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Skin anatomy

Skin  is made up by an epitelium called epidermis, by a connective tissue (elastic cells and collagen) called dermis and by an adipose tissue known as hypodermic. In dermis and in hypodermic are contained skin annexes (nails and body hair), vessels and nerves. Epidermis is divided into layers, that depend on the maturation of keratinotacytes. These are epithelial cells, that progressly migrate from the basal layer till the skin surface, undergoing a process of differentiation,  called keratinization. The origin of epidermis is due to the stratication of the epithelial cells. During its intraepidermal transit, thekeratinocyte takes some changes of its morphology, which allow to divide histologically  the epidermis in four layers: basal, spinous, granular, clear (only in palms and soles) and  cornified. The basal layer is adjacent to the dermis and consists in an unique layer of cylindrical keratinocytes. They represent the germ cells of epidermis, whose division depend on the continous renovation of epithelium. Interposed between basal keratinocytes, there are the melanocytes, cells whose main function is to produce and secrete melanin, moving it later to keratinicytes. We find more of them in photo exposed areas like face while less on the trunk. Above the basal cells, we find the spinous layer that, normally, has  thickness of about five cells. In areas, subjected to pressure, like elbows,  palms and soles, it is thicker; in other areas like face and forearm, it can be thinner. Above spinous cells, as they mature and  start to keratinized, we find the granolous layer, that usually has a thickness of about 1-2 cells.The clear layer is situated above the granolous one but it is clear only in some areas, above all palms and soles.The superficial layer of epidermis is the cornified layer usually with a thickness of  3-4 cells. In it, cells are mainly composed by keratin filaments and they aggregate to each other, giving an aspect of  woven basket. In under pressure or traumatised areas is thickened.

Dermis is made up of two parts: the papillary region, situated under the epidermis, and the reticular dermis, that is between the papillary and the hypodermis. It contains collagen (that allows a remarkable resistance to traction), elastic fibers (that give it a reversible extendibility), Glycosaminoglycans (that give hydratation) and the cutaneous annexes, that are the hair follicles and all structures that are similar to it  (sebaceous glands).