Skin is one of the most important organs of the body. Besides being an undeniable aesthetic asset, healthy skin also protects us from external aggressions such as ultraviolet (UV) rays, pollution, bacteria, viruses, and chemicals.
The skin consists of three main layers : the epidermis, the dermis, and the hypodermis.
The epidermis is the outer layer of the skin and is usually composed of four layers: the stratum corneum, the stratum granulosum, the stratum spinosum, and the stratum basale. The stratum lucidum is a fifth layer of the epidermis found only on the palms of the hands and on the soles of the feet. It is located between the stratum corneum and stratum granulosum.
The epidermis layers comprise four main types of cells: keratinocytes, melanocytes, Langerhans cells and Merkel cells.
Keratinocytes compose the majority of the cells of the epidermis. These cells produce keratin, a protein which gives the skin, i.a., its waterproof properties and protects it from external aggressions.
Keratinocytes are produced within the stratum basale, the deepest layer of the epidermis. More mature keratinocytes move progressively to the upper skin layer where they eventually die. The epidermis is regenerated every four to six weeks.
The upper skin layer, the stratum corneum, consists entirely of dead keratinocytes cells which protect us from external aggressions.
Melanocytes compose about one fourth of the cells in the basale layer. These cells synthesize melanin, a pigment that determines the color of the skin and varies from yellow to brown-black. When the skin is exposed to ultraviolet (UV) rays, the synthesis of melanin pigments is stimulated. Pigments are transferred to the keratinocytes where they form a shield protecting deoxyribonucleic acids (DNA) from UV radiation.
The epidermis also contains Langerhans cells which are associated with the immune system. They concentrate mainly in the stratum spinosum and in the stratum granulosum.
Finally, the epidermis includes some Merkel cells. They act as touch receptors and are found at the junction of the basale layer and the dermis. These cells are associated with the sensory nerve endings .
The epidermis is regenerated every four to six weeks.
The dermis consists of two regions: the papillary dermis and the reticular dermis.
The papillary area represents the upper region of the dermis. It is composed of loose connective tissue that contains collagenous and elastic fibers mainly oriented perpendicular to the skin surface. This region is very irregular and gives the surface of the dermis a bumpy surface.
The reticular dermis forms about four fifths of the dermal volume. This region is composed of dense connective tissue that contains collagen and elastic fibers oriented parallel to the surface of the skin. With age, collagen and elastic fibers degrade which generates a loss of skin elasticity and causes wrinkles.
The dermis also includes many nerve endings and a cell population consisting primarily of fibroblasts and immune system-associated cells. The dermis also includes sebaceous glands and sweat glands.
The vascularization of the dermis is important because it also supplies by diffusion the epidermis with nutrients. Blood vessels are divided into two networks: the superficial and deep vascular plexuses. Under the dermis is the hypodermis, an adipose tissue connecting the skin to the underlying organs.
Elizabeth Hartinger, Ph.D., M.A.Sc., B.Eng.
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