" Skin
Starting from the deep portion towards the surface, 5 distinct layers can be recognized: basal or germinative, thorny, granular or granular, 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 is made up of a single layer of cubic or cylindrical cells, anchored to the basement membrane by means of junctions called hemidesmosomes. The cells that form this layer are partially undifferentiated; in fact comparable to stem cells, they are therefore subject to intense mitotic activity.
Due to the fact of being undifferentiated, these cells are able to multiply, dividing by mitosis and replacing the superficial skin cells, lost or desquamated during the day.
The proliferative cells of the basal layer are also flanked by melanocytes and Merkel cells.
SPINY 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 is progressively filled with the precursors of the keratin (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". Such points of contact are called desmosomes.
The spinous layer also contains Langerhans cells, which arise from a precursor in the bone marrow and are involved in the immune response.
GRANULAR LAYER
The keratinocytes, more flattened than the underlying spinous layer, contain in their cytoplasm numerous keratoyalin granules, hence the name "granular layer".
The nuclei show signs of degeneration, the cells are less viable but continue to produce keratin, which accumulates in the cell itself making it less permeable. These cells also contain organelles, called Odland's granules or lamellar bodies, which are particularly rich in phospholipids.
GLOSSY LAYER
It is found only in thick skin (palm of the hand and soles of the feet). It is formed by keratinocytes filled with keratin and closely adhered to each other, now devoid of nucleus and organelles.
CORNEUM LAYER
It is the most superficial layer of the epidermis. Vulgarly called skin, it is made up of many layers of extremely flattened cells and imbricated between them (arranged, that is, like the tiles of a roof), generally dead and arranged on 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 due to desquamation.
The skin is an extremely dynamic organ, since, as we have seen, the cells of the epidermis are continually renewed. When a cell in the basal layer divides by mitosis, it gives rise to two daughter cells, which can maintain their proliferative capacity, or detach themselves from the basal lamina, rise to the surface and gradually differentiate into keratinocytes.
If the outermost layers of the epidermis are removed (wound, peeling), the proliferation rate of the basal cells 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 an "intense proliferative activity, the epidermis thickens and also the rate of desquamation of the corneocytes increases.
In a healthy skin, 14 days are instead necessary for a basal cell to rise to the surface, each time taking on the characteristics of the cells that characterize the crossed layer; arrived in the stratum corneum these cells remain there for another two weeks, before desquamating or being washed away.
In healthy skin the entire cycle therefore lasts 4 weeks.
CONTINUE: Differentiation of keratinocytes "
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