system fights germs. It is your body's defense system. In most allergic reactions, however, it is responding to a false alarm.

Allergies can cause a runny nose, sneezing, itching, rashes, swelling or asthma. Symptoms vary. Although allergies can make you feel bad, they usually won't kill you. However, a severe reaction called anaphylaxis is life-threatening.