Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Integumentum

C h a p t e r 4 5

the dermis is richly vascularized. It contains capillaries, end arterioles, and venules that nourish the epidermal layers of the skin. Lymph vessels and nerve tissue also are found in this layer. Reticular Dermis The reticular layer of the dermis is the thicker area of the dermis and forms the bulk of the dermal layer. This is the tough layer in animal hides from which leather is made. The reticular dermis is characterized by a com- plex meshwork of dense collagen bundles intercon- nected with large elastic fibers and ground substance, a viscid gel that is rich in mucopolysaccharides. The col- lagen fibers are oriented parallel to the body’s surface in any given area. Collagen bundles may be organized lengthwise, as on the abdomen, or in round clusters, as on the heel. The direction of surgical incisions is often determined by this organizational pattern. Immune Cells Once thought to be composed primarily of fibroblasts, it is now believed that the dermis is mainly composed of dendritic cells called dermal dendrocytes. Believed to be one of the main cell types of the dermis, dermal dendro- cytes are spindle-shaped cells that have both phagocytic and antigen-presenting functions and play an important part in the immunobiology of the dermis. In addition, it is possible that dermal dendrocytes may be able to initi- ate or respond to immunologic events in the epidermis. Dermal dendrocytes also are thought to be involved in processes such as wound healing, blood clotting, and inflammation. The dermis also contains macrophages, T cells and mast cells. Dermal macrophages and venular epithelial cells may present antigen to T cells in the dermis, most of which are previously activated or memory T cells. T-cell responses to macrophage- or endothelium-associated antigens in the dermis are probably more important in generating an immune response to antigen challenge in previously exposed persons than in initiating a response to a new antigen. The major type of T-cell–mediated immune response in the skin is delayed-type hypersensi- tivity (see Chapter 16). Mast cells, which have a prominent role in immu- noglobulin E–mediated immediate hypersensitivity, also are present in the dermis. These cells are strategically located at body interfaces such as the skin and mucous membranes and are thought to interact with antigens that come in contact with the skin. BloodVessels The arteries that nourish the skin form two plexuses (i.e., collections of blood vessels), one located between the dermis and the subcutaneous tissue and the other between the papillary and reticular layers of the dermis. The pink color of light skin results primarily from blood in the vessels of this latter plexus. Capillary flow that arises from vessels in this plexus also extends up and nourishes the overlying epidermis by diffusion. Blood leaves the skin through small veins that accompany the

subcutaneous arteries. The lymphatic system of the skin, which aids in combating certain skin infections, also is limited to the dermis. The skin is richly supplied with arteriovenous anasto- moses in which blood flows directly between an artery and a vein, bypassing the capillary circulation. These anastomoses are important for temperature regulation. They can open up, letting blood flow through the skin vessels when there is a need to dissipate body heat, and close off, conserving body heat if the environmental temperature is cold. Innervation The innervation of the skin is complex. The dermis is well supplied with sensory receptors for temperature, pain, and touch (see Chapter 35), as well as nerves that supply the blood vessels, sweat glands, and arrector pili muscles. The papillary layer of the dermis is supplied with free nerve endings that serve as nociceptors (i.e., pain receptors) and thermoreceptors. The dermis also con- tains encapsulated pressure-sensitive receptors that detect pressure and touch. The largest of these are the pacinian corpuscles, which are widely distributed in the dermis and subcutaneous tissue in the digits of the hands and breasts. These mechanoreceptors are spe- cialized to perceive pressure, touch, and vibration. Meissner corpuscles are encapsulated mechanorecep- tors specialized for tactile discrimination. They are concentrated on the fingertips and palms of the hands, where they account for about half of the tactile recep- tors. They are also located on the eyelids, lips, tongue, nipples, and skin of the foot and forearm. The deep dermis is supplied with small, oval mechanorecep- tors called Ruffini corpuscles. They are slowly adapt- ing receptors, responding to heavy pressure and joint movement. They are also believed to detect cold. The skin is also supplied by Krause end bulbs , nerve end- ings contained in a cylindrical or oval capsule. They are found most frequently in the oral cavity, conjunc- tiva, and genitalia. Although their function is uncer- tain, they are thought to act as mechanoreceptors and heat detectors. Most of the skin’s blood vessels are innervated by the sympathetic nervous system. The sweat glands are innervated by cholinergic fibers but controlled by the sympathetic nervous system. Likewise, the sympa- thetic nervous system controls the arrector pili (pilo- motor) muscles that cause elevation of hairs on the skin ( pilo- means hair). Contraction of these muscles tends to cause the skin to dimple, producing “goose bumps.” Basement Membrane Zone The basement membrane zone is a layer of intercellu- lar and extracellular matrices that serves as an inter- face between the dermis and the epidermis. It separates the epithelium from the underlying connective tissue, anchors the epithelium to the loose connective tissue underneath, and serves as a selective filter for molecules