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The skin covers the body and protects it from the external environment. It also prevents water loss, provides sensory function, plays a role in body temperature regulation, and is the site of vitamin D synthesis.
The skin is composed of 2 layers: the outer epidermis and the deeper dermis. The dermis is connected to underlying structures via a subcutaneous tissue, the hypodermis, which is not technically considered part of the skin.
The epidermis provides barrier and protection, it consists mainly of the protein keratin, a tough and water-insoluble structural protein.
The dermis constitutes the bulk of the skin, it provides support and flexibility. The dermis consists mainly of collagen, and to a lesser extent, elastin fibers. Loss of collagen and elastin, such as with aging, causes the skin to slack. The boundary surface between the epidermis and dermis is not flat but wavy, meaning the 2 tissues interlock, strengthening their connection. With age, this boundary flattens and the skin becomes more fragile. The dermis is well vascularized and contains sensory nerves, hair follicles, sebaceous glands and sweat glands. It has 2 zones: the upper papillary dermis with loose connective tissue, and the lower reticular dermis with denser connective tissue. The dermis houses immune cells and allows inflammatory response to activate upon exposure to invading organisms.
The hypodermis is composed of loose connective and adipose tissues. This is where most of the body fat is stored. The hypodermis provides thermal insulation, padding and serves as the body main energy storage.
The thickness and proportion of the epidermis and dermis vary greatly depending on their location on the body, but the skin is classified as thick or thin based on the thickness of the epidermis alone. Thick skin is found only in areas where there is a lot of abrasion: palms, soles, digits; and has 5 epidermal layers. Thin skin is everywhere else and has 4 epidermal layers.
Most cells of the epidermis are keratin-producing cells, or keratinocytes. New cells are constantly produced by mitotic cell division in the basal layer. They then move towards the skin surface as they age and differentiate, changing shape, from cuboidal to flat. The distinct epidermal layers represent different stages of keratinocyte differentiation, from their birth to their death.
The spinous layer is characterized by presence of abundant desmosomes which connect keratin filaments of adjacent cells, anchoring them together, providing resistance to physical stress.
The granular layer is loaded with keratohyalin granules. These granules release several substances that cross-link keratin filaments, converting them into an impermeable keratin matrix. This process is known as cornification or keratinization, the result of which is the most superficial layer, the cornified layer, about 30 cells thick. These fully keratinized dead cells form the skin barrier. They are shed periodically from the surface as new cells are moving up. The entire epidermis is replaced every 30 to 40 days. The renewal process becomes slower with age but faster in injured skin, when cell proliferation is accelerated for wound healing.
The epidermis also contains immune cells, touch sensory cells and melanocytes. Melanocytes produce the pigment melanin and transfer it to keratinocytes. The amount of melanin produced is the major determinant of skin color. Melanin synthesis is stimulated by UV light and is thought to be a protective mechanism against UV radiation damage.

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Eczema, or dermatitis, is a group of conditions characterized by inflammation of the skin. Among the many types of dermatitis, the most common is atopic dermatitis, also known as atopic eczema. Very often, when not specified otherwise, the term “eczema” is used to describe the atopic type.
Symptoms of atopic dermatitis include rashes, redness, scaling, and occasionally small blisters. Depending on the patient’s age, these patches may appear on the face, scalp, neck, inside the elbows, behind the knees, on the buttocks, hands and feet. The condition evolves in the form of recurrent inflammatory flare-ups followed by periods of remission. Flare ups can be triggered upon contact with irritants such as soap, detergents, rough fabric or certain foods. A dry atmosphere, changes in temperature, dental eruptions and stress are also common triggers. Over time, the skin can become thickened, bumpy and constantly itch, even when the inflammation is not flaring up. Atopic eczema usually starts in early childhood and MAY last into adult life. Most children outgrow the disease with age but their skin may remain dry and easily irritable.
Atopic dermatitis is an allergic disease. The cause is unknown but it is likely to involve genetic and environmental factors. Atopic eczema often runs in families whose members also tend to develop hay fever, asthma and certain food allergies. Most notable is the gene that encodes for filaggrin, a protein involved in water retention and is responsible for the skin barrier function. Mutations in the filaggrin gene cause dry skin and, as a result, a strong susceptibility to the disease. Eczema is NOT contagious.
There is no cure for atopic dermatitis. Treatments aim to relieve symptoms, reduce frequency of flare ups and prevent skin infection. A treatment plan may include:
– Lifestyle changes: bathe at least once a day but avoid soaps; wear silk clothing and avoid wool; avoid allergy triggers.
– Skincare: use oil-based, fragrance-free moisturizers to keep the skin hydrated during remissions.
– Medications: anti-inflammatory drugs such as steroid creams can be used during flare-ups. Antibiotics may be required if skin infection occurs.