Though many people probably regard the skin as just our outside layer, it is in reality a large and complex organ – in fact, it is the largest in the body, with a total surface area of about 2 square metres. It varies considerably in thickness from about 0.5mm on the eyelids to more than 5mm on the upper back, with an average of about 2mm. It grows constantly from inside, replacing the outer layers as they wear off: during a lifetime, each of us sheds about 50 kilos of skin cells.

Within the skin are other structures with specialised functions. These include hairs and hair follicles, finger and toe nails, sebaceous (fat-producing) glands, sweat glands, small arteries and veins, and a wide range of nerve types ending in receptors for detecting external stimuli such as heat and cold, pressure, vibration and pain.

In order to understand what can go wrong with the skin, it is first necessary to consider the structure and functions of healthy skin. We can then apply this knowledge in our exploration of acne, eczema, and psoriasis, and how they can be treated.

Structure of the healthy skin
From the outside inwards, the skin has three main layers, called the epidermis, the dermis and the hypodermis.

The epidermis: This outer layer is mostly very thin (about 0.1mm thick), except for the palms and soles, where it may reach as much as 1.4mm. Under the microscope, it can be seen to consist of several different layers – though these are hard to detect in very thin skin. The innermost layer of the epidermis is called the basal layer (stratum basale), where cell division occurs to make new skin cells. Scattered within this layer are specialised cells called melanocytes which make melanin, the brown substance that accumulates during a sun tan. We all have similar numbers of melanocytes, but those of dark-skinned people are genetically programmed to make far more melanin than those with a fair skin. Above the basal layer is the prickle cell layer, (stratum spinosum), so-called because the cells appear to be linked together by minute strands. Above this is the granular layer (stratum granulosum), in which the cells lose their nuclei and begin to flatten and die. Finally, on the outside is the horny layer (stratum corneum). In surface view, this appears as overlapping plates like roof tiles. The epidermis lacks blood vessels and nerve fibres but is traversed by the ducts of sweat glands and by hairs.

The dermis: Lying below the epidermis, this consists of two layers, a papillary layer and a lower reticular layer. The border between the epidermis and the papillary layer is not flat, but is very wavy, with projections reaching up into the epidermis. These upward waves contain small blood vessels that nourish the epidermis and nerve endings concerned with fine touch. Within the papillary layer are thin fibres of collagen, a tough fibrous protein, which provide strength and flexibility. The reticular layer, lying below the papillary layer, is much denser and contains thick collagen fibres which criss-cross to form a tough network, and also elastin fibres which give the skin its stretch and recoil properties. Just how tough the reticular layer is can be gauged from the fact that this is the part of animal skins that is used to make leather. Embedded in the dermis are various accessory structures such as nails and hair follicles, various glands, and nerve endings for sensations such as deep pressure and temperature.

The hypodermis: The borderline between the deepest layer and the dermis above is often indistinct. The hypodermis is made up of loose, fibrous tissue, rich in blood vessels, lymphatic vessels and nerves. The base of hair follicles and the coiled tubes of sweat glands may also project down into the hypodermis. On parts of the body with mobile skin, there are few hypodermal fibres, but where it is attached to underlying muscle or bone, there are tightly woven fibres. Finally, sheets of fat cells may be present in the hypodermis where extra padding is needed – the heels and the breasts in women.

Glands and accessory structures:
Embedded within the epidermis, dermis and hypodermis are a number of glands and other structures that form part of the skin.

There are two types of microscopic sweat glands called eccrine and apocrine, and the sebaceous (oil) glands. Millions of eccrine sweat glands are found all over the body, and are at their most abundant on the palms, fingers and soles of the feet. It has been estimated that the combined length of sweat gland ducts in an adult is about 10 kilometres! Everyone sweats. A passive person loses about 70 millilitres of fluid a day, but an athlete can lose as much as a litre in an hour’s strenuous exercise. This fluid is secreted through the coiled part of the gland embedded in the hypodermis, reaching the surface through a more or less straight duct. Less numerous are the apocrine sweat glands which are restricted to the armpits, the dark skin round the nipples and in the anal and genital regions. The parts involved with secretion are usually more deeply embedded in the skin than those of eccrine glands. Notably, the milk-producing breast tissues are modified apocrine glands.

The sebaceous glands are embedded in the dermis and are closely associated with the hair follicles (together called the pilosebaceous unit). They produce a semi-fluid oily secretion called sebum that is composed largely of fats and waxes. This reaches the surface through the duct of the hair root. The oily secretion can be oxidised by the air and turned black. ‘Blackheads’ are, therefore, oxidised sebum and not due to lack of washing or skin hygiene.

Hairs are considered to be accessory structures of the skin, as they are made from the same material as the outer dead skin, keratin. In the case of hairs, the keratinised cells are long and thin and thus give the elongated structure of the hair shaft. Hair pigment occupies the outer layer of the hair shaft, but production tends to diminish with age. Hairs are remarkably tough and have been shown to be stronger than strands of nylon or copper of similar thickness. Hairs grow from the base of hair follicles from a small structure called the bulb. Typically, scalp hairs grow at a rate of about 3 millimetres a week. Although men often appear hairier than women, the two sexes actually have similar numbers of hair follicles (about 20 million). The difference in appearance arises because male hair is much coarser than that of women (thus more obvious), and male facial hair has a much longer growth phase.

It may seem odd to list nails as a part of the skin, but they are in fact modifications of the epidermis, are made of the same hard keratin as hair, and can be affected in both eczema and psoriasis. Normal nails appear pink because they are translucent and allow the colour of the underlying blood vessels to be seen. The nail grows out of the nail fold at a rate of about 2 millimetres a month and arises from an actively dividing band of cells called the matrix.

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