Starting from the deep portion towards the surface, 5 distinct layers can be recognized: basal or germinative, thorny, granular or grainy, shiny and horny.
BASAL OR GERMINATIVE LAYER
It is the deepest layer of the epidermis and is supported by a basement membrane that separates it from the underlying dermis. It consists of a single layer of cubic or cylindrical cells, anchored to the basement membrane by junctions called hemidesmosomes. The cells forming this layer are partially undifferentiated; comparable in fact to stem cells, they are therefore the object of intense mitotic activity.
Precisely because they are undifferentiated, these cells are able to multiply, dividing by mitosis and replacing the superficial cells of the skin, lost or flaked during the day.
The proliferative cells of the basal layer are also flanked by melanocytes and Merkel cells.
LARGE LAYER
It is a thick layer, formed by several rows of polyhedral cells, given by the division of the underlying germinative layer. These cells (called keratinocytes) gradually rise towards the surface; during this migration the cytoplasm of the most superficial epithelial cells gradually fills with keratin precursors (basic component of hair and nails).
At the level of the junctions between the various cells, the keratin filaments vaguely resemble spines, hence the name "thorny layer". These points of contact are called desmosomes.
The spiny layer also contains Langerhans cells, which derive from a precursor in the bone marrow and are implicated in the immune response.
GRANULAR LAYER
The keratinocytes, more flattened than the underlying spinous layer, contain in their cytoplasm numerous granules of keratoialin, hence the name "grainy layer".
The nuclei show signs of degeneration, the cells are less vital but continue to produce keratin, which accumulates in the cell itself, making it less permeable. These cells also contain organelles, called Odland granules or lamellar bodies, particularly rich in phospholipids.
GLOSSY LAYER
It is found only in the thick skin (palm of the hand and soles of the feet). It is formed by keratinocytes filled with keratin and closely adherent to each other, now devoid of nucleus and organelles.
CORNEO LAYER
It is the most superficial layer of the epidermis. Commonly called skin, it is made up of many layers of extremely flattened and imbricated cells (arranged, that is, like the tiles of a roof), generally dead and arranged in several layers. In general, two portions can be considered: a deeper and more compact one in which the cells (corneocytes) are joined together, and a superficial one in which the cells (called horny scales) are disjointed and tend to detach by desquamation.
The skin is an extremely dynamic organ, since, as we have seen, the cells of the epidermis are continuously renewed. When a cell in the basal layer divides by mitosis it gives rise to two daughter cells, which can maintain the proliferative capacity, or break away from the basal lamina, rise towards the surface and differentiate gradually into keratinocytes. For a cell to differentiate, it is essential that this detachment from the basal lamina occurs.
If the outer layers of the epidermis are exported (wound, peeling), the speed of basal cell proliferation increases significantly.
The mitotic speed of these cells is therefore regulated by very specific factors; if this control fails, a rather common pathology called psoriasis arises, in which the basal layer of the affected skin areas is subject to intense proliferative activity, the epidermis thickens and the rate of desquamation of the corneocytes increases.
In a healthy skin, for a basal cell to rise to the surface, taking on the characteristics of the cells that characterize the layer crossed, it takes 14 days; arrived in the stratum corneum these cells remain there for another two weeks, before flaking or being washed away.
The whole cycle lasts 4 weeks in healthy skin.
CONTINUE: Keratinocyte differentiation »
