The Structure of the Skin

Guy Heynen, MD
March 20, 2002 (Reviewed: March 7, 2004)

Skin structure

The skin is composed of 3 main layers. The surface one visible to us is called the epidermis, which itself is laid on top of the dermis. The dermis contains connective tissue, blood vessels, occasional white blood cells (lymphocytes and other cells concerned with immunity), nerve endings, hair follicles, sweat glands and sebaceous glands (which secrete the oily substance sebum). The connective tissue is made up of fibroblasts - cells that are responsible for the manufacture of collagen and elastic fibers, which provide the skin with its toughness and elasticity.

The epidermis and the dermis are separated by a continuous membrane (the basal membrane), which is anchored to the underlying dermis by a network of small fibers; this constitutes the first physical barrier to the outer environment.

click on figure to enlarge it

There is a thin film of sebum covering the top layer of the epidermis (the stratum corneum). This is a waxy mixture of lipids and cell fragments secreted by sebaceous glands, which also contains a mixture of bacteria that do not normally invade the body any further. Sebum helps to keep the skin moist and the stratum corneum healthy. The epidermis doesn't contain blood vessels, but has nerve endings, occasional muscle fibers, melanocytes - cells that produce the melanin pigment that infiltrates epidermal cells (keratinocytes) to protect them from UV radiation damage - and special cells called Langerhans cells, which play an important role in atopic and allergic contact dermatitis.

The third layer - the hypodermis, or subcutis - contains fatty tissue, and serves to attach the dermis to the underlying tissues (e.g. the muscles).

The keratinization process and its acceleration in dermatitis

The deepest layer of epidermal cells - the basal-layer cells - which rests on the basal membrane of the epidermis, divides continuously to produce other cells that are progressively pushed up towards the surface. In normal skin, it takes approximately 2-3 weeks for the cells to arrive at the surface of the skin (the corneal layer, or the stratum corneum), and an additional 2-3 weeks for them to be rubbed off by the wear and tear process, or simply to flake off (desquamate or scale off). Millions of such cells are sloughed off each day and are replaced by keratinocytes from the intermediate layer - the stratum granulosum.

The migration of keratinocytes, which become tightly bound together by intercellular bridges or junctions called desmosomes, from the basal to the corneal layer, is called keratinisation. Close packing of the cells of the stratum corneum forms an effective physical barrier against potential outside aggressors and also helps retain moisture.

The process of keratinisation normally takes about 30 to 40 days to complete and is under the control of growth stimulating and inhibiting factors. These factors are either produced and secreted from distant sites (such as sex hormones, growth hormone, and steroid hormones) or produced and secreted locally by keratinocytes, T-lymphocytes and Langerhans cells (such as cytokines, and local growth factors).

In atopic dermatitis, where the epidermis is under the influence of increased production of cytokines and other local growth factors and inflammatory substances, the keratinization process is accelerated, the skin becomes thicker and drier, and visible scaling may occur. (These changes in the skin in atopic dermatitis are depicted in the graphic below). These substances also stimulate the nerve endings, sending an unpleasant itching sensation to the brain, which results in scratching. The mechanical effect of scratching is itself a powerful signal for keratinocytes to produce more local cytokines, starting the infamous itch-scratch vicious cycle. Worse, the scratch disrupts the physical barrier of the close-packed cells, allowing bacteria to invade the epidermis, causing additional inflammation. In this chain of events, early interruption of excess production of cytokines and/or their effects is obviously a priority goal. This offers the best chance of interrupting the symptoms and signs of dermatitis; it will be the subject of future articles in this series.

The figure below represents changes that occur in skin affected by atopic dermatitis. Looking at it from left to right shows the various stages of the disease. The bold numbers in parentheses refer to the same numbers in the figure. A few definitions have already been given in previous articles on the subject, but some new ones appear here.

click on figure to enlarge it

(1) Intercellular edema (spongiosis). Fluid accumulates in the epidermis as cells move out of the blood vessels and into the skin, during the inflammation of dermatitis.
(2) Acanthosis is a variable thickening of the cells of the stratum spinosum.
(3) Hyperplasia. An increase in cells of the stratum corneum, in response to local tissue damage by inflammation or scratching, which is more than sebum production can cope with. The skin becomes dry (xerosis), thick and inelastic (this skin is said to be lichenified). It is subject to painful fissures.
(4) Papule. A small, visible, palpable, flat-topped, firm lesion, raised above the skin surface (a papule larger than 1 cm diameter is called a plaque).
(5) Vesicle. A small blister (less than 1 cm diameter). Larger blisters, called bullae, are more likely to occur in irritant contact dermatitis than allergic contact dermatitis or atopic dermatitis.
(6) Erythema is reddened skin.
(7) Scaling becomes excessive and visible.
(8) Weeping lesions are scratched and ruptured lesions, which ooze serum, dried blood, scales and pus.
(9) Crust dried matter from a weeping lesion, yellow in color.
(10) Excoriations. Scratch marks on the skin's surface. In patients with chronic dermatitis, excoriations are clearly visible on the skin some time after the person has scratched themselves.
(11) Fissure. A linear crack which extends at least into the dermis of excessively dry skin. Occurs in chronic dermatitis.
(12) Pain receptors stimulated - itch is a sensation that only occurs in skin and it is a major feature of dermatitis.
(13) Cells migrating from the blood vessels of the dermis.
(14) Vesicle forming.

Sources

• An Atlas of Dermatology. L. Fry, Parthenon, London, 1997



• Atopic dermatitis: new insights and opportunities for therapeutic intervention. DY. Leung, J Allergy Clin Immunol, 2000, vol. 105, pp. 860--876

Related Links
The Many Functions of the Skin
What is Dermatitis?
What Causes Dermatitis?

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