Continuation of the Skin

‭Continuation of the skin.‬ ‭Merkel cells:‬ ‭ re cells found in the thick skin so palms of the hands soles of the feet and at the level of the lips‬ A ‭and are located at the level of the basal layer so between the basal keratinocytes to the basal lamina‬ ‭and therefore the interface between epidermis and dermis. Those are globular cells which contain‬ ‭small fine granulations and indeed they secrete substances like dopamine or chromogranine and‬ ‭these are therefore cells considered to belong to the diffuse endocrine system or the apud system‬ ‭which is made up of small scattered cells for example at the level of the bronchial and digestive‬ ‭mucosa and also at the skin level. Second particularity, there is in these cells keratin fibers 20 which‬ ‭are not found in keratinocytes and third particularity this Merkel cell is associated with nerve‬ ‭endings therefore the base of the cell, closest to the basal lamina /of the dermis is ultimately‬ ‭positioned inside nerve endings so it is in fact also a mechanoreceptor. So this cell has several‬ ‭potentialities: secreted (endocrine) and being a receptor for mechanical incentives.‬ ‭-Some properties and functions of the epidermis.‬ ‭ -‬ 1 ‭proliferation:‬‭As we have already seen, the epidermis‬‭is a structure where there are cells in‬ ‭mitosis located at the base of the epidermis, therefore at the level of the basal base and at the level‬ ‭of the first layers of the cell base.are thorny. And then these cells mature with migration from the‬ ‭base of the epidermis towards the surface. The renewal of the epidermis takes place in 15 days or 1‬ ‭month depending on the topography:there are areas of the epidermis which are renewed very‬ ‭quickly and of others a little more slowly and it also depends on age because the epidermis renews‬ ‭itself much more quickly in small children. In pathology, there are skin diseases which are directly‬ ‭linked to proliferation disorders: psoriasis which are skin abnormalities which manifest by red‬ ‭connections with more or less whitish scars and we see that in this psoriasis there are there is an‬ ‭increase in the number of proliferating cells at the level of the basal layers and the first layers of the‬ ‭spines and a reduction in the cycle time of these cells so all of this accelerates cell production and it‬ ‭causes faster migration of cells so these cells reach the surface without having had time to mature‬ ‭correctly. This insufficient maturation explains that there is a cohesion of insufficient cells relative‬ ‭to each other so they did not have time to correctly manufacture their junction system and therefore‬ ‭there is a weakening of the epidermis Whoexplains the skin lesions that can be seen.‬ ‭ -‬ 2 ‭Regeneration and healing:‬‭2 types of regeneration are‬‭described: the so-called first‬ ‭intentions, that is to say that there is no loss of substance and no inflammation therefore simply‬ ‭lesion cutanned but without removal of a piece of the epidermis so there will be a fibrin clot which‬ ‭forms and this will be accompanied by an increase in synthesis of different elements such as‬ ‭collagens and glycosaminoglycans and the regeneration of‬ ‭Epidermis will occur by sliding of the keratinocytes so at this level when there is no cell loss during‬ ‭the lesion, there will simply be sliding of the keratinocytes, which covers the injured area. On the‬ ‭other hand, if there is a loss of substances, a cell loss during the lesion, there will be an‬ ‭inflammatory exudate which will therefore cause inflammatory cells to appear in the dermis‬ ‭(polynuclear cells, lymphocytes) and the result goes between the production of a fibrous tissue‬ ‭made par the connective cells (fibroblasts) and this fibrous tissue will be covered by a thin‬ ‭epithelium so there is cellular production from stem cells which are put into cycle and which will‬ ‭proliferate. This second mechanism is much slower because the cells must arrive, the cell‬ ‭proliferation to achieve the restoration of what has been wronged.‬ ‭ -‬ 3 ‭Senescence of the skin:‬‭it is marked by the reduction‬‭in the thickness of the skin and by the‬ ‭progressive loss of elasticity therefore there are anomalies of the epidermis which are reduction of‬ ‭the epidermis then anomalies which concern the dermis since this loss of elasticity is linked in fact‬ ‭to alterations of collagen fibers and elastic fibers which are in the dermis.‬ ‭ -‬ 4 ‭Protection and defense:‬‭are provided precisely by the‬‭epidermis. There is mechanical‬ ‭protection provided by the thickness (these multiple layers which are superimposed) and by the fact‬ ‭that the cells are very linked to each other therefore it allows resistance to friction, abrasion. There‬ ‭is chemical protection which is linked to several elements: there melanin which protects DNA from‬ ‭the carcinogenic effect of radiation but there is also the content lamellar granules: this keratolain,‬ ‭substances which will diffuse from the keratinocytes to make a sort of cement between the different‬ ‭types of cells, therefore waterproofs the skin and helps protect the skin and underlying organs from‬ ‭a certain number of substances chimiques. This helps prevent evaporation so this protection is‬ ‭against the outside but it is also against a loss of fluid from the body for life because that these‬ ‭liquids cannot pass through the skin which is made impermeable. And there are immunological‬ ‭defenses linked to specific cells of the epidermis:langerhans cells and it is linked to cells of the‬ ‭dermis, connective tissue which we will talk about in a moment which are the lymphocytes.‬