The skin is a complex organ system capable of going through a wide variety of color changes. Normal skin color arises from a mixture of red, blue, yellow and brown colored pigments. In normal skin, melanin is the main pigment or color determinant, giving a color ranging from very light tan to dark brown or black, depending on the amount of melanin in the epidermis. It can be given a yellow color by carotenoids, red by oxygenated hemoglobin in capillaries, and blue by reduced hemoglobin in dermal venules and pigment in the dermis. Melanin is synthesized by a special cell, melanocyte, which is a dendritic cell located in the basal layer of the epidermis, but some melanocytes higher in the epidermis and a few in the dermis.
The melanin pigment system consists of millions of such melanocytes, each of which is functionally associated with 36 keratinocytes; This cluster of keratinocyte and associated melanocyte is referred to as the "epidermal melanin unit", which appears to be a structural and functional entity. Within each working melanocyte, melanin is synthesized and packaged in special pigment organelles called "melanosomes". The dendritic processes of melanocytes project between keratinocytes so that a single melanocyte supplies melanosomes to 36 keratinocyte groups.
Melanine migrate
These melanosomes migrate centrifugally through the dendritic processes of melanocytes and are then transferred to or captured by keratinocytes. Although the number of active epidermal melanin units per unit area varies considerably in various regions of human skin, the keratinocyte/melanocyte ratio remains constant. The epidermal melanin unit has been suggested to be the functional integrator of the multicellular melanin pigmentation system in humans and animals.
Although skin color can be conceptually thought of as a mixture of the colors red, blue, yellow, and brown above, racial differences in normal skin color and skin color are a function of the number, size, and distribution of melanin-laden organelles.
Melanosomes
It is the melanosomes that are distributed into the keratinocytes that give the skin its color. In the absence of disease, other color contributors play a minor or insignificant role. Therefore, in the absence of melanin, the skin remains essentially white, as in tyrosinase-negative albinism or vitiligo macules. Recent evidence that the movement of keratinocytes within the epidermis is more complex than originally thought is not inconsistent with the concept of the epidermal melanin unit.
Keratinocytes activite
Keratinocytes do not divide randomly in the germinal layer of certain types of human and mouse epidermis. Mitoses can also occur in suprabasilar keratinocytes. According to the epidermal proliferation unit concept, young basal cells divide and move peripherally in the epidermis before final division and formation of regular columns of cornified cells.
Each or several epidermal proliferative units can then be associated with a donor melanocyte. The study of the melanin-producing mechanism should be approached at five scientific levels: macromolecular (visual) - skin viewed as an organ system; multicellular (histological)-epidermal melanin unit; cellular-melanocytes as unicellular glands; subcellular (electron microscopic)- melanosome as a metabolic unit of melanogenesis; and macromolecular (biochemical)-tyrosinase, enzyme and melanoprotein, the end product of melanogenesis.
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