Biology 112

Chapter 6:  Skin & The Integumentary System

Skin & its Tissues
The Skin (Integument): composed of 2 regions:

• Epidermis: outermost layer composed of epithelial cells
• Dermis: underlying layer composed of fibrous connective tissue; vascularized (many blood vessels)

Subcutaneous layer (hypodermis): just deep to the dermis

• mostly adipose with some areolar connective tissue
• insulates, absorbs shocks, & anchors skin loosely to muscles

Epidermis: keratinized stratified squamous epithelium

• no blood vessels; cells in lower layers (close to dermis) have adequate blood supply; as cells divide, new cells are pushed toward surface away from blood & soon die
• keratinocytes: dominant skin cells; produce the fibrous protein keratin
• tightly connected by adhesion junctions
• outermost cells dead & keratinized; lifespan of 25-45 days
• thick skin has accelerated cell division & keratinization
• areas subject to regular pressure may develop even greater rate of cell division (calluses, corns)
• melanocytes: spider-shaped cells that produce the pigment melanin
• located in deepest layer of epidermis; melanosomes in melanocyte processes transfer melanin to keratinocytes
• melanin granules protect the cell nucleus from UV radiation

Layers of Epidermis:

• Thick skin has 5 layers (strata)
• Stratum basale, stratum spinosum, stratum granulosum, stratum lucidum & stratum corneum
• Thin skin has only 4 layers... missing stratum lucidum; stratum corneum thinner

Skin Color: dictated by 3 pigments: melanin, hemoglobin & carotene

• melanin: produced from amino acid tyrosine in melanocytes
• color ranges from yellow to reddish-brown to black
• amount of melanin produced is mostly genetically determined; differences in skin color generally result from differences in melanin production
• freckles & pigmented moles are due to local melanin accumulations
• protects cell nucleus from UV light-induced mutations; UV repair mechanisms may stimulate synthesis
• hemoglobin: oxygenated hemoglobin in dermal capillaries gives fair skin a pinkish color
• carotene: yellow-orange pigment found in plants (carrots...)
• accumulates in stratum corneum & hypodermis; most evident in thick skin

• biochemical imbalances (liver) may lead to accumulation of pigment bilirubin (breakdown product of red blood cells); produces jaundice

Dermis: strong, flexible connective tissue

• fibroblasts, macrophages, white blood cells with collagen & elastin fibers
• rich supply of nerve fibers, blood vessels & lymphatic vessels
• houses hair follicles, oil & sweat (sudoriferous) glands
• upper layer composed of areolar connective tissue
• dermal papillae: projections that indent the epidermis; contain capillary loops & touch receptors (Meissner's corpuscles)
• dermal ridges on palms of hand & soles of feet form epidermal ridges - genetically determined pattern of ridges; leaves fingerprint
• lower layer composed of dense irregular connective tissue 

Accessory Structures of the Skin
Nails: scalelike epidermal modifications at posterior ends of fingers & toes

• nails consist of a nail plate over a surface of skin called the nail bed
• epithelial cells at base of nail plate produce new growth - cells become keratinized to form new nail plate as they grow out over nail bed
• cuticle (eponychium): fold of skin over nail root
• lunula: whitish half moon shaped base resulting from thicker skin in the area
• blood in dermal blood vessels beneath nail give pink color to nails

Hair & hair follicles:

• hair are flexible strands of mostly dead, keratinized cells
• hair has shaft (above skin) & root (below skin)
• melanin from melanocytes at base of follicle produce color
• hair follicle: extends from epidermal surface to dermis or hypodermis
• hair cells at base of follicle/root divide to produce the hair; cells eventually die as they move further away from blood supply through root into shaft
• arrector pili muscle: smooth muscle bundle that contracts to raise hair
• alopecia: hair loss brought on by aging & hormones

Sebaceous glands: oil glands usually associated with hair follicles

• holocrine glands that secrete sebum (mix of oil & cellular debris) through ducts into hair follicles
• sebum keeps hair & skin soft & waterproof

Sweat glands (sudoriferous glands): exocrine glands that produce watery secretion

• coiled tubular glands in the dermis/hypodermis with epithelial cells that secrete sweat through ducts that open through pores at skin surface
• eccrine glands: most numerous sweat glands; throughout skin, but most numerous in forehead, neck & back
• sweat helps to regulate body temperature in response to heat or exercise
• sweat is mostly water, plus small amounts of salt & wastes (urea, uric acid)
• apocrine glands: become active at puberty; most numerous in axillary regions & groin; activated by emotion, fear, pain
• ceruminous glands: modified sweat glands in external ear; secrete earwax
• mammary glands: modified sweat glands in breast; secrete milk

Regulation of Body Temperature

• hypothalamus of brain is main integrating center for thermoregulation; includes heat-loss center & heat-promoting center
• heat-promoting mechanisms:
• vasoconstriction of cutaneous blood vessels (blood rerouted to internal organs)
• increase in metabolic rate
• shivering (contraction of skeletal muscle)
• enhanced thyroxine release (increases metabolism & heat)
• heat-loss mechanisms:
• vasodilation of cutaneous blood vessels (heat lost through skin)
• enhanced sweating

Healing of Wounds:

• epidermal wound healing: abrasions & minor burns cause damage to epidermis
• basal cells migrate to bridge gap in broken tissue
• epidermal growth factor stimulates basal stem cells to divide & replace damaged cells & new strata
• deep wound healing:
• blood clot forms to loosely unite wound edges
• inflammation occurs, accompanied by heat, redness, pain & swelling
• clot becomes a scab, epithelial cells begin to migrate beneath scab to bridge wound , & granulation tissue forms (blood capillaries form & fibroblasts secrete collagen fibers to fill gap)
• extensive growth of epithelial cells beneath scab, as well as growth of fibers & blood vessels
• fibrosis: scar tissue formation
• scab sloughs off once epidermis is restored

Homeostatic imbalances of skin:
Skin cancer

• basal cell carcinoma
• squamous cell carcinoma
• melanoma

Burns:

• first-degree: only part of epidermis is destroyed
• second-degree: portion of epidermis & part of dermis destroyed
• third-degree: epidermis & full thickness of dermis & associated structures destroyed