Chapter 5 – Skin!

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Chapter 5!The Integumentary System!

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Chapter 5 – Skin!

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1. A cutaneous membrane!•  Epidermis!•  Dermis!

2. Accessory structures!

•  E.g. hair, glands, nails!

3. Subcutaneous layer (hypodermis)!

The Integumentary System Consists of:!

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•  Protection of underlying tissues from:!Abrasion, chemicals, UV radiation, infection, etc.!

•  Excretion!Salts, wastes, water!

•  Production of melanin!UV protection!

•  Production of keratin!Resists abrasion, “waterproofs” skin!

•  Synthesis of vitamin D3!

Aids in Ca2+ absorption!

Integumentary System Functions – 1!

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Integumentary System Functions – 2!

•  Lipid storage!•  Detection of touch, pressure, pain, temperature!•  Body temperature regulation!

Insulation, evaporation, vasomotion!•  Blood “storage”!

Blood is diverted from skin to skeletal muscles during emergency situations.!

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Integumentary System Figure 5-1!

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SECTION 5-1 !The epidermis is composed of strata (layers) with various functions!

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Epidermis is dominated by layers of keratinocytes!•  Thin skin: four layers (strata)!•  Thick skin: a) five strata on palms, soles of feet!

b) stratum corneum very thick! c) contains stratum lucidum!

Thin Skin and Thick Skin Figure 5-3!

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From deepest to most superficial…!•  Stratum germinativum (basale)!•  Stratum spinosum!•  Stratum granulosum!•  Stratum lucidum (thick skin only)!•  Stratum corneum!

Layers of the Epidermis!

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Structure of the Epidermis Figure 5-3!

Thick Skin

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•  Keratinocytes!•  Langerhans (dendritic) cells!•  Merkel cells!•  Melanocytes!

Epidermal Cell Types!

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Keratinocytes (1 of 2)!

90% of epidermal cells are this type!A. Produce keratin (intermediate filaments)!•  Resists abrasion!•  Decreases water loss!

B. Produce keratohyalin!•  Dehydrates cell!•  Cross-links keratin fibers!

C. Cells have desmosomes (“spot welds”)!•  Intermediate filaments (keratin) – hold cells

together!

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Keratinocytes (2 of 2)!

D. Stratum germinativum (basale) cells have hemidesmosomes!•  Anchor epidermis to basal lamina!

E. Produce lamellar granules!•  Found primarily in stratum granulosum!•  Lipid-rich secretion!

Released from cells!“Waterproofing” function!

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Dendritic (Langerhans) Cells!

Immune function:!•  Migrate to skin from bone marrow!•  Function as antigen-presenting cells in lymph

nodes!•  Interact with T lymphocytes!

University of WashingtonDept. of Bioengineering

•  Very important immune system cells (Ch. 22)!

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Merkel Cells!

Found in stratum basale of hairless skin!•  Attached to keratinocytes by desmosomes!•  Produce receptor potentials, release

neurotransmitter!•  Activate Merkel discs (neuron cell processes)!

!Function in light touch and pressure reception!

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Melanocytes!

Cell body is part of stratum germinativum!Contain enzyme tyrosinaseTyrosine (amino acid) ⎯ tyrosinase → melanin

(pigment)•  Pigment packaged in melanosomes•  Colors = Black ↔ brown ↔ yellow!Keratinocytes engulf melanosomes!!Melanin protects DNA from UV radiation! Vitiligo Treatment Clinics

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Melanocytes Figure 5-4!

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SECTION 5-2 !Factors influencing skin color are epidermal pigmentation and dermal circulation!

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Skin Color Variation (1 of 3)!

Melanin is major factor:!

All races have about same number of melanocytes!

Melanin protects DNA in epidermal and dermal!cells from UV radiation!

Color differences depend upon:!•  Melanin type (color)!•  Amount of melanin produced!•  Degree of dispersal and persistence of

pigment!

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Skin Color Variation (2 of 3)!Importance of melanin dispersal/persistence:!

Lighter-skinned races!•  Melanosomes dispersed in:!

S. basale and s. spinosum!•  Melanin destroyed rapidly by lysosomes!

Darker-skinned races!•  Melanosomes dispersed in:!

S. basale, s. spinosum and s. granulosum!•  Melanin not as rapidly destroyed by

lysosomes!

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Skin Color Variation (3 of 3)!

Other factors:!A. Hemoglobin in blood vessels (dermal blood

flow pattern)!

B. Carotene (Vitamin A precursor)!•  Yellow-orange pigment from diet!•  Accumulates in s. corneum and in fat!

C. Albinism!•  Genetic absence of tyrosinase!

D. Melanocyte-stimulating hormone (abnormal)!

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SECTION 5-3 !Sunlight causes epidermal cells to convert a steroid into vitamin D3!

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Epidermal cells synthesize vitamin D3 (cholecalciferol) when exposed to UV!•  S. germinativum and spinosum!

!Conversion to active form involves liver and

kidneys!!

D3 ⎯ blood→ LIVER ⎯ blood→ KIDNEYS!!Vitamin D (Calcitriol - hormone from kidneys)

required for Ca2+ absorption from gut!

Epidermal Cells - Vitamin D!

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Vitamin D Synthesis!

Vitamin D3 inactive form

Calcitriol,active form

Liver

Kidney

www.clinuvel.com/en/blog/wp-content/uploads/2011/04/Vitamin_D_Synthesis.jpg

Skin

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Epidermal Growth Factor (EGF)!

A peptide released by:!•  Salivary glands!•  Duodenal glands!

Effects:!•  ↑ mitosis in s. basale and spinosum!•  ↑ keratin production rate!•  ↑ epidermal repair rate!•  ↑ activity by epithelial glands!

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SECTION 5-5!The dermis is the tissue layer that supports the epidermis!

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Reticular layer

Papillary layer

Figure 5-3a Contains blood vessels, lymphatics, sensory nerves!

•  Papillary layer !(dermal papillae)!Nearest epidermis!Loose CT!

•  Reticular layer !Dense irregular CT (→ leather)!•  Collagen fibers for strength!•  Elastic fibers for elasticity!

Dermal Organization, Strength, Elasticity!

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Dermal Circulation Figure 5-10!

Back to slide 65!

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Cutaneous plexus in reticular layer!Subpapillary plexus in papillary layer!•  “Plexus” = network!

Blood vessels are within CT, maintain avascular epidermis!•  Thermoregulation (via vasomotion)!•  “Stores” blood!

Dermal Circulation!

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SECTION 5-6 !The hypodermis is tissue beneath the dermis that connects it to underlying tissues

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Not technically part of cutaneous membrane (skin)!•  A.K.A. hypodermis, superficial fascia!

Contains loose (areolar) CT and adipose tissue!

Functions:!•  Anchors skin to underlying structures!•  Acts as a cushion (fat)!•  Acts as thermal insulation (fat)!•  Stores energy (fat)!

Hypodermis (Subcutaneous Layer)!

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SECTION 5-7 !Hair is composed of keratinized dead cells that have been pushed to the surface

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Hair Functions!

A. Shields scalp from UV!B. Cushions a light blow to the head!C. Senses external environment!

e.g. Root hair plexus - early warning system!D. Protection from particles!

e.g. Eyelashes, nose hairs!E.  Insulates cranium!

•  More important in other animal species: !Piloerection → ↑ insulating layer of “dead”

air!

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Hair is produced by hair follicles (epithelial structures)!•  Are keratinized invaginations of epidermis!

Associated with:!•  Sebaceous gland (epithelial)!•  Arrector pili (smooth muscle)!•  Root hair plexus (nervous)!

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We will skip the microanatomy of hairs.!

Hairs and Hair Follicles!

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Lanugo!•  Fine, unpigmented - usually shed before birth!

Vellus hairs (“peach fuzz”, translucent)!•  Covers much of body surface!

Terminal hairs!•  Heavy, pigmented, perhaps curly!

Club hairs (cessation of growth)!•  Shed and grow according to hair growth

cycle!

Hair Types!

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Hair Color!

Melanin type and amount determines normal color!

Aging:!↓ tyrosinase activity → ↓ melanin production → !↓ gray color!

!Air bubbles also accumulate in medulla → white!

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SECTION 5-8 !Sebaceous glands and sweat glands are exocrine glands found in the skin

Recall that glands are epithelial derivatives.!!The first four slides about glands are from Chapter 4.!

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Exocrine glands!•  Secrete into ducts that empty onto an

epithelial surface - E.g. sweat (sudoriferous) glands, oil (sebaceous)glands!

Endocrine glands!•  Release hormones into extracellular fluid →

blood - E.g. pituitary gland, thyroid gland!Heterocrine glands!•  Do both - E.g. pancreas, ovaries, testes!

Glandular Epithelia – from Chapter 4, pp. 123 - 125!

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Merocrine (eccrine)!•  Product released through exocytosis!•  E.g. merocrine sweat glands, sublingual salivary

gland!Apocrine!

•  Involves the loss of both product and cytoplasm!•  E.g. mammary glands!•  E.g. sweat glands in armpits, groin are called this*!

Holocrine!•  Cell fills with product, dies, releases product!•  E.g. oil (sebaceous) glands in skin!

Modes of Secretion!

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Mechanisms of Glandular Secretion Figure 4-6!

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Secretions by Exocrine Glands!

Serous glands!•  Secrete watery solution containing enzymes!•  E.g. parotid salivary glands, sweat glands!

Mucous glands!•  Secrete mucins (proteoglycans)!•  Mucin + water = mucus!•  E.g. sublingual salivary glands, Brunner’s

glands in duodenum!Mixed glands!•  Secrete both - submaxillary salivary gland!

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Holocrine secretion type!

Most connected to hair follicles!•  Others = sebaceous follicles!

e.g. on face, back, glans penis, labium minus, tarsal glands (eyelids)!

Secretion = sebum!•  Accumulated sebum = blackhead (comedo)!

Black = melanin + oil oxidized (bananas)!Bacteria feed on sebum → comedo!

Sebaceous (Oil) Glands (back to Chapter 5)!

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“Zits” FYI!

Bacterium (Propionibacterium acnes) is part of normal (commensal) bacteria on skin!

Becomes trapped in airless (plugged) hair follicle!Converts sebum into fatty acids that deactivate

histone deacetylases. These normally inhibit inflammation.!

Inhibition removed à inflammation occurs in keratinocytes (“zits”)!

Puberty: sex hormones à increased production of sebum à more chance for acne!

https://www.newscientist.com/article/2110826-how-lack-of-oxygen-makes-bacteria-cause-acne-and-how-to-stop-it/!

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Functions of Sebum!

Composed of triglycerides, cholesterol, proteins, electrolytes!

Secretion mechanism = holocrine!Moisturizes hair!•  Prevents keratin from becoming brittle!

Moisturizes skin!•  Helps “waterproof” skin!•  Softens skin!•  Inhibits bacterial growth!

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Sebaceous Glands and Follicles Figure 5-13!

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Sudoriferous (Sweat) Glands!

Sudoriferous = “to bear sweat”!Types!•  “Apocrine”!•  Merocrine (eccrine)!

!Do not blame me for the ridiculously confusing

terminology!!

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Method of secretion is actually merocrine (controversial)

Produce secretion that becomes odorous!•  Sticky, viscous sweat!•  Secreted onto hair follicle!

Secretion begins at puberty !•  Axilla, pubic region, areola of breast!•  Bacterial growth → odor!

Myoepithelial cells contract, promote discharge!

1. “Apocrine” Sweat Glands!

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2. Merocrine (Eccrine) Sweat Glands!

Most sweat glands are merocrine!•  Most dense on palms, soles!

Serous (watery) secretion = sensible perspiration!Functions:!•  Excretion !

Salts, urea, uric acid, amino acids, sugars, lactate!•  Thermoregulation!•  Protection!

Flush chemicals, bacteria from surface!Contains antibiotic = dermacidin!

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Sweat (Sudoriferous) Glands Figure 5-14!

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1. Mammary glands!•  Structurally related to apocrine sweat glands!•  Discussed in Chapter 28!

2.  Ceruminous glands!•  Modified sweat glands!•  In external ear canal!•  Mix with sebaceous secretions → cerumen!

60% keratin, 12-20% fatty acids, alcohols, oils, <10% cholesterol !Traps foreign particles!

Other Glands!

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Control of Glandular Secretion!

Sebaceous glands and apocrine glands!•  Regional control not possible!•  All on or all off!

Merocrine sweat glands!•  Regional control possible!•  Thermoregulation!

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Functions!•  Protect ends of digits!•  Facilitate manipulation with fingers!

Structure!•  Epithelial derivatives!

!We will not be covering the microanatomy of nails.!

Nails!

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Sensory Structures Figure 15-4!

Receptor! Location! Function(s)!Free nerve endings! Between epidermal

cells!Pain, touch, pressure, temperature!

Root hair plexus! Around hair root! Movement of hair!

Merkel cells and Merkel (tactile) discs!

S. basale of epidermis!

Fine touch and pressure!

Meissner’s (tactile) corpuscles!

Papillary layer of dermis!

Fine touch and pressure!

Pacinian (lamellated) corpuscles!

Throughout dermis, hypodermis, joint capsules!

Deep pressure, distortion!

Ruffini corpuscle! Reticular layer of dermis!

Deep pressure, distortion!

No short-answer questions on this

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Sensory Structures Figure 15-4!

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Repair is possible because skin contains stem cells.!

•  Epidermis - stratum basale (germinativum)!•  Dermis - mesenchyme cells

SECTION 5-10 !Several steps are involved in repairing the integument following an injury

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1. Epidermal Wound Healing!

Wound is to epidermis only!•  S. basale cells leave basement membrane!•  Cells enlarge!•  Cells migrate across wound to center!•  Stop when surrounded by similar cells!

(Contact inhibition)!•  Replace upper layers by mitosis!

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2. Deep Wound Healing!

Wound extends into dermis!Repair may leave a scar!Four general phases:!•  Inflammation phase!•  Migratory phase!•  Proliferative phase!•  Maturation phase!

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Integumentary Repair - 1 of 2 Figure 5-16!

Inflammation Phase!(Mast cells, basophils)

Migratory Phase

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Integumentary Repair - 2 of 2 Figure 5-16!

Proliferative Phase

Maturation (Scarring) Phase

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1. Inflammation Phase!

Area is red, swollen, hot, painful (Why?)!

A. Blood clot (scab) forming!•  Unites edges of wound (fibrin)!

Stop bleeding!!↓ bacterial invasion!

B. ↑ vasodilation and vessel permeability!•  Histamine from mast cells, basophils!•  Cytokines attract WBCs to wound!

C. Mesenchyme cells begin differentiating into fibroblasts!

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2. Migratory Phase!

Scab has formed from fibrin!•  Epithelial cells migrating under scab!•  Fibroblasts migrating in, making collagen!•  WBCs still migrating in!•  Blood vessels growing into site!

Clot + fibroblasts + blood vessels = granulation tissue!

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3. Proliferative Phase!

Growth and proliferation of epithelial cells under scab!•  Fibroblasts producing collagen fibers in

random patterns!•  More vessel growth occurring!

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4. Maturation Phase!

•  Scab falls off!•  Epithelium regains proper thickness!•  Collagen fibers become more organized!•  Fibroblast population returns to normal number!!Scar!•  CT grows faster than EPI!•  More than normal amount of CT growth!•  Collagen arranged in more regular pattern!•  Myofibroblasts!

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SECTION 5-11 !Effects of aging include dermal thinning, wrinkling, and reduced melanocyte activity

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1. ↓ S. basale activity → ↓ thickness of epidermis!•  More injuries, more and recurring infections!

2. ↓ Langerhans cell activity!•  More and recurring infections!

3. ↓ Vitamin D3 production → ↓ Ca2+ and PO42-

uptake!•  Bone loss, muscular weakness!

4. ↓ Glandular activity → !•  ↓ sebum → dry, scaly skin!•  ↓ merocrine secretion → overheating!

Changes with Aging (1 of 3)!

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Changes with Aging (2 of 3)!

5. ↓ Blood supply to dermis (see slide 28)!•  Skin cool → inappropriate sensation of cold!•  Less able to release heat when appropriate!

6.  Melanocyte activity!•  paler skin, ↑ sunburn, gray hair!

7. ↓ Number and thickness of collagen/elastin fibers!•  ↓ strength of dermis!•  ↓ elasticity of dermis, sagging skin!

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Changes with Aging (3 of 3)!

8. ↓ hair follicle function → hair loss!9.  Changes in sex hormones → hair and fat

distribution!Effects of Weightlessness on SkinPreliminary results from astronauts:!•  Collagen production in dermis stimulated (anti-

aging?)!•  Epidermis gets thinner!Being investigated. Would be important on long space flights – like to Mars.!http://uk.reuters.com/article/2015/07/20/us-germany-astronaut-skin-idUKKCN0PU13M20150720!