26 Integumentary Homeostasis

In earlier sections, you learned about the amazing regenerative capacity of skin: Basal cells divide and differentiate to form keratinocytes, which move superficially to the surface and change their structure along the way. There are some cases where this regenerative property can cause problems, and some traumas where regeneration is put to the test.

Wound Repair and Scarring

The first step to repairing damaged skin is the formation of a blood clot that scabs over with time. Many different types of cells are involved in wound repair, especially if the surface area that needs repair is extensive. Before the basal stem cells of stratum basale can re-create the epidermis, fibroblasts and mesenchymal cells mobilize and divide rapidly to repair the damaged tissue, forming a loose, highly vascular tissue called granulation tissue. This increased vascular network helps deliver the oxygen and nutrients necessary for further repair. Immune cells, like macrophages, roam the area and engulf any foreign matter, reducing the chance of infection while also clearing away any tissue debris.

Scars occur when there is repair of skin damage, but the healing process prevents regeneration of the original skin structure. Almost every cut or wound, with the exception of ones that only scratch the surface (epidermal wounds), leads to some degree of scar formation. As the tissue tries to repair itself to the original state, fibroblasts often generate a greater density of collagen fibers than were in the original tissue, which is what results in scar formation. This dense, fibrous structure (connective tissue) has few cells and does not allow for the regeneration of accessory structures (epithelial tissue) like hair follicles, sweat glands, or sebaceous glands.

Sometimes, scarring does not stop when the wound is healed. This results in the formation of a raised or hypertrophic scar called a keloid. In contrast, scars that result from acne and chickenpox have a sunken appearance and are called atrophic scars.

Scarring of skin after wound healing is a natural process and does not need to be treated further. Application of oil and lotions may reduce the formation of scar tissue by keeping the skin soft and pliable as it heals, allowing the separate edges to be pulled together. However, modern cosmetic procedures like dermabrasion, laser treatments, and filler injections have been invented as remedies for severe scarring. All of these procedures try to reorganize the structure of the epidermis and underlying collagen in the connective tissue to make it look more natural.

Burns

Burns are wounds that result when the skin is exposed to intense heat, radiation, electrical current, or caustic chemicals. Burns cause skin cells to die, disrupting the epithelial barrier that normally prevents fluid loss. Depending on the severity of the burn, dehydration, electrolyte imbalance, and renal and circulatory failure may follow. Burn patients are treated with intravenous fluids to offset the dehydration, as well as intravenous nutrients that enable the body to repair tissues and replenish lost proteins. Another serious threat to a burn patient is the high possibility of infection. Burned skin is extremely susceptible to bacteria and other pathogens, as the loss of body covering provides a direct pathway for the entry of microorganisms.

Burns are classified by the degree of their severity. In first-degree burns, only the epidermis is damaged. Although painful, these burns typically heal on their own within a few days. A typical sunburn is a first-degree burn. Second-degree burns destroy both the epidermis and the dermis. They result in painful blistering of the skin. It is important for individuals with second-degree burns to keep the area clean and sterile in order to prevent infection. If this is done, the burns heal on their own within several weeks.

Third-degree burns are more serious and penetrate the full thickness of the skin, including the subcutaneous (hypodermis) layer. The skin may appear white, red, or black. Fourth-degree burns are the most severe, affecting the underlying muscle and bone as well. Third- and fourth-degree burns are usually not as painful because the nerve endings themselves are damaged. However, a third- or fourth-degree burn results in rapid water loss and infection due to the exposure of the underlying tissue, and requires emergency trauma care. Full-thickness burns cannot be repaired locally because the local repair machinery itself is damaged. Full-thickness burns require excision of the affected tissue (amputation in severe cases), followed by grafting of skin from an unaffected part of the body, or from skin grown in tissue culture for grafting purposes.

Changes in the Integumentary System

Puberty

During puberty, the changes in the endocrine system produce hormones that regulate aspects of the integumentary system. Sebaceous glands, apocrine sweat glands and hair growth become active or more active.

Advanced Aging

All organ systems in the body, including the integumentary system, undergo subtle changes over time that add up as a person ages. Reductions in metabolic activity and cell division, lowered blood circulation, and decreased hormonal levels are some of the basic changes that occur in the human body as it ages.

The epidermis is made up of several layers of cells that are shed and replaced by basal stem cell division. With aging, these cells divide less reliably and aging skin becomes thinner. The dermis, which is responsible for the elasticity and resilience of the skin, also weakens since elastin fibers become cross-linked and don’t stretch. Repair of wounds occurs more slowly as a person ages. The hypodermis, which stores fats in its adipose tissue, loses its structure due to the reduction and redistribution of fat, which in turn is affected by hormonal levels. The accessory structures also have lowered activity, generating thinner hair and nails, and reduced amounts of sebum and sweat. Reduced sweating ability causes the elderly to be unable to tolerate extreme heat. Other cells in the skin, such as melanocytes and dendritic cells, also become less active, leading to a paler appearance and lowered immunity. Wrinkling of skin occurs mainly due to the breakdown of collagen and elastin, the weakening of muscles under the skin, and the inability of the skin to retain adequate moisture.

Many anti-aging products line the shelves of stores today. These products (mainly composed of tretinoin, or Retin-A) try to introduce more structure into the skin, first by rehydrating the skin with moisturizers, and then by triggering mitotic activity of the underlying cells to encourage tissue formation and regeneration. Some products contain epidermal growth factor, which induces collagen synthesis, the end-product of wound healing, to give the skin a healthier look.

Thermoregulation and Thermal Homeostasis

The integumentary system regulates body temperature through several different means. Recall that sweat glands — accessory structures to the skin — excrete water, salt, and other substances to cool the body when it becomes warm. This is termed “sensible perspiration.” Even if the body does not appear to be noticeably sweating, approximately 500 mL of sweat are secreted a day. If the body becomes excessively warm due to high temperatures, vigorous activity, or a combination of the two, the blood vessels in the integumentary system dilate and sweat glands produce large amounts of sweat — up to three gallons a day. As the sweat evaporates from the skin surface, the body is cooled; in a way, the skin surface acts like the radiator of a car. The dilated blood vessels in the dermis that account for the redness that many people experience when exercising on a hot day also help in the dissipation of body heat by increasing the superficial blood flow.

When temperatures are cold, the adipose tissue of the hypodermis helps insulate the body. Goosebumps and the arrector pili muscles help to insulate by trapping air on the surface of the skin. Additionally, the dermal blood vessels constrict to decrease blood flow (and heat loss) at the skin.

The Sense of Touch

The fact that you can feel an ant crawling on your skin, causing you to flick it off before it bites, is due to the fact that the sensory receptors in the skin (especially the hair root plexus associated with the hair in the follicles) can sense changes in the environment. This occurs via sensory receptors that convert physical or chemical stimuli into electrical signals that are sent to the central nervous system (CNS; brain and spinal cord). The CNS then processes this sensory input and generates a voluntary response to the ant. The skin acts as a sense organ because the dermis and hypodermis contain sensory receptors that extend into the epidermis. These receptors, such as Merkel discs, are more concentrated at the fingertips and lips, which are most sensitive to touch. Examples of other sensory receptors present in the skin are tactile (Meissner’s) corpuscles and lamellated (Pacinian) corpuscles that respond to pressure and vibration, respectively, and free nerve endings that are sensitive to temperature (hot or cold) and pain.

Skin Conditions and General Health

There are a variety of conditions that can change the appearance of the skin. In vitiligo, the melanocytes in certain areas lose the ability to produce melanin, possibly due to an autoimmune reaction, leading to loss of pigmentation in patches. Liver disease or liver cancer can cause an accumulation of bile and the yellow pigment bilirubin, leading to the skin appearing yellow or jaundiced (“jaune” is French for “yellow”). Tumors of the pituitary gland can result in the secretion of large amounts of corticotropin-releasing hormone (CRH), which can stimulate excessive levels of melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH). In Addison’s disease, excessive CRH stimulates production of adrenocorticotropic hormone (ACTH) and MSH. In these diseases, the skin takes on a deep bronze color.

The oxygen content of blood in the dermal vascular system is reflected in skin hue. Healthy individuals who have adequate supplies of the red-colored oxygen-bound hemoglobin usually have a pink hue to their cheeks and lips. Indeed, hemoglobin and the vasculature of the skin also contributes to skin color in people with fairer skin. A sudden drop in oxygenation of the skin can initially cause the skin to turn ashen (or white). If there is a prolonged reduction in oxygen levels, it makes the skin appear blue (cyanosis), which happens when a person has a limited oxygen supply to the body. The blue color is most obvious in areas where blood capillaries are close to the surface, like the lips and nail beds. Blue lips are a sign of hypoxia and hypothermia, which lead to drastically reduced peripheral circulation, or of any other condition that prevents adequate oxygenation of the blood, which can occur in a variety of lung diseases. At the other extreme, elevated blood levels of carbon monoxide can cause the lips and oral mucosa to appear bright cherry red because the carbon monoxide molecule binds to hemoglobin tighter than oxygen, giving the hemoglobin in the red blood cells the bright red color. Carbon monoxide poisoning is fatal, and prompt treatment with pure oxygen is indicated.