Skin Basics. About skin. Reviewed By: Kimberly Bazar, M.D., AAD Mary Ellen Luchetti, M.D., AAD

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1 Page 1 of 5 Skin Basics Reviewed By: Kimberly Bazar, M.D., AAD Mary Ellen Luchetti, M.D., AAD Summary The skin is the largest organ in the body. It consists of three layers the epidermis, the dermis and the subcutaneous tissue. Hair and nails are both modified forms of skin. The skin serves as the body s natural protective covering. It prevents germs and other potentially harmful agents from entering the body. It also shields internal organs, muscles, nerves, blood vessels and other structures from the sun s harmful rays and protects the body from injury. In addition to providing protection, the skin prevents the loss of essential fluids (e.g., blood, water), regulates body temperature and senses painful and pleasant stimuli. The skin is also a minor source of waste disposal and an indicator of health problems. Because the skin is located on the outside of the body, it is susceptible to a variety of diseases, disorders, discolorations and growths. Most skin problems, however, can be treated by a dermatologist with a minimal amount of pain and risk. The skin is also susceptible to damage from the environment and the body s natural aging process. To minimize damage, patients are advised to take steps to protect their skin, such as wearing hats and sunscreen when spending time in the sun. About skin Covering the entire outside of the body, the skin is the body s largest organ. It also grows faster than any other organ in the body. People continuously renew their skin throughout their lifetime. In a 150 pound (68 kilogram [kg]) person, the skin measures approximately 2 square yards (1.7 square meters) and weighs about 9 pounds (4 kg). The skin s average thickness is around a tenth of an inch (0.3 centimeters). The thickest skin is located on the upper back, the palms of the hands and the soles of the feet. The thinnest sections of skin are on the eyelids and the inner folds of the elbows. Hair and nails are modified versions of skin. Hair maintains warmth in the body by preserving heat. Hair located around the eyes and in the nose and ears protects these areas from small particles such as dust. It also cushions the body against injury. Hair is composed of a hair shaft, which projects from the skin s surface, and a root.

2 Page 2 of 5 Embedded in the skin, the root is a soft thickened bulb located at the base of the hair. Each root ends in a hair bulb, which sits in a tubular sac known as a hair follicle. There are more than 100,000 hair follicles located on the head alone. At the bottom of each hair follicle is a papilla, the area from which the hair sprouts. It contains an artery that nourishes the root of the hair. The hair grows in response to new cells forming at the base of the root. As the cells multiply they form a rod of tissue in the skin. The rod of cells then moves upward through the skin as new cells form under it. As the cells move up, they are cut off from their supply of nourishment. This causes the cells to die. The dead cells and keratin (a hard protein formed from the cells) form the shaft of the hair. When a hair grows and falls out, another hair takes its place. Hairs grow approximately half an inch (about 12.7 millimeters) each month, and continue to grow for about six years. The length of a hair is related to the growing phase of its follicle. Hair follicles are active for two to six years, followed by a rest period of about three months. Typically, hair grows more rapidly in the summer than in the winter. It also grows more quickly during the day than at night. The size of a person s hair follicles determines whether they have thick or thin hair. Large follicles produce thick hair, and narrow follicles generate thin hair. When these follicles become damaged or die they no longer produce hair, resulting in baldness. Hair color varies from person to person. It is determined by the amount of melanin (dark pigment) in each hair. People with light hair, such as blondes and redheads, have only a small amount of melanin in their hair while people with black or dark brown hair have a larger amount. As people age, their hair becomes grey because pigment no longer forms in the hair. Nails sprout from deep folds in the fingers and toes. They provide protection and support for the sensitive fingertips and toes. They also provide insight into a person s general health, as illness can impact the appearance and growth rate of the nails. As cells below the nail root move up to the skin s surface, the cells located closest to the nail root flatten out and become tightly pressed together. The cells transform into thin plates, which stack into layers to form the nail. As these cells accumulate, the nail is gradually pushed forward. The skin located beneath the nail is known as the matrix. Nails grow from the matrix, and if the matrix is damaged, the nail will grow ridged. The larger pink part of the nail is known as the nail plate. Its pink appearance is due to the network of blood vessels in the underlying layer of skin. The white, crescent shaped area at the base of the nail is known as the lunula. A nail that is torn will regrow, as long as its matrix is not badly injured. On average, it takes nine to 12 months for fingernails to grow and 12 to 18 months for toenails to grow. As with hair, nails typically grow more rapidly in the summer than in the winter. The skin is the body s natural protective covering. Its external location, however, makes it vulnerable to a variety of diseases, disorders, discolorations and growths. The majority of skin problems can be treated by a dermatologist with a minimal amount of pain and risk. Dermatologists are physicians that specialize in diagnosing and treating conditions of the skin, mouth, hair, nails and external genitalia. People with skin problems such as rashes, itchiness (pruritus) or growths should seek medical treatment from a dermatologist soon after the condition develops. The skin is susceptible to damage from the environment. To minimize external damage to the skin, people should take steps to protect their skin. This involves wearing hats, sunscreen, long-sleeved shirts and pants when spending time in the sun. In addition, the natural aging process of the skin is damaging because the dermis loses moisture, elasticity and thickness as individuals grow older. It is impossible to prevent all of the changes that occur to the skin as a person ages. However, older individuals can take steps to slow the pace of these changes, or to treat symptoms that result from these changes. Avoiding sun exposure, moisturizing the skin, maintaining a healthy diet and

3 Page 3 of 5 consuming adequate fluids can be very beneficial. Anatomy of the skin Each square inch of skin contains approximately 19,000,000 skin cells, as well as many other structures, such as nerve endings, hair follicles and glands. The skin consists of three layers: the epidermis, the dermis and the subcutaneous tissue. The epidermis is the thin yet tough top layer of skin. Measuring as thick as a sheet of paper, the epidermis has a number of sublayers. The bottom sublayer is made up of column-shaped cells known as basal cells. As basal cells divide, they form flat, scale-like skin cells called squamous cells. Also known as keratinocytes, squamous cells manufacture keratin, a protein which protects the skin from damaging substances (e.g., chemicals) as well as viral, bacterial and fungal infections. During cell division, basal cells push newly formed squamous cells up into the higher sublayers of the epidermis. As these new cells move upward and away from their source of nourishment, the cells shrink, flatten out and ultimately die. The top sublayer of the epidermis is known as the stratum corneum or horny layer. When undamaged, this waterproof portion of the skin helps prevent bacteria, viruses and other foreign substances from entering the body. It consists of dead squamous cells, which are mostly composed of keratin. These cells continuously flake off during friction and activities such as washing. Approximately 30,000 to 40,000 dead skin cells are shed each minute. The cells are then replaced by new cells. Due to this process, the skin is constantly being replenished. The epidermis cells are replaced about every 28 days. Although basal cells and squamous cells comprise the majority of the epidermis, other cells are also present in the layer. Melanocytes are cells that manufacture melanin, a dark brown pigment that filters out ultraviolet radiation from sunlight and provides color to the skin and the hair. The more melanin that is produced in a person s body, the darker the skin. Langerhans cells are also present in the epidermis. Part of the immune system, these cells help protect the body from infection. They also play a part in the development of skin allergies. Melanocytes and Langerhans cells are both concentrated at the basal layer of the epidermis. However, they both contain dendritic processes, long, thin, arm-like structures that interact with squamous cells. These structures enable them to transfer pigment and monitor the immune status of the skin. The epidermis also has supplemental areas that extend into the dermis and subcutaneous tissue. These areas are lined with epithelial cells, which have the potential for cell division and can become any type of epidermal tissue. The areas include the follicles, sebaceous glands and sweat glands. If the epidermis is removed or destroyed, the epithelial cells can basically reconstruct the epidermis. The thick middle layer of the skin is called the dermis. It constitutes the majority of the skin. It contains strong white fibers (collagen) and elastic yellow fibers (elastin), which intertwine to provide the skin with its strength and elasticity. A number of other structures are also located in the dermis, including: Blood vessels. These structures provide the skin with oxygen and nutrients and help regulate body temperature. They also remove waste from skin cells. Nerve endings. These structures sense pain, touch, pressure and temperature. Some areas of skin, such as the fingertips and toes, have more nerve endings than other areas. Hair follicles. Tubular sacs located throughout the skin, with the exception of the lips, palms and soles. Each living follicle produces one strand of hair. They develop in the basal cell area of the epidermis and are rooted in the subcutaneous tissue. During periods of active growth, the follicles extend into the dermis.

4 Page 4 of 5 Oil glands. These structures branch off the walls of the hair follicles. Also known as sebaceous glands, oil glands secrete an oily substance called sebum. The sebum, which rises up through the hair follicle and out through a pore, provides lubrication for the skin and hair and keeps the skin soft and moist. This lubrication is important because it provides protection against friction. It also gives the skin and hair more of a waterproof quality and acts as a barrier against foreign substances. Oil glands are located throughout the skin, but large numbers are found in the scalp, face, midchest and genitals. Overactive oil glands can cause excessively oily skin (seborrhea). Sweat glands. Sweat is a liquid composed of salt, water and chemicals. There are two types of sweat-producing glands located in the skin eccrine glands and apocrine glands. Eccrine glands are located throughout the skin, but the highest concentrations of the glands are found in the palms, soles, scalp and underarms. The glands produce sweat when a person is hot, stressed or feeling strong emotions. Sweat escapes through pores (tiny holes in the skin). When this sweat evaporates from the surface of the skin, it has a cooling effect on the body. Apocrine glands are specialized sweat glands that develop when a person reaches puberty. Located in high concentrations under the arms and around the nipples and genitals, apocrine glands secrete a thick, oily sweat when a person is under stress or experiencing strong emotions. When the sweat is digested by skin bacteria in these areas, body odor is produced. Sweat glands control nerve fibers that enable the brain to control sweating and regulate heat. This reaction is activated when a person s core body temperature reaches or rises above a certain temperature. Consisting mostly of fat, the bottom layer of the skin is called the subcutaneous tissue. It contains the skin s nerves and blood vessels. The roots of the oil glands and sweat glands are also located here. This layer is responsible for conserving the body s heat and insulating the body. It also provides protective padding for the body and acts as an energy storage area. The subcutaneous layer varies in thickness around the body. Though it may measure only a fraction of an inch on the eyelids, it can be several inches thick on the abdomen and buttocks. Function of the skin The skin has several important functions, including: Protection. The skin serves many protective functions. It prevents infectious agents, such as germs, and other harmful substances from entering the body. It also shields internal organs, muscles, nerves, blood vessels and other structures from the sun s harmful ultraviolet rays and protects the body from injury.

5 Page 5 of 5 Preventing the loss of essential fluids and substances. The skin prevents the loss of blood, water, other life-sustaining fluids, fat, and vitamin D. Regulation of body temperature. The skin contains blood vessels that expand and contract in response to a person s body temperature. Sweat is a natural response to feeling hot. The evaporation of sweat on the skin causes a reduction in body temperature. When a person is cold, the blood vessels in their skin narrow. As a result, the skin becomes pale and cold. Because the blood flow in the skin decreases, the person loses less heat. This conserves the heat for the body s vital organs. Sensation. The skin senses painful and pleasant stimuli. It contains sensory nerves that send messages to the brain about dangers, such as injury, extreme heat or extreme cold. Nerve cells also make the skin sensitive to pressure and touch. In addition, the skin contains a group of specialized nerve endings that stimulate the nerve and endocrine systems. This stimulation results in sexual excitement. Waste disposal. The skin is a minor source of waste disposal. Sweat glands located in the skin excrete waste products such as urea (a byproduct of protein metabolism) therefore eliminating them from the body. Indication of health problems. The skin s characteristics (e.g., texture, temperature, color, clarity) provide information about a person s general health and suggest a number of disorders. For instance, jaundice (a yellow color in the skin) may be a sign of liver disease or anemia (a condition characterized by too few red blood cells). Questions for your doctor regarding skin Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following skin-related questions: 1. What changes in my skin may indicate an illness? 2. How can I minimize the amount of damage to my skin? 3. Why is it important to take care of my skin? 4. Does my skin problem require a visit to a dermatologist? 5. How often should I see a dermatologist? 6. What skin conditions am I at risk for? 7. What steps can I take to ensure healthy hair? 8. What steps can I take to ensure healthy nails?