The largest and heaviest body system - the skin and its appendages (the hair, nails, and certain glands) performs many vital functions. They protect the inner organs, bones, muscles, and blood vessels; help to regulate body temperature; and provide sensory information. They also prevent body fluids from escaping and eliminate body wastes through more than 2 million pores.
Two layers of skin (integument), the epidermis and dermis, lie above a third layer of subcutaneous tissue. Numerous epidermal appendages exist throughout the skin, including hair, nails, sebaceous glands, and two types of sweat glands: eccrine glands (located over most of the body except the lips) and apocrine glands (found in the axilla and groin near hair follicles). The integumentary system covers an area of 10¾ to 21½ ft² (1 to 2 m²) and accounts for about 15% of body weight.
The epidermis, the outermost skin layer, varies in thickness from less than 0.1 mm on the eyelids to more than 1 mm on the palms and soles. It's composed of avascular, stratified squamous (scaly or plate-like) epithelial tissue that contains multiple layers: a superficial, keratinized, horny layer of cells (stratum corneum) - composed of several layers of cells in various stages of change as they migrate upward - and a deeper, germinal (basal cell) layer.
After mitosis occurs in the basal cell layer, epithelial cells undergo a series of changes as they migrate to the outermost part of the stratum corneum, made up of tightly arranged layers of cellular membranes and keratin. Interspersed among the keratinized cells below the stratum corneum are the specialized Langerhans' cells. These cells have a function in the immune response and assist in the initial processing of antigens that enter the epidermis. Epidermal cells usually are shed from the surface as epidermal dust. Differentiation of cells from the basal cell layer to the stratum corneum takes up to 28 days.
Basal cell layer
The basal cell layer produces new cells to replace the superficial keratinized cells that are continuously shed or worn away. The layer's deepest part contains melanocytes, which produce the brown pigment melanin and disperse it to the surrounding epithelial cells. Melanin primarily serves to filter ultraviolet radiation (light). Exposure to ultraviolet light can stimulate melanin production.
The second layer of skin, the dermis - or corium is an elastic system that contains and supports blood vessels, lymphatic vessels, nerves, and epidermal appendages (hair, nails, and eccrine and apocrine glands). The dermis consists of two layers: the superficial papillary dermis and the reticular dermis.
The papillary dermis is studded with fingerlike projections (papillae) that nourish the epidermal cells. The epidermis lies over these papillae and bulges downward to fill the spaces. A collagenous membrane known as the basement membrane lies between the epidermis and dermis, holding them together.
The reticular dermis covers a layer of subcutaneous tissue (adipose layer, or panniculus adiposus), a specialized layer primarily composed of fat cells. It insulates the body to conserve heat, acts as a mechanical shock absorber, and provides energy.
Intercellular material called matrix makes up most of the dermis. Matrix contains connective tissue fibers called collagen, elastin, and reticular fibers. Collagen, a protein, gives strength to the dermis; elastin makes the skin pliable; and reticular fibers bind the collagen and elastin together.
The matrix and connective tissue fibers are produced by spindle-shaped connective tissue cells (dermal fibroblasts), which become part of the matrix as it forms. Fibers are loosely arranged in the papillary dermis but more tightly packed in the deeper reticular dermis.
The epidermal appendages include hair, nails, sebaceous glands, eccrine glands, and apocrine glands.
The skin performs many functions: protection of underlying structures, sensory perception, temperature and blood pressure regulation, vitamin synthesis, and excretion.
The epidermis protects against trauma, noxious chemicals, and invasion by microorganisms.
The skin maintains body surface integrity by cell migration and by shedding and can repair surface wounds by intensifying normal cell replacement mechanisms. Regeneration doesn't occur if the dermal layer is destroyed.
The sebaceous glands produce sebum, a mixture of keratin, fat, and cellulose debris. Combined with sweat, sebum forms a moist, oily, acidic film that is mildly antibacterial and antifungal and protects the skin surface.
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