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Chapter 5

The Integumentary System

Lecture Outline

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INTRODUCTION

• The skin and its accessory structures make up the integumentary system.

• The integumentary system functions to guard the body’s physical and biochemical integrity, maintain a constant body temperature, and provide sensory information about the surrounding environment.

• Skin and its accessory structures– structure– function– growth and repair– development– aging

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STRUCTURE OF THE SKIN (Figure 5.1)

• The superficial portion of the skin is the epidermis and is composed of epithelial tissue.

• The deeper layer of the skin is the dermis and is primarily composed of connective tissue.

• Deep to the dermis is the subcutaneous layer or hypodermis. (not a part of the skin)– It consists of areolar and adipose

tissue.– fat storage, an area for blood vessel

passage, and an area of pressure-sensing nerve endings.

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Overview of Epidermis

• Stratified squamous epithelium• avascular (contains no blood vessels)

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Four Principle Cells of the Epidermis – Figure 5.2

• keratinocytes

– produce the protein keratin, which helps protect the skin and underlying tissue from heat, microbes, and chemicals, and lamellar granules, which release a waterproof sealant

• melanocytes

– produce the pigment melanin which contributes to skin color and absorbs damaging ultraviolet (UV) light

• Langerhans cells

– derived from bone marrow

– participate in immune response

• Merkel cells

– contact a sensory structure called a tactile (Merkel) disc and function in the sensation of touch

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Layers (Strata) of the Epidermis• Stratum corneum• Stratum lucidum• Stratum granulosum• Stratum spinosum• Stratum basale

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Stratum Basale (stratum germinativum)

• Stratum corneum– Barrier to light, heat, water, chemicals &

bacteria– Lamellar granules in this layer make it

water-repellent.• Stratum lucidum

– Contains precursor of keratin• Stratum granulosum

– transition between the deeper, metabolically active strata and the dead cells of the more superficial strata

• Stratum spinosum

– provides strength and flexibility to the skin

• Stratum basale– Deepest single layer of epidermis

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Dermis (Figure 5.1)• Connective tissue layer composed of collagen & elastic fibers,

fibroblasts, macrophages & fat cells

• Contains hair follicles, glands, nerves & blood vessels

• Two major regions of dermis

– papillary region

– reticular region

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Dermis - Papillary Region

• Top 20% of dermis• areolar connective tissue containing fine elastic fibers, corpuscles of touch

(Meissner’s corpuscles), adipose cells, hair follicles, sebaceous glands, sudoriferous glands – The collagen and elastic fibers provide strength, extensibility (ability to

stretch), and elasticity (ability to return to original shape after stretching) to skin.

• Finger like projections are called dermal papillae– anchors epidermis to dermis– contains capillaries that feed epidermis– contains Meissner’s corpuscles (touch) & free nerve endings for

sensations of heat, cold, pain, tickle, and itch

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Dermis - Reticular Region

• Dense irregular connective tissue

• Contains interlacing collagen and elastic fibers

• Packed with oil glands, sweat gland ducts, fat & hair follicles

• Provides strength, extensibility & elasticity to skin

– stretch marks are dermal tears from extreme stretching

• Epidermal ridges form in fetus as epidermis conforms to dermal papillae

– fingerprints are left by sweat glands open on ridges

– increase grip of hand

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Dermis -- Structure

• Epidermal ridges increase friction for better grasping ability and provide the basis for fingerprints and footprints. The ridges typically reflect contours of the underlying dermis.

• Lines of cleavage in the skin indicate the predominant direction of the underlying collagen fibers. Knowledge of these lines is especially important to plastic surgeons.

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Basis of Skin Color

• The color of skin and mucous membranes can provide clues for diagnosing certain problems, such as – Jaundice

• yellowish color to skin and whites of eyes• buildup of yellow bilirubin in blood from liver disease

– Cyanosis• bluish color to nail beds and skin• hemoglobin depleted of oxygen looks purple-blue

– Erythema• redness of skin due to enlargement of capillaries in

dermis• during inflammation, infection, allergy or burns

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Skin Color Pigments• Melanin produced in epidermis by melanocytes

– melanocytes convert tyrosine to melanin• UV in sunlight increases melanin production

• The wide variety of colors in skin is due to three pigments - melanin, carotene, and hemoglobin (in blood in capillaries) - in the dermis.

• Carotene in dermis– yellow-orange pigment (precursor of vitamin A)– found in stratum corneum & dermis

• Hemoglobin– red, oxygen-carrying pigment in blood cells– if other pigments are not present, epidermis is translucent so pinkness will

be evident

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Accessory Structures of Skin

• develop from the embryonic epidermis

• Cells sink inward during development to form:

– hair

– oil glands

– sweat glands

– nails

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Structure of Hair

• Shaft -- visible• Root -- below the surface• Follicle surrounds root

• Functions of Hair– Prevents heat loss

– Decreases sunburn

– Eyelashes help protect eyes

– Touch receptors (hair root plexus) senses light touch

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Glands of the Skin

• Specialized exocrine glands found in dermis

• Sebaceous (oil) glands

• Sudiferous (sweat) glands

• Ceruminous (wax) glands

• Mammary (milk) glands

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Sudoriferous (sweat) glandsEccrine sweat glands have an extensive distribution most areas of skin

– secretory portion is in dermis with duct to surface– ducts terminate at pores at the surface of the epidermis.– regulate body temperature through evaporation (perspiration)– help eliminate wastes such as urea.

Apocrine sweat glands are limited in distribution to the skin of the axilla, pubis, and areolae; their duct open into hair follicles.– secretory portion in dermis– duct that opens onto hair follicle– secretions are more viscous

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Ceruminous Glands

• Ceruminous glands are modified sudoriferous glands that produce a waxy substance called cerumen.

– found in the external auditory meatus

– contains secretions of oil and wax glands

– barrier for entrance of foreign bodies

• An abnormal amount of cerumen in the external auditory meatus or canal can result in impaction and prevent sound waves from reaching the ear drum (Clinical Application).

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Structure of Nails (Figure 5.5)

• Tightly packed keratinized cells• Nail body

– visible portion pink due to underlying capillaries

– free edge appears white• Nail root

– buried under skin layers– lunula is white due to thickened

stratum basale• Eponychium (cuticle)

– stratum corneum layer

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Sebaceous (oil) glands• Sebaceous (oil) glands are usually connected to hair

follicles; they are absent in the palms and soles

• Secretory portion of gland is located in the dermis

– produce sebum

• contains cholesterol, proteins, fats & salts

• moistens hairs

• waterproofs and softens the skin

• inhibits growth of bacteria & fungi (ringworm)

• Acne

– bacterial inflammation of glands

– secretions are stimulated by hormones at puberty

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TYPES OF SKIN• Thin skin

– covers all parts of the body except for the palms and palmar surfaces of the digits and toes.

– lacks epidermal ridges

– has a sparser distribution of sensory receptors than thick skin.

• Thick skin (0.6 to 4.5 mm)

– covers the palms, palmar surfaces of the digits, and soles

– features a stratum lucidum and thick epidermal ridges

– lacks hair follicles, arrector pili muscles, and sebaceous glands, and has more sweat glands than thin skin.

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FUNCTIONS OF SKIN -- thermoregulation• Perspiration & its evaporation

– lowers body temperature– flow of blood in the dermis is adjusted

• Exercise– in moderate exercise, more blood brought to surface helps lower

temperature– with extreme exercise, blood is shunted to muscles and body

temperature rises

• Shivering and constriction of surface vessels– raise internal body temperature as needed

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FUNCTIONS OF SKIN• blood reservoir

– extensive network of blood vessels

• protection - physical, chemical and biological barriers– tight cell junctions prevent bacterial invasion– lipids released retard evaporation– pigment protects somewhat against UV light– Langerhans cells alert immune system

• cutaneous sensations – touch, pressure, vibration, tickle, heat, cold, and pain arise in the

skin

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FUNCTIONS OF SKIN• Synthesis of Vitamin D

– activation of a precursor molecule in the skin by UV light

– enzymes in the liver and kidneys modify the activated molecule to produce calcitriol, the most active form of vitamin D.

– necessary vitamin for absorption of calcium from food in the gastrointestinal tract

• excretion

– 400 mL of water/day, small amounts salt, CO2, ammonia and urea

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Epidermal Wound Healing• Abrasion or minor burn• Basal cells migrate across the wound • Contact inhibition with other cells stops migration• Epidermal growth factor stimulates basal cells to divide

and replace the ones that have moved into the wound• Full thickness of epidermis results from further cell

division

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Deep Wound Healing• When an injury extends to tissues deep to the epidermis, the repair process is more complex than epidermal healing, and scar formation

results.• Healing occurs in 4 phases

– inflammatory phase has clot unite wound edges and WBCs arrive from dilated and more permeable blood vessels– migratory phase begins the regrowth of epithelial cells and the formation of scar tissue by the fibroblasts– proliferative phase is a completion of tissue formation– maturation phase sees the scab fall off

• Scar formation– hypertrophic scar remains within the boundaries of the original wound– keloid scar extends into previously normal tissue

• collagen fibers are very dense and fewer blood vessels are present so the tissue is lighter in color

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Deep Wound Healing

• Phases of Deep Wound Healing

– During the inflammatory phase, a blood clot unites the wound edges, epithelial cells migrate across the wound, vasodilatation and increased permeability of blood vessels deliver phagocytes, and fibroblasts form

– During the migratory phase, epithelial cells beneath the scab bridge the wound, fibroblasts begin scar tissue, and damaged blood vessels begin to grow. During this phase, tissue filling the wound is called granulation tissue.

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Phases of Deep Wound Healing

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Deep Wound Healing

• Phases of Deep Wound Healing

– During the proliferative phase, the events of the migratory phase intensify.

– During the maturation phase, the scab sloughs off, the epidermis is restored to normal thickness, collagen fibers become more organized, fibroblasts begin to disappear, and blood vessels are restored to normal

– Scar tissue formation (fibrosis) can occur in deep wound healing.

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Age Related Structural Changes

• Collagen fibers decrease in number & stiffen • Elastic fibers become less elastic• Fibroblasts decrease in number• decrease in number of melanocytes (gray hair, blotching)• decrease in Langerhans cells (decreased immune

responsiveness)• reduced number and less-efficient phagocytes• Most of the changes occur in the dermis

– wrinkling, slower growth of hair and nails– dryness and cracking due to sebaceous gland atrophy– Walls of blood vessels in dermis thicken so decreased

nutrient availability leads to thinner skin as subcutaneous fat is lost.