epidermis

skin

skin, the flexible tissue (integument) enclosing the body of vertebrate animals. In humans and other mammals, the skin operates a complex organ of numerous structures (sometimes called the integumentary system) serving vital protective and metabolic functions. It contains two main layers of cells: a thin outer layer, the epidermis, and a thicker inner layer, the dermis. Along the internal surface of the epidermis, young cells continuously multiply, pushing the older cells outward. At the outer surface the older cells flatten and overlap to form a tough membrane and gradually shed as calluses or collections of dead skin. Horns, hoofs, hair (fur), feathers, and scales are evolutionary adaptations of the epidermis. Although the epidermis has no blood vessels, its deeper strata contain melanin, the pigment that gives color to the skin. The underlying dermis consists of connective tissue in which are embedded blood vessels, lymph channels, nerve endings, sweat glands, sebaceous glands, fat cells, hair follicles, and muscles. The nerve endings, called receptors, perform an important sensory function. They respond to various stimuli, including contact, heat, and cold. Response to cold activates the erector muscles, causing hair or fur to stand erect; fright also causes this reaction. From the outer surface of the dermis extend numerous projections (papillae) that fit into pits on the inner surface of the epidermis so that the two layers are firmly locked together. In humans, whorls on the fingers show where the epidermis falls between rows of papillae, making the patterns used in fingerprinting. The skin provides a barrier against invasion by outside organisms and protects underlying tissues and organs from abrasion and other injury, and its pigments shield the body from the dangerous ultraviolet rays in sunlight. It also waterproofs the body, preventing excessive loss or gain of bodily moisture. Human skin performs several functions that help maintain normal body temperature: its numerous sweat glands excrete waste products along with salt-laden moisture, the evaporation of which may account, in certain circumstances, for as much as 90% of the cooling of the body; its fat cells act as insulation against cold; and when the body overheats, the skin's extensive small blood vessels carry warm blood near the surface where it is cooled. The skin is lubricated by its own oil glands, which keep both the outside layer of the epidermis and the hair from drying to brittleness. Human skin has remarkable self-healing properties, particularly when only the epidermis is damaged. Even when the injury damages the dermis, healing may still be complete if the wounded area occurs in a part of the body with a rich blood supply. Deeper wounds, penetrating to the underlying tissue, heal by scar formation. Scar tissue lacks the infection-resisting and metabolic functions of healthy skin; hence, sufficiently extensive skin loss by widespread burns or wounds may cause death.

Epidermis (plant)

The outermost layer (occasionally several layers) of cells on the primary plant body. Its structure is variable; this article singles out five structural components of the tissue: (1) cuticle; (2) stomatal apparatus (including guard cells and subsidiary cells); (3) bulliform (motor) cells; (4) trichomes; and (5) root hairs.

Trichomes

Leaves, herbaceous stems, and floral organs usually retain the epidermis through life. Most woody stems retain it for one to many years, after which it is replaced. In roots it is usually short-lived. See Leaf, Periderm

Cutin is a mixture of fatty substances characteristically found in epidermal cells. It impregnates the outer cell walls and occurs as a continuous layer (cuticle) on the outer surface. The cuticle covers the surfaces of young stems, leaves, floral organs, and even apical meristems. Waxes appear as a deposit on the outside of the cuticle in many plants; the bloom on purple grapes and plums is an example. Most often the waxes are present in small quantity, but the leaves of some plants may be almost white with wax (Echeveria subrigida). The waxes of a few species are of great commercial value in the manufacture of polishes for floors, furniture, automobiles, and shoes. Other substances, such as gums, resins, and salts, usually in crystalline form, may be deposited on the outside of the cuticle.

The apertures in the epidermis which are surrounded by two specialized cells, the guard cells, are known as stomata. The singular form, stoma, is derived from the Greek word for mouth. However, some authorities prefer to include both aperture and guard cells within the concept of stoma. The apertures of stomata are contiguous with the intercellular space system of underlying tissues and thus permit gas exchange between internal cells and the external environment. The opening and closing of the stomatal aperture is caused by relative changes in turgor between the guard cells and surrounding epidermal cells.

Bulliform (motor) cells are large, highly vacuolated cells that occur on the leaves of many monocotyledons but are probably best known in grasses. They are thought to play a role in the unfolding of developing leaves and in the rolling and unrolling of mature leaves in response to alternating wet and dry periods.

Appendages derived from the protoderm are known as trichomes; the simplest are protrusions from single epidermal cells. Included in the concept, however, are such diverse structures as uniseriate hairs, multiseriate hairs (Begonia, Saxifraga), anchor hairs, stellate hairs, branched (candelabra) hairs, peltate scales, stinging hairs, and glandular hairs (see illustration). Cotton and kapok fibers are unicellular epidermal hairs.

Root hairs are thin-walled extensions of certain root epidermal cells. They develop only on growing root tips and may arise from any epidermal cell, or from specialized cells known as trichoblasts. The life of a given root hair is usually numbered in days. See Root (botany), Secretory structures (plant)

Epidermis

epidermis

epidermis