Outer appearance, inner self
We are surrounded by our skin on every side. This envelope and frontier is part of our outer appearance, but it can also serve to reveal our inner selves. We blush with emotion, fear gives us goose pimples, stress is reflected in skin disorders, we sweat from heat or fright. Indeed certain European languages describe our general psychological condition in terms of ‘feeling good/bad in our own skin'.
An adult's skin measures 1.5 to 2 square metres and makes up 10% of its body mass. In all it is about four millimetres thick. The epidermis, or outer layer (one-tenth of a millimetre), ensures its impermeability and resistance. This is renewed every four weeks or so through the sloughing off of dead surface cells. Below it is the dermis, a ‘lining' five to nine-tenths of a millimetre thick, consisting of different elements, some of them well-known to cosmetics users, like collagen (produced by fibroblasts) and elastin fibres (which give skin its elasticity and resistance).
The dermis also contains blood vessels and hair follicles - the roots of which feed on average one hundred thousand head hairs, plus a million other skin hairs on women's bodies and five million on men's - as well as sweat glands and sebaceous glands. The subcutaneous hypodermis, in turn, is a tissue made up of adipocytes (greasy cells) which form a protective mattress providing thermal insulation, and an energy reservoir of lipids and fatty acids.
This ensemble plays an extraordinary sentry role, forming a biological barrier which makes us as good as impenetrable - when all goes well - against life-threatening UV radiation bacteria, viruses and fungi. This ‘shell' is made up of battalions of cells, the largest contingent of which being epidermal keratinocytes (which have a maximum thickness fifty thousandths of a millimetre). Organised in strata, these usually number between ten and 100 per cell, depending on whether they are in ‘fragile' (neck, inside arms) or more exposed parts of the body (palms of the hands and feet). This prodigious tissue is constantly renewed. In fact, we lose one twenty-first of our keratinocytes every day. During this same period of time new cells are being produced at the bottom of the subcutaneous stratum, to provide the inevitable replacement. Whenever a cell rises to the skin surface, becomes ‘worn' and is sloughed off, another cell is ready to take its place.
The skin has its own defence system against micro-organism attack. Its lining of Langerhans cells, produced in the bone marrow, and the mucosa (200 to 700 per square millimetre) detect the intruder, carry its chemical trace to the lymph glands and transmit the information to the lymphocytes, the specialist cells of the immune system.
Climate controller and neurotransmitter
In its role of regulating body temperature, the skin can - within certain limits - make light work of temperature changes. It resists heat by secreting through the sweat glands the sweat which enables body heat to be expelled and evaporate. Nor is it afraid of severe cold, at least for short periods. In this case the blood circulation in the dermis is limited, putting a brake on heat loss.
Even more surprisingly, the skin is a veritable network for the reception and transmission of sensory information and is in constant relation with the brain thanks to the blood capillaries and the nerve endings running through its entire length and breadth. At any point in time the brain can locate and define an event taking place on our outer envelope (collision, sting, contact, heat, fluid, etc.). But communication also passes in the opposite direction, from the central nervous system to the skin. Going white with fear or blushing with shame, for example, is simply the effect of contraction or dilating of the cutaneous vessels.
Despite all its strong points, our outer envelope remains fragile and the number of ailments to which it can fall victim runs into thousands.
Skin ages, dehydrates, wrinkles, sags, and with age develops red blotches, known in French as ‘cemetery flowers'. The collagen fibres, which provide skin with its firmness, and the elastin fibres that give it its flexibility, are produced up to the age of 15 or 20, after which this precious capital erodes year after year.
Precious tissue for research
This strange mechanism fascinates researchers. Skin was the first tissue to be reconstructed in the laboratory - by Howard Green in the 1970s - after noticing that skin cells placed in a box containing nutritive environments began to multiply. His research led to cultured epidermis, used for skin grafts for patients with severe burns - two square centimetres of tissue expand to a square metre within a fortnight. Another aspect has focused scientists' attention in recent years: human skin is one of the ‘basic organs' that can produce stem cells which offer major hope for regeneration therapies.
Mastering the biological process could provide an alternative to the use of foetal stem cells, which are the only stem cells available right now.
In pharmacology, other advances are based on the skin's function as a therapeutic vector.
Patches and gels, the uses of which are still limited, offer the advantage of not upsetting the liver or the stomach.
The link between certain skin disorders (psoriasis, acne) and stress is also being examined in the workplace context, revealing more of the relationship between the skin and the nervous system. Researchers are also developing skin sensors for detecting early stage infections among dialysis patients and the appearance of bedsores among the bedridden.
The epidermis is also the ideal location for fixing microsensors that track pulmonary or cardiac malfunction, also in remote mode.
Skin as a revealer
Beyond its physical peculiarities, skin also tells us a lot about our psyche, sociability and feelings. Touch is a powerful medium. Immediately below the skin, our nerve fibre ends function as sensory receptors that react to pressure, temperature, pain, and also gentleness.
The skin carries our reactions, but also provokes those of others. It has a colour which has given - and still gives - rise to many deviant forms of behaviour. Racism is not dead, even when we know that everything is a question of melanosomes, melanocytes and melanin. The latter is a natural pigment which darkens the epidermis to a greater or less extent depending on its concentration. A human is born with 3.5 grams of melanic pigments, divided into two families: phenomelanins (orange-red, which yields blond-reddish hair and light skins) and eumelanins (brown-blacks, the intensity of which vary from very light to very dark). People are born with one or another skin colour. This has varied in the course of evolution, depending on the natural environment.
Populations exposed to intense sunlight develop a natural ‘tan' which protects them against UV rays. In fact, all Homo sapiens have the same origin, and until we learn otherwise, their oldest ancestor, Toumaï, lived in Africa some 7 million years ago - and was, we can suppose, black.
Whatever its colour, the skin's outer appearance can be changed. The intended message varies with time, civilisation and fashion. In ‘primitive' societies, skin painting, tattoos, scarification (geometric scars) can indicate membership to an ethnic group or a society - secret or not - or mark an event like setting off hunting or for war. Used to signify religious affiliation, circumcision is also recommended for hygienic reasons. ‘Marking', as practiced by the Nazis or slave traders, can also serve to dehumanise people and turn them into ‘human cattle'. All this symbolism was (and in certain cases still is) imposed from outside. Nowadays, even if desired by the group as an identity reference - like among hooligans or football supporters - skin-deep signs more often than not reflect individual choices. Tattooing and piercing are part of the process of seduction, or even provocation.
Skin is an asset one hides or reveals. "Is not skin the most erotic part of the body wherever the clothing affords a glimpse," French intellectual Roland Barthes once wrote.