The Integumentary System
Chapter Overview
1. Skin Composition
2. Epidermis
3. Dermis
4. Skin Color
5. Accessory Structures of the Skin
6. Skin Damage and Repair
7. Functions of Skin
Introduction
Skin
1. covers the external surface of body
2. largest organ of body in surface area and
weight
3. consists of two main layers – epidermis
and dermis
4. associated with many other structures –
accessory parts
5. skin with its accessory parts together
forms the integumentary system
6. performs numerous functions
Skin Composition
Layers of Integument
•
Epidermis
– most superficial
•
Dermis
– deep to epidermis
•
Hypodermis
– anchors the dermis
Figure 1.1a
Figure 1.1b
Skin Structure
Two principal parts to skin
1. epidermis
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thin
composed of epithelial tissue
2. dermis
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thicker
composed of connective tissue
Hypodermis
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deep to dermis but not part of skin
AKA subcutaneous layer
areolar and adipose tissues
fibers from dermis anchor it to hypodermis
Thin Skin
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covers all parts of body except for palms,
palmar surfaces of fingers, and soles of
feet
epidermis is thin
has hair and sebacous glands
fewer sudoriferous glands than thick skin
sparcer distribution of sensory receptors
Thick Skin
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covers palms
palmar surfaces of fingers
soles of feet
thick epidermis
lacks hair and sebacous glands
has more sudoriferous glands
sensory receptors more densely
clustered
Epidermis
Epidermis
•
•
composed of keratinized stratified
squamous epithelium
four principal types of cells
1.
2.
3.
4.
keratinocytes
melanocytes
Langerhans cells
Merkel cells
Figure 5.2a
Keratinocytes
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most numerous
arranged in 4 or 5 layers
produce keratin
produce lamellar granules
– waterproofing sealant
Melanocytes
•
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found in deepest layer of epidermis
produce melanin
– pigment that blocks UV radiation
– clusters around nucleus of cells
•
•
long slender projections extend between
keratinocytes and transfer melanin to
them
melanocytes provide protective melanin
to keratinocytes but vulnerable to UV
radiation themselves
Langerhans cells
•
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small fraction of epidermal cells
participate in immunity
easily damaged by UV radiation
Merkel Cells
•
•
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least numerous of epidermal cells
located in deepest layer
contact flattened process of sensory
neuron a tactile (Merkel) disc
function together with the disc in
sensation of touch
Layers of Epidermis
Deepest to most superficial
• stratum basale
• stratum spinosum
• statum granulosum
• stratum lucidum (only in thick skin)
• stratum corneum
Stratum Basale
•
single row of cuboidal or columnar
keratinocytes
– some are stem cells
•
•
continually undergo cell division
melanocytes and Merkel cells found
scattered here also
Stratum Spinosum
•
•
•
8 to 10 layers tightly packed many-sided
keratinocytes
most superficial become somewhat
flattened
microscopic preparation shrinks cells
– they pull apart and appear covered with
spines
– living cells are rounded and plump, however
Stratum Granulosum
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middle most layer
5 layers of tightly packed many sided
keratinocytes
flattened
undergoing apoptosis
granules are membrane-enclosed
lamellar granules
– release lipid-rich secretion water-repellent
Stratum Lucidum
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present only in thick skin
3 to 5 layers of clear flat dead
keratinocytes
– cells contain large amounts of keratin
Stratum Corneum
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most superficial layer
25 to 30 layers of dead flat keratinocytes
continually shed and replaced by cells
from deeper strata
interior of cells mostly keratin
lipids from lamellar granules is between
cells
protects against injury and microbes
serves as effective water-repellent
Keratinization and Growth
•
•
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cells of stratum basale pushed toward
surface as new cells appear
more keratin produced along the way
takes about 4 weeks in average
epidermis
Dermis
Dermis
• deep to epidermis
• composed mainly of connective tissue
– containing collagen and elastic fibers
• embedded in dermis
– blood vessels, nerves, glands, hair follicles
• cells of dermis
– fibroblasts
– macrophages
– adipocytes
Layers of Dermis
• papillary region
– 1/5th of total dermis
– areolar connective tissue with fine elastic
fibers
– surface area enhanced by dermal papillae
– contain
• capillary loops
• corpuscles of touch (Meissner corpuscles)
• free nerve endings
Layers of Dermis
• reticular region
– 80% of dermis
– attached to hypodermis
– consists of denser irregular connective tissue
• collagen fibers and some coarse elastic fibers
– between fibers are
hair follicles
nerves
sebacous glands
sudoiferous glands
– provides extensibility and elasticity
– extreme stretching of dermis causes striae (stretch
marks)
Figure 5.1a
Figure 5.1b
Layers of Dermis
• epidermal ridges
– either straight lines or loops
– caused by epidermis projecting into dermis
between dermal papillae
– increase surface area
– function to increase grip of hands and feet
– glands open on tops of ridges leaving
behind fingerprints and footprints when
smooth objects are touched.
Skin Color
Skin color
•
Three main pigment producing molecules
1. melanin
2. carotene
3. hemoglobin
Melanin
produced by melanocytes
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number of melanocytes same in most people
activity of melanocytes differs
exposure to UV light increases production
Carotene
precursor of vitamin A
•
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vitamin A used to synthesize pigments
needed for vision
found in stratum corneum and fatty areas of
the dermis and hypodermis
without melanin and carotene, skin
appears translucent
•
European-Americans have pink skin because
hemoglobin shows through
Accessory Structures
of Skin
Hair
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present on most skin surfaces except
palms, palmar surfaces of fingers, soles,
and plantar surfaces of toes
adults’ hair most heavily distributed on
scalp, eyebrows, armpits, and external
genitalia
genetic and hormonal influences
determine thickness and patterns of
distribution
largely protective functions
Anatomy of Hair
• composed of columns of dead,
keratinized cells bonded together by
extracellular proteins
• shaft is superficial portion
• transverse section varies
– perfectly rounded = straight hair
– oval shaped section = wavy hair
– kidney shaped section = curly hair
Anatomy of Hair
• three concentric layers
– inner medulla
• pigment granules and air spaces
– middle cortex
• pigment in dark hair
• mostly air in gray or white hair
– outer cuticle
• single layer of thin, flat, heavily keratinized
cells
• arranged like shingles on a roof with free
edge pointing toward end of hair
Hair Roots
•
•
hair follicle surrounds root of hair
external root sheath
– downward continuation of epidermis
• internal root sheath
– produced by matrix
• dermal root sheath
– dense dermis surrounding hair follicle
• bulb of hair enlarged at root
– has germinal layer of cells called matrix
Hair Roots
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•
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sebacous glands associated with follicle
arrector pilli muscle makes hair stand on
end
hair root plexuses
- free nerve endings that surround hair
follicle making dermis
sensitive to hair movement
Figure 5.3a
Figure 5.3b
Figure 5.3c
Figure 5.3d
Hair Growth
• growth stage
– matrix cells differentiate, keratinize, and
die
– forms root sheath and hair shaft
– as new cells areise, hair grows longer
– scalp hair 2 to 6 years
– average 85% of hair in this stage
• resting stage
– old hair is pushed out of follicle
– 3 months
Hair Color
• melanin produced by melanocytes in
matrix of hair bulb
• dark hair has true melanin deposited
in cells
• blond and red hair have melanin
variants with more iron and sulfur
• gray hair has less melanin
• white hair has air bubbles and no
melanin
Glands of Skin
• sebaceous glands
– secrete oil onto shaft of hair or surface
of skin
• sudoiferous glands
– secrete sweat
• eccrine or apocrine
• ceruminous glands
– ears
• mammary glands
– breasts
Sebaceous Glands
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also known as oil glands
connected to hair follicles
secreting portion in dermis
found everywhere except palms and
soles
• secrete sebum
– triglycerides, cholesterol, proteins,
inorganic salts
– keeps skin soft and pliable, prevents
excessive water evaporation from skin
– inhibits growth of certain skin bacteria
Sudoriferous Glands
• also known as sweat glands
• eccrine
– most common type
– evenly distributed
• most common on forehead, palms, and
souls (up to 450 per square cm)
– secrete sweat with water, ions, urea,
uric acid, ammonia, amino acids,
glucose, and lactic acid
– main function is temperature regulation
– also mimimal participation in waste
elimination
Sudoriferous Glands
•
appocrine sweat glands
– less common
– found mainly
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•
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axilla
groin
areolae
bearded region of male faces
– secretory portion in hypodermis
– opens mainly onto hair follicle
– same components as eccrine PLUS lipids and
proteins
– function begins in puberty
– stimulated during emotional stress and sexual
excitement
Sudoriferous Glands
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ceruminous
– secrete cerumen (ear wax)
– found in external auditory canal
– secrete products onto surface of canal or into
sebaceous glands in ear
– function to provide sticky barrier to impede
entrance of foreign bodies into ear
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Nails
plates of tightly packed, hard, keratinized
epidermal cells
parts
– nail body
•
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visible portion
appears pink due to underlying blood capillaries
lunula near root is white because of thickened
matrix
– free edge
•
extends past finger or nail body
– nail root
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portion buried in fold of skin at base of nail
matrix with mitotic cells here
– cuticle
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stratum corneum
Figure 5.4a
Figure 5.4b
Functions of Skin
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thermoregulation
blood storage
protection
cutaneous sensations
excretion and absorption
synthesis of calcitrol (vitamin D)
Functions of Skin
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thermoregulation
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regulates body temperature
liberating sweat allowing evaporative cooling of blood
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reducing flow of blood in dermis when cool to conserve
heat
blood reservoir
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increasing blood flow in dermis
8 to 10% of total blood flow in resting adults found in
dermal blood vessels
cutaneous sensations
– tactile (touch, pressure, vibration, tickling)
– thermal (warmth and coolness)
– pain (usually indication of impending or actual
tissue damage)
Functions of Skin
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protection
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from invasion by microbes
from abrasion
from heat
from chemicals
from water evaporation through cells
from influx of water through skin
sebum and pH of sweat are antimicrobial
melanin protects from UV radiation
Langerhans cells are active in immunity
macrophages phagocytize bacteria and viruses
that manage to penetrate
Functions of Skin
•
excretion and absorption
– minor role in eliminating wastes
– minor role in absorbing materials from external
environment
– sweat excretes water, salts, carbon dioxide,
ammonia and urea
•
synthesis of calcitrol
– most active form of vitamin D
– synthesized in response to UV exposure
– aids in absorption of calcium from foods