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CHAPTER 5

The Integumentary System

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INTRODUCTION• Integument = skin & accessory structures

• Functions:– Guards body’s physical & biochemical integrity– Maintains constant body temperature– Provides sensory information about surrounding

environment

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STRUCTURE OF THE SKIN• Cutaneous membrane• Superficial portion = epidermis

– composed of epithelial tissue• Deeper layer of skin = dermis

– primarily connective tissue• Deep to dermis = subcutaneous layer (hypodermis)

– not a part of skin– areolar & adipose tissue– fat storage– area for blood vessel passage– area of pressure-sensitive nerve endings

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Principle Cells In Epidermis• Keratinocytes (90% of epidermal cells)

– keratin: protect skin & underlying tissue from heat, microbes, & chemicals

– lamellar granules: produce waterproof sealant • Melanocytes (8% of epidermal cells)

– produce melanin • contributes to skin color • absorbs damaging ultraviolet (UV) light

– sensitive to UV light

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Principle Cells In Epidermis

• Langerhans cells (small % of epidermal cells) – derived from bone marrow immune response– extremely sensitive to UV light

• Merkel cells (least numerous cells)– contact flattened sensory neuron (Merkel disc) – function in sensation of touch

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Layers of the Epidermis• Four or five layers, depending upon degree of

friction & mechanical pressure applied to skin

• From deepest to most superficial, the layers of the epidermis are:

– stratum basale (stratum germinativum)– stratum spinosum– stratum granulosum– stratum lucidum (only in palms and soles)– stratum corneum

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Stratum Basale• Also called stratum germinativum

• Deepest layer of epidermis – Single layer of cuboidal or columnar keratinocytes– Merkel cells, melanocytes, & Langerhans cells– Tonofilaments eventually will form keratin in

superficial layers– Desmosomes & hemi-desmosomes attach cells to

each other & to basement membrane– Rapidly dividing layer

• When destroyed, new skin cannot regenerate without a skin graft

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Stratum Spinosum• Superficial to stratum

basale

• 8-10 layers of keratinocytes

• Tonofilaments & desmosomes provide strength & flexibility

• Langerhans cells & melanocytes also found in this layer

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Stratum Granulosum• 3-5 layers of flat apoptotic keratinocytes

• Tonofilaments more apparent

• Lamellar granules release lipid that repels water

• Keratohyalin converts tonofilaments into keratin

• Keratinocytes die

• **Transition between the metabolically active strata & dead superficial layer**

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Stratum Lucidum • 3-5 layers of clear, dead, flattened keratinocytes

• Present ONLY in fingertips, palms & soles ‘thick skin’

• Large amounts of keratin

• Thickened plasma membranes

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Stratum Corneum

• 25 to 30 layers of flattened, dead keratinocytes

• Continuously shed & replaced

• Barrier to light, heat, water, chemicals & bacteria– Lamellar granules repel water

• Callus = abnormal thickening of stratum corneum– from constant exposure to friction

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Keratinization & Epidermal Growth• Stem cells in stratum basale divide to produce

keratinocytes

• Keratinocytes slowly pushed towards surface accumulate keratin @ each step

• keratinization – occurs as cells move to skin surface (deep to

superfic)– about 4 weeks from beginning to end

• Epidermal growth factor (EGF) and other hormone-like proteins regulate epidermal growth

• Psoriasis– Premature shedding of keratinocytes (7-10d) – Treated with UV light & topical ointment

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Dermis• Connective tissue layer composed of collagen &

elastic fibers, fibroblasts, macrophages & fat cells

• Contains hair follicles, glands, nerves & blood vessels

• Two major regions:– papillary region– reticular region

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Dermis - Papillary Region• Top 20% of dermis

• Mostly areolar CT – contains fine elastic fibers

• Dermal papillae = finger-like projections– anchors epidermis to dermis– capillary loops feed epidermis– contains Meissner’s corpuscles & free nerve

endings • touch sensations (Meissner’s)• heat, cold, pain, tickle, and itch

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Dermis - Reticular Region• Attached to subcutaneous (sub-Q) layer• Dense irregular connective tissue

– interlacing collagen bundles– coarse elastic fibers

• Some adipose, oil glands, sweat glands, & hair follicles• Provides strength, extensibility & elasticity to skin

– stretch marks = dermal tears from extreme stretching (striae)

• Epidermal ridges form in fetus as epidermis conforms to dermal papillae fingerprints– genetically determined – increase grip of hand

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Skin Pigments• Melanin produced in epidermis by melanocytes

– tyrosinase converts tyrosine to melanin• UV light increases melanin production

– differences in skin color determined by **AMOUNT** of pigment present

• Clinical observations– freckles or liver spots = accumulation of

melanocytes– mole = benign overgrowth of melanocytes– albinism = inherited lack of tyrosinase; no pigment– vitiligo = autoimmune loss of melanocytes in areas

of skin produces white patches

• Three pigments in dermis yield variety of skin colors– range from yellow to red & tan to black

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Skin Pigments• Melanin

– Pheomelanin (yellow to red)– Eumelanin (brown to black)– Increased synthesis results in “tan” protects

from further damage (within limits)

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

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

present, pinkness will be evident

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Skin Color as a Diagnostic Tool• Color of skin and mucous membranes can provide clues

for diagnosing certain problems– Jaundice

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

disease– Cyanosis

• bluish color in nail beds and skin• hemoglobin depleted of oxygen looks purple-blue

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

dermis• caused by inflammation, infection, allergy or burns

– Pallor = paleness of skin resulting from shock or anemia

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Accessory Structures of Skin

• Develop from embryonic epidermis

• Cells sink inward during development to form:– hair– oil glands– sweat glands– nails

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HAIR (PILI)• Present on all skin surfaces except palms, soles, &

palmar/plantar surfaces of the digits

• Eyebrows & lashes protect from foreign particles

• Sense light touch

• Anatomy– shaft – root– hair follicle

• New hairs develop from cell division of matrix in the bulb

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Structure of Hair• Shaft = superficial (visible) portion

• Root = below the surface

• 3 concentric layers– medulla

• 2-3 rows of irregularly. shaped cells– cortex

• elongated cells– cuticle

• single layer of thin, flat cells• heavily keratinized

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Structure of Hair• Follicle surrounds root

– Epithelial root sheath• external root sheath• internal root sheath

• Dermal root sheath• surrounds follicle

• Bulb = base of follicle• blood vessels (in papilla)• germinal cell layer (matrix)

– arise from str. basale– **site of cell division**– gives rise to internal root sheath

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Hair-Related Structures• Arrector pili

– smooth muscle associated w/ hair– contraction causes goosebumps as hair is pulled

vertically

• Hair root plexus– surrounds follicle– touch-sensitive dendrites

• detect hair movement

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Hair Growth• Growth cycle = growth stage & resting stage

– Growth stage • lasts for 2 to 6 years• matrix cells at base of hair root divide length

– Resting stage• lasts for 3 months•matrix cells inactive & follicle atrophies• Old hair falls out as growth stage begins again

– normal hair loss is 70 to 100 hairs per day

• Rate of growth & replacement can be altered by illness, diet, blood loss, severe emotional stress, & gender

• Chemotherapeutic agents affect rapidly dividing matrix cells

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ACCESSORY STRUCTURES: Glands

Specialized exocrine glands found in dermis:

• Sebaceous (oil) glands

• Sudiferous (sweat) glands

• Ceruminous (wax) glands

• Mammary (milk) glands

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Sebaceous glands• Usually connected to hair follicles• Absent in palms and soles• Secretory portion of gland is located in the dermis• Produce sebum

– lipid-rich, oily substance– moistens hairs– waterproofs and softens the skin– inhibits growth of bacteria & fungi

• Acne– bacterial inflammation of sebaceous glands– caused by increased production of sebum

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Sudoriferous glands• Simple, coiled tubular glands

• Eccrine sweat glands (merocrine) = most common– secretory portion is deep in dermis– excretory duct terminates as pore at surface

of epidermis– regulate body temp thru evaporation

(perspiration)– help eliminate wastes such as urea

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• Apocrine sweat glands – limited distribution in body

•found in armpit & groin regions – secretory portion in subcutaneous layer

•secrete via exocytosis (merocrine manner)

– excretory duct opens into hair follicle– more viscous sweat includes lipids &

proteins– begin functioning @ puberty– responsible for cold sweats

Sudoriferous glands

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Ceruminous Glands• Modified sudoriferous glands in external ear

– produce waxy substance called cerumen– secretory portion in subcutaneous layer– excretory ducts open into ear canal or sebaceous

gland– secretions = combination of oil & wax

glands• sticky barrier against foreign substances

• Impacted cerumen– abnormal amount of cerumen in external auditory

meatus or canal – prevents sound waves from reaching ear drum

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ACCESSORY STRUCTURES: Nails• Tightly packed, dead keratinized cells• Nail body

– pink, visible portion• Free edge

– extends past distal end of finger• Nail root

– portion buried in fold of skin • Lunula

– crescent-shaped area near proximal end• Hyponychium (nail bed)

– secures nail to fingertip• Eponychium (cuticle)

– stratum corneum layer• Nail matrix = growth region of nail

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FUNCTIONS OF SKIN• Thermoregulation

– Liberation of sweat lowers body temperature– Adjusts blood flow in dermis

• Constriction of vessels = warming effect• Dilation of vessels = cooling effect

– During exercise• moderate exercise: more blood brought to

surface to lower temperature• extreme exercise: blood shunted to muscles &

body temperature rises

• Blood reservoir – extensive network of blood vessels– 8-10% total blood flow in adult

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FUNCTIONS OF SKIN• Protection

– physical, chemical & biological barriers– keratin & tight cell junctions prevent bacterial invasion– lipids retard evaporation– pigment protects somewhat against UV light– Langerhans cells alert immune system to presence of

microbes, etc.

• Cutaneous sensations – touch– pressure – vibration– tickle

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FUNCTIONS OF SKIN• Synthesis of Vitamin D

– UV light activates precursor molecule in skin– enzymes in liver & kidneys modify activated

molecule to produce active form of vitamin D– necessary for absorption of calcium in the GI tract

• Excretion/Absorption– 400 mL of water/day– 200 mL/day as sweat (for sedentary person)

• excrete NH3, urea, salts via sweat

– minimal absorption of fat-soluble vitamins

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Transdermal Drug Administration

• Method of drug absorption across epidermis & into blood vessels of dermis– drug contained in adhesive skin patch– drug absorption most rapid in areas of thin

(scrotum, face and scalp)

• Examples:– nitroglycerin (prevention of chest pain from

coronary artery disease)– scopolamine (motion sickness)– estradiol (estrogen replacement therapy)– nicotine (stop smoking alternative)

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Age-Related Structural Changes • Most age-related changes occur in dermis

– Collagen fibers decrease in number & stiffen – Elastic fibers lose elasticity & thicken– Fibroblasts decrease in number

• decreased production of collagen & elastic fibers• wrinkles

• Decrease in number of melanocytes (gray hair, blotching)

• Decrease in Langerhans cells (decreased immune responsiveness)

• Reduced number and less-efficient phagocytes• Dermal blood vessels thicken

– decreased nutrient availability – loss of subcutaneous fat

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Photodamage• Ultraviolet light (UVA and UVB) both damage the

skin

• Acute overexposure causes sunburn

• DNA damage in epidermal cells can lead to skin cancer

• UVA produces oxygen free radicals that damage collagen and elastic fibers and lead to wrinkling of the skin

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Skin Cancer• 1 million cases diagnosed per year• 3 common forms of skin cancer

– basal cell carcinoma (metastasis rare)– squamous cell carcinoma (may metastasize)– malignant melanomas (rapid metastasis)

• can result in death within months of diagnosis

• key to treatment is early detection– ‘ABCD’ acronym

• risks factors: skin color, sun exposure, family history, age & immunological status