To carry out its protective task, the skin must be a resistant and impermeable barrier. This role falls in large part on the epidermis, directly exposed as it is to the external environment.
Impermeable.... but not impenetrable
While the dermis is mechanically strong, it is hydrophilic, i.e. it avidly takes up water. It is the superficial layer of the skin or horny layer which makes the skin impermeable and hydrophobic, protecting the dermis from massive water entry. This layer also resists chemical attacks, thanks to the corneocytes filled with the hard insoluble protein keratin and the lipid cement which ensures cohesion between the corneocytes, and therefore impermeability.
However, the epidermis is not an impenetrable barrier. Free water from the dermis continually crosses the epidermis by capillarity and evaporates from the surface of the skin, known as the Trans Epidermal Water Loss, or TEWL (on average 9g/m2 of skin per hour). In the reverse direction, the skin lets water in, and active molecules too.
This property of cutaneous absorption is used to hydrate the skin and promote the action of cosmetics or medical products.
Scanning electron microscopy of skin bacteria
Bacteria - Stop!
The superficial layer of the skin is covered by the acidic mantle (pH between 4 and 5.5). This is the hydrolipidic film which has all the properties required to prevent non-resident bacteria from developing and maintain the skin's barrier. It is composed of a mixture of sweat, sebum and lipids to which antibiotic peptides antibiotiques appels dfensines et dermcidines
are added called defensins and dermcidins synthesised by the sweat glands. The horny layer is not a sterile place and numerous resident bacteria (1012 bacteria/m2)
are sheltered there and prosper in the inter-corneocytic spaces. Three species of bacteria are particularly well adapted to withstand the acid environment and antibiotic peptides: Staphylococcus, Propionibacterium and Corynebacterium. They feed on corneocyte debris or on sebum cells and they prevent other undesirable bacteria from developing. They therefore play a fundamental role as "cleaners".
Maintaining the physiological pH of the skin is essential to avoid the proliferation of pathogens and an excess or lack of hygiene upsets the equilibrium of this cutaneous flora.
Confocal Microscopy of a Langerhans cells.
Sentries lying in wait
The skin has a very elaborate defence system, where different types of cells act together or successively. It is therefore not surprising to find that the skin has an immunological function. This function is in the first instance carried out by the Langerhans cells, which are dendritic cells originating from the bone marrow, then by T-lymphocytes and macrophages.
Langerhans cells which make up 2 % of the epidermal cells are located in the Malphigian layer and are responsible for immune response against an antigen
applied locally to the skin. If an antigen penetrates and crosses the horny layer, these cells take charge of it and leave the epidermis. Within less than 6 hours they may be found in dermal lymphatic vessels where they pass the antigen to T-lymphocytes Real killer cells, the T-lymphocytes release the immunological cascade reaction to eliminate the antigen and keep it in the "immunological memory". If the same antigen again penetrates the epidermis, even years later, it will be immediately recognised and the immunological reaction will be more rapid and effective.
If the antigen however manages to cross this first line of defence, the macrophages of the dermis detect it and eliminate it. This duo is complemented by the keratinocytes which, in the case of an infection, produce more cytokine IL1, which stimulates T-lymphocyte activity.
Thermoregulation
Like all mammals, man is homeothermic: he maintains his internal body temperature constant, near 37C, even if the external temperature varies. When subjected to temperature variations, no matter how small, the organism reacts by starting a series of processes to restore its optimum temperature. This is known as
thermoregulation.
The organism is capable of eliminating excess heat by perspiration. To perform this task, a thermoregulatory centre is needed the hypothalamus, as well as radiators (the blood vessels) and finally a cooling system (the sweat glands). Thermolysis is said to be the phenomenon which aims to lower a raised body temperature and the phenomenon aiming to raise body temperature is known as thermogenesis.
Acne is a skin disorder involving the sebaceous gland. The canal leading from this gland becomes blocked leading to the formation of spots. Male hormones play a significant role in triggering acne.
A genetic illness characterised by the absence of melanin pigments in the skin and hair.
Small molecule of 10 to 15 atoms, the basic component of the proteins of all living things. 20 in number, they join together in chains to form the protein. As an illustration they might be compared with the letters of the alphabet which are the building blocks of our writing system.
Male hormone which in particular regulate the appeance and the development of the male charcteristics (e.g. testosterone)
A substance destroying the fungi and yeast-like organisms responsible for mycosis.
Any substance capable of triggering an immune response.
Used to describe a cell without a nucleus.
Describes a sweat gland whose secretions flow into the pilary canal. They are only found in specific zones of the organism (armpit, pubic area, perineum, etc.).
Or programmed cell death. Biological process during which the cells commit suicid, by fragmentation of their genetic matter.
The deepest layer of the epidermis containing Merkel cells, melanocytes and keratinocytes. It is in this layer that keratinocytes divide before migrating to the upper layers of the epidermis.
Is described as being the interface between the cells and the extracellular matrix.
Sample from a living being of a fragment of tissue for microscopic examination.
Specific organelles found in Langerhans cells, which allow their caracteriation.
Very thin blood vessel organised in a network un every tissues. Gas, nutriment exchange and waste elimination in tissue take place at their level.
Process by which an unspecialized cell becomes specialized into one of the many cells that make up the body, such as a heart, liver or muscle or skin cell.
Usual method of cell multiplication in which a mother cell divides into two identical daughter cells, each containing a set of chromosomes identical to those of the mother cell.
Particular class of lipids naturally present in the skin and hair.
Method of microscopy using Laser light beams, enabling the same object to be observed at successive depths.
Tissue which has the role of supporting the other tissues (or organs of the body) and protecting, nourishing and holding them together.
An anucleate, biologically dead cell, that characterises the distinctive end state of the keratincocytes.
Desmosomes that maintain cohesion between the corneocytes in the horny layer.
Device for measuring level of skin moisturisation.
Sulphur containing amino-acid, a constituent of proteins.
Small protein secreted by the macrophages, responsible for regulating the immune response.
Branching prolongations of nerve cells, or cells of nervous origin.
Device for measuring skin creasing.
Deep layer of the skin, covered by the epidermis and formed from connective tissue.
Connecting zone between two epithelial cells.
Elimination of superficial layers of the epidermis in the form of small flakes.
Layer in the epidermis located between the mitotic layer and the horny layer. This is the layer in which keratinisation takes place.
Deoxyribonucleic acid located in the nucleus of each cell. DNA is structured in a double helix, and is the support of the genetic information.
Describes a small sweat gland that leaves its secretions on the skin's surface. Eccrine sweat gland are ubiquitous.
Property of certain bodies of returning to their original shape and volume when the force exerted on them ceases to act.
Protein which accelerates a biochemical reaction such as the chemical transformation of a given substance into another substance.
Superficial layer of the skin, covering the dermis and formed from epithelial tissue.
Tissue formed from one or more layers of cells which covers the body (epidermis), the internal cavities (mucosae) or which makes up the glands.
Red or brown melanin pigment. The true protective melanin.
Organic acid, a major constituent of lipids. A fatty acid has a long hydrocarbon chain and a terminal carboxyl group.
All the genes carried by the chromosomes and which make up an individual's genetic inheritance.
Protein linked to one or more sugars.
Invisible part of the hair buried 4 mm under the scalp and which is the place where the hair is produced.
Plant from the Middle-East and North Africa. Dried and ground henna leaves produce a powder used to dye hair and nails.
The outermost layer of the epidermis which acts as the skin's barrier (stratum corneum).
Deepest layer of the skin essentially made up of fatty cells, the adipocytes, and connective tissue.
Part of the central nervous system that controls sleep and sexual activity as well as hunger, thirst, body temperature control mechanisms and certain emotions.
(Latin words: "in glass") Used to describe any biological experiment carried out outside of the organism. As opposed to in vivo.
(Latin words: "in the living") Used to describe any biological experiment carried out within the living organism. As opposed to in vitro.
Major part of the sun's rays, with a long wavelength, responsible for the heat effect.
Extra-cellular substance which ensures the cohesion of cells within a tissue.
Sulphur rich fibrous protein, insoluble in water and very strong. It forms the essential part of hair fibre, the skin, nails, horns, hooves, feathers and wool. It is also present in the superficial cells of the epidermis.
About an epithelium having keratin as its main protein component.
Cell of the epidermis or of hair follicles, responsible for the synthesis of keratin.
Epidermal dendritic cell involved in the immune response.
Fatty chemical substances, found in all living organisms, playing a considerable role in cell structure and with an important energetic role as well.
Device for measuring the skin's lipid content.
Organic yellow liquid essentially made up of water, lymphocytes and various organic substances (cholesterol) and minerals.
Swelling in a lymph vessel in which the lymph is relieved of waste and infectious agents that it contains by lymphocytes and macrophages.
Membranous duct punctuated with nodes in which the lymph circulates. They have a very similar structure to that of blood vessels.
Cell found in the lymph nodes, thymus, bone marrow and spleen playing a role in the immune defence system.
Layer of the epidermis 5 to 6 cells deep that become flatter in the superficial part, often called the spinous layer or spinosum stratum.
Sensory cell located in the dermis, on the boundary of the epidermis. These receptors are also involved in the sense of touch.
Coloured pigment giving colour to the hair and skin.
Cells of the epidermis or of hair follicles, responsible for the synthesis of melanin and transmitting it to the keratinocytes.
Sensory cell located on the baal layer of the epidermis, involved in the sense of touch.
Unit of measurement equal to one millionth of a metre or a thousandth of a millimetre (formerly: micron).
Usual method of cell multiplication in which a mother cell divides into two identical daughter cells, each containing a set of chromosomes identical to those of the mother cell.
Magnetic resonance imaging. Non-invasive medical imaging method.
Neuro-physiological component of pain, that perceives potentially threatening stimuli.
Procedure of exploring the organism without any risk of its integrity being affected.
Part of an organised body that carries out one or more defined functions.
Any structure differentiated from the cell having the role of an organ, for example: mitochondria, Golgi bodies.
Any structure differentiated from the cell having the role of an organ, for example: mitochondria, Golgi bodies.
(Lat. = mouth) The ostia are openings at the skin surface from which hair grows.
Sensory receptor highly sensitive to pressure and vibrations. They are located deep in the dermis and the hypodermis.
Molecule made up of a small number of amino acids. It is small protein.
Abbreviation of "hydrogen potential".) Index expressing the activity (or concentration) of hydrogen ions in a solution. If the pH is less than 7, the solution is acidic. If it is equal to 7, it is neutral. If it is greater than 7, it is alkaline.
All the apparent characteristics of an individual, which correspond both to the expressed part of the genotype and phenomena defined by the external environment.
Classification of sensitivity to solar radiation based on skin, eye and hair colour.
Science involving the study of an organ's functions and properties, and the tissue of a plant or animal.
Inserting a jewel or decoration into the skin by perforation.
Coloured matter in various plant or animal tissue and organs giving it a particular colour.
Network of inter-woven nerves and blood vessels.
Molecule made up of an association between 4 to several thousand amino acids and forming the essential component of cells (the word "protein" coming from the Greek prtos which means "first").
Biological process during which the cells commit suicid, by fragmentation of their genetic matter (apoptosis).
Method enabling imprints of the skin's relief to be made.
Sensory receptor that are confined to the dermis and are stimulated by variations caused by tension and stretching.
Serious neurological illness due to an intoxication from lead or lead salts.
Observation method using electron beams enabling surfaces to be examined and relief to be shown. Possible enlargement from 10 to 100,000 times.
Symbolic ritual marking indicating ethnic membership or initiation.
Skin appendage responsible for producing sebum.
Greasy material produced by the sebaceous glands associated with the hair follicles and which naturally lubricates the hair.
Used to describe a squamous or pavement epithelium has a superficial layer of cells, which are flat. It looks like a pavement.
Used to describe an epithelium made up of several layers of cells (stratum = layer).
Horny layer.
All the systems that defend the skin against the harmful effects of the sun. This degree of tolerance is acquired at birth, is not renewable and depends on an individual's phototype.
Chemical molecules capable of enhancing wetting by water (such as detergents and soaps).
Skin appendage responsible for producing sweat.
Cultural tradition aiming at introducing pigments into the upper dermis so as to creates coloured designs. Tattoos are indelible.
Regulatory mechanism by which the internal temperature of the body of mammals and birds is maintained constant.
Device for measuring skin torsion.
Minority part of solar radiation, short wave but highly energetic. UV rays cause a lot of damage to the epidermis and to the dermis.
Part of the sun's spectrum visible to the human eye.
Rare genetic illness in which the enzymes that repair DNA are deficient. This dillness causes people to be highly sensitive to solar irradiation and are proned to develop cancers.
