Transcript Chpa

Chapter 6: Skin and Its
Appendages
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INTRODUCTION
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Skin (integument) is the body’s largest organ
Approximately 1.6 to 1.9 m2 in average-sized
adult
Integumentary system describes the skin
and its appendages: hair, nails, and skin
glands
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STRUCTURE OF THE SKIN
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Skin classified as a cutaneous membrane
Two primary layers: epidermis and dermis;
joined by dermoepidermal junction (Figures
6-1 and 6-2)
Hypodermis lies beneath dermis
Thin and thick skin (Figure 6-3)
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Thin skin covers most of the body’s surface (1 to 3
mm thick)
Thick skin covers soles and palms (4 to 5 mm
thick)
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STRUCTURE OF THE SKIN:
EPIDERMIS
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Epidermis
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Cell types (Figure 6-4)
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Keratinocytes: constitute more than 90% of cells present;
principal structural element of the outer skin; sometimes called
corneocytes after they are fully keratinized
Melanocytes: pigment-producing cells (5% of the total);
contribute to skin color and filter ultraviolet (UV) light
Epidermal dendritic cells: branched antigen-presenting cells;
play a role in immune response; also called Langerhans cells
Tactile epithelial cells (Merkel cells): attach to sensory nerve
endings to form “light touch” receptors
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STRUCTURE OF THE SKIN:
EPIDERMIS (cont.)
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Cell layers
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Stratum basale (base layer): single layer of columnar
cells; only these cells undergo mitosis and then migrate
through the other layers until they are shed
Stratum spinosum (spiny layer): cells arranged in eight
to 10 layers with desmosomes that pull cells into spiny
shapes; cells rich in RNA
Stratum germinativum (growth layer): another name for
stratum basale or stratum spinosum and stratum basale
together
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STRUCTURE OF THE SKIN:
EPIDERMIS (cont.)
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Stratum granulosum (granular layer): cells arranged in
two to four layers and filled with keratohyalin granules;
contains high levels of lysosomal enzymes
Stratum lucidum (clear layer): cells filled with keratin
precursor called eleidin; absent in thin skin
Stratum corneum (horny layer): most superficial layer;
thin squamous cells, dead cells filled with keratin
(barrier area – to environment and water loss)
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STRUCTURE OF THE SKIN:
EPIDERMIS (cont.)
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Epidermal growth and repair
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Turnover or regeneration time refers to time required for
epidermal cells to form in the stratum basale and migrate to
the skin surface—approximately 35 days
Epidermal growth factor regulates epidermal growth and
repair
Shortened turnover time due to abrasion will increase the
thickness of the stratum corneum and result in callus
formation
Normally 10% to 12% of all cells in stratum basale enter
mitosis daily
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STRUCTURE OF THE SKIN:
EPIDERMIS (cont.)
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Dermoepidermal junction
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A basement membrane with unique fibrous
elements and a polysaccharide gel “glue” the
epidermis to the dermis below
The junction is a partial barrier to the passage of
some cells and large molecules
Separation of the epidermis from the DEJ
result in blisters
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STRUCTURE OF THE SKIN: DERMIS
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Dermis
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Sometimes called “true skin”—much thicker than the
epidermis and lies beneath it
Gives strength to the skin
Serves as a reservoir storage area for water and
electrolytes
Contains various structures
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Arrector pili muscles and hair follicles (Figure 6-5)
Sensory receptors (Figure 6-6)
Sweat and sebaceous glands
Blood vessels
Rich vascular supply plays a critical role in temperature
regulation
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STRUCTURE OF THE SKIN:
DERMIS (cont.)
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Layers of dermis
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Papillary layer: composed of dermal papillae that
project into the epidermis; contains fine collagenous
and elastic fibers and the dermoepidermal junction;
forms a unique pattern that gives individual fingerprints
Reticular layer: contains dense, interlacing white
collagenous fibers and elastic fibers to make the skin
tough yet stretchable; when processed from animal
skin, produces leather
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STRUCTURE OF THE SKIN:
DERMIS (cont.)
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Dermal growth and repair
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The dermis does not continually shed and regenerate
itself as does the epidermis
During wound healing, fibroblasts begin forming an
unusually dense mass of new connective fibers; if not
replaced by normal tissue, this mass remains a scar
Cleavage lines: patterns formed by the collagenous
fibers of the reticular layer of the dermis; also called
Langer’s lines (Figure 6-7)
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STRUCTURE OF THE SKIN:
HYPODERMIS
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Hypodermis
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Also called the subcutaneous layer or superficial fascia
Located deep to the dermis; forms connection between
skin and other structures
Not part of the skin
Rich blood supply makes it a perfect site for fast a
relatively pain free absorption of injected material – insulin
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SKIN COLOR
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Melanin
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Basic determinant is quantity, type, and distribution of
melanin
Types of melanin
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Melanin formed from tyrosine by melanocytes (Figure 6-8)
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Eumelanin: group of dark brown, almost black, melanins
Pheomelanin: group of reddish and orange melanins
Melanocytes release melanin in packets called melanosomes
Melanosomes are ingested by surrounding keratinocytes and
form a cap over the nucleus
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SKIN COLOR (cont.)
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Albinism: congenital absence of melanin
Expression of melanin genes regulated by tyrosinase,
exposure to sunlight (UV radiation), and certain hormones,
including melanocortins (adrenocorticotropic hormone,
alpha melanocyte stimulating hormone) and endothelin-1
(Figures 6-9 and 6-10)
Cumulative effects of UV ray exposure may produce age
spots (Figure 6-11)
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SKIN COLOR: OTHER PIGMENTS
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Other pigments
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Beta-carotene (group of yellowish pigments from food) can
also contribute to skin color
Hemoglobin: color changes also occur as a result of
changes in blood flow
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Redder skin color when blood flow to skin increases
Cyanosis: bluish color caused by darkening of hemoglobin
when it loses oxygen and gains carbon dioxide (Figure 6-12)
Bruising can cause a rainbow of different colors to appear in
the skin (Figure 6-13)
Other pigments: from cosmetics, tattoos, and bile
pigments in jaundice (Box 6-4)
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FUNCTIONS OF THE SKIN
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Protection (Table 6-2)
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Physical barrier to microorganisms
Barrier to chemical hazards
Reduces potential for mechanical trauma
Prevents dehydration
Protects from excess UV ray exposure (melanin function)
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FUNCTIONS OF THE SKIN (cont.)
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Surface film
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Emulsified protective barrier formed by mixing of
residue and secretions of sweat and sebaceous
glands with sloughed epithelial cells from skin
surface; shedding of epithelial elements is called
desquamation
Functions
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Antibacterial, antifungal activity
Lubrication
Hydration of skin surface
Buffer of caustic irritants
Blockade of toxic agents
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FUNCTIONS OF THE SKIN (cont.)
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Chemical composition
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From epithelial elements: amino acids, sterols,
and complex phospholipids
From sebum: fatty acids, triglycerides, and
waxes
From sweat: water, ammonia, urea, and lactic
and uric acid
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FUNCTIONS OF THE SKIN (cont.)
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Sensation
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Skin acts as a sophisticated sense organ
Somatic sensory receptors detect stimuli that
detection of pressure, touch, temperature, pain,
and other general senses
Flexibility
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Skin is supple and elastic, thus permitting change
in body contours without injury
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FUNCTIONS OF THE SKIN (cont.)
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Excretion
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Water
Urea/ammonia/uric acid
Hormone (vitamin D) production (Figure 6-14)
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Exposure of skin to UV light converts 7-dehydrocholesterol
to cholecalciferol, a precursor to vitamin D
Blood transports precursor to liver and kidneys, where
vitamin D is produced
Process and end result fulfill the necessary steps required
for vitamin D to be classified as a hormone
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FUNCTIONS OF THE SKIN (cont.)
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Immunity
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Phagocytic cells destroy bacteria
Epidermal dendritic cells trigger helpful immune reaction
working with helper T cells
Homeostasis of body temperature
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To maintain homeostasis of body temperature, heat
production must equal heat loss; skin plays a critical role in
this process
Heat production
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By metabolism of foods in skeletal muscles and liver
Chief determinant of heat production is the amount of
muscular work being performed
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FUNCTIONS OF THE SKIN (cont.)
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Homeostasis of body temperature
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Heat loss: approximately 80% of heat loss occurs through
the skin; remaining 20% occurs through the mucosa of the
respiratory, digestive, and urinary tracts (Figure 6-15)
 Evaporation: to evaporate any fluid, heat energy must
be expended; this method of heat loss is especially
important at high environmental temperatures when it is
the only method heat can be lost from the skin
 Radiation: transfer of heat from one object to another
without actual contact; important method of heat loss in
cool environmental temperatures
 Conduction: transfer of heat to any substance in contact
with the body; accounts for relatively small amounts of
heat loss
 Convection: transfer of heat away from a surface by
movement of air; usually accounts for a small amount of
heat loss
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FUNCTIONS OF THE SKIN (cont.)
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Homeostatic regulation of heat loss (Figure 6-16)
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Heat loss by the skin is controlled by a negative
feedback loop
Receptors in the hypothalamus monitor the body’s
internal temperature
If body temperature is increased, the hypothalamus
sends a nervous signal to the sweat glands and blood
vessels of the skin
The hypothalamus continues to act until the body’s
temperature returns to normal
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APPENDAGES OF THE SKIN
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Hair (Figure 6-17)
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Development of hair
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Distribution is over entire body except palms of hands and
soles of feet and a few other small areas
Fine and soft hair coat present before birth called lanugo
Coarse pubic and axillary hair that develops at puberty called
terminal hair
Papilla: cluster of capillaries under germinal matrix
Root: part of hair embedded in follicle in dermis
Shaft: visible part of hair
Medulla: inner core of hair
Cortex: outer portion
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APPENDAGES OF THE SKIN: HAIR
(cont.)
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Appearance of hair
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Color: result of different amounts, distribution, types of
melanin in cortex of hair (Figure 6-18)
Growth: growth and rest periods alternate; hair on head
averages 5 inches of growth per year
Sebaceous glands attach to and secrete sebum (skin
oil) into follicle
Male pattern baldness (androgenic alopecia) results
from combination of genetic tendency and male sex
hormones (Figure 6-19)
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APPENDAGES OF THE SKIN (cont.)
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Nails (Figure 6-20)
 Consist of epidermal cells converted to hard keratin
 Nail body: visible part of each nail
 Root: part of nail in groove hidden by fold of skin, the cuticle
 Lunula: moon-shaped white area nearest root
 Nail bed: layer of epithelium under nail body; contains abundant
blood vessels
 Appears pink under translucent nails
 Nails may have pigmented streaks (Figure 6-21)
 Separation of a nail from the nail bed is called onycholysis
(Figure 6-22)
 Growth: nails grow by mitosis of cells in stratum basale beneath
the lunula; average growth about 0.5 mm/week, or slightly over 1
inch per year
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APPENDAGES OF THE SKIN (cont.)
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Skin glands (Figure 6-23)
 Two types of sweat glands
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Eccrine glands
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Apocrine glands
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Most numerous sweat glands; quite small
Distributed over total body surface with exception of a few small areas
Simple, coiled, tubular glands
Function throughout life
Secrete perspiration or sweat; eliminate wastes and help maintain a
constant core temperature
Located deep in subcutaneous layer
Limited distribution: axilla, areola of breast, and around anus
Large (often more than 5 mm in diameter)
Simple, branched, tubular glands
Begin to function at puberty
Secretion shows cyclic changes in female with menstrual cycle
Odor is from bacteria feeding off gland production
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APPENDAGES OF THE SKIN: SKIN
GLANDS (cont.)
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Skin glands (cont.)
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Sebaceous glands
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Ceruminous glands
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Secrete sebum: oily substance that keeps hair and skin soft and
pliant; prevents excessive water loss from skin
Lipid components have antifungal activity
From hair follicles
Found in dermis except in palms and soles
Secretion increases in adolescence; may lead to formation of
pimples and blackheads
Modified apocrine sweat glands
Empty contents into external ear canal alone or with sebaceous
glands
Mixed secretions of sebaceous and ceruminous glands called
cerumen (wax)
Function of cerumen to protect area from dehydration; excess
secretion can cause blockage of ear canal and loss of hearing
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CYCLE OF LIFE: SKIN
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Children
 Skin is smooth, unwrinkled, and characterized by elasticity and
flexibility
 Few sweat glands
 Rapid healing
Adults
 Development and activation of sebaceous and sweat glands
 Increased sweat production
 Body odor
 Increased sebum production
 Acne
Old age
 Decreased sebaceous and sweat gland activity
 Wrinkling (Figure 6-24)
 Decrease in body’s ability to cool itself
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THE BIG PICTURE: SKIN AND THE
WHOLE BODY
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Skin is a major component of the body’s
structural framework
Skin defines the internal environment of the
body
Primary functions are support and protection
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Severity of Burns
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First-degree burns
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Second-degree burns AKA partial thickness
burns
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Only epidermis is damaged
Skin is red and swollen
Epidermis and upper dermis are damaged
Skin is red with blisters
Third-degree burns
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Destroys entire skin layer
Burn is gray-white or black
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