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5 Integumentary System
5.1 Introduction to the Integumentary System
In this chapter you will learn about the structure and functions of the integumentary system, along with its relationships to culture, evolution, and health. Specifically, you will learn about:
The functions of the organs of the integumentary system — the skin and hair — including protecting the body, helping to regulate homeostasis, and sensing and interacting with the external world.
The two main layers of the skin: the thinner outer layer (called the epidermis) and the thicker inner layer (called the dermis).
The cells and layers of the epidermis and their functions, including synthesizing vitamin D and protecting the body against injury, pathogens, UV light exposure, and water loss.
The composition of epidermal cells and how the epidermis grows.
The composition and layers of the dermis and their functions, including cushioning other tissues, regulating body temperature, sensing the environment, and excreting wastes.
The specialized structures in the dermis, which include sweat and sebaceous (oil) glands, hair follicles, and sensory receptors that detect touch, temperature, and pain.
The structure and biological functions of hair, which include retaining body heat, detecting sensory stimuli, and protecting the body against UV light, pathogens, and small particles.
Skin cancer — which is the most common form of cancer — and its types and risk factors.
10.2 THE INTEGUMENTARY SYSTEM
Figure 5.1 The body as a canvas.
ART FOR ALL ERAS
Pictured in Figure 5.1 is Maud Stevens Wagner, a tattoo artist from 1907. Tattoos are not just a late 20th and early 21st century trend. They have been popular in many eras and cultures. Tattoos literally illustrate the biggest organ of the human body: the skin. The skin is very thin, but it covers a large area — about 2 m2 in adults. The skin is the major organ in the integumentary system.
WHAT IS THE INTEGUMENTARY SYSTEM?
In addition to the skin, the integumentary system includes the hair and nails, which are organs that grow out of the skin. Because the organs of the integumentary system are mostly external to the body, you may think of them as little more than accessories, like clothing or jewelry, but they serve vital physiological functions. They provide a protective covering for the body, sense the environment, and help the body maintain homeostasis.
THE SKIN
The skin is remarkable not only because it is the body’s largest organ: the average square inch of skin has 20 blood vessels, 650 sweat glands, and more than 1,000 nerve endings. Incredibly, it also has 60,000 pigment-producing cells. All of these structures are packed into a stack of cells that is just 2 mm thick. Although the skin is thin, it consists of two distinct layers: the epidermis and dermis, as shown in the diagram (Figure 5.2).
Figure 5.2 The epidermis is the thinner outer layer of skin, and the dermis is the thicker inner layer of skin. The latter contains structures such as blood vessels and sweat glands.
OUTER LAYER OF SKIN
The outer layer of skin is the epidermis. This layer is thinner than the inner layer (the dermis). The epidermis consists mainly of epithelial cells, called keratinocytes, which produce the tough, fibrous protein keratin. The innermost cells of the epidermis are stem cells that divide continuously to form new cells. The newly formed cells move up through the epidermis toward the skin surface, while producing more and more keratin. The cells become filled with keratin and die by the time they reach the surface, where they form a protective, waterproof layer. As the dead cells are shed from the surface of the skin, they are replaced by other cells that move up from below. The epidermis also contains melanocytes, the cells that produce the brown pigment melanin, which gives skin most of its color. Although the epidermis contains some sensory receptor cells, it contains no nerves, blood vessels, or other structures.
INNER LAYER OF SKIN
The dermis is the inner, thicker layer of skin. It consists mainly of tough connective tissue and is attached to the epidermis by collagen fibers. The dermis contains many structures (as shown in Figure 5.2), including blood vessels, sweat glands, and hair follicles, which are structures where hairs originate. In addition, the dermis contains many sensory receptors, nerves, and oil glands.
HYPODERMIS
Below the epidermis and dermis is a fatty layer of tissue, the hypodermis. It contains blood vessels, connective tissue, and the axons of sensory neurons. The hypodermis contains approximately 50 percent of the body’s fat and insulates the body.
FUNCTIONS OF THE SKIN
The skin has multiple roles in the body. Many of these roles are related to homeostasis. The skin’s main functions are preventing water loss from the body and serving as a barrier to the entry of microorganisms. Another function of the skin is synthesizing vitamin D, which occurs when the skin is exposed to ultraviolet (UV) light. Melanin in the epidermis blocks some of the UV light and protects the dermis from its damaging effects.
Another important function of the skin is helping to regulate body temperature. When the body is too warm, for example, the skin lowers body temperature by producing sweat, which cools the body when it evaporates. The skin also increases the amount of blood flowing near the body surface through vasodilation (widening of blood vessels), bringing heat from the body core to radiate out into the environment. The sweaty hair and flushed skin of the young man pictured in Figure 5.3 reflect these skin responses to overheating.
The skin and other parts of the integumentary system work with other organ systems to maintain homeostasis.
The skin works with the immune system to defend the body from pathogens by serving as a physical barrier to microorganisms.
Vitamin D is needed by the digestive system to absorb calcium from food. By synthesizing vitamin D, the skin works with the digestive system to ensure that calcium can be absorbed.
To control body temperature, the skin works with the cardiovascular system to either lose body heat, or to conserve it through vasodilation or vasoconstriction.
To detect certain sensations from the outside world, the nervous system depends on nerve receptors in the skin.
Figure 5.3 Both sweating and flushing of the skin are signs that the skin is working to cool the body.
5.3 EPIDERMIS
Figure 5.3 Feel the burn!
FEEL THE BURN
The person in Figure 5.3 is no doubt feeling the burn — sunburn, that is. Sunburn occurs when the outer layer of the skin is damaged by UV light from the sun or tanning lamps. Some people deliberately allow UV light to burn their skin, because after the redness subsides, they are left with a tan. A tan may look healthy, but it is actually a sign of skin damage. People who experience one or more serious sunburns are significantly more likely to develop skin cancer. Natural pigment molecules in the skin help protect it from UV light damage. These pigment molecules are found in the layer of the skin called the epidermis.
WHAT IS THE EPIDERMIS?
The epidermis is the outer of the two main layers of the skin. The inner layer is the dermis. It averages about 0.10 mm thick, and is much thinner than the dermis. The epidermis is thinnest on the eyelids (0.05 mm) and thickest on the palms of the hands and soles of the feet (1.50 mm). The epidermis covers almost the entire body surface. It is continuous with — but structurally distinct from — the mucous membranes that line the mouth, anus, urethra, and vagina.
STRUCTURE OF THE EPIDERMIS
There are no blood vessels and very few nerve cells in the epidermis. Without blood to bring epidermal cells oxygen and nutrients, the cells must absorb oxygen directly from the air and obtain nutrients via diffusion of fluids from the dermis below. However, as thin as it is, the epidermis still has a complex structure. It has a variety of cell types and multiple layers.
CELLS OF THE EPIDERMIS
There are several different types of cells in the epidermis. All of the cells are necessary for the important functions of the epidermis.
The epidermis consists mainly of stacks of keratin -producing epithelial cells called keratinocytes. These cells make up at least 90 per cent of the epidermis. Near the top of the epidermis, these cells are also called squamous cells.
Another eight per cent of epidermal cells are melanocytes. These cells produce the pigment melanin that protects the dermis from UV light.
About one per cent of epidermal cells are immune cells that detect and fight pathogens entering the skin.
Less than one per cent of epidermal cells are Merkel cells, which respond to light touch and connect to nerve endings in the dermis.
LAYERS OF THE EPIDERMIS
The epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs in the palms of the hands and soles of the feet, where the epidermis is thicker than in the rest of the body. The layers of the epidermis are shown in Figure 5.4 and described in the following text.
Figure 5.4 The epidermis has multiple layers, and structures (such as hairs from the dermis below it) pass through them. This diagram illustrates the five layers that exist on the palms and soles of the feet.
Stratum Basale
The stratum basal is the innermost (or deepest) layer of the epidermis. It is separated from the dermis by a membrane called the basement membrane. The stratum basal contains stem cells — called basal cells— which divide to form all the keratinocytes of the epidermis. When keratinocytes first form, they are cube-shaped and contain almost no keratin. As more keratinocytes are produced, previously formed cells are pushed up through the stratum basal. Melanocytes and Merkel discs are also found in the stratum basal. The Merkel cells are especially numerous in touch-sensitive areas, such as the fingertips and lips.
Stratum Spinosum
Just above the stratum basal is the stratum spinosum. This is the thickest of the four epidermal layers. The keratinocytes in this layer have begun to accumulate keratin, and they have become tougher and flatter. Spiny cellular projections form between the keratinocytes and hold them together. In addition to keratinocytes, the stratum spinosum contains the immunologically active Langerhans cells.
Stratum Granulosum
The next layer above the stratum spinosum is the stratum granulosum. In this layer, keratinocytes have become nearly filled with keratin, giving their cytoplasm a granular appearance. Lipids are released by keratinocytes in this layer to form a lipid barrier in the epidermis. Cells in this layer have also started to die, because they are becoming too far removed from blood vessels in the dermis to receive nutrients. Each dying cell digests its own nucleus and organelles, leaving behind only a tough, keratin-filled shell.
Stratum Lucidum
Only on the palms of the hands and soles of the feet, the next layer above the stratum granulosum is the stratum lucidum. This is a layer consisting of stacks of translucent, dead keratinocytes that provide extra protection to the underlying layers.
Stratum Corneum
The uppermost layer of the epidermis everywhere on the body is the stratum corneum This layer is made of flat, hard, tightly packed dead keratinocytes that form a waterproof keratin barrier to protect the underlying layers of the epidermis. Dead cells from this layer are constantly shed from the surface of the body. The shed cells are continually replaced by cells moving up from lower layers of the epidermis. It takes a period of about 48 days for newly formed keratinocytes in the stratum basal to make their way to the top of the stratum corneum to replace shed cells.
FUNCTIONS OF THE EPIDERMIS
The epidermis has several crucial functions in the body. These functions include protection, water retention, and vitamin D synthesis.
PROTECTIVE FUNCTIONS
The epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.
Protection from Physical Damage
Most of the physical protection of the epidermis is provided by its tough outer layer, the stratum corneum. Because of this layer, minor scrapes and scratches generally do not cause significant damage to the skin or underlying tissues. Sharp objects and rough surfaces have difficulty penetrating or removing the tough, dead, keratin-filled cells of the stratum corneum. If cells in this layer are pierced or scraped off, they are quickly replaced by new cells moving up to the surface from lower skin layers.
Protection from Pathogens
Figure 5.5 This scrape on the knee provides an opportunity for bacteria to enter the body through the broken skin.
When pathogens such as viruses and bacteria try to enter the body, it is virtually impossible for them to enter through intact epidermal layers. Generally, pathogens can enter the skin only if the epidermis has been breached, for example by a cut, puncture, or scrape (like the one pictured in Figure 5.5). That’s why it is important to clean and cover even a minor wound in the epidermis. This helps ensure that pathogens do not use the wound to enter the body. Protection from pathogens is also provided by conditions at or near the skin surface. These include relatively high acidity (pH of about 5.0), low amounts of water, the presence of antimicrobial substances produced by epidermal cells, and competition with non-pathogenic microorganisms that normally live on the epidermis.
Protection from UV Light
UV light that penetrates the epidermis can damage epidermal cells. In particular, it can cause mutations in DNA that leads to the development of skin cancer, in which epidermal cells grow out of control. UV light can also destroy vitamin B9 (in forms such as folate or folic acid), which is needed for good health and successful reproduction. In a person with light skin, just an hour of exposure to intense sunlight can reduce the body’s vitamin B9 level by 50 per cent.
Melanocytes in the stratum basal of the epidermis contain small organelles called melanosomes which produce, store, and transport the dark brown pigment melanin. As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to keratinocytes in the epidermis, where they absorb UV light that strikes the skin. This prevents the light from penetrating deeper into the skin, where it can cause damage. The more melanin there is in the skin, the more UV light can be absorbed.
WATER RETENTION
Skin’s ability to hold water and not lose it to the surrounding environment is due mainly to the epidermis. Lipids arranged in an organized way among the cells of the stratum corneum form a barrier to water loss from the epidermis. This is critical for maintaining healthy skin and preserving proper water balance in the body.
Although the skin is impermeable to water, it is not impermeable to all substances. Instead, the skin is selectively permeable, allowing certain fat-soluble substances to pass through the epidermis. The selective permeability of the epidermis is both a benefit and a risk.
Selective permeability allows certain medications to enter the bloodstream through the capillaries in the dermis. This is the basis of medications that are delivered using topical ointments, or patches (see Figure 5.6) that are applied to the skin. These include steroid hormones, such as estrogen (for hormone replacement therapy), scopolamine (for motion sickness), nitroglycerin (for heart problems), and nicotine (for people trying to quit smoking).
Selective permeability of the epidermis also allows certain harmful substances to enter the body through the skin. Examples include the heavy metal lead, as well as many pesticides.
Figure 5.6 This skin patch delivers small amounts of nicotine through the skin of a person in a smoking cessation program.
Vitamin D Synthesis
Vitamin D is a nutrient that is needed in the human body for the absorption of calcium from food. Molecules of a lipid compound named 7-dehydrocholesterol are precursors of vitamin D. These molecules are present in the stratum basal and stratum spinosum layers of the epidermis. When UV light strikes the molecules, it changes them to vitamin D3. In the kidneys, vitamin D3 is converted to calcitriol, which is the form of vitamin D that is active in the body.
WHAT GIVES SKIN ITS COLOR?
Melanin in the epidermis is the main substance that determines the color of human skin. It explains most of the variation in skin colour in people around the world. Two other substances also contribute to skin color, however, especially in light-skinned people: carotene and hemoglobin.
The pigment carotene is present in the epidermis and gives skin a yellowish tint, especially in skin with low levels of melanin.
Hemoglobin is a red pigment found in red blood cells. It is visible through skin as a pinkish tint, mainly in skin with low levels of melanin. The pink color is most visible when capillaries in the underlying dermis dilate, allowing greater blood flow near the surface.
BACTERIA ON SKIN
Figure 5.7 The bacterium Staphylococcus epidermidis is a common microorganism living on healthy human skin.
The surface of the human skin normally provides a home to countless numbers of bacteria. Just one square inch of skin normally has an average of about 50 million bacteria. These generally harmless bacteria represent roughly one thousand bacterial species (including the one in Figure 5.7) from 19 different bacterial phyla. Typical variations in the moistness and oiliness of the skin produce a variety of rich and diverse habitats for these microorganisms. For example, the skin in the armpits is warm and moist and often hairy, whereas the skin on the forearms is smooth and dry. These two areas of the human body are as diverse to microorganisms as rainforests and deserts are to larger organisms. The density of bacterial populations on the skin depends largely on the region of the skin and its ecological characteristics. For example, oily surfaces, such as the face, may contain over 500 million bacteria per square inch. Despite the huge number of individual microorganisms living on the skin, their total volume is only about the size of a pea.
In general, the normal microorganisms living on the skin keep one another in check, and thereby play an important role in keeping the skin healthy. If the balance of microorganisms is disturbed, however, there may be an overgrowth of certain species, and this may result in an infection. For example, when a patient is prescribed antibiotics, it may kill off normal bacteria and allow an overgrowth of single-celled yeast. Even if skin is disinfected, no amount of cleaning can remove all of the microorganisms it contains. Disinfected areas are also quickly recolonized by bacteria residing in deeper areas (such as hair follicles) and in adjacent areas of the skin.
MYTH
REALITY
“Sunblock and sunscreen are just different names for the same type of product. They both work the same way and are equally effective.”
Sunscreens and sun blocks are different types of products that protect the skin from UV light in different ways. They are not equally effective. Sun blocks are opaque, so they do not let light pass through. They prevent most of the rays of UV light from penetrating to the skin surface. Sun blocks are generally stronger and more effective than sunscreens. Sun blocks also do not need to be reapplied as often as sunscreens. Sunscreens, in contrast, are transparent once they are applied the skin. Although they can prevent most UV light from penetrating the skin when first applied, the active ingredients in sunscreens tend to break down when exposed to UV light. Sunscreens, therefore, must be reapplied often to remain effective.
“The skin needs to be protected from UV light only on sunny days. When the sky is cloudy, UV light cannot penetrate to the ground and harm the skin.”
Even on cloudy days, a significant amount of UV radiation penetrates the atmosphere to strike Earth’s surface. Therefore, using sunscreens or sun blocks to protect exposed skin is important even when there are clouds in the sky.
“People who have dark skin, such as African Americans, do not need to worry about skin damage from UV light.”
No matter what color skin you have, your skin can be damaged by too much exposure to UV light. Therefore, even dark-skinned people should use sunscreens or sun blocks to protect exposed skin from UV light.
“Sunscreens with an SPF (sun protection factor) of 15 are adequate to fully protect the skin from UV light.”
Most dermatologists recommend using sunscreens with an SPF of at least 35 for adequate protection from UV light. They also recommend applying sunscreens at least 20 minutes before sun exposure and reapplying sunscreens often, especially if you are sweating or spending time in the water.
“Using tanning beds is safer than tanning outside in natural sunlight.”
The light in tanning beds is UV light, and it can do the same damage to the skin as the natural UV light in sunlight. This is evidenced by the fact that people who regularly use tanning beds have significantly higher rates of skin cancer than people who do not. It is also the reason that the use of tanning beds is prohibited in many places in people who are under the age of 18, just as youth are prohibited from using harmful substances, such as tobacco and alcohol.
Review
What is the epidermis?
Identify the types of cells in the epidermis.
Describe the layers of the epidermis.
Explain three ways the epidermis protects the body.
What makes the skin waterproof?
Why is the selective permeability of the epidermis both a benefit and a risk?
How is vitamin D synthesized in the epidermis?
Describe bacteria that normally reside on the skin, and explain why they do not usually cause infections.
Explain why the keratinocytes at the surface of the epidermis are dead, while keratinocytes located deeper in the epidermis are still alive.
Which layer of the epidermis contains keratinocytes that have begun to die?
Explore More
The science of skin colour – Angela Koine Flynn, TED-Ed, 2016.
5.4 DERMIS
Figure 5.8 Goose bumps!
GOOSE BUMPS
No doubt you’ve experienced the tiny, hair-raising skin bumps called goose bumps, like those you see in Figure 5.8. They happen when you feel chilly. Do you know what causes goose bumps, or why they pop up when you are cold? The answers to these questions involve the layer of skin known as the dermis.
WHAT IS THE DERMIS?
The dermis is the inner of the two major layers that make up the skin, the outer layer being the epidermis The dermis consists mainly of connective tissue. It also contains most skin structures, such as glands and blood vessels. The dermis is anchored to the tissues below it by flexible collagen bundles that permit most areas of the skin to move freely over subcutaneous (“below the skin”) tissues. Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes.
ANATOMY OF THE DERMIS
The basic anatomy of the dermis is a matrix, or sort of scaffolding, composed of connective tissues. These tissues include collagen fibres — which provide toughness — and elastin fibres, which provide elasticity. Surrounding these fibres, the matrix also includes a gel-like substance made of proteins. The tissues of the matrix give the dermis both strength and flexibility.
Figure 5.9 This photomicrograph shows a cross-section of the dermis.
STRUCTURES IN THE DERMIS
The dermis contains numerous sensory receptors which make the skin the body’s primary sensory organ for the sense of touch. The dermis also contain blood vessels. They provide nutrients to remove wastes from dermal cells, as well as cells in the lowest layer of the epidermis. The circulatory components of the dermis are shown in Figure 5.10 below.
Figure 5.10 Both the papillary layer and the reticular layer of the dermis contain blood vessels, as shown in this diagram.
GLANDS
Glands of the dermis include sweat glands and sebaceous (oil) glands. Both are exocrine glands, which are glands that release their secretions through ducts to nearby body surfaces. The diagram in Figure 5.11 shows these glands, as well as several other structures in the dermis.
Figure 5.11 The dermis contains sweat and oil (sebaceous) glands, as well as hair follicles and blood vessels.
Sweat Glands
Sweat glands produce the fluid called sweat, which contains mainly water and salts. The glands have ducts that carry the sweat to hair follicles, or to the surface of the skin. There are two different types of sweat glands: eccrine glands and apocrine glands.
Eccrine sweat glands occur in skin all over the body. Their ducts empty through tiny openings called pores onto the skin surface. These sweat glands are involved in temperature regulation.
Apocrine sweat glands are larger than eccrine glands, and occur only in the skin of the armpits and groin. The ducts of apocrine glands empty into hair follicles, and then the sweat travels along hairs to reach the surface. Apocrine glands are inactive until puberty, at which point they start producing an oily sweat that is consumed by bacteria living on the skin. The digestion of apocrine sweat by bacteria causes body odor.
Sebaceous Glands
Sebaceous glands are exocrine glands that produce a thick, fatty substance called sebum. Sebum is secreted into hair follicles and makes its way to the skin surface along hairs. It waterproofs the hair and skin and helps prevent them from drying out. Sebum also has antibacterial properties, so it inhibits the growth of microorganisms on the skin. Sebaceous glands are found in every part of the skin — except for the palms of the hands and soles of the feet, where hair does not grow.
HAIR FOLLICLES
Hair follicles are the structures where hairs originate (see the diagram above). Hairs grow out of follicles, pass through the epidermis, and exit at the surface of the skin. Associated with each hair follicle is a sebaceous gland, which secretes sebum that coats and waterproofs the hair. Each follicle also has a bed of capillaries, a nerve ending, and a tiny muscle called an arrector pili.
FUNCTIONS OF THE DERMIS
The main functions of the dermis are regulating body temperature, enabling the sense of touch, and eliminating wastes from the body.
TEMPERATURE REGULATION
Several structures of the dermis are involved in regulating body temperature. For example, when body temperature rises, the hypothalamus of the brain sends nerve signals to sweat glands, causing them to release sweat. An adult can sweat up to four liters an hour. As the sweat evaporates from the surface of the body, it uses energy in the form of body heat, thus cooling the body. The hypothalamus also causes dilation of blood vessels in the dermis when body temperature rises. This allows more blood to flow through the skin, bringing body heat to the surface, where it can radiate into the environment.
When the body is too cool, sweat glands stop producing sweat, and blood vessels in the skin constrict, thus conserving body heat. The arrector pili muscles also contract, moving hair follicles and lifting hair shafts. This results in more air being trapped under the hairs to insulate the surface of the skin. These contractions of arrector pili muscles are the cause of goose bumps.
SENSING THE ENVIRONMENT
Sensory receptors in the dermis are mainly responsible for the body’s tactile senses. The receptors detect such tactile stimuli as warm or cold temperature, shape, texture, pressure, vibration, and pain. They send nerve impulses to the brain, which interprets and responds to the sensory information. Sensory receptors in the dermis can be classified on the basis of the type of touch stimulus they sense. Mechanoreceptors sense mechanical forces such as pressure, roughness, vibration, and stretching Thermoreceptors sense variations in temperature that are above or below body temperature. Nociceptors sense painful stimuli. Figure 5.12 shows several specific kinds of tactile receptors in the dermis. Each kind of receptor senses one or more types of touch stimuli.
Free nerve endings sense pain and temperature variations.
Merkel cells sense light touch, shapes, and textures.
Meissner’s corpuscles sense light touch.
Pacinian corpuscles sense pressure and vibration.
Ruffini corpuscles sense stretching and sustained pressure.
Figure 5.12 A variety of types of tactile receptors are located in the dermis of the skin.
EXCRETING WASTES
The sweat released by eccrine sweat glands is one way the body excretes waste products. Sweat contains excess water, salts (electrolytes), and other waste products that the body must get rid of to maintain homeostasis. The most common electrolytes in sweat are sodium and chloride. Potassium, calcium, and magnesium electrolytes may be excreted in sweat, as well. When these electrolytes reach high levels in the blood, more are excreted in sweat. This helps to bring their blood levels back into balance. Besides electrolytes, sweat contains small amounts of waste products from metabolism, including ammonia and urea. Sweat may also contain alcohol in someone who has been drinking alcoholic beverages.
FEATURE: MY HUMAN BODY
Figure 5.13 Acne can be embarrassing, but most people will experience it at one point in their lives.Acneis the most common skin disorder in Canada. At least 20% of Canadians have acne at any given time and it affects approximately 90% of adolescents (as in Figure 5.13). Although acne occurs most commonly in teens and young adults, but it can occur at any age. Even newborn babies can get acne.
The main sign of acne is the appearance of pimples (pustules) on the skin, like those in the photo above. Other signs of acne may include whiteheads, blackheads, nodules, and other lesions. Besides the face, acne can appear on the back, chest, neck, shoulders, upper arms, and buttocks. Acne can permanently scar the skin, especially if it isn’t treated appropriately. Besides its physical effects on the skin, acne can also lead to low self-esteem and depression.
Acne is caused by clogged, sebum-filled pores that provide a perfect environment for the growth of bacteria. The bacteria cause infection, and the immune system responds with inflammation. Inflammation, in turn, causes swelling and redness, and may be associated with the formation of pus. If the inflammation goes deep into the skin, it may form an acne nodule.
Mild acne often responds well to treatment with over-the-counter (OTC) products containing benzoyl peroxide or salicylic acid. Treatment with these products may take a month or two to clear up the acne. Once the skin clears, treatment generally needs to continue for some time to prevent future breakouts.
If acne fails to respond to OTC products, nodules develop, or acne is affecting self-esteem, a visit to a dermatologist is in order. A dermatologist can determine which treatment is best for a given patient. A dermatologist can also prescribe prescription medications (which are likely to be more effective than OTC products) and provide other medical treatments, such as laser light therapies or chemical peels.
What can you do to maintain healthy skin and prevent or reduce acne? Dermatologists recommend the following tips:
Wash affected or acne-prone skin (such as the face) twice a day, and after sweating.
Use your fingertips to apply a gentle, non-abrasive cleanser. Avoid scrubbing, which can make acne worse.
Use only alcohol-free products and avoid any products that irritate the skin, such as harsh astringents or exfoliants.
Rinse with lukewarm water and avoid using very hot or cold water.
Shampoo your hair regularly.
Do not pick, pop, or squeeze acne. If you do, it will take longer to heal and is more likely to scar.
Keep your hands off your face. Avoid touching your skin throughout the day.
Stay out of the sun and tanning beds. Some acne medications make your skin very sensitive to UV light.
Review
What is the dermis?
Describe the basic anatomy of the dermis.
Name the two types of sweat glands in the dermis and explain how they differ.
What is the function of sebaceous glands?
Describe the structures associated with hair follicles.
Explain how the dermis helps regulate body temperature.
Identify three specific kinds of tactile receptors in the dermis, along with the type of stimuli they sense.
How does the dermis excrete wastes? What waste products does it excrete?
What are subcutaneous tissues? Which layer of the dermis provides cushioning for subcutaneous tissues? Why does this layer provide most of the cushioning, instead of the other layer?
For each of the functions listed below, describe which structure within the dermis carries it out.
Brings nutrients to and removes wastes from dermal and lower epidermal cells
Causes hairs to move
Detects painful stimuli on the skin
5.5 HAIR
WHAT IS HAIR?
Hair is a filament that grows from a hair follicle in the dermis of the skin. It consists mainly of tightly packed, keratin-filled cells called keratinocytes. The human body is covered with hair follicles, with the exception of a few areas, including the mucous membranes, lips, palms of the hands, and soles of the feet.
STRUCTURE OF HAIR
The part of the hair located within the follicle is called the hair root. The root is the only living part of the hair. The part of the hair that is visible above the surface of the skin is the hair shaft. The shaft of the hair has no biochemical activity and is considered dead.
FOLLICLE AND ROOT
Hair growth begins inside a follicle (see Figure 5.14 below). Each hair follicle contains stem cells that can keep dividing, which allows hair to grow. The stem cells can also regrow a new hair after one falls out. Another structure associated with a hair follicle is a sebaceous gland that produces oily sebum. The sebum lubricates and helps to waterproof the hair. A tiny arrector pili muscle is also attached to the follicle. When it contracts, the follicle moves, and the hair in the follicle stands up.
Figure 5.14 A hair follicle has a sebaceous gland and an arrector pili muscle.
SHAFT
The hair shaft is a hard filament that may grow very long. Hair normally grows in length by about half an inch a month. In cross-section, a hair shaft can be divided into three zones, called the cuticle, cortex, and medulla.
The cuticle (or outer coat) is the outermost zone of the hair shaft. It consists of several layers of flat, thin keratinocytes that overlap one another like shingles on a roof. This arrangement helps the cuticle repel water. The cuticle is also covered with a layer of lipids, just one molecule thick, which increases its ability to repel water. This is the zone of the hair shaft that is visible to the eye.
The cortex is the middle zone of the hair shaft, and it is also the widest part. The cortex is highly structured and organized, consisting of keratin bundles in rod-like structures. These structures give hair its mechanical strength. The cortex also contains melanin, which gives hair its color.
The medulla is the innermost zone of the hair shaft. This is a small, disorganized, and more open area at the center of the hair shaft. The medulla is not always present. When it is present, it contains highly pigmented cells full of keratin.
FUNCTIONS OF HAIR
In humans, one function of head hair is to provide insulation and help the head retain heat. Head hair also protects the skin on the head from damage by UV light.
The function of hair in other locations on the body is debated. One idea is that body hair helps keep us warm in cold weather. When the body is too cold, arrector pili muscles contract and cause hairs to stand up (shown in Figure 5.15), trapping a layer of warm air above the epidermis. However, this is more effective in mammals that have thick hair or fur than it is in relatively hairless human beings.
Figure 5.15 Arrector pili muscles will make hairs stand erect, more commonly recognized as goose bumps. (1) Epidermis (2) Arrector pili muscle (3) Hair follicle (4) Dermis
Human hair has an important sensory function, as well. Sensory receptors in the hair follicles can sense when the hair moves, whether it moves because of a breeze, or because of the touch of a physical object. The receptors may also provide sensory awareness of the presence of parasites on the skin.
Figure 5.16 This young child is using her eyebrows to good effect to convey her displeasure.
Some hairs, such as the eyelashes are especially sensitive to the presence of potentially harmful matter. The eyelashes grow at the edge of the eyelid and can sense when dirt, dust, or another potentially harmful object is too close to the eye. The eye reflexively closes as a result of this sensation. The eyebrows also provide some protection to the eyes. They protect the eyes from dirt, sweat, and rain. In addition, the eyebrows play a key role in nonverbal communication (see Figure 5.16). They help express emotions such as sadness, anger, surprise, and excitement.
Review
Compare and contrast the hair root and hair shaft.
Describe hair follicles.
Why do some people go bald? – Sarthak Sinha, TED-Ed, 2015.
5.6 SKIN CANCER
Figure 5.17 I hope they’re wearing sunscreen!
BATHING IN SUNSHINE
Summer sun may feel good on your body, but its invisible UV rays wreak havoc on your skin. Exposing the skin to UV light causes photo-aging: premature wrinkling, brown discolorations, and other unattractive signs of sun exposure. Even worse, UV light increases your risk of skin cancer.
WHAT IS SKIN CANCER?
Skin cancer is a disease in which skin cells grow out of control. It is caused mainly by excessive exposure to UV light, which damages DNA Therefore, skin cancer most often develops on areas of the skin that are frequently exposed to UV light. However, it can also occur on areas that are rarely exposed to UV light. Skin cancer affects people of all skin colours, including those with dark skin. It also affects more people altogether than all other cancers combined. One in five Canadians develops skin cancer in his or her lifetime.
TYPES OF SKIN CANCER
Skin cancer begins in the outer layer of skin, the epidermis. There are three common types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma.
BASAL CELL CARCINOMA
Figure 5.18 Basal cell carcinoma
Basal cell carcinoma occurs in basal cells of the epidermis. Basal cells are stem cells in the stratum basal layer that divide to form all the keratinocytes of the epidermis. Basal cell carcinoma is the most common form of skin cancer and 1 in 8 Canadians will develop basal cell carcinoma during their lifetime. A basal cell carcinoma may appear as a pearly or waxy sore, like the one shown in Figure 5.18. Basal cell carcinomas rarely spread (or undergo metastasis) so they can generally be cured with a biopsy, in which the lesion is cut out of the skin and analyzed in a medical lab.
Squamous Cell Carcinoma
Figure 5.19 Squamous cell carcinoma
Squamous cell carcinoma occurs in squamous cells of the epidermis. Squamous cells are flattened, keratin-filled cells in upper layers of the epidermis. Squamous cell carcinoma is the second most common form of skin cancer. More than two million cases occur in the United States each year. A squamous cell carcinoma may appear as a firm, red nodule, or as a flat lesion with a scaly or crusty surface, like the one pictured in Figure 5.19. Squamous cell carcinomas are generally localized and unlikely to metastasize, so they are usually curable surgically.
Melanoma
Figure 5.20 Melanoma
Melanoma occurs in melanocytes of the epidermis. Melanocytes are the melanin-producing cells in the stratum basal of the epidermis. Melanoma is the rarest type of skin cancer, accounting for less than one per cent of all skin cancer cases. Melanoma, however, is the deadliest type of skin cancer. It causes the vast majority of skin cancer deaths, because melanoma is malignant. If not treated, it will metastasize and spread to other parts of the body. If melanoma is detected early and while it is still localized in the skin, most patients survive for at least five years. If melanoma is discovered only after it has already metastasized to distant organs, there is only a 17% of patients surviving for five years. You can see an example of a melanoma in Figure 5.20.
Melanoma can develop anywhere on the body. It may develop in otherwise normal skin, or an existing mole may become cancerous. Signs of melanoma may include a:
Mole that changes in size, feel, or colour.
Mole that bleeds.
Large brown spot on the skin sprinkled with darker specks.
Small lesion with an irregular border and parts that appear red, white, blue, or blue-black.
Dark lesion on the palms, soles, fingertips, toes, or mucous membranes.
SKIN CANCER RISK FACTORS
Exposure to UV radiation causes about 90 per cent of all skin cancer cases. The connection between skin cancer and UV light is so strong that the World Health Organization has classified UV radiation (whether from tanning beds or the sun) as a Group 1 carcinogen (cancer-causing agent). Group 1 carcinogens are those carcinogens that are known with virtual certainty to cause cancer. In addition to UV light, Group 1 carcinogens include tobacco and plutonium. In terms of numbers of cancers caused, UV radiation is far worse than tobacco. More people develop skin cancer because of UV light exposure than develop lung cancer because of smoking. The increase in cancer risk due to UV light is especially great if you have ever had blistering sunburns as a child or teen.
Besides UV light exposure, other risk factors for skin cancer include:
Having light coloured skin.
Having a lot of moles.
Being diagnosed with precancerous skin lesions.
Having a family history of skin cancer.
Having a personal history of skin cancer.
Having a weakened immune system.
Being exposed to other forms of radiation or to certain toxic substances such as arsenic.
FEATURE: MY HUMAN BODY
As with most types of cancer, skin cancer is easiest to treat and most likely to be cured the earlier it is detected. The skin is one of the few organs that you can monitor for cancer yourself, as long as you know what to look for. A brown spot on the skin is likely to be a harmless mole, but it could be a sign of skin cancer. As shown in Figure 5.21 below, unlike moles, skin cancers may be asymmetrical, have irregular borders, may be very dark in color, and may have a relatively great diameter. These characteristics can be remembered with the acronym ABCD.
Figure 5.21 ABCDs of skin cancer
With the help of mirrors, you should check all of your skin regularly. Look for new skin growths or changes in any existing moles, freckles, bumps, or birthmarks. Report anything suspicious or different to your doctor.
If you have risk factors for skin cancer, it’s a good idea to have an annual skin check by a dermatologist. This helps ensure that cancerous or precancerous lesions will be detected before they grow too large and become difficult to cure, or in the case of melanoma, before they metastasize.
Review
What is skin cancer?
How common is skin cancer?
Compare and contrast the three common types of skin cancer.
Identify factors that increase the risk of skin cancer.
How does exposure to UV light cause skin cancer?
In which layer of the skin does skin cancer normally start?
Which two skin cancers described in this section start in the same sub-layer? Include the name of the sub-layer and the cells affected in each of these cancers.
Which type of skin cancer is most likely to spread to other organs? Explain your answer.
Which form of skin cancer is the deadliest?
5.7 CASE STUDY: WEARING HIS HEART ON HIS SLEEVE
Figure 5.22 Tattoos can last forever.
Aiko, 22, and Larissa, 23, met through mutual friends and hit it off right away. They began dating and just four months later, they are now madly in love. They spend as much time as they can with each other, and have decided to move in together when Larissa’s roommate moves out. They are even discussing getting married one day.
Inspired by his passion for Larissa, Aiko is considering getting her name tattooed on his arm. As you probably know, tattoos are designs on the skin created by injecting pigments into the skin with a needle. Aiko looks up different tattoo styles online, and starts to envision what he would want in a tattoo.
One day at a street festival, Aiko sees a sign that says “Henna Tattoos.” Henna tattoos are not technically tattoos — they are temporary designs that artists can create on the skin using a paste made out of the leaves of the henna plant. The henna stains the skin a reddish-brown colour, and once the paste is scraped off, the design typically remains on the skin for a few weeks. The use of henna to create designs on the skin is called mehndi. It is traditionally used by people in and from regions such as India, Pakistan, the Middle East, and Africa to celebrate special occasions, particularly weddings. Mendhi is often done on the palms of the hands and soles of the feet, where the designs usually come out darker than on other areas of the skin. You can see some examples of henna art in the images below.
Figure 15.23 Example of henna art.
Aiko asks the mehndi artist to inscribe Larissa’s name on his arm, so that he can see whether he likes it without making the permanent commitment of a real tattoo. Two days later, Aiko visits his parents. They are not familiar with mehndi, and they have a moment of panic when they think he got a real tattoo. Aiko reassures them that it is temporary but tells them that he is thinking about getting a real tattoo.
His parents are concerned. His father points out that he has not known Larissa long — what if they break up and he regrets the tattoo? His mother additionally worries about whether tattoos are safe. Aiko says that he doesn’t think he will regret the decision, but if he does, he can cover it up with another tattoo or get it removed with laser treatments. He also tells them that he would go to an artist and shop that are reputable and take appropriate safety precautions. His parents warn him that getting a tattoo removed may not be as simple as he thinks, and that he should think very carefully before making such a permanent decision.
Humans have long decorated and adorned their skin with tattoos, makeup, and piercings. They also color, cut, straighten, curl, and remove their hair; and paint, grow, and cut their nails. The skin, hair, and nails make up the integumentary system.
Figure 5.24 Let’s hope this couple lasts as long as this tattoo.
Are you still wondering whether Ayk, actually got a tattoo of his new girlfriend’s name on his arm? Figure 5.24 is your answer! Let’s hope his love for Larissa — and for the artwork — lasts as long as his tattoo. According to a poll conducted for Global TV by Ipsos Reid in 2012, 10% of Canadian and 11% of American adults regret getting a tattoo. Although laser tattoo removal is available, it does not always work fully, can cause pain and scarring, and is expensive and time-consuming. Some people who regret a tattoo opt instead (or additionally) to cover it with another tattoo.
Why are tattoos essentially permanent? Tattoos are created by inserting a needle containing pigment through the epidermis and into the dermis of the skin. The pigment is injected into the dermal layer, creating the design. The pigment can remain in the dermal layer for a person’s lifetime for a few reasons. One, unlike the thinner outer epidermal layer, the dermis is not continually shed and replaced, so the pigment generally stays put. Two, the pigments used in tattooing mainly consist of large particles. When you get a tattoo, the penetration of the skin and insertion of foreign particles causes an immune response in which white blood cells attempt to engulf and remove the pigment. Because most of the pigment particles are so large, however, they cannot be removed from the dermis by the immune cells, and the design remains.
In laser tattoo removal, pulses from a high-intensity laser are applied to the tattoo and absorbed by the pigments. This breaks up the large pigment particles into particles that are small enough to be removed by the immune system. The pigments may then be excreted out of the body, or moved to other areas of the body, such as the lymph nodes. Different wavelengths of laser energy are often required to remove different colors of pigments, because they absorb different wavelengths of light. Generally, blue and black are the easiest colors to remove. Green, red, and yellow tend to be the hardest to remove. It may take as many as six to ten laser treatments — with a few weeks of recovery time in between — to remove a tattoo. Some tattoos can never be completely removed.
Why are mehndi designs (like Ayko’s trial “henna tattoo”) not permanent? Unlike real tattoos, henna paste is applied on the surface of the skin (shown below in Figure 5.25), and not injected into the skin with a needle. The dye molecules simply migrate from the paste into the top layer of the epidermis, the stratum corneum.
Figure 5.25 Henna paste being applied to create a mehndi design.
As you have learned, the stratum corneum consists of dead, keratin-filled keratinocytes, which are continually shed and replaced with new cells from the layers below. As a result, mehndi is not permanent. The design is lost as the cells that contain the dye are shed and replaced.
As you read in the beginning of this chapter, mehndi is often applied to the palms of the hands and soles of the feet, which generally results in a darker stain than other areas of the body. This is because the stratum corneum is thicker in these regions, so the dye penetrates through more layers of cells, making the design appear darker. What else is different about the epidermis of the palms and soles? You may recall that these regions are the only place where there is a fifth layer of epidermis — the stratum lucidum — making the skin in these areas even thicker and tougher.
Hopefully, Ayko thought carefully about the potential emotional and social implications of getting a tattoo — and learned how difficult they are to remove — before getting a real one. Health and safety should also be of utmost concern to anyone considering getting a tattoo. As you have learned in this chapter, the skin acts as a barrier against dangerous pathogens and substances. When you penetrate the skin using a needle, it can introduce harmful viruses and bacteria directly into the dermis, where the blood vessels are. Tattoo artists and shops need to take precautions to protect their clients against diseases that can be transmitted through blood (such as HIV and hepatitis), as well as bacterial infections. The tattoo artist should wear disposable gloves and a mask, use new and unopened needles and ink tubes, and properly sterilize other equipment. Even if the artist takes all the proper precautions, there is still a chance that the unopened ink could have been contaminated with pathogens during the production process. The shop should be aware of any ink recalls. Anyone getting a tattoo should make sure their artist and shop strictly adhere to all local health and safety regulations.
The risk of disease is not the only risk from tattoos. The pigments in tattoos may contain heavy metals and other potentially toxic substances. Tattoo parlors are regulated by provincial guidelines in Canada, and these guidelines vary from province to province — but these guidelines are mainly concerned with sterilization of equipment and don’t address anything about pigments. A recent study published in the scientific journal Nature (Scientific Reports) showed that pigments from tattoos may migrate from a person’s tattoos into their lymph nodes. Among the substances that make up the tattoo ink that migrated were aluminum, chromium, iron, nickel and copper – all considered “toxic”.
Additionally, people can sometimes have an allergic reaction to the pigments or develop scarring or granulomas (small bumps of tissue due to an immune response) around the tattoo. Rarely, people can experience temporary swelling or burning of their tattoos when they get scanned in an MRI machine for a medical procedure. Clearly, people should think carefully about the potential health implications before getting a tattoo.
Fortunately, Ayko found a reputable and safe tattoo artist, and is not experiencing any ill effects from his tattoo. He is happy with his tattoo, at least for now. Tattoos — and other kinds of decoration of the integumentary system — are forms of artistic, personal, and cultural expression that have been used by many cultures over the course of human history. The system that protects us from the elements, helps us maintain homeostasis, and mediates our interactions with the outside world also happens to be easily modifiable! Whether it is a haircut, makeup, beard style, nail polish, piercing or a tattoo, humans have a variety of ways of altering our integumentary system, which changes our outward appearance and what we communicate to others.
Attributions
This chapter is composed of text taken from of the following sources:
