Week 5- The Integumentary System PDF

The integumentary System The Skin • It covers the entire body, has a surface area of 1.2 to 2.2 square meters, and accounts for about 7% of total body weight in the average adult. • Varying in thickness from 1.5 to 4.0 millimeters or more in different parts of the body, the skin is composed of tw...

The integumentary System The Skin • It covers the entire body, has a surface area of 1.2 to 2.2 square meters, and accounts for about 7% of total body weight in the average adult. • Varying in thickness from 1.5 to 4.0 millimeters or more in different parts of the body, the skin is composed of two distinct layers: 1. The epidermis , composed of epithelial cells, is the outermost protective shield of the body. 2. The underlying dermis, making up the bulk of the skin, is a tough, leathery layer composed mostly of dense connective tissue. **Only the dermis is vascularized. Nutrients reach the epidermis by diffusing through the tissue fluid from blood vessels in the dermis. The subcutaneous tissue • The subcutaneous tissue lies just deep to the skin . It is also known as the hypodermis or the superficial fascia, because it is superficial to the tough connective tissue wrapping (fascia) of the skeletal muscles. • The subcutaneous tissue is not part of the skin, but it shares some of the skin's protective functions. • It consists mostly of adipose tissue with some areolar connective tissue. • Besides storing fat, the subcutaneous tissue anchors the skin to the underlying structures (mostly to muscles), but loosely enough that the skin can slide relatively freely over those structures. • Sliding skin protects us by ensuring that everyday bumps often just glance off our bodies. • Because of its fatty composition, the subcutaneous tissue also acts as a shock absorber and an insulator that reduces heat loss. The epidermis is a keratinized stratified squamous epithelium The epidermis consists of four distinct cell types and four or five distinct layers: 1. Stratum Basale (Basal Layer) 2. Stratum Spinosum (Prickly Layer) 3. Stratum Granulosum (Granular Layer) 4. Stratum Lucidum (Clear Layer) 5. Stratum Corneum (Horny Layer) **In thick skin , which covers areas subject to abrasion-the palms, fingertips, and soles of the feet- the epidermis consists of five layers. Stratum Basale (Basal Layer) • The deepest epidermal layer, is also called the stratum germinativum "germinating layer". • It is attached to the underlying dermis along a wavy borderline that resembles corrugated cardboard. • It consists of a single row of stem cells-a continually renewing cell population-representing the youngest keratinocytes. • The many mitotic nuclei seen in this layer reflect the rapid division of these cells. Each time one of these basal cells divides, one daughter cell is pushed into the cell layer just above to begin its specialization into a mature keratinocyte. • The other daughter cell remains in the basal layer to continue the process of producing new keratinocytes. • Some 10-25% of the cells in the stratum basale are melanocytes, and their branching processes extend among the surrounding cells, reaching well into the more superficial stratum spinosum layer. Stratum Spinosum (Prickly Layer) • Is several cell layers thick. This layer gets its name from the many spine-like extensions of its keratinocytes as seen under a microscope. • However, these spines do not exist in living cells: They are artifacts created during tissue preparation because the cells shrink while holding tight at their many desmosomes. • Cells of the stratum spinosum contain thick bundles of intermediate filaments, which consist of a tension-resisting protein, pre-keratin. • These intermediate filaments resist tension in the cell and are anchored to the desmosomes. • Scattered among the keratinocytes are dendritic cells, which are most abundant in this epidermal layer. Stratum Granulosum (Granular Layer) • The thin layer consists of one to five cell layers in which keratinocyte appearance changes drastically, and the process of keratinization (in which the cells fill with keratin) begins. These cells flatten, their nuclei and organelles begin to disintegrate, and they accumulate two types of granules: 1. The keratohyaline granules, help to form keratin in the upper layers. 2. The lamellar granules, contain a water-resistant glycolipid that is secreted into the extracellular space. Together with tight junctions, the glycolipid plays a major part in slowing water loss across the epidermis. • Proteins within the keratinocytes and lipids deposited outside them make these cells tough and water resistant. This is why the outermost layers of skin are strong and water-tight. • Like all epithelia, the epidermis relies on capillaries in the underlying connective tissue (the dermis in this case) for its nutrients. • Above the stratum granulosum, the epidermal cells are too far from the dermal capillaries to survive. Glycolipids coating their external surfaces cut them off from nutrients. Stratum Lucidum • (Clear Layer) found only in thick skin, is visible through a light microscope as a thin translucent band just above the stratum granulosum. • It consists of a few rows of flat, dead keratinocytes. • Electron microscopy reveals that its cells are identical to those at the bottom of the next layer, the stratum corneum. Stratum Corneum • (Horny Layer) An abrupt transition occurs between the nucleated cells of the stratum granulosum and the flattened, anucleate cells of the stratum corneum. • This outermost epidermal layer is a broad zone 20 to 30 cell layers thick that accounts for up to three-quarters of the epidermal thickness • Keratin consists of the pre-keratin intermediate filaments embedded in a "glue" from the keratohyaline granules. • Keratin and proteins that accumulate just inside the plasma membrane of cells in this stratum protect the skin against abrasion and penetration. • The glycolipid between its cells helps keep this layer nearly waterproof. It is amazing that even a layer of dead cells can still perform so many functions. • The cells of the stratum corneum are shed regularly. These cells are the dandruff shed from the scalp and the flakes that come off dry skin. • In a lifetime, the average person sheds 18 kg (40 lb) of these flakes. The saying "Beauty is only skin deep" is especially interesting in light of the fact that when we look at someone, nearly everything we see is dead! Cells od Epidermis • The cells populating the epidermis include keratinocytes, melanocytes, dendritic cells, and tactile epithelial cells. Keratinocytes: • The chief role of keratinocytes is to produce keratin, the fibrous protein that helps give the epidermis its protective properties. • keratinocytes are tied together by desmosomes for strength and, in some layers, by tight junctions to hinder movement of water between cells. • Keratinocytes arise in the deepest part of the epidermis layer-the stratum basale. • Newly formed keratinocytes are pushed upward by the production of new cells beneath them, all the while making the keratin that eventually fills them . By the time the keratinocytes approach the skin surface, they are dead, scale-like flat sacs completely filled with keratin. • Millions of dead keratinocytes rub off every day, giving us a totally new epidermis every 25 to 45 days. Where the skin experiences friction, both cell production and keratin formation are accelerated. Persistent friction (from a poorly fitting shoe, for example) causes a thickening of the epidermis called a Callus. Melanocytes • Melanocytes, the spider-shaped epithelial cells that synthesize the pigment melanin found in the deepest layer of the epidermis, • Melanin is made in membrane-bound granules called melanosomes and then transferred through the cell processes (the "spider legs") to nearby keratinocytes. • As a result, the basal keratinocytes contain more melanin than do the melanocytes themselves. This melanin clusters on the superficial, or "sunny," side of the keratinocyte nucleus, forming a pigment shield that protects the nucleus from the damaging effects of ultraviolet (UV) radiation in sunlight. Cells of the Epidermis Dendritic Cells • The star-shaped dendritic cells arise from bone marrow and migrate to the epidermis. • Also called Langerhans cells, they ingest foreign substances and are key activators of our immune system. • Their slender processes extend among the surrounding keratinocytes, forming a more or less continuous network Tactile Epithelial Cells • Occasional tactile epithelial cells (or Merkel cells) are present at the epidermal-dermal junction. • Shaped like a spiky hemisphere, each tactile epithelial cell is intimately associated with a disclike sensory nerve ending. • The combination functions as a sensory receptor for touch. The dermis • The dermis consists of papillary dermis and reticular dermis • The dermis, is made up of strong, flexible connective tissue. Its cells are typical of those found in any connective tissue proper: fibroblasts, macrophages, and occasional mast cells and white blood cells • Its semifluid matrix, embedded with fibers, binds the entire body together like a body stocking. It is your "hide" and corresponds to animal hides used to make leather. Papillary Dermis • The thin, superficial papillary dermis. Is Areolar Connective Tissue in which fine interlacing collagen and elastic fibers form a loosely woven mat with many small blood vessels. • The looseness of this connective tissue allows phagocytes and other defensive cells to wander freely as they patrol the area for bacteria that have penetrated the skin. • Peglike projections from its surface, called dermal papillae, indent the overlying epidermis. • Many dermal papillae contain capillary loops. Others house free nerve endings (pain receptors) and touch receptors called tactile corpuscles or Meissner's corpuscles. • In thick skin, such as the palms of the hands and soles of the feet, these papillae lie top larger mounds called dermal ridges, which in turn cause the overlying epidermis to form epidermal ridges. • Collectively, these skin ridges, referred to as friction ridges, may enhance our ability to grip certain kinds of surfaces. They also contribute to our sense of touch by enhancing vibrations detected by the large lamellar corpuscles (receptors) in the dermis. • Friction ridge patterns are genetically determined and unique to each of us. Because sweat pores open along their crests, our fingertips leave identifying films of sweat called fingerprints on almost anything we touch. ***Note that tactile epithelial cells and tactile corpuscles are different structures Reticular Dermis • The deeper reticular dermis, accounting for about 80% of the thickness of the dermis, is coarse, Dense Irregular Connective Tissue. • The network of blood vessels that nourishes this layer, the dermal vascular plexus, lies between this layer and the subcutaneous tissue. • The extracellular matrix of the reticular dermis contains thick bundles of interlacing collagen fibers. • The reticular dermis is named for its network of collagen fibers the name does not imply any special abundance of reticular fibers. • The collagen fibers run in various planes, but most run parallel to the skin surface. Separations, or less dense regions, between these bundles form cleavage (tension) lines in the skin. • These externally invisible lines tend to run longitudinally in the skin of the limbs and in circular patterns around the neck and trunk. • Cleavage lines are important to surgeons because when an incision is made parallel to these lines, the skin gapes less and heals more readily. The collagen fibers of the dermis give skin strength and resiliency that prevent minor jabs and scrapes from penetrating the dermis. Elastic fibers provide the stretch-recoil properties of skin. • Flexure lines are dermal folds that occur at or near joints, where the dermis is tightly secured to deeper structures. • Since the skin cannot slide easily to accommodate joint movement in such regions, the dermis folds and deep skin creases form. Flexure lines are also visible on the wrists, fingers, soles, and toes. Melanin, carotene, and hemoglobin determine skin color • Melanin is a polymer made of an amino acid called Tyrosine. Its two forms range in color from reddish yellow to brownish black. • Melanin synthesis depends on an enzyme in melanocytes called Tyrosinase. • Melanin is transported from melanocytes to the basal keratinocytes. Eventually, lysosomes break down the melanosomes, so melanin pigment is found only in the deeper layers of the epidermis. • Human skin comes in different colors. However, distribution of those colors is not random-populations of darker-skinned people tend to be found nearer the equator (where greater protection from the sun is needed), and those with the lightest skin are found closer to the poles. • Since all humans have the same relative number of melanocytes, differences in skin coloring reflect the kind and amount of melanin made and retained. • Melanocytes of black- and brown-skinned people produce many more and darker melanosomes than those of fair-skinned individuals, and their keratinocytes retain it longer. • Freckles and pigmented nevi (moles) are local accumulations of melanin. When we expose our skin to sunlight, keratinocytes secrete chemicals that stimulate melanocytes. Prolonged sun exposure causes a substantial melanin buildup, which helps protect the DNA of skin cells from UV radiation by absorbing the rays and dissipating the energy as heat. • Indeed, the initial signal for speeding up melanin synthesis seems to be a faster repair rate of DNA that has suffered photodamage. Melanin, carotene, and hemoglobin determine skin color Carotene • Is a yellow to orange pigment found in certain plant products such as carrots. • It tends to accumulate in the stratum corneum and in the fat of the subcutaneous tissue. • In the body, carotene can be converted to vitamin A, a vitamin that is essential for normal vision, as well as for epidermal health. Hemoglobin • The pinkish hue of fair skin reflects the crimson color of the oxygenated pigment hemoglobin in the red blood cells circulating through the dermal capillaries. • Because light-skinned people have only small amounts of melanin in their skin, the epidermis is nearly transparent and allows hemoglobin's color to show through. Associated Structures • Along with the skin itself, the integumentary system includes a number of associated structures. These skin appendages include hair and hair follicles, nails, sweat glands, and sebaceous (oil) glands. • Each plays a unique role in maintaining body homeostasis. • Although they all derive from the epithelial cells of the epidermis (i.e., they are epidermal derivatives), they all extend into the dermis. Structure of a Hair • Hairs, or pili, are flexible strands produced by hair follicles and consist largely of dead, keratinized cells. • The hard keratin that dominates hairs and nails has two advantages over the soft keratin found in typical epidermal cells: 1) It is tougher and more durable 2) its individual cells do not flake off. Structure of a Hair • The chief regions of a hair are the root, the part embedded in the skin, and the shaft, the part that projects above the skin's surface. • If the shaft is flat and ribbon-like in cross section, the hair is kinky; if it is oval, the hair is silky and wavy; if it is perfectly round, the hair is straight and tends to be coarse. • A hair has three concentric layers of keratinized cells: the medulla, cortex, and cuticle Epithelial root sheath External Internal fibrous sheath The medulla, its central core, consists of large cells and air spaces. The medulla, the only part of the hair that contains soft keratin, is absent in fine hairs. • The Cortex, a bulky layer surrounding the medulla, consists of several layers of flattened cells. • The outermost Cuticle is formed from a single layer of cells overlapping one another like shingles on a roof. • This arrangement helps separate neighboring hairs so the hair does not mat. (Hair conditioners smooth out the rough surface of the cuticle and make hair look shiny.) • The cuticle is the most heavily keratinized part of the hair, providing strength and keeping the inner layers tightly compacted. • Because it is subjected to the most abrasion, the cuticle tends to wear away at the tip of the hair shaft. This allows keratin fibrils in the cortex and medulla to frizz, creating "split ends." • Hair pigment is made by melanocytes at the base of the hair follicle and transferred to the cortical cells. • Different concentrations of melanin produce hair color from blond to brown to black. Red hair is colored by a related pigment called pheomelanin. • When melanin production decreases and air bubbles replace melanin in the hair shaft, hair turns gray or white Structure of a Hair Follicle • Hair follicles, fold down from the epidermal surface into the dermis (and sometimes into the subcutaneous tissue). • The deep end of the follicle, located about 4 mm below the skin surface, expands to form a hair bulb. • A knot of sensory nerve endings called a hair follicle receptor, or root hair plexus, wraps around each hair bulb. Bending the hair stimulates these endings. Consequently, our hairs act as sensitive touch receptors. • A papilla of a hair follicle (or hair papilla) is a dermal papilla that protrudes into the hair bulb. This papilla contains a knot of capillaries that supplies nutrients to the growing hair and signals it to grow. • If the hair papilla is destroyed by trauma, the follicle permanently stops producing hair. • It has a dermal component and an epidermal component. These components are described from external to internal: • Peripheral connective tissue sheath (fibrous sheath). This connective tissue sheath is derived from the dermis. It forms the external layer of the follicle wall. • Glassy membrane. is at the junction of the fibrous sheath and the epithelial root sheath. It is, in essence, the basement membrane of the follicle epithelium. • Epithelial root sheath. The epithelial root sheath is derived from the epidermis. It has two components: The external root sheath, a direct continuation of the epidermis, and the internal root sheath, which is derived from the matrix cells. The wall of a Hair Follicle The wall of a Hair Follicle • Hair grows because cells in the bulb of the follicle divide rapidly. These dividing cells make up the hair matrix and lie immediately adjacent to the hair papilla. • As the matrix produces new hair cells, the older part of the hair is pushed upward, and its fused cells become increasingly keratinized and die. • As they divide, hair matrix cells are replenished by stem cells that migrate down to the bulb from a region closer to the skin surface called the hair bulge. • Associated with each hair follicle is a bundle of smooth muscle cells called an Arrector Pili muscle. most hair follicles approach the skin surface at a slight angle. • The Arrector Pili muscle is attached in such a way that its contraction pulls the hair follicle upright and dimples the skin surface to produce goose bumps in response to cold temperatures or fear. • The more important role of the hair in humans is that its contractions force sebum out of hair follicles to the skin surface where it acts as a skin lubricant. Types and Growth of Hair • Hairs can be classified as Vellus or Terminal. • The body hair of children and adult females is pale, fine vellus hair. • The coarser, longer hair of the eyebrows, eyelashes, and scalp is terminal hair, which may also be darker. • At puberty, terminal hairs appear in the axillary and pubic regions of both sexes and on the face and chest of males. These terminal hairs grow in response to the stimulating effects of Androgens • Hairs grow an average of 2 mm per week, although this rate varies widely among body regions as well as with sex and age. • Each follicle goes through growth cycles. In each cycle, an active growth phase is followed by a resting phase in which the hair matrix is inactive and the follicle shrinks somewhat. • At the start of each active phase, the newly growing hair pushes out the old hair, which is shed. • The life span of hairs varies. In the scalp, the follicles stay active for an average of four years, so individual hairs grow quite long before being shed. • The follicles in eyebrows, in contrast, are active for only a few months so the eyebrows never grow very long. • Fortunately, the cycles of adjacent hair follicles are not synchronized. For this reason, humans shed only a small percentage of their hairs at any one time. Hair Thinning and Baldness • • • • • Given ideal conditions, hair grows fastest from the teen years to the 40s. When hairs are no longer replaced as quickly as they are shed, the hair thins. By age 60 to 65, both sexes usually experience some degree of balding. Coarse terminal hairs are replaced by vellus hairs, and the hair becomes increasingly wispy. True baldness is different, The most common type is male pattern baldness, a genetically determined, gender-influenced condition that changes the response of the hair follicles to androgens. • The hair follicles respond to androgens with shorter and shorter growth cycles. The cycles become so short that many hairs never even emerge from their follicles before being shed, and those that do are fine vellus hairs that look like peach fuzz in the "bald" area. • The drugs used to treat male pattern baldness either inhibit the production of androgens or increase blood flow to the skin and hair follicles. These treatments are only partly successful. The structure of nails • A nail forms a clear protective covering on the dorsal surface of the distal part of a finger or toe. • In contrast to soft keratin of the epidermis, nails (like hairs) contain hard keratin. • Each nail has a proximal root (embedded in the skin), a nail plate or body (visible attached portion), and a free edge. • The nail rests on a bed of epidermis called the nail bed. This bed contains only the deeper layers of the epidermis, because the nail itself corresponds to the superficial keratinized layers. • The thickened proximal portion of the nail bed, the nail matrix, is responsible for nail growth. • As the nail cells produced by the matrix become heavily keratinized, the nail body slides distally over the nail bed. • Nails normally appear pink because of the rich bed of capillaries in the underlying dermis. However, the region that lies over the thick nail matrix appears as a white crescent called the lunule "little moon". • The proximal and lateral borders of the nail are overlapped by skin folds, called nail folds. • The proximal nail fold projects onto the nail body as the cuticle or eponychium • The thickened region beneath the free edge of is the hyponychium ("below nail"). The structure of nails White crescent called the Lunule Each nail has a proximal root (embedded in the skin), a nail plate or body (visible attached portion), and a free edge. The nail rests on a bed of epidermis called the nail bed. The proximal and lateral borders of the nail are overlapped by skin folds, called nail folds. The proximal nail fold projects onto the nail body as the cuticle or eponychium. The thickened region beneath the free edge of is the hyponychium ("below nail"). Cutaneous Glands Sweat glands • Also called Sudoriferous glands are distributed over the entire skin surface except the nipples and parts of the external genitalia. • Their number is staggering-up to 3 million per person. • We have two types of sweat glands: eccrine and apocrine. In both types, the secretory cells are associated with myoepithelial cells, specialized cells that contract when stimulated by the nervous system. • Their contraction forces the sweat into and through the gland's duct system to the skin surface. Sweating is regulated by a branch of the autonomic (involuntary) nervous system. Eccrine Sweat Glands • Also called ,Merocrine sweat glands, are far more numerous than apocrine sweat glands and are particularly abundant on the palms, soles of the feet, and forehead. • Each is a simple, coiled, tubular gland. The secretory part lies coiled in the dermis, and the duct extends to open in a funnel-shaped pore at the skin surface • These sweat pores are different from the so-called pores of a person's complexion, which are openings of hair follicles.) • Eccrine gland secretion, commonly called sweat, is a hypotonic filtrate of the blood that passes through the secretory cells of the sweat glands and is released by exocytosis. • It is 99% water, with some salts (mostly sodium chloride), traces of metabolic wastes (urea, uric acid, and ammonia), and a microbe-killing peptide called dermcidin. Normally, sweat is acidic with a pH between 4 and 6. • Sweating's major role is to prevent the body from overheating. Heat-induced sweating begins on the forehead and spreads inferiorly over the remainder of the body. • Emotionally induced sweating-the so-called "cold sweat" brought on by fright or nervousness-begins on the palms, soles, and axillae (armpits) and then spreads to other body areas. Apocrine Sweat Glands • The approximately 2000 apocrine sweat glands are largely confined to the axillary and anogenital areas. • They are merocrine glands, which release their product by exocytosis like the eccrine sweat glands. • Larger than eccrine glands, they lie deeper in the dermis or even in the subcutaneous tissue, and their ducts empty into hair follicles. • Apocrine secretion contains the same basic components as true sweat, plus fatty substances and proteins. Consequently, it is viscous and sometimes has a milky or yellowish color. • The secretion is odorless, but when bacteria on the skin decompose its organic molecules, it takes on a musky and generally unpleasant odor, the basis of body odor. • Apocrine glands begin functioning at puberty under the influence of the male sex hormones (androgens) and play little role in maintaining a constant body temperature. • Three lines of evidence suggest that they may be the human equivalent of other animals' sexual scent glands: (I) Sexual foreplay increases their activity; (2) they enlarge and recede with the phases of a woman's menstrual cycle; and (3) behavioral studies show that their secretions may act as pheromones (chemical messengers released by one individual that trigger a response in other members of the same species). Two important types of modified apocrine glands • Ceruminous glands: are modified apocrine glands found in the lining of the external ear canal. Their secretion mixes with sebum produced by nearby sebaceous glands to form a sticky, bitter substance called cerumen, or earwax, that is thought to deter insects and block entry of foreign material. • Mammary glands: another type of specialized sweat gland, secrete milk. Sebaceous Glands • Oil glands, are simple branched alveolar glands that are found all over the body EXCEPT in the thick skin of the palms and soles. • They are small on the body trunk and limbs, but quite large on the face, neck, and upper chest. • These glands secrete an oily substance called Sebum. The central cells of the alveoli accumulate oily lipids until they become so engorged that they burst, so functionally these glands are holocrine glands. The accumulated lipids and cell fragments constitute sebum. • Most, but not all, sebaceous glands develop as outgrowths of hair follicles and secrete sebum into a hair follicle, or occasionally to a pore on the skin surface. • Arrector pili contractions force sebum out of the hair follicles to the skin surface. • Sebum softens and lubricates the hair and skin, prevents hair from becoming brittle, and slows water loss from the skin. Perhaps even more important is its bactericidal (bacterium-killing) action. • Sebaceous glands increase their activity during puberty under the influence of male sex hormones. Functions of the skin • First and foremost, the skin is a barrier. • The skin and its appendages perform a variety of functions, including: 1. Protection, 2. Body Temperature Regulation 3. Cutaneous Sensation 4. Metabolic Functions 5. Blood Reservoir 6. Excretion Protection • Protection Given its superficial location, the skin is our most vulnerable organ system, exposed to microorganisms, abrasion, temperature extremes, harmful chemicals, and UV radiation. • The skin constitutes at least three types of barriers: Chemical, Physical, and Biological. Chemical Barriers: • include skin secretions and melanin . Although the skin's surface teems with bacteria, the low pH of skin secretions-the acid mantle-retards their multiplication. • In addition, dermcidin in sweat and bactericidal substances in sebum kill many bacteria outright. • Skin cells also secrete natural antibiotics called defensins that literally punch holes in bacteria, making them leak like sieves. • Wounded skin releases large quantities of protective peptides called cathelicidins that are particularly effective in preventing infection by group A streptococcus bacteria. • Morover, melanin provides a chemical pigment shield to prevent UV damage to skin cells. Physical Barriers: • The continuity of skin and the hardness of its keratinized cells provide physical barriers. • As a physical barrier, the skin is a remarkable compromise. A thicker epidermis would be more impenetrable. • The outstanding barrier capacity of the skin arises from the structure of its stratum corneum, which has been compared to bricks and mortar. Multiple layers of dead flat cells are the bricks and the glycolipids surrounding them are the mortar. • Epidermal continuity works hand in hand with the acid mantle and chemicals in skin secretions to ward off bacterial invasion. • The water-resistant glycolipids of the epidermis block most diffusion of water and watersoluble substances between cells, preventing both their loss from and entry into the body through the skin. However, there is a continual small loss of water through the epidermis. Substances that do penetrate the skin in limited amounts include: 1. Lipid-soluble substances, such as oxygen, carbon dioxide, fat-soluble vitamins (A, D, E, and K), and steroids (estrogens) 2. Oleoresins of certain plants, such as poison ivy and poison oak 3. Organic solvents, such as acetone, dry-cleaning fluid, and paint thinner, which dissolve the cell lipids 4. Salts of heavy metals, such as lead and mercury 5. Selected drugs (nitroglycerine, seasickness medications) 6. Drug agents called penetration enhancers that help ferry other drugs into the body Biological Barriers Include the Dendritic Cells of the epidermis and Macrophages in the dermis: • Dendritic cells are part of the immune system. They patrol beneath the skin's surface, engulfing any foreign invaders that have penetrated the epidermis. Once they have captured their victim, they leave the skin and migrate to the nearest lymph node. There they display their prey to other immune cells. This triggers an immune response. • Dermal Macrophages constitute a second line of defense. They dispose of viruses and bacteria that manage to penetrate the epidermis. Like dendritic cells, macrophages can initiate an immune response. Body Temperature Regulation • The body works best when its temperature remains within homeostatic limits. Like car engines, we need to get rid of the heat generated by our internal reactions. As long as the external temperature is lower than body temperature, the skin surface loses heat to the air and to cooler objects in its environment, just as a car radiator loses heat to the air and other nearby parts. • Under normal resting conditions, and as long as the environmental temperature is below 31-32°C (88-90°F), sweat glands secrete about 500 ml (0.5 L) of sweat per day. This routine and unnoticeable sweating is called insensible perspiration. • When body temperature rises, the nervous system stimulates dermal blood vessels to dilate and the sweat glands into vigorous secretory activity. • On a hot day, sweat becomes noticeable and can account for the loss of up to 12 L (about 3 gallons) of body water in one day. This visible output of sweat is called sensible perspiration. Evaporation of sweat from the skin surface dissipates body heat and efficiently cools the body, preventing overheating. • When the external environment is cold, dermal blood vessels constrict. Their constriction causes the warm blood to bypass the skin temporarily and allows skin temperature to drop to that of the external environment. This slows passive heat loss from the body, saving body heat. Cutaneous Sensation • The skin is richly supplied with cutaneous sensory receptors, which are actually part of the nervous system. The cutaneous receptors are classified as Exteroceptors because they respond to stimuli arising outside the body. For example: • Tactile (Meissner's) corpuscles (in the dermal papillae) and tactile epithelial cells with their associated sensory nerve endings allow us to become aware of a caress or the feel of our clothing against our skin. • Lamellar (also called Pacinian) corpuscles (in the deeper dermis or subcutaneous tissue) alert us to bumps or contacts involving deep pressure. • Hair follicle receptors report on wind blowing through our hair and a playful tug on a ponytail. • Free nerve endings that meander throughout the skin sense painful stimuli (irritating chemicals, extreme heat or cold, and others). Metabolic Functions • The skin is a chemical factory, fueled in part by the sun's rays. When sunlight bombards the skin, modified cholesterol molecules are converted to a vitamin D precursor. • This precursor is transported via the blood to other body areas to be converted to vitamin D, which plays various roles in calcium metabolism. For example, calcium cannot be absorbed from the digestive tract without vitamin D. • Among its other metabolic functions, the epidermis makes chemical conversions that supplement those of the liver. For example, keratinocyte enzymes can: 1. "Disarm" many cancer-causing chemicals that penetrate the epidermis 2. Activate some steroid hormones- for instance, they can transform cortisone applied to irritated skin into hydrocortisone, a potent anti-inflammatory drug • Skin cells also make several biologically important proteins, including collagenase, an enzyme that aids the natural turnover of collagen. Blood Reservoir • The dermal vascular supply is extensive and can hold about 5% of the body's entire blood volume. • When other body organs, such as vigorously working muscles, need a greater blood supply, the nervous system constricts the dermal blood vessels. This constriction shunts more blood into the general circulation, making it available to the muscles and other body organs. Excretion • The body eliminates limited amounts of nitrogen-containing wastes (ammonia, urea, and uric acid) in sweat, although most such wastes are excreted in urine. • Profuse sweating is an important avenue for water and salt (sodium chloride) loss. Skin cancer and burns are major challenges to the body Skin Cancer • Is the most common kind of cancer- one in five of us will develop skin cancer at some point. • The single most important risk factor for skin cancer is exposure to the UV radiation in sunlight and tanning beds, which damages DNA bases. • Adjacent Pyrimidine Bases often respond by fusing, forming lesions called Dimers. • UV radiation also appears to disable a Tumor Suppressor Gene. • In limited numbers of cases, frequent irritation of the skin by infections, chemicals, or physical trauma seems to be a predisposing factor. • The three major forms of skin cancer are Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma. Basal cell carcinoma • The least malignant and most common, accounts for nearly 80% of cases. • Stratum basale cells proliferate, invading the dermis and subcutaneous tissue. • The cancer lesions occur most often on sun-exposed areas of the face and appear as shiny, dome-shaped nodules that later develop a central ulcer with a pearly, beaded edge. • Basal cell carcinoma is relatively slow-growing, and metastasis rarely occurs. • Full cure by surgical excision is the rule in 99% of cases. Squamous cell carcinoma • The second most common skin cancer, arises from the keratinocytes of the Stratum Spinosum. • The lesion appears as a scaly reddened papule (small, rounded elevation) that arises most often on the head (scalp, ears, and lower lip), and hands. • It tends to grow rapidly and metastasize if not removed. • If it is caught early and removed surgically or by radiation therapy, the chance of complete cure is good. Melanoma • Cancer of melanocytes, is the most dangerous skin cancer because it is highly metastatic and resistant to chemotherapy. • It accounts for only about 1% of skin cancers, but most skin cancer deaths. Its incidence is increasing rapidly. • Melanoma can begin wherever there is pigment. Most such cancers appear spontaneously, and about one-third develop from preexisting moles. • It usually appears as a spreading brown to black patch that metastasizes rapidly to surrounding lymph and blood vessels. ABCD rule for recognizing melanoma: • Asymmetry: The two sides of the pigmented do not match. • The key to surviving melanoma is early detection. Survival rates • Border irregularity: the lesion exhibit decline with increasing thickness of the melanoma, degree of indentations. involvement of nearby lymph nodes, and extent of metastasis. • Color: (blacks, browns, tans, and • In advanced cases, surgical treatment is followed by immunotherapy, sometimes blues and reds). • Diameter: larger than 6 mm in diameter radiation therapy, or, more recently, targeted gene therapy that has had promising results by shrinking tumors and/ or prolonging life. Burns • Devastating threat to the body primarily because of their effects on the skin. • A burn is tissue damage inflicted by intense heat, electricity, radiation, or certain chemicals, all of which kill cells in the affected areas. • The immediate threat to life resulting from severe burns is a catastrophic loss of body fluids containing proteins and electrolytes. • This leads to dehydration and electrolyte imbalance, and then renal failure (kidney shutdown) and circulatory shock (inadequate blood circulation due to reduced blood volume). • To save the patient, the lost fluids must be replaced immediately by administration of intravenous (IV) fluids. Evaluating Burns: Classified according to their severity (depth) as First-, Second-, Or Third-degree Burns. • In First-degree Burns, only the epidermis is damaged. Symptoms include localized redness, swelling, and pain. First-degree burns tend to heal in two to three days without special attention. Sunburn is usually a first-degree burn. • Second-degree Burns injure the epidermis and the upper region of the dermis. Symptoms mimic those of first-degree burns, but blisters also appear. The burned area is red and painful, but skin regeneration occurs with little or no scarring within three to four weeks if care is taken to prevent infection. First- and second-degree burns are considered partial-thickness burns Third-degree Burns: are full-thickness burns, involving the entire thickness of the skin. • The burned area appears gray-white, cherry red, or blackened, and initially there is little or no edema. • Since the nerve endings have been destroyed, the burned area is not painful. • Although skin might eventually regenerate, it is usually impossible to wait that long because of fluid loss and infection. Skin grafting is advised. • ln general, burns are considered critical if any of the following conditions exists: • Over 25% of the body has second-degree burns • Over 10% of the body has third-degree burns • There are third-degree burns of the face, hands, or feet (facial burns introduce the possibility of burned respiratory passageways, which can swell and cause suffocation) In adults, the percentage of body surface burned is estimated using The Rule Of Nines. This method divides the body into 11 areas, each accounting for 9% of total body area, plus an additional area surrounding the genitals accounting for 1 % of body surface area. The rule of nines is only approximate, so special tables are used when greater accuracy is desired. Treating Burns • Patients with severe burns need thousands of extra food calories daily to replace lost proteins and allow tissue repair. • No one can eat enough food to provide these calories, so burn patients are given supplementary nutrients through Gastric Tubes and IV Lines. • After the initial crisis has passed, infection becomes the main threat and sepsis (widespread bacterial infection) is the leading cause of death in burn victims. Bacteria, fungi, and other pathogens easily invade areas where the skin barrier is destroyed, and they multiply rapidly in the nutrient-rich environment of dead tissues. Antibiotics play an important role in bum treatment. • Longer-term treatment of full-thickness burns usually involves a Skin Graft. To prepare a burned area for a skin graft, the eschar or burned skin, must first be debrided (removed). • Then healthy skin is transplanted to the burned site. If possible, the skin is the patient's own (an Autograft). Otherwise, there is a good chance that the patient's immune system will reject the foreign skin. • Alternatively, Synthetic Skin-a Silicone "epidermis" bound to a spongy "dermal" layer composed of collagen and ground cartilage-can be applied to the debrided area. • In time, the patient's own dermal tissue absorbs and replaces the artificial one. Then the silicone sheet is peeled off and replaced with a network of epidermal cells cultured from the patient's own skin. Developmental Aspects of the Integumentary System • The epidermis develops from the embryonic ectoderm and the dermis and subcutaneous tissue develop from mesoderm. • By the end of the fourth month of development, the skin is fairly well formed. The epidermis has all its strata, dermal papillae are obvious, fingerprints have developed, and rudimentary epidermal appendages have formed by downward projections of cells from the basal layer. • During the fifth and sixth months, the fetus is covered with a downy coat of delicate colorless hairs called the Lanugo coat. This hairy cloak is shed by the seventh month, and Vellus Hairs Appear. • When a baby is born, its skin is covered with vernix caseosa , a white, cheesy looking substance produced by the sebaceous glands that protects the fetus's skin within the water-filled amnion. The newborn 's skin is very thin and often has accumulations in the sebaceous glands on the forehead and nose that appear as small white spots called Milia. These normally disappear by the third week after birth. • During infancy and childhood, the skin thickens, and more subcutaneous fat is deposited. Although we all have approximately the same number of sweat glands, the number that function increases in the first two years after birth and is determined by climate. • During adolescence, the skin and hair become oilier as sebaceous glands increase their activity, and acne may appear. Acne generally subsides in early adulthood, and skin reaches its optimal appearance when we reach our 20s and 30s. Thereafter, the skin starts to show the effects of cumulative environmental assaults (abrasion, wind, sun, chemicals). Scaling and various kinds of skin inflammation, or Dermatitis become more common. Aging Skin • The rate of epidermal cell replacement slows, the skin thins, and its susceptibility to bruises and other injuries increases. • The lubricating substances produced by the skin glands become deficient. • Skin becomes dry and itchy, although people with naturally oily skin seem to postpone this dryness until later in life. • Elastic fibers clump, and collagen fibers become fewer and stiffer. The subcutaneous fat layer diminishes, leading to the intolerance to cold so common in elderly people. • Additionally, declining levels of sex hormones result in similar fat distribution in elderly men and women. • The decreasing elasticity of the skin, along with the loss of subcutaneous tissue, inevitably leads to wrinkling. • Decreasing numbers of melanocytes and dendritic cells enhance the risk and incidence of skin cancer in this age group. • As a rule, redheads and fair-skinned individuals, who have less melanin to begin with, show age-related changes more rapidly than do those with darker skin and hair. • By age 50, the number of active hair follicles has declined markedly and continues to fall, resulting in hair thinning. • Hair loses its luster in old age, and the genes responsible for graying and male pattern baldness become active. • Although there is no known way to avoid skin aging, one of the best ways to slow the process is to shield your skin from the sun's rays of both UVA (aging rays) and UVB (rays that burn) light. References and Further Reading • Elaine N. Marieb and Katja N. Hoehn (2019). Human Anatomy and Physiology. 11th Edition. Pearson Education. Chapter 5