Integumentary system

ANP 1106

Structure of skin

• Is 1.5 -4 mm consisting of 2 distinct regions

• includes: epidermis, dermis, and hypodermis

Epidermis

• Superficial epithelial region: layered-thick

• keratinized stratified(multilayered) squamous(flat) epithelium

• The outermost protective shield of the body

• avascular

Dermis

• Dense connective tissue ; vascularized

• makes up most of the skin

• leathery layer is made up of dense connective tissue

Hypodermis

• Superficial fascia(connective tissue that wraps around)

• not part of the skin, hence subcutaneous

• Mostly adipose tissue/areolar connective tissue

• anchors skin to muscles with ability to slide

• acts as shock absorber and an insulator to reduce heat loss

• stores fat

4 types of epidermal cells

• Keratinocytes

• Melanocytes

• Dendritic (Langerhans) cells

• Tactile (merkel) cells

Keratinocytes

• Make up 95% of thin cells

• arise from stratum basale

What is the main function of keratinocytes? To make keratin

What is the lifespan of a keratinocyte? 28-56 days

What is epidermal growth factor? is a protein that stimulates cell growth and differentiation by binding to its receptor, EGFR

Melanocytes

• Produce melanin which is packed into melanosomes;

• deepest layer of epidermis(basale)

• numerous branching processes for melanosomes transfer to adjacent cells.

• Why is melanin important? To protect nucleus from damaging UV rays of the sun

Dendritic (Langerhans) cells

• epidermal dendritic cells (star-shaped);

• present in stratum spinosum

• migrate to epidermis from bone marrow.

• Can differentiate into macrophages

• macrophages activate immune system and ingest foreign substances

Tactile (Merkel) Cells

• present at the epidermis/dermis boundary

• have disc-like sensory nerve ending forms touch receptors

Epidermal cells and layers of the epiderm

stratum = layers

5 distinct layers of epidermis

Thick Skin: Contains 5 layers (strata) and is found in high abrasion(hands, feet)

Thin skin contains only 4 strata(omit stratum lucidum)

5 layers of skin:

• Stratum corneum

• Stratum lucidum

• Stratum granulosum

• Strum spinosum

• Stratum Basale

Stratum basale (basal layer)

• the deepest epidermal layer, is also called the stratum germinativum

• consists of a single row of stem cells that continuously proliferate and differentiate to maintain epidermis

• contains youngest keratinocytes, melanocytes & tactile epithelial cells

Stratum Spinosum (prickly layer)

• contain dendritic cells and many rows of flattened dividing keratinocytes

• its cells contain thick bundles of intermediate filaments, consisting of prekeratin anchored to desmosomes

Stratum granulosum (granular layer)

• consist of 1-5 layers and where keratinization (hardening) begins

• granules promote hardening and waterproofing

• these cells flatten, their nuclei and organelles begin to disintegrate and forms 2 types of granules

  • keratohyaline granules

  • lamellar granules

• keratinocytes stop dividing and die due to distance from capillaries and glycolipids coating

keratohyaline granules

• provides ‘glue’ that binds prekeratin intermediate filaments to from keratin

lamellar granules

• contain a water-resistant glycolipid that is secreted into the extracellular space.

• Together with tight junctions, the glycolipid slow water loss across the epidermis.

Stratum Lucidum (clear layer)

• found only in THICK skin

• is a thin transulecent band above the stratum granulosum

• consists of rows of flat, dead keratinocytes

• its cells are identical to those of stratum corneum

Stratum Corneum (horny layer)

• the outermost epidermal layer that accounts for ~3/4 of epidermal thickness

• keratin and proteins accumulate inside the plasma membrane of its cells to protect skin from abrasion/penetration

• glycolipid between its cells keeps it waterproof

• is a layer of completely dead cells and are shed to be replaced (ie. dandruff, flakes)

Dermis

• Strong, flexible connective tissue

• cells include fibroblasts, macrophages, some mast cells. and WBCs

• Semi-fluid matrix heavily embedded with collagen, elastin, and reticular fibers

  • Fibers in matrix bind body together

    • Makes up the hide used to make leather

• Contains nerves, blood vessels, and lymphatic vessels, epidermal hair follicles, oil glands and sweat glands

• Has only 2 layers

2 layers of the dermis

• Papillary dermis

• Reticular dermis

Papillary dermis

• made up of interwoven mat of areolar CT fibers interspersed with blood vessels

• has peg-like projections on its surface, AKA dermal papillae

  • is the superficial region of dermis that indents the overlying epidermis

  • contains either capillary loops or free nerve endings (tactile/meissner corpuscles and pain receptors)

  • create friction ridges

Reticular dermis

• largest layer of skin

• made up of deeper, thick dense irregular CT (thick bundles of collagen fibers running in different directions but mostly parallel to skin surface)

  • source of lines of cleavage (tension) lines(AKA collagen-free areas)

    • important for surgery bc it takes less time to heal

• has Pacinian corpuscles (pressure receptors), sweat/oil glands, hair root

• collagen fibers give strength/resiliency & maintain skin hydration

• elastic fibers provide stretch-recoil properties of skin

Friction ridges

• On palms of hands (& fingers), soles of feet,

• formed when dermal papillae lie on top of dermal ridges, which extend from the surface as epidermal ridges known as friction ridges

  • Definitively develop pre-birth

  • Persistent during life except for permanent scarring

  • Details are unique and never repeat

  • Overall patterns may vary within limits allowing classification

function of friction ridges

• enhance gripping ability

• contribute to sense of touch

• sweat pores in ridges leave unique fingerprint pattern

What is the physiological basis of stretch marks or striae?

• occurs due to extreme stretching during short period of time

• AKA scar tissue resulting from the tears in collagen and elastic fibers

• *keloids are deep scarring in tissue and dermis is projected and covered. a severe form of striae

What happens when you get a blister?

• bubble of fluid has accumulated in separated epidermis and underlying dermis

• heals fairly easily

flexure lines

• part of reticular layer that are dermal folds at or near joints, where dermis is tightly secured to deeper structures

• occurs due to skin’s inability to slide easily for joint movement causing deep skin crease

• common in wrist, toes, etc

Melanin

• only pigment made in the skin; derived from tyrosine; two forms that range in color from reddish yellow to brownish black

  • Eumelanin: darker brown

  • Pheomelanin: less effective type (reddish brown)

• melanin production is made by tyrosinase, an enzyme in melanocytes

What is the role of melanocytes contributing to skins of different colors/tanning ability

skin colour dependent on type and relative amount of melanin & keratinocyte retention of the pigment

What damage does sun do to the skin

• Elastic fibers clump, causing skin to become leathery

• Temporarily depresses immune system

• Cause alterations(mutation) in DNA that may lead to skin cancer

• UV light destroys folic acid

  • folic acid is needed for DNA synthesis

How does sunscreen protect your skin?

• exogenous form of protection similar to what melanin does

• it reflects the UV light away from cells them from UV damage

Carotene

• yellow to orange pigment found in plant products - e.g.: carrots

• deposits in keratinocytes (esp. stratum corneum) & hypodermis

  • carotenoderma: excess carotene

• Most obvious in palms and soles

Hemoglobin

• from capillary circulation & gives skin a pinkish hue (explains the transparent epidermis of Caucasian skin).

• localized in dermis

What is cyanosis?

• When hemoglobin is poorly oxygenated, giving the skin a bluish-gray tint.

• especially obvious in the oral mucous membranes and nail beds, particularly in darker-skinned individuals.

• can be a sign of respiratory or cardiovascular problems.

Accessory structures of the skin

• Hair

• hair follicles

• nails

• sweat glands

• sebaceous glands

Hair

• AKA pili

• flexible strands of dead, keratinized cells produced by hair follicles

• none on palms, soles, lips, nipples, & portions of external genitalia

function of hair

• sense insects on skin

• guard head from physical trauma, heat loss, sun

• shield eyes

• filter particles from inhaled air

composition of hair

• hard keratin

  • (more durable, doesn’t flake) – more cysteine-cysteine bonds that gives it less flexibility, more strength

• Soft keratin

6 main parts of hair

• shaft

  • projects above the skin’s surface (keratinization complete).

  • Has 3 layers: cuticle, medulla, cortex

  • shape/size determines hair texture

• root

  • the part embedded in the skin (contained within hair follicles

• Bulb

  • expanded deep end of follicle- has papilla/root hair plexus

• Follicle: outer CT root sheath and inner epithelial root sheath; hair matrix

• Arrector pili muscle: 1/follicle; contract to pull hair up and dimple skin. causes goosebumps

  • found outside the wall of hair follicle

• Sebaceous gland: holocrine gland that secretes sebum (oily- lubrication and waterproofing; bactericidal)

coily hair

• shaft is flat and ribbonlike in cross section

silky and wavy hair

• shaft is oval

straight hair shaft shape

round

3 layers of hair shaft

• cuticle: single layer of overlapping cells

• Cortex: several layers of flattened keratinocytes; pigment is here

• medulla: large cells separated by air spaces-absent in fine (vellus) hair

  • absence of it makes hair softer in females than males

What are split ends

• caused by damage to cuticle

  • normally sits flat, but become raised or missing scales

• ends of hair are not as smooth

What happens when hair is turning gray or white?

• happens bc of change in color due to melanocytes

• melanocytes produce less melanin until they stop completely

  • are replaced by air bubbles, which lack color =gray hair

• caused by age, stress, vitamin deficiency

Structure of hair follicles

• hair papilla

  • Dermal papilla containing a knot of capillaries that supplies nutrient to growing hair

  • if destroyed=no more hair growth

• hair matrix

  • Actively dividing area of bulb that produces hair cells

  • as matrix makes new cells, it pushes older ones upward

Vellus hair

• pale, fine body hair of children and adult females

terminal hair

• coarse, long hair

• found on scalp and eyebrows

• at puberty

  • appears in axillary and pubic regions of both sexes

  • also on face and neck of males

• nutrition and hormones affect hair growth

  • growth rate=~2mm/week

growth cycles of follicles

• active growth phase followed by regression/resting phase

• each follicle has only certain number of growth cycles before it is done

which hair has longer active phase-eyebrow or head hair?

• head hair has longer active phases (~4 yrs), which is why they grow long before they are shed

• eyebrow hair is active for only few months so they never grow very long

What is hirsutism

• excessive hair growth

• must be terminal hair on face, neck, chest particularly women (ie. PCOS)

  • occurs due to excess androgen production

Balding(alopecia/hair loss) with age

• hair grows fastest between teen years and 40s

• after 40s, hair shed faster than replaces

• by 60-65, hair thins

• later years, terminal hair replaced by vellus hair causing wispy hair

male pattern baldness

• a type of true baldness

• genetically determined, gender influenced

• altered response of hair follicle to androgen that shortens growth cycles

Nails

• scale-like modification of epidermis that contain hard keratin-protective useful tool

• free edge, body, nail folds,-2 lateral and 1 proximal

Parts of a nail

• nail root: embedded in skin

• nail plate: body visible attached portion

• nail bed: epidermis underneath keratinized nail plate

• nail matrix: thickened portion of bed responsible for nail growth

  • lunule: thickened nail matrix, appears white,

• nail folds

  • eponychium: nail fold that projects onto surface of nail body AKA cuticle

  • hyponychium: area under free edge of plate that accumulates dirt

Sweat glands(sudoriferous glands)

• all skin surfaces except nipples & parts of external genitalia contain sweat glands/~3mil/person

• 2 types

  • eccrine and apocrine

what is sweat

• 99% of water + salts, vitamin C, antibodies, dermcidin, traces of metabolic wastes like urea, uric acid, ammonia

• pH ~4-6

• regulated by ANS

• prevents body from overheating

Eccrine(merocrine)

• simple coiled tubular glands with pore at surface to release sweat

• more numerous than apocrine

• abundant on palms, soles and forehead

• function in thermoregulation

• regulated by the SNS

• secrete sweat

Apocrine

• axillary & anogenital areas; larger; ducts empty into follicles

• lies deeper in dermis

• are also merocrine glands

• same as sweat but with fatty substances and some proteins -odorless until decomposed by skin bacteria leading to body odor

  • viscous with mily/yellowish color

• begin functioning at puberty due to androgrens

• activated by SNS in times of stress

2 types of modified apocrine sweat glands

• ceruminous: secrete wax (cerumen) in external ear canal. produced by nearby sebaceous gland

• mammary: secrete milk

Sebaceous (oil glands)

• are simple branched alveolar glands

• widely distributed holocrine glands, except for thick skin of palms and soles

  • most develop from hair follicles and secrete into hair follicles

  • relatively inactive until puberty

• Secretes sebum

  • oily secretion

  • bactericidal (bacteria-killing) properties

  • softens hair and skin

Acne

• usually an infectious inflammation of sebaceous glands, resulting in pimples (pustules or cysts)

• whiteheads, or closed comedones, are blocked sebaceous glands

  • if secretion is oxidized, whitehead becomes blackhead, AKA open comedones

• overactive sebaceous glands in infants can cause seborrhea, known as ‘cradle cap’

  • begin as pink, raised lesions on scalp that turn yellow/brown and slough off

6 Major function of skin

• protection

• thermoregulation

• cutaneous sensation

• metabolic function

• excretion: some N-containing wastes; NaCl

• blood reservoir

Protection as function of skin

• chemical barrier:

  • acidic skin secretions (acid mantle) retards bacterial replication

  • sweat also contains dermcidin and other anti-bacterial agents

  • melanin protects against UV-induced damage

• Physical barrier:

  • barrier to trauma and bacterial invasion; also waterproofing

• biological barrier:

  • Langerhans cells of epidermis and macrophages in dermis

    • not impermeable to gases, fat-soluble vitamins, steroids

Thermoregulation as function of skin

• sweating (o.5-12L fluid/day)

• insensible perspiration: routine and unnoticeable sweating

• sensible: visible output of sweat

• evaporation of the sweat is what cools the body

Cutaneous Sensation as function of skin

• Skin is supplied with cutaneous sensory receptors that are part of NS. are AKA exteroceptors

  • they respond to external stimuli on the body

Metabolic function of the skin

• vit D. synthesis needed for absorption of Ca 2+

• keratinocyte enzymes aid in conversion of topically-applied cortisone by hydrocortisone

Blood reservoir as function of skin

• dermis can hold about 5% of total blood volume

• NS can constrict dermal blood vessels to make it more available to other organs and muscles

Excretion as a function of the skin

• body eliminates limited amounts of nitrogen-containing wastes (ammonia, urea, and uric acid) in sweat

• Profuse sweating is an important avenue for water and salt (sodium chloride) loss

define burn

tissue damage caused by heat, electricity, radiation, chemicals

Types of burn severity(depth)

• First degree: only epidermis damaged

  • redness, swelling, pain. heals quickly(ie. sunburn); partial thickness burn

• Second degree: epidermis & upper dermis

  • blisters, redness; partial thickness burn

• Third degree: entire thickness of skin (epidermis + dermis)

  • appears gray-white, cherry red, or blackened

  • nerve endings/sensory receptors are destroyed so it doesn’t feel painful

What are the 2 main concerns for burns

1. fluid loss

   • loss of fluid→ loss of blood volume→ hypovolemic shock

2. infection

potential for repair of third degree burn

• depends on severity

  • ie. skin grafting=for smaller area of burn

  • artificial skin=stem cells are grown and transplanted for larger burns

  • skin bank= stored/frozen for ~2 years

Rule of Nines for evaluating burns

• Used to estimate volume of fluid loss

• Body is broken into 11 sections, with each section representing 9% of body surface (except genitals, which account for 1%)

• Ex: 41/2% means that amount of fluid is lost in that area and must be restored