3.6: Dermis

The skin layer under the epidermis is called the dermis (Figure 3.1 ). It is considerably thicker than the epidermis and contains many different types of structures, include vessels, nerve cells, and small muscles. These structures are embedded in a foundation material consisting mostly of fibers and some cells suspended in a small quantity of soft gel. The gel consists mostly of water with some complex proteins and carbohydrates. Because of the variety of its structures, the dermis makes many contributions to three of the four main functions of the skin discussed earlier in this chapter.

Foundation Material

Fibers Fibers made of protein are the most abundant material in the dermis. The protein fibers are mainly of two types. Collagen fibers constitute approximately 80 percent of the fiber materials, and the others consist of elastin fibers.

The collagen fibers are tangled with each other to form a dense mat. This ensures that the skin will not split open or tear when it is subjected to pulling or twisting forces or is cut. Therefore, it is like a rip‑stop fabric. Still, the mat is very flexible so that parts of the body can bend freely.

Elastin fibers are mixed among the collagen fibers. Because of their elasticity, these fibers cause the skin automatically to return to its original position after it has been pulled, bent, or twisted out of shape.

Cells Scattered among the dermal fibers are cells of various types. The most abundant type, accounting for about 60 percent of these cells, is the fibroblast. Fibroblasts produce and secrete the proteins that form collagen and elastin fibers. They are regulated by chemical and physical signals. Sunlight inhibits collagen production.

Macrophages ("large eaters") constitute 20 to 40 percent of the dermal cells. They wander about among the fibers, engulfing and digesting unwanted materials, including cellular debris, foreign substances, and bacteria. Macrophages are also important as defense cells because they function like the Langerhans cells of the epidermis.

Additional defense cells in the fibers include white blood cells that attack and remove harmful materials in a variety of ways (see Chaps. 4 and 15). Mast cells release a substance called histamine. Whenever there is injury to the dermis. Histamine starts the process of inflammation (Figure 3.3) (Erythrocytes are red blood cells (RBCs), leucocytes are white blood cells (WBCs), and thrombocytes are platelets.)

Figure 3.3 Inflammation (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Inflammation in any part of the body involves an increase in the diameter of blood vessels and in the porosity of the smallest vessels (capillaries). These changes deliver more white blood cells to the injured area to protect the body from infection and remove damaged body cells. More oxygen and nutrients are also brought to the area to supply body cells with all the materials they need to repair the injury. The extra fluids that arrive cause swelling and usually help flush away toxins and debris. In more serious vessel damage, movement of the fluid is inhibited by clotting materials that leak out of the vessels. The redness, swelling, and pain that accompany inflammation serve as warning signs that an injury has occurred. In addition, the pain encourages the person to avoid the circumstances that caused the injury and limit the use of the damaged area until healing has occurred.

Beyond their role in defense, mast cells release a substance called heparin, which stimulates the migration of cells that form new blood vessels in the dermis. These new vessels are important when new dermal components are formed during the healing of a wound and when the skin grows.

Gel As was mentioned above, the fibers and cells of the dermis are surrounded by a small amount of soft gel material. This gel is made up mostly of water but also contains a variety of large and small molecules dissolved in the water. The water provides a favorable environment for the cells, allows materials to get to and from the cells, and maintains the firmness of the skin. The large molecules provide firmness by keeping enough water in the dermis. They also bind together the other structures in the dermis as a soft glue would. Most of the small dissolved molecules are either nutrients moving to the cells or waste products moving to the blood vessels for removal from the skin. Some inactive vitamin D is also in the water. The vitamin D moves out of the dermis after being acted upon by light (see Vitamin D, below).

Age Changes in Fibers and Cells

The collagen fibers of the dermis undergo substantial changes with increasing age. These changes have a profound effect on the properties of the dermis and the ability of the skin to perform normally. One change is a gradual decrease in the amount of collagen. The remaining collagen fibers become thicker and less organized and form larger bundles of fibers. These changes may be due to the increase in the number of cross‑links between the fibers. The increasing cross‑links make the fibers stiffer and less able to move, leaving the skin stiffer and less able to stretch. Pulling forces are then more likely to cause injury to the skin because the skin yields less when pulled.

The progressive cross‑linking of collagen does not continue throughout life, however. In very old age enzymes in the skin break down the cross‑links faster than they can form. Then the strength of the collagen mat decreases, and the skin can be torn more easily.

Age changes in elastin fibers are not as well documented, partly because of the difficulty of distinguishing age changes from changes caused by exposure to sunlight. In any event, elastin fibers become thicker, stiffer, more tightly bound by cross‑links, less regular in their arrangement, and, sometimes, impregnated with calcium. The changes in dermal elastin fibers are virtually identical to the changes that occur in the elastic fibers in arteries altered by atherosclerosis (Chapter 4).

While changes in elastin fibers do not alter the ability of the skin to be stretched, they reduce its tendency to return to its original shape and size after being pulled. The skin also does not regain its normal thickness as well after being compressed. Overall, then, the skin seems to become a looser covering that hangs from the body.

With advancing age, the number of fibroblasts increases. This increase may result from an accumulation of old cells and a decrease in the ability to produce new ones. The old fibroblasts may also have less ability to produce new fibers to replace older fibers. Therefore, age changes in the fibroblasts may permit the accumulation of age changes in the fibers and the resulting alterations in the properties of the skin. Furthermore, the deterioration of fibroblasts seems to contribute to the gradual reduction in the speed and strength of skin healing.

Unlike the fibroblasts, the numbers of dermal macrophages and white blood cells seem to decrease with age. The result is a reduction in the defense functions performed by the skin, including a lowered ability to prevent infection, remove harmful chemicals and debris, and initiate immune responses. Therefore, the healthy survival of the entire body is at greater risk.

The age‑related decline in the number of dermal mast cells, which may reach 50 percent, also reduces the defense function of the skin. With fewer mast cells, there is a lowering of both the speed and the intensity of inflammatory responses. Therefore, there is both less warning that injury is occurring and reduced defense against further damage. Furthermore, the declining population of mast cells cannot produce as much heparin. This results in a declining ability to produce new blood vessels in areas of healing and may be a main factor contributing to the normal decrease in the number of dermal blood vessels.

Another age change in the dermal foundation material involves large molecules called mucopolysaccharides, which hold much water. The amount of mucopolysaccharide in the dermis decreases slightly. Therefore, the amount of bound water also declines, and so the firm consistency of the skin diminishes. The skin becomes more easily compressed and returns to its original thickness more slowly. The decrease in bound water may also reduce the movement of small molecules through the dermis. This means that skin cells are not as well serviced. Finally, the reduction in the amount of water held in the dermis probably contributes to the general thinning of the dermis and the thinner appearance any elderly people.

Blood Vessels

The blood vessels in the dermis are numerous, although some areas of the body (e.g., scalp) have more of them. Like all vessels that carry blood, the dermal vessels deliver useful materials to the cells and carry away manufactured substances that can be used elsewhere in the body. These vessels also remove wastes produced by the cells. Furthermore, blood flow in these vessels delivers white blood cells and antibodies for defense of the area they serve.

Dermal vessels also help regulate body temperature. They widen when the temperature in the body rises above the desirable level. This widening is called dilation (or vasodilation), and it allows more warm blood to flow close to the surface of the body. Much of the heat in the blood passes out of the body to the cooler surrounding environment. The result is a lowering of the body temperature so that it is again in the desirable range.

Conversely, if body temperature drops below the normal level, the dermal vessels become narrower. This is called constriction (or vasoconstriction), and it reduces blood flow through the vessels. With less warm blood flowing near the surface of the body, the rate of heat loss is reduced. The body can then become warmer as its muscles and other active cells produce more heat.

Age Changes in Dermal Vessels

With aging, the number of dermal blood vessels decreases substantially, particularly in the layer just below the epidermis and in skin chronically exposed to sunlight. The remaining vessels often show irregularities in structure. These changes cause a decrease in blood flow to the dermis.

This reduction in blood flow decreases the delivery of nutrients to all dermal structures. This may be a main reason for the age‑related shrinkage and decline in function of many skin structures. There is also slower removal of material (e.g., wastes, vitamin D), and the delivery of white blood cells and antibodies declines. Even the epidermis, which has no vessels of its own and therefore depends on blood flow in the dermis, is serviced less well. Reduced blood flow can also cause paleness of the skin. Furthermore, reduced dermal blood flow can be of great importance for elderly individuals who use topically applied medications, which can reach dangerously high concentrations in the skin. Meanwhile, the rest of the body, which may need the medication, receives less because the medication remains in the skin.

Adding to the problems caused by reduced dermal blood flow is the increase in thickness of a layer of material that surrounds the capillaries. This layer, the basement membrane, is normally quite thin. It allows certain white blood cells and many substances to enter and leave the capillaries freely so that the areas near the capillaries are well serviced.

The age‑related thickening of the basement membrane inhibits the movement of white blood of the circulatory system to provide for the needs of the skin.

The aging of dermal vessels also adversely affects the thermoregulatory function of the skin. As blood flow declines, there is a reduction in the ability to release excess heat from the body. The vessels also constrict and dilate more slowly and to a lesser degree. The result is a reduced ability not only to release heat but to slow heat loss when the body begins to get chilled. These changes constitute a major reason the elderly have difficulty maintaining normal body temperature when the external temperature deviates from a moderate level or when activities such as vigorous exercise cause an alteration in body temperature.

Sweat Glands

The dermis contains two types of sweat glands. Eccrine sweat glands secrete a watery material that is the visible perspiration (i.e., sweat) seen when a person becomes uncomfortably warm. The other type of sweat gland is the apocrine sweat gland.

Each eccrine sweat gland is a tubular gland with a highly coiled portion, in the dermis, which produces most of the perspiration (Figure 3.1 ). The coiled portion leads into a fairly straight portion that extends upward through the dermis and epidermis and finally opens onto the surface of the skin.

The purpose of the perspiration produced by these glands is to cool the body. When the brain detects an abnormal rise in body temperature, it sends nerve impulses to the glands, stimulating them to secrete perspiration. The water in the perspiration evaporates when it reaches the surface of the skin. As it evaporates, it carries heat away from the skin, resulting in a lowering of body temperature.

Perspiration also contains a number of substances, including useful ones such as salt, which are dissolved in the water. However, the secretion of these useful substances is not beneficial to the body. Serious problems such as muscle cramps and dizziness can develop if a person perspires abundantly without replacing the useful substances by drinking beverages or eating foods that contain more of these substances.

Unlike the eccrine sweat glands, which are widespread, most apocrine sweat glands are in the skin of the armpits and in the genital area. These glands secrete a small amount of thick materials that do little to promote healthy survival. This secretion is a main source of unpleasant body odor.

Apocrine gland activity is controlled largely by the level of sex hormones in the body, though the nervous system may increase the secretion during periods of stress or intense emotions.

With aging, the number of eccrine sweat glands decreases dramatically in all parts of the body except the scalp. The remaining glands are reduced in size and produce perspiration at a decreasing rate.

The result of these changes is a reduction in the ability of the glands to cool the body. This puts the elderly at an ever increasing risk of becoming overheated in particularly warm environments or during vigorous exercise.

Though changes in the number of apocrine sweat glands have not been well studied, it is known that they shrink and that their rate of secretion diminishes significantly. This is probably due to age‑related reductions in sex hormone levels. This is one of the few alterations with advancing age that most people agree is desirable since diminishing apocrine gland secretion leads to a substantial decrease in unpleasant body odor. While apparently having no biological importance, this change can have positive effects on other parameters (e.g., social).

Sebaceous Glands

Besides sweat glands, the dermis contains glands that produce an oily substance called sebum; these glands are called sebaceous glands (Figure 3.1 ). Sebaceous glands are usually found beside hair follicles and secrete sebum into the follicles. The sebum coats the hair, and as it leaves the follicles, it spreads out to form a thin coating on the epidermis.

Sebum contributes to the maintenance of homeostasis mainly by removing the ability of the skin to act as a barrier. Because sebum is an oily material, it helps make the keratin of the epidermis more impermeable to water. It also helps keep the keratin pliable so that the stratum corneum does not crack when it is bent. Cracks in the keratin could allow water and other chemicals to leak into and out of the body and permit the entrance of harmful microbes. In addition, certain materials in the sebum inhibit the growth of fungi that could break down keratin.

Sebum is also cosmetically important because it gives skin keratin a smoother appearance and adds luster to the hair. Finally, by keeping keratin pliable, sebum reduces the breaking and splitting of hair.

Age Changes in Sebaceous Glands

Though there seems to be no age change in the number of sebaceous glands and though they increase in size, there is a decrease in the production of sebum. This decline seems to result from declining levels of the sex hormones that normally stimulate sebum production.

The reduction in sebum production lowers its contributions, though the amount produced is usually sufficient to prevent serious biological problems. However, the cosmetic contributions made by sebum decline substantially. The results are the clearly visible signs of aging of the skin and hair. The widespread use of skin and hair lotions that augment the diminished contributions of sebum attests to the importance of these cosmetic changes.

Nerves

The two types of nerve cells (neurons) are sensory neurons and motor neurons (Figure 3.1 ). Sensory neurons monitor conditions in and around the dermis, including conditions in the epidermis and the external environment. Sensory neurons send information about these conditions to the brain and spinal cord. Motor neurons control the functioning of blood vessels and eccrine sweat glands by relaying instructions from the brain and spinal cord to the skin.

Sensory Neurons

There are several types of sensory neurons in the dermis, each of which is specialized to monitor a single kind of stimulus (e.g., light touch, heat, pressure). An additional type is activated when conditions vary greatly from normal or there is injury to the skin. This type of sensory neuron warns of the danger by providing the sensation of pain.

Beyond determining if there has been a change in conditions near the surface of the body, sensory neurons provide information about what type of change has occurred and the location of that change. The nervous system can then initiate the proper type of response to preserve the well‑being of the body.

Motor Neurons

Information about the structure and function of the motor neurons of the skin and the effects of aging on these neurons is presented in Chapter 6.

Age Changes in Sensory Neurons

As a person ages, there is little change in the number or structure of the sensory neurons for pain and the touch receptors connected to hair follicles. Conversely, the numbers of touch receptors not connected to hair follicles and of pressure receptors decrease dramatically. In addition, there are alterations and distortions in the structure of both types of receptors. Little is known about the effects of aging on the other types of sensory neurons.

As a result of age changes in sensory neurons, there is decreased sensitivity to touch, pressure, and vibration. This is especially evident in the fingers, the palms of the hands, and other areas of the body lacking hair (e.g., the penis). In addition, there is a decreased ability to detect the exact location of touch and pressure stimuli and therefore to determine the shapes of objects by touching them. One practical consequence is a reduction in manual dexterity. Interestingly, the thinning of the skin with age compensates somewhat for changes in sensory neurons allowing stimuli to reach these neurons more easily.

These sensory decrements reduce the ability of the skin to inform the body about conditions on and just outside its surface. The person is then less able to respond negatively to dangerous or harmful stimuli and positively to helpful or pleasurable stimuli.

The ability of the skin to perform its monitoring function is adversely affected by many factors beyond changes in the number and shape of its sensory neurons. Some of these factors include the consistency of the skin and the subcutaneous layer; the ability of the neurons to conduct impulses to the brain and spinal cord; and the ability of the brain and spinal cord to process and interpret those impulses (Chapter 6).