What do you think of when you look at your skin in the mirror? Do you think about cleaning and caring for it, checking for damage from the sun, or maybe adding a tattoo? Or do you think about the fact that the skin is an organ belonging to one of the body’s most essential and dynamic systems, the integumentary system?  The skin makes up about 16% of the body weight of an adult and covers an area of 1.5 to 2 m², making it the largest organ system in the human body.

The integumentary system refers to the skin, as well as accessory structures, such as hair, nails, and glands. The skin is responsible for much more than your outward appearance, beginning with acting as a protective covering between your internal and external environments.  This chapter will introduce the structure and many other functions of the integumentary system.

Part 1: Layers of the Skin

Although you may not typically think of the skin as an organ, it is in fact composed of different types of tissues that work together to perform unique and critical functions. The skin is composed of two main layers: an outer epidermis and a deeper dermis (Figure 9.1). The deeper layer of skin is well vascularized (i.e., has numerous blood vessels). It also has numerous sensory and nerve fibers, ensuring communication to and from the brain.  The skin is ultimately affixed to deeper body structures, such as muscle, by connective tissue in a subcutaneous layer known as the hypodermis.

 

Epidermis

The epidermis (Figures 9.2 & 9.3) is composed of keratinized, stratified squamous epithelium. Like all epithelial tissue, it is avascular. The dark-staining epithelial cells of the epidermis are arranged in multiple strata with the outermost cells appearing the most squamous in shape. The majority of skin areas is classified as thin skin, meaning that it has four visible layers (i.e., strata) of cells (Figure 9.2). From deepest to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. Thick skin, however, is found exclusively on the palms of the hands and the soles of the feet. This type of epidermis has an additional fifth layer, called the stratum lucidum, located between the stratum corneum and the stratum granulosum (Figures 9.3 & 9.4).

 

 

Regardless of whether skin is thick or thin, the dominant cells found in the epidermis are called keratinocytes. A keratinocyte is a cell that manufactures and stores the protein keratin. Keratin is an intracellular fibrous protein that confers hardness and water-resistance to skin, hair, and nails. Because the skin covers so much surface area, the presence of keratin helps prevent excessive water loss across the skin surface. The maturation of keratinocytes proceeds from the innermost toward the outermost layers. By the point the keratinocytes reach the stratum corneum, they have died and regularly slough away but will be replaced continuously  by cells originating from the deeper layers (Figure 9.4).

1. Stratum Basale: The stratum basale (also called the stratum germinativum), is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis. The cells in the stratum basale bond to the dermis via intertwining collagen fibers that make up the basement membrane.

A finger-like projection, or fold, known as the dermal papilla (plural = dermal papillae) is found in the superficial portion of the dermis. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connection made (Figure 9.6). In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer (papillary layer), resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Why these ridges form in the pattern that you see is a result of genetics and the fetal environment. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes.

The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through growth (mitosis) to produce new cells.

As new cells are formed, the existing cells are pushed superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin its color and also helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage.

The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum.

3. Stratum Granulosum: The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum. The cells (three to five layers deep) become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin, which is fibrous, and keratohyalin, which accumulates as granules within the cells (Figure 9.4). These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of hair and nails.

4. Stratum Lucidum: The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened (Figure 9.4). These cells are densely packed with eleiden, a clear substance derived from keratohyalin that gives these cells their transparent (i.e., lucid) appearance and provides a barrier to water.

5. Stratum Corneum: The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment (Figure 9.4). The increased keratinization (also called cornification) of the cells in this layer gives it its name. There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues and provides a mechanical protection against abrasion for the more delicate, underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum (or stratum lucidum in the case of the palms and soles of feet). The entire layer is replaced during a period of about four weeks. Cosmetic procedures, such as microdermabrasion, help remove some of the dry, upper layer and aim to keep the skin looking “fresh” and healthy.

Dermis

The dermis might be considered the “core” of the integumentary system (derma- = “skin”), as distinct from the epidermis (epi- = “upon” or “over”) and hypodermis (hypo- = “below”). It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts (Figure 9.5).

1. Papillary Layer: The papillary layer is made of loose, areolar connective tissue, which means the collagen and elastin fibers of this layer form a loose mesh. This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like dermal papillae (Figure 9.5). Within the papillary layer are fibroblasts, a small number of fat cells (adipocytes), and an abundance of small blood vessels.

In addition, the papillary layer contains phagocytes, defensive cells that help fight bacteria or other infections that have breached the skin. This layer also contains lymphatic capillaries, nerve fibers, and touch receptors called the tactile (Meissner) corpuscles (Figure 9.11).

2. Reticular Layer: Underlying the papillary layer is the much thicker reticular layer, composed of dense, irregular connective tissue. This layer is well vascularized and has a rich sensory and nerve supply. The reticular layer appears reticulated (net-like) due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. Collagen fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis. In addition, collagen binds water to keep the skin hydrated.

 

Hypodermis

The hypodermis (also called the subcutaneous layer or superficial fascia) is a layer directly below the dermis and serves to connect the skin to the underlying fascia (fibrous tissue) of the bones and muscles. It is not considered a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.

Part 2: Accessory Structures of the Skin

Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. These structures embryologically originate from the epidermis and can extend down through the dermis into the hypodermis.

Hair

Hair is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the hair follicle (Figures 9.7 & 9.8).

 

The hair shaft is the part of the hair not anchored to the follicle, and much of this is exposed at the skin’s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the hair root. The hair root ends deep in the dermis at the hair bulb (which is the deep end of the hair follicle). The root includes a layer of actively growing basal cells called the hair matrix. The hair bulb surrounds the hair papilla, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis (Figure 9.7).

Just as the basal layer of the epidermis forms the layers of epidermis that get pushed to the surface as the dead skin on the surface sheds, the basal cells of the hair bulb divide and push cells outward in the hair root and shaft. The external hair is completely dead and composed entirely of keratin. For this reason, hair does not have sensation. Furthermore, you can cut your hair or shave without damaging the hair structure because the cut is superficial. Most chemical hair removers also act superficially; however, electrolysis and plucking both attempt to destroy the hair bulb so hair cannot grow.

 

Hair serves a variety of functions in the human body, including protection, sensory input, and communication. For example, hair on the head protects the skull from the sun. The hair in the nose and ears and around the eyes (eyelashes) defends the body by trapping and excluding dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat and other particles from dripping into and bothering 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, much 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 connected to a smooth muscle called the arrector pili that contracts in response to nerve signals from the nervous system, making the external hair shaft “stand up” (Figure 9.7).  The primary purpose for this would be to trap a layer of air to add insulation, but in humans, the hairs are too far away from each other to be effective as insulation. The erection of the hair shafts in response to low body temperature nevertheless persists in humans and is visible as “goose bumps.”

 

Hair grows and is eventually shed and replaced by new hair. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced and can happen due to hormonal or dietary changes. Hair loss can also result from the aging process, or the influence of hormones.

Nails

The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes. The nail body is formed on the nail bed and protects the tips of our fingers and toes, as they are the farthest extremities and the parts of the body that experience the maximum mechanical stress (Figure 9.9). In addition, the nail body forms a back support for picking up small objects with the fingers. The nail body is composed of densely packed dead keratinocytes. The nail body forms at the nail root, which has a matrix of proliferating cells from the stratum basale that enables the nail to grow continuously.

 

Sweat Glands

When the body becomes warm, sudoriferous glands produce sweat to cool the body. Sweat glands develop from epidermal projections into the dermis and are classified as merocrine glands; that is, the secretions are excreted by exocytosis through a duct without affecting the cells of the gland. There are two main types of sweat glands, each secreting slightly different products.

An eccrine sweat gland is a type of gland that produces a hypotonic (relative to blood plasma) sweat for thermoregulation. 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 (Figure 9.10).

 

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, released by exocytosis, is composed mostly of water, with some salt, antibodies, traces of metabolic waste, and dermicidin, an antimicrobial peptide. Eccrine glands are a primary component of thermoregulation in humans and thus help to maintain homeostasis.

An apocrine sweat gland is usually associated with hair follicles in densely hairy areas, such as the armpits and anogenital regions. Apocrine sweat glands are larger than eccrine sweat glands and lie deeper in the dermis, sometimes even reaching the hypodermis, with the duct normally emptying into the hair follicle. In addition to water and salts, apocrine sweat includes organic compounds that make the sweat thicker and subject to bacterial decomposition and subsequent smell. The release of this sweat is under both nervous and hormonal control and plays a role in the poorly understood 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 precipitate due to a change in pH and form a physical block in the duct, which prevents sweat from coming out of the pore.

There are several different types of modified apocrine glands that have become specialized to serve particular functions. One example is the mammary gland, which allows mammals to produce and secrete milk (a mixture of water, salt, and organic compounds) in appropriate concentrations to nourish growing offspring. Another example is the ceruminous gland, which is found in the external auditory meatus (ear canal) and secretes a pigmented mixture of lipids and proteins that combines with the sebum secreted from sebaceous glands (see below) and dead keratinocytes to form cerumen (earwax). This sticky substance is used to trap small foreign bodies (e.g., dirt, small insects) and help prevent damage to the tympanic membrane (eardrum).

Sebaceous Glands

A sebaceous gland is a type of oil gland that is found all over the body and helps to lubricate and waterproof the skin and hair. Most sebaceous glands are associated with hair follicles (Figures 9.7 & 9.8). They generate and excrete sebum, a mixture of lipids, onto the skin surface, thereby naturally lubricating the dry and dead layer of keratinized cells of the stratum corneum, keeping it pliable. The fatty acids of sebum also have antibacterial properties and prevent water loss from the skin in low-humidity environments. The secretion of sebum is stimulated by hormones, many of which do not become active until puberty. Thus, sebaceous glands are relatively inactive during childhood.

 

Part 3: Functions of the Integumentary System

The skin and accessory structures perform 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. The underlying hypodermis has important roles in storing fats, forming a “cushion” over underlying structures and providing insulation from cold temperatures.

Protection: The skin protects the rest of the body from the basic elements of nature such as wind, water, and UV sunlight. It acts as a protective barrier against water loss, due to the presence of layers of keratin and glycolipids in the stratum corneum. It also is the first line of defense against abrasive activity due to contact with grit, microbes, or harmful chemicals. Sweat excreted from sweat glands deters microbes from over-colonizing the skin surface by generating dermicidin, which has antibiotic properties.

Sensation: The fact that you can feel an ant crawling on your skin, allowing you to flick it off before it bites, is because the skin, and especially the hairs projecting from hair follicles in the skin, can sense changes in the environment. The hair root plexus surrounding the base of the hair follicle senses a disturbance and then transmits the information to the central nervous system (brain and spinal cord), which can then respond by activating the skeletal muscles of your eyes to see the ant and the skeletal muscles of the body to act against the ant.

The skin acts as a sense organ because the epidermis, dermis, and the hypodermis contain specialized sensory nerve structures that detect touch, surface temperature, and pain. These receptors are more concentrated on the tips of the fingers, which are most sensitive to touch, especially the tactile (Meissner) corpuscle (Figure 9.11), which responds to light touch, and the lamellated (Pacinian) corpuscle, which responds to vibration.

Merkel cells (Figure 9.4), seen scattered in the stratum basale, are also touch receptors. In addition to these specialized receptors, there are sensory nerves connected to each hair follicle, pain and temperature receptors scattered throughout the skin, and motor nerves that innervate the arrector pili muscles and glands. This rich innervation helps us sense our environment and react accordingly.

 

Thermoregulation: The integumentary system helps regulate body temperature through its tight association with the nervous system. The nervous system is continuously monitoring body temperature and initiating appropriate motor responses. Recall that sweat glands, accessory structures to the skin, secrete water, salt, and other substances to cool the body when it becomes warm.

Even when the body does not appear to be noticeably sweating, approximately 500 mL of sweat (insensible perspiration) are secreted a day. If the body becomes excessively warm due to high temperatures, vigorous activity (Figure 9.12ac), or a combination of the two, sweat glands will be stimulated by the sympathetic nervous system to produce large amounts of sweat, as much as 0.7 to 1.5 L per hour for an active person. When the sweat evaporates from the skin surface, the body is cooled as body heat is dissipated.

In addition to sweating, arterioles in the dermis dilate so that excess heat carried by the blood can dissipate through the skin and into the surrounding environment (Figure 9.12b). This accounts for the skin redness that many people experience when exercising.

When body temperatures drop, the arterioles constrict to minimize heat loss, particularly in the ends of the digits and tip of the nose. This reduced circulation can result in the skin taking on a whitish hue. Although the temperature of the skin drops as a result, passive heat loss is prevented, and internal organs and structures remain warm. If the temperature of the skin drops too much (such as environmental temperatures below freezing), the conservation of body core heat can result in the skin actually freezing, a condition called frostbite.

 

Other Functions: The epidermal layer of human skin synthesizes vitamin D when exposed to UV radiation. In the presence of sunlight, a precursor of vitamin D is synthesized from a derivative of the cholesterol in the skin. Further biochemical reactions in the liver and kidney then produce calcitriol, the active chemical form of the vitamin, which has important roles in the absorption of calcium from the small intestine. In present day society, vitamin D is added as a supplement to many foods, including milk and orange juice, compensating for the need for sun exposure.

The skin is also a minor component of the excretory system, which is used to remove metabolic waste products from the body. Most metabolic waste products are removed from the body via the urinary and respiratory systems. However, the skin does release in sweat some of the metabolic waste products that are found in blood plasma, albeit at relatively low concentrations.

 

Part 4: Pigmentation

Most cells in the body are naturally colorless, but the integumentary system appears colorful due in part to the presence of pigments.  The color of skin is influenced by a number of pigments, including melanin, carotene, and hemoglobin. Recall that melanin is produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The melanin is transferred in the keratinocytes via a cellular vesicle called a melanosome (Figure 9.13).

Dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning salon causes melanin to be manufactured and built up in keratinocytes, as sun exposure stimulates keratinocytes to secrete chemicals that stimulate melanocytes. The accumulation of melanin in keratinocytes results in the darkening of the skin, or a tan. This increased melanin accumulation protects the DNA of epidermal cells from UV ray damage. In contrast, too much melanin can interfere with the production of vitamin D, an important nutrient involved in calcium absorption.

 

It requires about 10 days after initial sun exposure for melanin synthesis to peak, which is why pale-skinned individuals tend to suffer sunburns of the epidermis initially. Dark-skinned individuals can also get sunburns but are more protected than are pale-skinned individuals. Melanosomes are temporary structures that are eventually destroyed by fusion with lysosomes; this fact, along with melanin-filled keratinocytes in the stratum corneum sloughing off, makes tanning impermanent.

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 sufficient DNA damage to result in skin cancer.

Freckles and Moles

Dermatology is the medical specialty that studies the integument. In a dermatological exam, special attention is given to spots of pigmentation called freckles and moles. Freckles are flat spots of pigmentation that develop as a result of overactivity of melanocytes in these areas. They are prevalent in sun-exposed areas of the body, such as the face, arms, neck, and chest, but will fade seasonally.

Moles (also known as nevi) are larger masses of melanocytes, and although most are benign, they should be monitored for changes that might indicate the presence of cancer (Figure 8). Cancerous moles may have an irregular border or uneven halves. In contrast to freckles, moles are raised and do not disappear with seasons.