Skin

The Integumentary System

•Skin and its accessory structures

•General Functions of the Skin

•Regulation of body temperature

•Protection as physical barrier

•Sensory receptors

•Excretion and absorption

•Synthesis of vitamin D

•

•2 Major layers of skin

–epidermis is epithelial tissue

–dermis  is layer of connective tissue, nerve & muscle

•Subcutaneous tissue (hypodermis) is layer of adipose & areolar tissue

–subcutaneous injection

–intradermal = within the skin layer

•Development of the Skin

•Epidermis develops from ectoderm

•Dermis develops from mesoderm

•Slippery coating of oil and sloughed off skin called vernix caseosa is present at birth

•Cell types of the Epidermis

• Keratinocytes--90%

–produce keratin

• Melanocytes-----8 %

–produces melanin pigment

–long cell processes

•Langerhan cells

–from bone marrow

–phagocytes

    Merkel cells

–touch receptor

•1. Stratum Basale

•Deepest single layer of cells

•Stratum germinativum

•Combination of merkel cells, melanocytes, keratinocytes & stem cells that divide repeatedly

•2. Stratum Spinosum

•8 to 10 cell layers held together by desmosomes

•At slide cells shrink and look spiny

•Melanin

•3. Stratum Granulosum

•3 - 5 layers of flat dying cells

•Nuclear degeneration

•Contain dark-staining keratohyalin granules

•4. Stratum Lucidum

•Seen only in thick skin on palms & soles of feet

•Three to five layers of clear, flat, dead cells

•Contains precursor of keratin

•5. Stratum Corneum

•25 to 30 layers of flat dead cells filled with keratin

•Continuously shed

•Barrier to light, heat, water, chemicals & bacteria

•Friction stimulates callus formation

•Keratinization & Epidermal Growth

•Stem cells divide to produce keratinocytes

•As keratinocytes are pushed up towards the surface, they fill with keratin

•Dermis

•Connective tissue layer

•Contains hair follicles, glands, nerves & blood vessels

•Major layers of dermis

–papillary

–reticular

•Papillary l.

•Top 20% of dermis

•Composed of loose CT

•Finger like projections called dermal papillae

•Functions:

–contains capillaries that feed epidermis

–contains Meissner’s corpuscles (touch) & free nerve endings (pain and temperature)

• Reticular l.

•Dense irregular connective tissue

•Collagen and elastic fibers

•Contains glands & hair follicles

•Provides strength, extensibility & elasticity to skin

•Epidermal ridges conforms to dermal papillae   =  fingerprints

•Types of Skin

•Thin skin - covers most of body

–thin epidermis (.1 to .15 mm.) that lacks stratum lucidum

–lacks epidermal ridges, has fewer sweat glands and sensory receptors

•Thick skin - only on palms and soles

–thick epidermis (.6 to 4.5 mm.) with distinct stratum lucidum & thick stratum corneum

–lacks hair follicles and sebaceous glands

–-thick hypodermis

•Accessory Structures of Skin

•Epidermal derivatives

•Epithelium sinks inward during development to form:

–hair

–glands

–nails

• Structure of Hair

• Shaft -- visible

–medulla, cortex & cuticle

–Root -- below the surface

•Follicle surrounds root:

–external root sheath

–internal root sheath

–base of follicle is bulb

•germinal cell layer

•C.t.Papilla contains blood vessels

•

•Hair Related Structures

• Muscle arrector pili

–smooth muscle in dermis contracts with cold or fear.

–forms goosebumps as hair is pulled vertically

•

•Hair Color and Texture

•Glands of the Skin

•Sebaceous

•Sweat

•Ceruminous (wax) glands

•Mammary (milk) glands

•

•Sebaceous (oil) glands

•Secretory portion in the dermis

•Most open onto hair shafts

•Sebum

–combination of cholesterol, proteins, fats & salts

–keeps hair and skin from soft & pliable

•Acne

–bacterial inflammation of glands

–secretions stimulated by hormones at puberty

• Sweat glands

• Eccrine (sweat) glands

–most areas of skin

–regulate body temperature with perspiration

• Apocrine (sweat) glands

–armpit and pubic region

–duct opens onto hair follicle

–secretions more viscous

• Nails

•Tightly packed keratinized cells

•Nail body

–visible portion pink due to underlying capillaries

–free edge appears white

•Nail root

–buried under skin layers

–lunula is white due to thickened stratum basale

•Eponychium (cuticle)

–stratum corneum layer

•Skin Color

•melanin – most significant factor in skin color

–produced by melanocytes

–accumulate in the keratinocytes of stratum basale and 

stratum spinosum

–eumelanin – brownish black

–pheomelanin - a reddish yellow sulfur-containing pigment

–

•people of different skin colors have the same 

number  of melanocytes

–dark skinned people

•produce greater quantities of melanin

•melanin granules in keratinocytes more spread out than 

tightly clumped

•melanin breaks down more slowly

•melanized cells seen throughout the epidermis

–light skinned people 

•melanin clumped near keratinocyte nucleus

•melanin breaks down more rapidly

•little seen beyond stratum basale

•

•amount of melanin also varies with exposure to

ultraviolet (UV) rays of sunlight

•Thermoregulation

•Releasing of sweat onto the skin

–perspiration & its evaporation lowers body temperature

•Adjusting flow of blood to the body surface

–in moderate exercise, more blood brought to surface helps lower temperature

–with extreme exercise, blood is shunted to muscles and body temperature rises

•Shivering and constriction of surface vessels

–raise internal body temperature as needed

•

•Protection

•Physical, chemical and biological barrier

–tight cell junctions prevent bacterial invasion

–lipids released retard evaporation

–pigment protects somewhat against UV light

–langerhans cells alert immune system

•Cutaneous Sensations

•Touch, temperature, pressure, vibration, tickling and some pain sensations arise from the skin.

•

•Excretion and Absorption

•Only a minor role is played by the skin

•400 mL of water evaporates from it daily

•Small amounts salt, CO2, ammonia and urea are excreted

•Lipid soluble substances can be absorbed through the skin

–vitamins A, D, E and K, Oxygen and CO2

–acetone and dry-cleaning fluid, lead, mercury, arsenic, poisons in poison ivy and oak

•Synthesis of Vitamin D

•Sunlight activates a precursor to vitamin D

•Enzymes in the liver and kidneys transform that molecule into calcitriol (most active form of vitamin D)

•Necessary vitamin for absorption of calcium from food in the gastrointestinal tract

•

•Epidermal Wound Healing

•Abrasion or minor burn

•Basal cells migrate across the wound

•Contact inhibition with other cells stops migration

•Epidermal growth factor stimulates cell division

•Full thickness of epidermis results from further cell division

• Deep Wound Healing

•If an injury reaches dermis, healing occurs in 4 phases

–inflammatory phase has clot unite wound edges and WBCs arrive from dilated and more permeable blood vessels

–migratory phase begins the regrowth of epithelial cells and the formation of scar tissue by the fibroblasts

–proliferative phase is a completion of tissue formation

–maturation phase sees the scab fall off

•Scar formation

–hypertrophic scar remains within the boundaries of the original wound

–keloid scar extends into previously normal tissue

•collagen fibers are very dense and fewer blood vessels are present so the tissue is lighter in color

•Phases of Deep Wound Healing

•Development of the Skin

•Epidermis develops from ectodermal germ layer

•Dermis develops from mesodermal germ layer

–at 8 weeks, fetal “skin” is simple cuboidal epithelium

–nails begin to form at 10 weeks, but do not reach the fingertip until the 9th month

–dermis forms from mesoderm by 11 weeks

–by 16 weeks, all layers of the epidermis are present

–oil and sweat glands form in 4th and 5th month

–by 6th months, delicate fetal hair (lanugo) has formed

•Slippery coating of oil and sloughed off skin called vernix caseosa is present at birth

•Skin Cancer

•1 million cases diagnosed per year

•3 common forms of skin cancer

–basal cell carcinoma  (rarely metastasize)

–squamous cell carcinoma  (may metastasize)

–malignant melanomas   (metastasize rapidly)

•most common cancer in young women 

•arise from melanocytes ----life threatening

•key to treatment is early detection watch for changes in symmetry, border, color and size

•risks factors include-- skin color, sun exposure, family history, age and immunological status