Outer Skin (Epidermis)

The epidermis, which in some places is only paper-thin, is formed of so-called keratinizing squamous epithelium and consists of 5 layers. The bottom layer, the germ or basal cell layer (stratum germinativum, stratum basale), constantly produces new cells that gradually move up to the surface over a period of 3 to 4 weeks and become increasingly flatter. These newly formed spinous or prickle cells are transformed into granular cells, the shiny cells of the stratum lucidum (only on the hands and feet) and the horny cells, which have no nucleus. Ultimately, these cells lie as horny scales on the surface of the skin, from where they are then shed. Each layer of cells is at the same stage of transformation.
Altogether, most of the cells in the epidermis are in the process of keratinizing (becoming horny). These cells (keratinocytes) account for up to 95 % of the cells in the epidermis. In the deeper part of the epidermis, there are also 3 other kinds of cells: the melanocytes (pigment cells), Langerhans’ cells (body defence cells) and Merkel’s cells (touch cells).

The cells of the epidermis are firmly interconnected with each other by so-called desmosomes (cell processes) and tonofibrils (fibres). The tonofibrils help to make the skin elastic and together with the desmosomes provide the enormous tensile strength of healthy skin (capable of bearing a tensile load of up to 90 kg per cm²).

On the outer surface, the gaps between the cells (intercellular spaces) of the horny cell layer and the upper granular layer are closed by sealing substances (lipid lamellae or “grease layer”). Each single cell in these layers of the epidermis is surrounded by lipids. These consist of ceramides, cholesterol and fatty acids. A major component of this fatty substance is linoleic acid. Besides their protective function (skin barrier), the surface lipids betwen the corneocytes and the cells of the granular layer also act as an adhesive, holding the upper cell layers together.

The skin barrier is formed by the hornycell layer and the lipids in the intercellular spaces. It prevents the uncontrolled penetration and escape of water and electrolytes. If the spaces between the cells were not sealed by lipids in this way, the body would loose up to 20 litres of water per day. Water accounts for 60 % of body weight.

The absorption of care creams and medication through the skin depends on the solvent in which they are contained (e.g. water or alcohol) or the chemical structure. The effectiveness of the skin barrier varies from body region to body region. The skin of the face, eyelids, backs of the hands and genitals is more absorbent than that of the trunk, arms and legs. Except for water, the palms of the hands are impervious to virtually all other substances.

The lipid layers of the epidermis also provide protection from penetration by microorganisms such as bacteria, viruses and skin fungi. By shedding the scales on the surface, the skin also effectively removes such microorganisms. The process of scaling is caused by the breakdown of the lipid lamellae and loss of the remaining desmosome-tonofibril links in the intercellular spaces.

The bacteria on the surface of the skin live in symbiosis with the skin; they are needed and tolerated. The number of bacteria varies strongly depending on body region. In the dry areas of skin on the forearms, some 300 to 5,000 bacteria per square centimetre of skin can be found. On the hairy areas of the face, chest and back, they number some 50,000 per square centimetre, on the forehead 200,000, on the hairy scalp 1 to 2 million, and in the armpits 2 to 4 million.

The colonization of the skin with normal bacteria (skin flora) is of great benefit for the human body. These skin bacteria produce bacterial inhibitors, protecting the skin from harmful bacteria, and they also produce free fatty acids from the skin lipids. Through the production of fatty acids, the bacteria help to protect the skin and therefore the body as a whole (acid protection barrier, pH value).

Pigment cells (melanocytes)

About one cell in ten in the germ layer of the epidermis is a pigment cell (melanocyte) that produces the pigment melanin. The number of melanocytes varies between 1100/mm² on the inside of the forearm to 2900/mm² in the face. The total number of pigment cells in the epidermis of the human body is estimated to be about 2 billion (2 thousand million). One melanocyte supplies about 36 epidermis cells with pigment particles (melanosomes). Melanocytes are found in almost every tissue, but are most numerous in the epidermis, the dermis, the hair follicles, the eyes, and around blood vessels and superficial nerves. There are particularly large numbers of pigment cells in the face and genital region.

Melanin is what primarily determines a person’s skin colour. Eumelanin produces a black-brown skin colour, phaeomelanin a reddish-brown colour. Besides melanin, the normal skin colour is also influenced by haemoglobin (blood pigment) and the carotinoids (e.g. the pigment contained in carrots). Pigment production is activated by ultraviolet light (sunshine, phototherapy). The melanocytes release the pigment produced in them through long cell processes (i.e. extensions, like the tentacles of an octopus) to the surrounding cells of the epidermis in the form of pigment particles.

The differences in individual human skin colour from pale to yellow and dark depend on the size, density, distribution and concentration of pigment particles in the skin cells. People with dark skin do not have more melanocytes than those with pale skin; their melanocytes are simply more active and produce greater quantities of pigment. So dark-skinned people possess melanocytes that produce melanin faster and release it in bigger quantities to the surrounding cells of the epidermis than pale-skinned people. The suntan acquired by pale-skinned people is ultimately the visible result of processes in the skin whereby exposure to strong sunlight causes the pigment cells to produce increased quantities of melanin pigment in order to protect the skin. The melanin is released to the neighbouring cells, where it clusters round the cell nucleus to form a protective barrier. Damage to the nuclei of the cells of the epidermis can be the first step towards skin cancer.

Two types of melanin pigmentation occur in humans. The first is the constitutional skin colour and is the result of the genetically inherited melanin pigmentation. It is not dependent on exposure to sunlight and other influences (e.g. hormones). The other is the stimulable (facultative) skin colour, which is triggered by exposure to sunlight.

In the case of albinos, melanin pigmentation is lacking in the skin, hair and eyes. The normal number of melanocytes is present in the basal cell layer, but they do not produce any pigment.

The number of melanocytes decreases with age. The melanocyte density falls by about 6 – 8 % per decade.

Increased pigmentation during pregnancy is due to the activation of hormones. Melanocyte-stimulating hormone (alpha-MSH) is formed in the pituitary gland and controls the production of melanin in the melanocytes. Oestrogens (female hormones) can also lead to an increase in skin pigmentation, e.g. during pregnancy. Androgens (male hormones) do not have this effect.

Langerhans’ cells

Langerhans’ cells are specific defence cells and are therefore part of the body’s immune system. They make up about 3 – 5 % of the cells of the epidermis. With their cell processes (extensions) they are similar to the pigment cells and are located in the middle layers of the epidermis. Langerhans’ cells can detect pathogens (bacteria, viruses, fungi) or chemical substances and absorb them. They migrate through the epidermis to the dermis and from there via the lymphatic system to the lymph nodes. In the lymph nodes, the foreign body may be destroyed or, after contact with specific lymphocytes (white blood corpuscles of the body’s defence system), may trigger allergies. These specific lymphocytes are then carried back to the skin through the blood system. Whether the exchange of the foreign substance (antigen) between the Langerhans’ cells and the specific lymphocytes already takes place in the skin or only in the peripheral lymph nodes is not yet known.

Merkel’s cells

Merkel’s cells are touch cells and only occur in the skin and the mucous membranes with several layers of squamous epithelium. They are located in the germ layer of the epidermis and are broad and flat nerve endings. They are present in increased numbers in the skin of the palms of the hands, the soles of the feet, and in the hair sheaths. In adults they are particularly numerous on the finger tips, in infants and small children on the lips and tongue.