Integumentary System Notes

Integumentary System

• Integumentary system consists of:
  • Skin
  • Hair
  • Nails
  • Sweat glands
  • Sebaceous (oil) glands

Structure of Skin

• Skin consists of two distinct regions:
  • Epidermis: superficial region
    • Consists of epithelial tissue and is avascular
  • Dermis: underlies epidermis
    • Mostly fibrous connective tissue, vascular
  • Hypodermis (superficial fascia)
    • Subcutaneous layer deep to skin
    • Not part of skin but shares some functions
    • Mostly adipose tissue that absorbs shock and insulates
    • Anchors skin to underlying structures: mostly muscles

Epidermis

• Epidermis consists mostly of keratinized stratified squamous epithelium
• Four cell types found in epidermis:
  • Keratinocytes
    • Produce fibrous keratin (protein that gives skin its protective properties)
    • Major cells of epidermis
    • Tightly connected by desmosomes
    • Millions slough off every day
  • Melanocytes
    • Spider-shaped cells located in deepest epidermis
    • Produce pigment melanin, which is packaged into melanosomes
    • Melanosomes are transferred to keratinocytes, where they protect nucleus from UV damage
  • Dendritic (Langerhans) cells
    • Star-shaped macrophages that patrol deep epidermis
    • Are key activators of immune system
  • Tactile (Merkel) cells
    • Sensory receptors that sense touch

Layers of the Epidermis

• Epidermis is made up of four or five distinct layers
  • Thick skin contains five layers (strata) and is found in high-abrasion areas (hands, feet)
  • Thin skin contains only four strata
• Five layers of skin
  • Stratum basale
  • Stratum spinosum
  • Stratum granulosum
  • Stratum lucidum (only in thick skin)
  • Stratum corneum

Stratum Basale (Basal Layer)

• Deepest of all epidermal layers (base layer)
• Layer that is firmly attached to dermis
• Consists of a single row of stem cells that actively divide (mitotic), producing two daughter cells each time
  • One daughter cell journeys from basal layer to surface, taking 25–45 days to reach surface
    • Cell dies as it moves toward surface
  • Other daughter cell remains in stratum basale as stem cell
• Layer also known as stratum germinativum because of active mitosis
• 10–25% of layer also composed of melanocytes

Stratum Spinosum (Prickly Layer)

• Several cell layers thick
• Cells contain weblike system of intermediate prekeratin filaments attached to desmosomes
  • Allows them to resist tension and pulling
• Keratinocytes in this layer appear spikey, so they are called prickle cells
• Scattered among keratinocytes are abundant melanosomes and dendritic cells

Stratum Granulosum (Granular Layer)

• Four to six cells thick, but cells are flattened, so layer is thin
• Cell appearance changes
  • Cells flatten, nuclei and organelles disintegrate
  • Keratinization begins
    • Cells accumulate keratohyaline granules that help form keratin fibers in upper layers
    • Cells also accumulate lamellar granules, a water-resistant glycolipid that slows water loss
  • Cells above this layer die
    • Too far from dermal capillaries to survive

Stratum Lucidum (Clear Layer)

• Found only in thick skin
• Consists of thin, translucent band of two to three rows of clear, flat, dead keratinocytes
• Lies superficial to the stratum granulosum

Stratum Corneum (Horny Layer)

• 20–30 rows of flat, anucleated, keratinized dead cells
• Accounts for three-quarters of epidermal thickness
• Though dead, cells still function to:
  • Protect deeper cells from the environment
  • Prevent water loss
  • Protect from abrasion and penetration
  • Act as a barrier against biological, chemical, and physical assaults
• Cells change by going through apoptosis (controlled cell death)
  • Dead cells slough off as dandruff and dander
  • Humans can shed ~50,000 cells every minute

Dermis

• Strong, flexible connective tissue
• Cells include fibroblasts, macrophages, and occasionally mast cells and white blood cells
• Fibers in matrix bind body together
  • Makes up the “hide” that is used to make leather
• Contains nerves, blood vessels, and lymphatic vessels
• Contains epidermal hair follicles, oil glands, and sweat glands
• Two layers
  • Papillary
  • Reticular

Papillary Layer

• Superficial layer of areolar connective tissue consisting of loose, interlacing collagen and elastic fibers and blood vessels
  • Loose fibers allow phagocytes to patrol for microorganisms
• Dermal papillae: superficial region of dermis that sends fingerlike projections up into epidermis
  • Projections contains capillary loops, free nerve endings, and touch receptors (tactile corpuscles, also called Meissner’s corpuscles)
• In thick skin, dermal papillae lie on top of dermal ridges, which give rise to epidermal ridges
  • Collectively ridges are called friction ridges
    • Enhance gripping ability
    • Contribute to sense of touch
    • Sweat pores in ridges leave unique fingerprint pattern

Reticular Layer

• Makes up ~80% of dermal thickness
• Consists of coarse, dense fibrous connective tissue
  • Many elastic fibers provide stretch-recoil properties
  • Collagen fibers provide strength and resiliency
    • Bind water, keeping skin hydrated
• Cutaneous plexus: network of blood vessels between reticular layer and hypodermis
• Extracellular matrix contains pockets of adipose cells
• Cleavage (tension) lines in reticular layer are caused by many collagen fibers running parallel to skin surface
  • Externally invisible
  • Important to surgeons because incisions parallel to cleavage lines heal more readily
• Flexure lines of reticular layer are dermal folds at or near joints
  • Dermis is tightly secured to deeper structures
  • Skin’s inability to slide easily for joint movement causes deep creases
  • Visible on hands, wrists, fingers, soles, toes

Clinical – Homeostatic Imbalance 5.1

• Extreme stretching of skin can cause dermal tears, leaving silvery white scars called striae
  • Also known as “stretch marks”
• Acute, short-term traumas to skin can cause blisters, fluid-filled pockets that separate epidermal and dermal layers

Skin Color

• Three pigments contribute to skin color
  • Melanin
    • Only pigment made in skin; made by melanocytes
    • Made from amino acid tyrosine by tyrosinase
    • Packaged into melanosomes that are sent to shield DNA of keratinocytes from damaging UV sunlight
    • The more sun, the more need for protective shield, the more melanin will be produced
    • Two forms: reddish yellow to brownish black
    • Skin color differences are due to amount and form of melanin
    • Freckles and pigmented moles are local accumulations of melanin
  • Carotene
    • Yellow to orange pigment
    • Most obvious in palms and soles
    • Accumulates in stratum corneum and hypodermis
    • Can be converted to vitamin A for vision and epidermal health
  • Hemoglobin
    • Pinkish hue of fair skin is due to lower levels of melanin
    • Skin of Caucasians is more transparent, so color of hemoglobin shows through

Clinical – Homeostatic Imbalance 5.2

• Excessive sun exposure damages skin
  • Elastic fibers clump, causing skin to become leathery
  • Can depress immune system and cause alterations in DNA that may lead to skin cancer
  • UV light destroys folic acid
    • Necessary for DNA synthesis, so insufficient folic acid is especially dangerous for developing embryos
  • Photosensitivity is increased reaction to sun
    • Some drugs (e.g., antibiotics, antihistamines) and perfumes cause photosensitivity, leading to skin rashes

Clinical – Homeostatic Imbalance 5.3

• Alterations in skin color can indicate disease
  • Cyanosis
    • Blue skin color: low oxygenation of hemoglobin
  • Pallor (blanching or pale color)
    • Anemia, low blood pressure, fear, anger
  • Erythema (redness)
    • Fever, hypertension, inflammation, allergy
  • Jaundice (yellow cast)
    • Liver disorders
  • Bruises (black-and-blue marks)
    • Also referred to as ecchymoses or hematomas, are a result of clotted blood beneath skin
    • As clot is broken down, color of bruise changes
  • Brown or black “necklace” or bruises
    • Hyperpigmented dark areas in axillae and around neck may be a sign of insulin resistance and elevated blood glucose levels

Hair

• Consists of dead keratinized cells
• None located on palms, soles, lips, nipples, and portions of external genitalia
• Functions:
  • Warn of insects on skin
  • Hair on head guards against physical trauma
  • Protect from heat loss
  • Shield skin from sunlight

Structure of a Hair

• Hairs (also called pili): flexible strands of dead, keratinized cells
• Produced by hair follicles
• Contains hard keratin, not like soft keratin found in skin
  • Hard keratin is tougher and more durable, and cells do not flake off
• Regions:
  • Shaft: area that extends above scalp, where keratinization is complete
  • Root: area within scalp, where keratinization is still going on
• Three parts of hair shaft:
  • Medulla: central core of large cells and air spaces
  • Cortex: several layers of flattened cells surrounding medulla
  • Cuticle: outer layer consisting of overlapping layers of single cells
• Hair pigments are made by melanocytes in hair follicles
  • Combinations of different melanins (yellow, rust, brown, black) create all the hair colors
    • Red hair has additional pheomelanin pigment
    • Gray/white hair results when melanin production decreases and air bubbles replace melanin in shaft

Structure of a Hair Follicle

• Extends from epidermal surface to dermis
• Hair bulb: expanded area at deep end of follicle
• Hair follicle receptor (or root hair plexus): sensory nerve endings that wrap around bulb
  • Hair is considered a sensory touch receptor
• Wall of follicle composed of:
  • Peripheral connective tissue sheath
    • Derived from dermis
    • Also called fibrous sheath
  • Glassy membrane: thickened basal lamina
  • Epithelial root sheath
    • Derived from epidermis
• Hair matrix: actively dividing area of bulb that produces hair cells
  • As matrix makes new cells, it pushes older ones upward
• Arrector pili: small band of smooth muscle attached to follicle
  • Responsible for “goose bumps”
• Hair papilla
  • Dermal tissue containing a knot of capillaries that supplies nutrients to growing hair

Types and Growth of Hair

• Vellus hair: pale, fine body hair of children and adult females
• Terminal hair: coarse, long hair
  • Found on scalp and eyebrows
    • At puberty
      • Appear in axillary and pubic regions of both sexes
      • Also on face and neck of males
• Nutrition and hormones affect hair growth
• Follicles cycle between active and regressive phases
  • Average 2.25 mm growth per week
  • Lose 90 scalp hairs daily

Clinical – Homeostatic Imbalance 5.4

• In women, ovaries and adrenal glands produce small amounts of androgens (male sex hormones), but tumors on these organs can cause abnormally large amounts of androgens
  • Ex: polycystic ovary syndrome
    • Can result in excessive hairiness, called hirsutism, as well as other signs of masculinization
• In many cases, no clinical problem is found

Hair Thinning and Baldness

• Alopecia: hair thinning in both sexes after age 40
• True (frank) baldness
  • Genetically determined and sex-influenced condition
  • Male pattern baldness caused by follicular response to DHT (dihydrotestosterone)

Clinical - Homeostatic Imbalance 5.5

• Telogen effluvium (TE) is abrupt hair thinning caused by an abundance of hair follicles entering resting phase at same time
  • Can be due to stress-induced factors such as: hormonal changes, surgery, severe emotional trauma, or crash dieting
• Other causes or thinning and/or texture change:
  • Drugs (ex: antidepressants, chemotherapeutic drugs)
  • Hypothyroidism
  • Protein-deficient diets

Nails

• Scale-like modifications of epidermis that contain hard keratin
• Act as a protective cover for distal, dorsal surface of fingers and toes
• Consist of free edge, nail plate, and root
• Nail bed is epidermis underneath keratinized nail plate
• Nail matrix: thickened portion of bed responsible for nail growth
• Nail folds: skin folds that overlap border of nail
• Eponychium: nail fold that projects onto surface of nail body
  • Also called cuticle
• Hyponychium: area under free edge of plate that accumulates dirt
• Nails normally appear pink because of underlying capillaries
  • Lunule: thickened nail matrix, appears white
• Abnormal color or shape can be an indicator of disease

Clinical - Homeostatic Imbalance 5.6

• Nail appearance can help in diagnosing some diseases such as:
  • Yellow-tinged may indicate respiratory or thyroid gland disorder
  • Thickened yellow nails can be due to fungal infection of nail
  • Koilonchya, also called “spoon nail” – an outward concavity of nail may signal iron deficiency
  • Beau’s lines – horizontal lines across nails may indicate severe illnesses such as uncontrolled diabetes, heart attack or cancer chemotherapy

Sweat Glands

• Also called sudoriferous glands
• All skin surfaces except nipples and parts of external genitalia contain sweat glands
  • About 3 million per person
• Two main types
  • Eccrine (merocrine) sweat glands
  • Apocrine sweat glands
• Contain myoepithelial cells
  • Contract upon nervous system stimulation to force sweat into ducts

Eccrine (Merocrine) Sweat Glands

• Most numerous type
• Abundant on palms, soles, and forehead
• Ducts connect to pores
• Function in thermoregulation
  • Regulated by sympathetic nervous system
• Their secretion is sweat
  • 99% water, salts, vitamin C, antibodies, dermcidin (microbe-killing peptide), metabolic wastes

Apocrine Sweat Glands

• Confined to axillary and anogenital areas
• Secrete viscous milky or yellowish sweat that contains fatty substances and proteins
  • Bacteria break down sweat, leading to body odor
• Larger than eccrine sweat glands with ducts emptying into hair follicles
• Begin functioning at puberty
  • Function unknown but may act as sexual scent gland
• Modified apocrine glands
  • Ceruminous glands: lining of external ear canal; secrete cerumen (earwax)
  • Mammary glands: secrete milk

Sebaceous (Oil) Glands

• Widely distributed, except for thick skin of palms and soles
• Most develop from hair follicles and secrete into hair follicles
• Relatively inactive until puberty
  • Stimulated by hormones, especially androgens
• Secrete sebum
  • Oily holocrine secretion
  • Bactericidal (bacteria-killing) properties
  • Softens hair and skin

Clinical – Homeostatic Imbalance 5.7

• Acne is usually an infectious inflammation of the sebaceous glands, resulting in pimples (pustules or cysts)
  • Associated with Propionibacterium acne infection
• Whiteheads are blocked sebaceous glands
  • If secretion is oxidized, whitehead becomes a blackhead
• Overactive sebaceous glands in infants can lead to seborrhea, known as “cradle cap”
  • Begins as pink, raised lesions on scalp that turn yellow/brown and flake off

Functions of Skin

• Skin is first and foremost a barrier
• Its main functions include:
  • Protection
  • Body temperature regulation
  • Cutaneous sensations
  • Metabolic functions
  • Blood reservoir
  • Excretion of wastes

Protection

• Skin is exposed to microorganisms, abrasions, temperature extremes, and harmful chemicals
• Constitutes three barriers:
  • Chemical barrier
  • Physical barrier
  • Biological barrier
• Chemical barrier
  • Skin secretes many chemicals, such as:
    • Sweat, which contains antimicrobial proteins
    • Sebum and defensins, which kill bacteria
    • Cells also secrete antimicrobial defensin
  • Acid mantle: low pH of skin retards bacterial multiplication
  • Melanin provides a chemical barrier against UV radiation damage
• Physical barrier
  • Flat, dead, keratinized cells of stratum corneum, surrounded by glycolipids, block most water and water-soluble substances
  • Some chemicals have limited penetration of skin
    • Lipid-soluble substances
    • Plant oleoresins (e.g., poison ivy)
    • Organic solvents (acetone, paint thinner)
    • Salts of heavy metals (lead, mercury)
    • Some drugs (nitroglycerin)
    • Drug agents (enhancers that help carry other drugs across skin)
• Biological barriers
  • Epidermis contains phagocytic cells
    • Dendritic cells of epidermis engulf foreign antigens (invaders) and present to white blood cells, activating the immune response
  • Dermis contains macrophages
    • Macrophages also activate immune system by presenting foreign antigens to white blood cells
  • DNA can absorb harmful UV radiation, converting it to harmless heat

Clinical – Homeostatic Imbalance 5.8

• If organic solvents or heavy metals pass through skin and enter blood, multiple organ systems can be devastated which can be lethal
  • Examples:
    • Kidneys can shut down
    • Brain can be damaged
    • Absorption of lead can result in anemia and neurological defects.
• Care must always be used when handling organic solvents or heavy metals
  • Should never be touched with bare hands

Body Temperature Regulation

• Under normal, resting body temperature, sweat glands produce about 500 ml/day of unnoticeable sweat
  • Called insensible perspiration
• If body temperature rises, dilation of dermal vessels can increase sweat gland activity to produce 12 L (3 gallons) of noticeable sweat
  • Called sensible perspiration; designed to cool body
• Cold external environment
  • Dermal blood vessels constrict
  • Skin temperature drops to slow passive heat loss

Cutaneous Sensations

• Cutaneous sensory receptors are part of the nervous system
  • Exteroreceptors respond to stimuli outside body, such as temperature and touch
  • Free nerve endings sense painful stimuli

Metabolic Functions

• Skin can synthesize vitamin D needed for calcium absorption in intestine
• Chemicals from keratinocytes can disarm some carcinogens
• Keratinocytes can activate some hormones
  • Example: convert cortisone into hydrocortisone
• Skin makes collagenase, which aids in natural turnover of collagen to prevent wrinkles

Blood Reservoir

• Skin can hold up to 5% of the body’s total blood volume
• Skin vessels can be constricted to shunt blood to other organs, such as an exercising muscle

Excretion

• Skin can secrete limited amounts of nitrogenous wastes, such as ammonia, urea, and uric acid
• Sweating can cause salt and water loss

Skin Cancer and Burns

• Skin can develop over 1000 different conditions and ailments
• Many internal diseases reveal themselves on skin
• Most common disorders are infections
• Less common, but more damaging, are:
  • Skin cancer
  • Burns

Skin Cancer

• Most skin tumors are benign (not cancerous) and do not spread (metastasize)
• Risk factors
  • Overexposure to UV radiation
  • Frequent irritation of skin
• Some skin lotions contain enzymes that can repair damaged DNA
• Three major types of skin cancer
  • Basal cell carcinoma
  • Squamous cell carcinoma
  • Melanoma
• Basal cell carcinoma
  • Least malignant and most common
  • Stratum basale cells proliferate and slowly invade dermis and hypodermis
  • Cured by surgical excision in 99% of cases
• Squamous cell carcinoma
  • Second most common type; can metastasize
  • Involves keratinocytes of stratum spinosum
  • Usually is a scaly reddened papule on scalp, ears, lower lip, or hands
  • Good prognosis if treated by radiation therapy or removed surgically
• Melanoma
  • Cancer of melanocytes; is most dangerous type because it is highly metastatic and resistant to chemotherapy
  • Treated by wide surgical excision accompanied by immunotherapy
  • Key to survival is early detection: ABCD rule
    • A: asymmetry; the two sides of the pigmented area do not match
    • B: border irregularity; exhibits indentations
    • C: color; contains several colors (black, brown, tan, sometimes red or blue)
    • D: diameter; larger than 6 mm (size of pencil eraser)

Burns

• Tissue damage caused by heat, electricity, radiation, or certain chemicals
  • Damage caused by denaturation of proteins, which destroys cells
• Immediate threat is dehydration and electrolyte imbalance
  • Leads to renal shutdown and circulatory shock
• To evaluate burns, the Rule of Nines is used
  • Body is broken into 11 sections, with each section representing 9% of body surface (except genitals, which account for 1%)
  • Used to estimate volume of fluid loss
• Burns can be classified by severity
  • First-degree
    • Epidermal damage only
    • Localized redness, edema (swelling), and pain
  • Second-degree
    • Epidermal and upper dermal damage
    • Blisters appear
    • First- and second-degree burns are referred to as partial-thickness burns because only the epidermis and upper dermis are involved
  • Third-degree
    • Entire thickness of skin involved (referred to as full-thickness burns)
    • Skin color turns gray-white, cherry red, or blackened
    • No edema is seen and area is not painful because nerve endings are destroyed
    • Skin grafting usually necessary
• Burns are considered critical if:
  • >25% of body has second-degree burns
  • >10% of body has third-degree burns
  • Face, hands, or feet bear third-degree burns
• Treatment includes:
  • Debridement (removal) of burned skin
  • Antibiotics
  • Temporary covering
  • Skin grafts

Developmental Aspects of the Integumentary System

• Fetal: by end of 4th month, skin of fetus is developed
  • Lanugo coat: delicate hairs in 5th and 6th month
  • Vernix caseosa: sebaceous gland secretion that protects skin of fetus while in watery amniotic fluid
• Infancy to adulthood: skin thickens and accumulates more subcutaneous fat; sweat and sebaceous gland activity increases, leading to acne
  • Optimal appearance during 20s and 30s
  • After age 30, effects of cumulative environmental assaults start to show
  • Scaling and dermatitis become more common
• Aging skin
  • Epidermal replacement slows; skin becomes thin, dry, and itchy (decreased sebaceous gland activity)
  • Subcutaneous fat and elasticity decrease, leading to cold intolerance and wrinkles
  • Increased risk of cancer due to decreased numbers of melanocytes and dendritic cells
  • Hair thinning
• Ways to delay aging:
  • UV protection, good nutrition, lots of fluids, good hygiene