INTEGUMENTARY SYSTEM OUTLINE 5.1 Structure and Function of the Integument 119 5.1a Integument Structure 119 5 5.1b Integument Functions 120 5.2 Epidermis 121 5.2a Epidermal Strata 121 5.2b Variations in the Epidermis 122 Integumentary 5.3 Dermis 125 5.3a Papillary Layer of the Dermis 126 5.3b Reticular Layer of the Dermis 126 5.3c Stretch Marks, Wrinkles, and Lines of Cleavage 126 System 5.3d Innervation and Blood Supply 127 5.4 Subcutaneous Layer (Hypodermis) 128 5.5 Epidermal Accessory Organs 129 5.5a Nails 129 5.5b Hair 130 5.5c Exocrine Glands of the Skin 133 5.6 Integument Repair and Regeneration 136 5.7 Aging of the Integument 138 5.7a Skin Cancer 139 5.8 Development of the Integumentary System 140 5.8a Integument Development 140 5.8b Nail Development 140 5.8c Hair Development 140 5.8d Sebaceous and Sweat Gland Development 140 5.8e Mammary Gland Development 141 MODULE 4: INTEGUMENTARY SYSTEM mck78097_ch05_118-145.indd 118 2/11/11 3:02 PM Chapter Five Integumentary System 119 he integument (in-teg ́ū -ment; integumentum = a covering) The integument, or skin, is the body’s largest organ. T is the skin that covers your body. Skin is also known as the Although the skin is not as complex as most other organs, it does cutaneous (kū -tā ń ē -ŭ s) membrane, or cutaneous layer. The integu- consist of different tissue types that collectively perform specific mentary (in-teg-ū -men t́ ă-r ē) system consists of the skin and its activities. Its surface is covered by an epithelium that protects derivatives—nails, hair, sweat glands, and sebaceous glands. We underlying body layers. The connective tissues that underlie are most conscious of this highly visible and overexamined body the epithelium contain blood vessels, which provide nutrients to system, because it characterizes our self-image and reflects our the epithelial cells and give strength and resilience to the skin. emotions. Our skin is a vulnerable barrier to the outside world; it Smooth muscle controls blood vessel diameter and hair position is subjected to trauma, harmful chemicals, pollutants, microbes, for these integumentary structures. Finally, nervous tissue sup- and damaging sunlight. Still, it usually remains strong and pliable, ports and monitors sensory receptors in the skin, which provide is easily cleaned, is self-renewing, and serves as a visual indicator information about touch, pressure, temperature, and pain. of our physiology and health. Changes in the color of the skin may reflect body disorders or anomalies; skin changes or lesions some- 5.1a Integument Structure times indicate systemic infections or diseases. The scientific study The integument covers the entire body surface, an area that ranges and treatment of the integumentary system is called dermatology between about 1.5 and 2.0 square meters (m2) and accounts for 7% to (der-mă-tol ́ō -jē; derma = skin, logos = study). 8% of the body weight. Its thickness ranges between 1.5 and 4 mil- limeters (mm) or more, depending on body location. The integument consists of two distinct layers: a layer of stratified squamous epi- thelium called the epidermis, and a deeper layer of dense irregular 5.1 Structure and Function and areolar (loose) connective tissue called the dermis (figure 5.1). of the Integument Deep to the dermis is a layer composed of areolar and adipose connective tissue called the subcutaneous layer, or hypodermis. Learning Objectives: The subcutaneous layer is not part of the integumentary system; 1. Describe the general structure of the integument. however, it is described in this chapter because it is closely involved 2. Identify the varied functions of the integument. with both the structure and function of the skin. Hair shaft Sweat pore Epidermis Epidermal ridge Dermal papilla Papillary layer Arrector pili muscle Sebaceous (oil) gland Dermis Sweat gland duct Reticular Merocrine sweat gland layer Sensory nerve fiber Apocrine sweat gland Vein Artery Subcutaneous layer Adipose connective tissue Hair follicle Sensory Areolar receptors connective tissue Figure 5.1 Layers of the Integument. A diagrammatic sectional view through the integument shows the relationship of the cutaneous membrane (skin) to the underlying subcutaneous layer. mck78097_ch05_118-145.indd 119 2/11/11 3:02 PM 120 Chapter Five Integumentary System The integument meets the mucous membranes within the flows through these dermal vessels, the warmth from the blood nostrils, lips, anus, urethral opening, and vaginal opening. At dissipates through the skin, and the body cools off by evapora- these sites, the transition is seamless, and the epithelial defenses tion of the sweat. Conversely, when the body is cold and needs to remain intact and functional. conserve heat, the blood vessels in the dermis constrict to reduce blood flow. In an effort to conserve heat, more blood is shunted to 5.1b Integument Functions deeper body tissues, and relatively less blood flows in the dermal The integument is more than just a wrapping around the body. It blood vessels. serves many varied functions, including protection, prevention of Metabolic Regulation water loss, temperature regulation, metabolic regulation, immune defense, sensory reception, and excretion. Vitamin D3 is a cholesterol derivative synthesized from cholecal- ciferol (kō ́lē -kal-sif er-ol),́ which is produced by some epidermal Protection cells when they are exposed to ultraviolet radiation. Calcitriol (kal- The skin acts as a physical barrier that protects the entire body si-trı̄ ol)́ is synthesized from the cholecalciferol by some endocrine from physical injury, trauma, bumps, and scrapes. It also offers cells in the kidney. Calcitriol, the active form of vitamin D3, is a protection against harmful chemicals, toxins, microbes, and exces- hormone that promotes calcium and phosphorus absorption from sive heat or cold. Paradoxically, it can absorb certain chemicals and ingested materials across the wall of the small intestine. Thus, the drugs (such as estrogen from a birth control patch or nicotine from synthesis of vitamin D3 is important in regulating the levels of cal- a nicotine patch). Thus, the skin is said to be selectively permeable cium and phosphate in the blood. As little as 15 minutes of sunlight because some materials are able to pass through it, while others a day will provide your body with its daily vitamin D requirement! are effectively blocked. The epidermis is designed to withstand Immune Defense stresses and regenerate itself continuously throughout a person’s lifetime. The skin also protects deeper tissues from solar radiation, The epidermis contains a small population of immune cells. especially ultraviolet rays. When exposed to the sun, the melano- These immune cells (derived from a type of white blood cell), cytes become more active and produce more melanin, thus giving called epidermal dendritic (den-drit ́ik) cells, or Langerhans the skin a darker, tanned look. Even when you get a sunburn, the cells, play an important role in initiating an immune response by deeper tissues (muscles and internal organs) remain unaffected. phagocytizing pathogens that have penetrated the epidermis and also against epidermal cancer cells. Prevention of Water Loss Sensory Reception The epidermis is water resistant and helps prevent unnecessary water loss. (If the skin were not water resistant, each time you took The skin contains numerous sensory receptors. These receptors a bath you would swell up like a sponge as your skin absorbed are associated with nerve endings that detect heat, cold, touch, water!) Water cannot easily enter or exit the skin, unless it is pressure, texture, and vibration. For example, tactile cells (or specifically secreted by the sweat glands. The skin also prevents Merkel cells) are large, specialized epithelial cells that stimulate the water within the body cells and in the extracellular fluid (the specific sensory nerve endings when they are distorted by fine fluid outside of cells) from “leaking out.” When the skin is severely touch or pressure. Because your skin is responsible for perceiving burned, a primary danger is dehydration, because the individual many stimuli, it needs different sensory receptor types to detect, has lost the protective skin barrier, and water can escape from distinguish, and interpret these stimuli. body tissues. Excretion by Means of Secretion Although the integument is water resistant, it is not entirely waterproof. Some interstitial fluids slowly escape through the Skin exhibits an excretory function when it secretes substances epidermis to the surface, where they evaporate into the surround- from the body during sweating. Sweating, or sensible perspiration, ing air, a process called transepidermal water loss (TEWL). occurs when the body needs to cool itself off. Notice that sweat Approximately 500 milliliters (mL) (approximately 1 pint) of sometimes feels “gritty” because of the waste products being water is lost daily by evaporation of moisture from the skin or secreted onto the skin surface. These substances include water, from respiratory passageways during breathing. Insensible per- salts, and urea, a nitrogen-containing waste product of body cells. spiration is the release of water vapor from sweat glands under In addition, the skin contains sebaceous glands that secrete an oily “normal” circumstances when we are not sweating. In contrast, material called sebum, which lubricates the skin surface and hair. sensible perspiration is visible sweating. On most parts of the skin, water vapor released from sweat glands during insensible WHATW DID YOU LEARN? perspiration mixes with sebaceous secretions (sebum) to produce ●1 What are the two major layers of the integument and the a thin, slightly acidic film (pH 4–6) over the surface of the epider- components of each? mis. This film helps slow down TEWL by forming an oily barrier ●2 What is the relationship between exposure to sunlight and the over the surface of the skin.