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The epidermis


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
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Corneocyte paving in scanning electron microscopy

Corneocyte paving in scanning electron microscopy.

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 stratified squamous 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.

  The horny layer
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Pilled up corneocytes of the Horny layer.

Pilled up corneocytes of 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 : a pigment factory
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Melanocytes in culture

Melanocytes in culture

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.

  Guardians of the skin
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Optical microscopy of a Langerhans cell.

Optical microscopy of a Langerhans cell.

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.



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