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Biology 231
Human Anatomy and Physiology
Chapter 5 Lecture Outline
Integumentary System – skin and accessory structures
Dermatology – medical specialty for diagnosing and treating skin disorders
Skin (cutaneous membrane) – boundary between body and external
environment
2 layers of skin:
epidermis – superficial layer of epithelial tissue
dermis – deep layer of connective tissue
subcutaneous layer (subQ or hypodermis) not part of the skin; areolar
and adipose tissue layer deep to skin
Epidermis – keratinized stratified squamous epithelium
no blood vessels – nourished by diffusion from dermis
4 Primary Cell Types:
keratinocytes – 90% of cells; arranged in layers
keratin – tough, fibrous protein produced by keratinocytes;
protective barrier to heat, microbes and chemicals
melanocytes – 8% of cells; have slender projections running between
keratinocytes; produce melanin
melanin – black-brown pigment produced in melanosomes by
an enzyme (tyrosinase – converts tyrosine to melanin)
enzyme activity increases with exposure to UV radiation
absorbs UV radiation to protect keratinocyte DNA
melanoma – cancer of melanocytes
Merkel cells – associated with a nerve ending in the dermis;
detects touch and pressure
Langerhans cells – phagocytic cells of epidermis
Keratinocyte Growth and Development
cells divide in basal layer and are pushed toward surface;
cells become keratinized (accumulate keratin) as they approach the
surface;
cells undergo apotosis (programmed death);
surface cells are constantly sloughed and replaced;
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average time for process is 4-6 weeks
damage to surface layers stimulates faster growth
epidermal growth factor – protein which stimulates growth
Keratinocyte Developmental Layers (strata) – deep to superficial
Stratum germinativum (basale)– deepest layer, anchored to basement
membrane
basal cells (stem cells) – keratinocytes that undergo division
melanocytes, Merkel cells
Stratum spinosum – 8-10 layers of living cells; protein fibers connected
to desmosomes give spiny appearance
Langerhans cells
Stratum granulosum – 3-5 layers of flattened cells undergoing apotosis
keratin
keratohyalin – dark-staining protein granules that dehydrate cells
and cross-link keratin molecules
Stratum lucidum – only apparent in thick skin of fingertips, palms and
soles; 3-5 layers of clear, flat, dead cells; thick plasma membrane
and lots of keratin
Stratum corneum – 15-30 layers of dead, flat keratinocytes
full of keratin; continuously shed and replaced;
callus – thickening of stratum corneum due to constant friction
Dermis – connective tissue layer supporting epidermis; contains fibroblasts,
macrophages, some adipocytes, blood vessels, nerves, glands and hair follicles;
provides skin with strength, extensibility and elasticity
2 regions:
1) Papillary layer – areolar connective tissue; highly vascular (blood vessels)
dermal papillae – finger-like projections extending into epidermis
epidermal ridges – cover large dermal papillae on thick skin
provide stronger adhesion to basement membrane
sweat on surface causes fingerprints
sensory innervation
tactile corpuscles (touch receptors)
free nerve endings (detect heat & cold, pain, itching & tickling)
(lamellated corpuscles – detect pressure in deeper layers)
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2) Reticular layer – deep region of dense irregular connective tissue
gives skin its strength and elasticity
contains adipose cells, hair follicles, nerves, sebaceous and sweat
glands
stretch marks – tears in dermis
Skin Types
Thin skin (hairy) –most of body; thickness of sandwich bag
no stratum lucidum; has hair and sebaceous glands; fewer sensory
receptors and sweat glands
Thick skin (hairless) – palms, palmar surfaces of digits, soles of feet;
thickness of paper towel
visible stratum lucidum; large epidermal ridges; no hair or sebaceous
glands present; more sensory receptors and sweat glands
Skin color – due to 3 pigments in normal skin
1) melanin – amount and darkness variable in individual
freckles – patches of melanin
2) carotene – yellow-orange pigment; in stratum corneum and adipose tissues
3) hemoglobin – in red blood cells; red when carrying oxygen
cyanosis – bluish color due to oxygen depleted hemoglobin
erythema – redness due to engorged capillaries
albinism –inherited inability to produce melanin; melanocytes unable to produce
enzyme (tyrosinase)
jaundice – yellow color due to buildup of bilirubin in blood; usually due to
liver disease
ACCESSORY STRUCTURES OF SKIN – modifications of epidermis
1) Hair – found on thin skin
Structure – dead, keratinized cells bonded by extracellular proteins
2 regions of hair:
Shaft – superficial portion; projects from skin
Root – deep portion; penetrates dermis and anchors hair
3 layers of hair:
medulla – innermost; has pigment & soft keratin
cortex – middle; contains pigment & soft keratin
cuticle – outermost; scale-like cells with hard keratin
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Hair Follicle – produces and nourishes growing hair
3 layers of follicle:
external root sheath – continuation of epidermis
internal root sheath – produced by hair matrix
glassy membrane – thick basement membrane
hair bulb – at base of follicle
matrix – stem cells that divide to form hair line inner surface of
hair bulb
papilla – surrounded by bulb; connective tissue with blood vessels
that nourish growing hair
arrector pili muscle – smooth muscle that makes hair stand up
root hair plexus – nerve endings around hair root, sensitive to touch
Hair Growth Cycle
growth stage – matrix cells divide, differentiate, keratinize,
and die; lasts 2-5 years for scalp hair
resting stage – growth stops; about 3 months
old hair falls out when growth stage resumes
Hair Color – melanocytes in matrix
dark hair – true melanin
blond or red – melanin contains sulfur & iron
Functions of hair – prevents heat loss and sunburn; protects from foreign
particles (eyelashes, nose); senses light touches (root plexus)
2) Skin Glands – exocrine glands
Sebaceous glands (oil glands) – lie in dermis; open into hair follicles
coat hair and skin to moisturize and inhibit bacteria
sebum – mixture of triglycerides, cholesterol, proteins and salts
coats hair and skin to moisturize and inhibit bacteria
acne – bacterial infection of sebaceous glands and hair follicles
Sweat glands (sudoriferous glands) – produce watery secretions
2 types:
Merocrine sweat glands – located widely in dermis and open at
surface pores; function in evaporative cooling
Apocrine sweat glands – axillae, pubic region; mainly lie in
subQ and open into hair follicles
stimulated by emotions and hormones
secretions thicker and more odorous
(sensible vs. insensible perspiration)
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Ceruminous glands – modified sweat glands in ear canal; lie in subQ and
open in ear canal or sebaceous gland ducts;
cerumen (ear wax) – waxy combination of ceruminous &
sebaceous secretions; traps foreign material entering ear
impacted cerumen – accumulation that effects hearing
3) Nails – plates of tightly packed, highly keratinized epidermal cells
protect distal digits
nail anatomy:
nail body – visible portion; free edge extends beyond digit
nail root – portion buried in skin fold
nail matrix – deep to nail root; stem cells of nail
eponychium (cuticle) – stratum corneum band over nail base
FUNCTIONS OF SKIN
Thermoregulation – homeostatic regulation of body temperature
sweat – increases with heat or exercise; evaporative cooling
adjusting blood flow – blood vessels in skin dilate during moderate
exercise; increases radiant heat loss; constriction of surface
vessels conserves heat
Protection
keratinized cells with extensive cell junctions - physical barrier to
microbes, abrasion, heat and chemical damage
sebum – inhibits dehydration and water absorption
contains bactericidal chemicals
melanin – protects from UV radiation
Langerhans cells – protection from microbes
Cutaneous sensation – detection of tactile, thermal and pain sensations on
body surface
Merkel cells
tactile corpuscles
free nerve endings
hair root plexuses
lamellated corpuscles
Excretion and Absorption – passage of waste materials out and other materials in
through skin
excretion – salts, carbon dioxide, urea and ammonia
absorption – lipid-soluble materials; vitamins (A,D,E,K), drugs
(hydrocortisone, estradiol, nicotine), O2, toxins (lead,
mercury, dry-cleaning fluid, acetone, poison ivy or oak)
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Synthesis of vitamin D3 – UV rays convert steroids to vitamin D3
further modified in liver and kidneys to calcitriol (necessary for absorbing
dietary calcium)
SKIN WOUND HEALING
Epidermal wound healing – only epidermis is damaged (eg. abrasions, minor
burns), no bleeding
basal cells migrate across wound
basal cell division replaces migrating cells
epidermal growth factor (EGF) – stimulates basal cell division
basal cells divide to regenerate layers
Deep wound healing – injuries extending beneath epidermis (dermis and subQ)
4 phases:
1. clotting – bleeding brings platelets, which form a blood clot
fills wound
2. migratory phase – clot becomes scab and epithelial cells migrate
beneath scab
granulation tissue – tissue filling damaged dermis
blood clot
fibroblasts - synthesize collagen and ground substance
regenerating blood vessels
phagocytic cells – remove debris
3. proliferative phase – continued growth and activity of all cells
epidermal layers reestablished
fibroblasts fill in hole in dermis
clot shrinks
4. maturation phase – scab sloughs, new tissue organizes
fibrosis –scar formation; extra collagen fibers and reduction in blood
vessels causes lighter appearance; specialized structures usually
much reduced – loss of some tissue function
Skin Aging – increases past age 40
collagen reduced and has less strength and organization = wrinkles
elastic fibers lose elasticity and less fat in subQ = sagging skin
thinner skin, especially dermis = easily injured
slower basal cell migration = slower healing
smaller sebaceous glands = dry skin and hair
reduced and altered melanocyte function = gray hair and age spots
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