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The skin covers the surface of the human body. The
area of an adult skin is about 1.5 ~ 2.0 square meters
with a thickness of 0.5 ~ 4.0 millimeters. The skin
is the largest organ of the body with diverse functions
such as protection against external irritation and injury,
regulation of body temperature, secretion, excretion
and absorption. It is also an organ of senses to cold
and heat, touch and pain. The normal human skin is divided
into 3 layers, the epidermis, dermis and the subcutaneous
tissue (Fig. 1A). There are skin appendages in the skin,
such as hair and hair follicles, sebaceous glands, sweat
glands and nails. The skin is also rich in blood vessels,
lymphatics and nerves. Under the microscope, the skin
is composed of many cells of diverse functions and morphology.
The epidermis is mainly composed of keratinocytes. Situated
in the basal layer is a population of dendritic cells,
called melanocytes (Fig. 1B), the main function of which
is the production of melanin, as well as the transfer
of the synthesized melanins into the keratinocytes.
After the uptake the keratinocytes store the melanins
within the cytoplasm and also gradually degradate these
melanins. By this way, the quantity of melanin within
the skin is maintained at a normal level. The function
of melanin apart from the maintenance of skin color
is the protection and reflection of light-waves to shield
the deeper cells and tissue from damage. Although the
skin color of Negroids, Caucasians and Mongoloids are
quite different the number of melanocytes within the
skin does not differ significantly. It is only the quantity
and the color of the melanin produced by the melanocytes
that determine the color difference between these peoples.
The color of the normal skin is basically even, although
there are slight variations in color in different parts
of the body. The color of the skin will also change
to a certain extent upon exposure to sunlight, vary
in different seasons, e.g., the color of the skin is
darker in outdoor workers than indoor workers, and the
skin color is darker in summer than in winter. But overall
the color of the skin is even. If there is metabolic
abnormality after melanocyte damage, white patches on
the skin in certain part of the body will appear, it
is pathological.
Figure 1 B The physiologic process
of melanin metabolism
¡ý -------¡ý
melanocyte-
keratinocyte
The physiologic process of melanin metabolism is carried
out in the basal layer of the epidermis. Melanin synthesis
by melanocytes at the basal layer ¡ú transfer to keratinocytes
after maturation ¡ú melanin uptake by keratinocytes ¡ú
protection and decrease of the biological damage of
the human body by ultraviolet light and sunlight ¡ú degradation
of melanin within the phagolysosomes of the keratinocytes
¡¶Lesson Two£ºPathogenesis
of vitiligo ¡·¡¡¡¡¡¡>Back<
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