Although the keratinocytes have the absolute majority, other cells - melanocytes, Langerhans cells and Merkel cells - co-exist with them in the epidermis. Each of these types have specific, and no less vital, functions.
A journey lasting 4 to 6 weeks
The epidermis (From the Greek epi, on top and derma, the skin) is the skin's outer structure serving a protective function However, the part we can see, the horny layer, is only a minute part of this stratifiedsquamous layer. It is the ultimate result of the keratinisation process and marks the final stage of a 4 to 6 week journey undertaken by the keratinocytes. Human skin is continually being renewed, in contrast with that of reptiles who moult. The desquamation of cells on the skin's surface should naturally be compensated for by renewal of the epidermis, a process undertaken by the keratinocytes (85% of the cells in the epidermis). These possess two properties which successively come into action - the ability to actively divide and the ability to differentiate.
The keratinocytes divide in the skin's mitotic layer. The innermost part of the epidermis, this is made up of a single line of keratinocytes held together and to the underlying dermis by desmosomes, a sort of "press stud" structure. Each keratinocyte divides to produce two identical daughter cells. One remains static in order to divide again while the other migrates to the upper layer, the differentiation layer, where it will undergo a number of morphological and biochemical changes.
The differentiation layer, between eight and fifteen cells deep, is the thickest layer of the epidermis. This is the level at which the keratinocytes grow and flatten. In an initial stratum formed of 6 layers of cells, the Malpighian layer, the cells' activity is mainly concentrated on generating keratin, a fibrous protein which progressively fills the cell. As they move towards the upper layers, the keratinocytes become flatter and their nucleus begins to degenerate. They secrete a cement made up of lipids, cholesterol, free saturated fatty acids and ceramides into the intercellular spaces, which increases cohesion between the cells and thereby contributes to making the epidermis an effective barrier.
Once they arrive at the outermost layer of the epidermis, the horny layer, the keratinocytes become corneocytes, anucleate flattened cells, filled with keratin. This is a water insoluble protein accounting for 95% of all proteins present in the epidermis and it is in part this substance which gives the epidermis, and therefore the skin itself, its protective function. Under the programmed action of specific enzymes, the corneocytes lose their cohesion and separate from the surface one by one in the desquamation process.
Pilled up corneocytes of the Horny layer.
The horny layer
The biological machinery produced by the epidermis has the sole function of producing a horny layer or stratum corneum. While thick on the palms of the hands and soles of the feet, it hardly exceeds 1/100th of a millimetre elsewhere. This layer, the most superficial of the epidermis, is in direct contact with the external environment and therefore plays a vital role as a barrier against all sorts of aggression.
This layer is composed of corneocytes, completely flattened cells, with neither nucleus nor cytoplasmic organelles. These cells are therefore dead, biologically speaking, but nevertheless remain active: the result of the final phase of keratinocyte differentiation, they are filled with keratin and other products such as lipids, fatty acids and ceramides.
On looking closely, two distinct layers of corneocytes can be distinguished: the compact layer, where the corneocytes are linked one to another with corneodesmosomes, has the role of a barrier, and the outer sloughing layer, where the breakdown of these corneodesmosomes provokes the phenomenon of desquamation. The corneocytes are thus continually eliminated by this natural phenomenon or following external aggression such as rubbing, washing or detergents.
Still in this layer, the corneocytes, as they gradually detach, provide numerous spaces where the bacteria living on the skin (1012 bacteria/m2) find refuge and thrive. Staphylococcus epidermis and aureus, Propionibacterium, and Corynebacterium, particularly adapted to the acid environment of the hydrolipidic film, feed on the remains of corneocytes or cells from the sebaceous gland. Other bacteria, known as transients, may be present on the surface of the skin, but the pH conditions are not favourable for them and they do not develop.
The horny layer is generally thin (thin skin), except on the surface of the palms of the hands and the soles of the feet (thick skin) where its thickness is considerable. Thick skin contains no hair follicles but has many sweat glands. Its surface is covered with furrows and crevices visible to the naked eye due to folds of the dermis.
Melanocytes in culture
Melanocytes: a pigment factory
The melanocytes are dendritic cells only found in the deepest layer of the epidermis. Their function is to produce melanin, the pigment which gives the skin its colour, and to transfer it to the surrounding keratinocytes by means of cytoplasmic processes. Since they account for 5% of the cells in the epidermis, each melanocyte has to supply melanin to 35 keratinocytes.
Optical microscopy of a Langerhans cell.
Guardians of the skin
Discovered in 1868 by a young Berlin doctor, Paul Langerhans, the Langerhans cells (2-5% of the cells in the epidermis) are derived from bone marrow and found in the Malphigian layer. Like the melanocytes, Langerhans cells are dendritic cells but are differentiated from them by the presence of specific organelles in their cytoplasm, Birbeck granules. These make up essential elements in the organism's defence system. The function of these cells is to detect foreign bodies (antigens) which have penetrated the epidermis, capturing intruders and then carrying them to the lymph nodes in the dermis, where they are presented to the lymphocytes. A cellular type of immune response is then triggered, neutralising and finally eliminating the antigen.
Tactile cells
Merkel cells (6-10% of the cells in the epidermis), situated between the keratinocytes in the renewal layer, remain in contact with a nerve ending. They can be isolated or grouped together in clusters called Merkel corpuscles. They serve as mechanoreceptors and are involved in the function of touch.
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.
