14.2 Basic Integumentary Concepts

The integumentary system includes the skin, hair, and nails. The skin is an organ that performs a variety of essential functions, such as protecting the body from invasion by microorganisms, chemicals, and other environmental factors; preventing dehydration; acting as a sensory organ; modulating body temperature and electrolyte balance; and synthesizing vitamin D.^[1]

Skin

The skin is made of multiple layers of cells and tissues, which are held to underlying structures by connective tissue. See Figure 14.1^[2] for an image of the layers of the skin. The skin is composed of two main layers: the uppermost thin layer called the epidermis made of closely packed epithelial cells, and the inner thick layer called the dermis that houses blood vessels, hair follicles, sweat glands, and nerve fibers. Beneath the dermis lies the hypodermis that contains connective tissue and adipose tissue (stored fat) to connect the skin to the underlying bones and muscles. The skin acts as a sense organ because the epidermis, dermis, and hypodermis contain specialized sensory nerve structures that detect touch, surface temperature, and pain.^[3]

The color of skin is created by pigments, including melanin, carotene, and hemoglobin. Melanin is produced by cells called melanocytes that are scattered throughout the epidermis. See Figure 14.2^[4] for an illustration of melanin and melanocytes. When there is an irregular accumulation of melanocytes in the skin, freckles appear. Dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning bed causes additional melanin to be manufactured and built up, resulting in the darkening of the skin referred to as a tan. Increased melanin accumulation protects the DNA of epidermal cells from UV ray damage, but it requires about ten days after initial sun exposure for melanin synthesis to peak. This is why pale-skinned individuals often suffer sunburns during initial exposure to the sun. Darker-skinned individuals can also get sunburns, but they are more protected from their existing melanin than pale-skinned individuals.^[5]

Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin, and in severe cases, can cause DNA damage resulting in skin cancer. Moles are larger masses of melanocytes, and although most are benign, they should be monitored for changes that indicate the presence of skin cancer. See Figure 14.3^[6] for an image of moles.

Patients are encouraged to use the ABCDE mnemonic to watch for signs of early-stage melanoma developing in moles. Consult a health care provider if you find these signs of melanoma when assessing a patient’s skin:

• Asymmetrical: The sides of the moles are not symmetrical
• Borders: The edges of the mole are irregular in shape
• Color: The color of the mole has various shades of brown or black
• Diameter: The mole  is larger than 6 mm. (0.24 in.)
• Evolving: The shape of the mole has changed

Hair

Hair is made of dead, keratinized cells that originate in the hair follicle in the dermis. For these reasons, there is no sensation in hair. See Figure 14.4^[7] for an image of a hair follicle. Hair serves a variety of functions, including protection, sensory input, thermoregulation, and communication. For example, hair on the head protects the skull from the sun. Hair in the nose, ears, and around the eyes (eyelashes) defends the body by trapping any dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat and other particles from dripping into the eyes.

Hair also has a sensory function due to sensory innervation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, even more so than the skin surface. This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface. Each hair root is also connected to a smooth muscle called the arrector pili that contracts in response to nerve signals from the sympathetic nervous system, making the external hair shaft “stand up.” This movement is commonly referred to as goose bumps. The primary purpose for this movement is to trap a layer of air to add insulation.^[8]

Nails

The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes. See Figure 14.5^[9] for an illustration of a fingernail. The nail body is formed on the nail bed and protects the tips of our fingers and toes as they experience mechanical stress while being used. In addition, the nail body forms a back-support for picking up small objects with the fingers.^[10]

Sweat Glands

When the body becomes warm, sweat glands produce sweat to cool the body. There are two types of sweat glands that secrete slightly different products. An eccrine sweat gland  produces hypotonic sweat for thermoregulation. See Figure 14.6^[11] for an illustration of an eccrine sweat gland. These glands are found all over the skin’s surface, but are especially abundant on the palms of the hand, the soles of the feet, and the forehead. They are coiled glands lying deep in the dermis, with the duct rising up to a pore on the skin surface where the sweat is released. This type of sweat is composed mostly of water and some salt, antibodies, traces of metabolic waste, and dermicidin, an antimicrobial peptide. Eccrine glands are a primary component of thermoregulation and help to maintain homeostasis.^[12]

Apocrine sweat glands are mostly found in hair follicles in densely hairy areas, such as the armpits and genital regions. In addition to secreting water and salt, apocrine sweat includes organic compounds that make the sweat thicker and subject to bacterial decomposition and subsequent odor. The release of this sweat is controlled by the nervous system and hormones and plays a role in the human pheromone response. Most commercial antiperspirants use an aluminum-based compound as their primary active ingredient to stop sweat. When the antiperspirant enters the sweat gland duct, the aluminum-based compounds form a physical block in the duct, which prevents sweat from coming out of the pore.^[13]

Skin Lesions

A lesion is an area of abnormal tissue. There are many terms for common skin lesions that may be described in a patient’s chart. These terms are defined in Table 14.2. See Figure 14.7^[14] for illustrations of common skin lesions.

Table 14.2 Medical Terms Associated with Skin Lesions and Rashes^[15]