Skin and Body Membranes

Human Anatomy and Physiology

General Characteristics of Body Membranes

Cover surfaces of body/ line body cavities Form protective and often lubricating sheets

around organs 2 major groups

Epithelial Cutaneous/ Mucous/ Serous Membranes

Connective Synovial Membranes

Epithelial Membranes: covering & lining membranes

Cutaneous (skin) Mucous (lines body cavities that open to

exterior) Serous (lines body cavities closed to exterior) Remember all include both epithelial tissue

AND an underlying layer of connective tissue…so these membranes are actually simple organs

Cutaneous Membrane

Epidermis on outside Stratified squamous Keratinizing

Dermis under epidermis Dense (fibrous)

connective tissue

Cutaneous membrane is exposed to air so its considered a “dry membrane”

Mucous Membranes (mucosa)

Lines body cavities opening to outside: respiratory/ digestive/ urinary/ reproductive tracts

Epithelium resting on loose connective tissue

“wet membranes” Adapted for absorption

or secretion

Serous Membranes (serosa) Line body cavities that are

not exposed to outside Simple squamous

epithelium resting on thin areolar connective tissue

Occurs in pairs: parietal and visceral

Peritoneum = abdominal cavity

Pericardium = around heart

Pleura = around lungs

Parietal layer lines the wall of ventral body/ Visceral layer covers the outside of the organ in that cavity

Connective Tissue membranes: Synovial

No epithelial cells Soft areolar connective

tissue Line fibrous capsules

surrounding joints Cushion organs

rubbing against each other

Secrete lubricating fluid Also line bursae

The Integumentary System (your skin!)

List 4 important functions of the integumentary system, and explain how these functions are accomplished.

Label a diagram of the skin recognizing the following: epidermis, dermis (papillary and reticular layers), hair and hair follicle, sebaceous gland, and sweat gland

Describe the distribution and function of sebaceous glands, sweat glands and hair

The Integumentary System (continued)

Name the factors that determine skin color and describe the function of melanin

Describe syndromes/ infections/ allergic reactions in skin

Differentiate first, second and third degree burns

Explain the importance of the “rule of nines” Summarize the characteristics of basal cell

carcinoma, squamous cell carcinoma and malignant melanoma

What specifically is the integumentary system?

Cutaneous membrane

All its derivatives: Sweat glands Oil glands Hair Nails

Functions of the Skin1. Controls internal body temperature:

1. Heat loss: activates sweat glands and allows blood to flush into skin capillary beds so heat can radiate from skin surface

2. Heat retention: not allowing blood to flush to skin capillary beds

2. Aids in excretion of urea and uric acid: perspiration by sweat glands

3. Synthesizes vitamin D: modified cholesterol molecules in skin

Functions of Skin (cont.)

Protects deeper tissue from Mechanical damage: physical barrier, keratin, fat

cells, pressure receptors to stimulate movement Chemical damage: impermeable keratin,

chemoreceptors, nociceptors Bacterial damage: skin secretions Ultraviolet radiation: melanin Thermal damage: thermoreceptors/ nociceptors Desiccation: water proofing glycolipid and keratin

Integumentary system provides a wealth of sensory data

Receptors are classified by the following: Stimulus type Location Structural complexity

Classification by Stimulus Type

Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch

Thermoreceptors—sensitive to changes in temperature

Photoreceptors—respond to light energy (e.g., retina)

Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry)

Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

Classification by Location

1.Exteroceptors Respond to stimuli arising outside the body Receptors in the skin for touch, pressure, pain, and temperature Most special sense organs

2. Interoceptors (visceroceptors) Respond to stimuli arising in internal viscera and blood vessels Sensitive to chemical changes, tissue stretch, and temperature

changes

3. Proprioceptors Respond to stretch in skeletal muscles, tendons, joints, ligaments,

and connective tissue coverings of bones and muscles Inform the brain of one’s movements

Classification by Structural Complexity

1. Complex receptors (special sense organs) Vision, hearing, equilibrium, smell, and taste

(Chapter 15)

2. Simple receptors for general senses: Tactile sensations (touch, pressure, stretch,

vibration), temperature, pain, and muscle sense Unencapsulated (free) or encapsulated dendritic

endings

Unencapsulated Dendritic Endings

Thermoreceptors Cold receptors (10–40ºC); in superficial dermis Heat receptors (32–48ºC); in deeper dermis

Nociceptors Respond to:

Pinching Chemicals from damaged tissue Temperatures outside the range of thermoreceptors Capsaicin

Light touch receptors Tactile (Merkel) discs Hair follicle receptors

Table 13.1

Encapsulated Dendritic Endings

All are mechanoreceptors Meissner’s (tactile) corpuscles—discriminative touch Pacinian (lamellated) corpuscles—deep pressure and

vibration Ruffini endings—deep continuous pressure Muscle spindles—muscle stretch Golgi tendon organs—stretch in tendons Joint kinesthetic receptors—stretch in articular

capsules

Table 13.1

Skin (Integument)

Consists of three major regions1. Epidermis—superficial region

2. Dermis—middle region

3. Hypodermis (superficial fascia)—deepest region Subcutaneous layer deep to skin (not technically

part of skin) Mostly adipose tissue

Epidermis

Keratinized stratified squamous epithelium Cells of epidermis

Keratinocytes—produce fibrous protein keratin Melanocytes

10–25% of cells in lower epidermis Produce pigment melanin

Epidermal dendritic (Langerhans) cells—macrophages that help activate immune system

Tactile (Merkel) cells—touch receptors

Keratin Fibrous protein that

helps give the epidermis its protective properties

Found not only in skin, but also hair, nails, claws, horns, scales, shells, feathers, even baleen plates of whales

Strong, waterproof, contains sulfur

Melanocytes

Spider-shaped cells found in stratum basale.

Produce pigment melanin which accumulates in granules called melanosomes

Melanosomes are taken up by keratinocytes where they accumulate on the sunny side of the nucleus

Layers of the Epidermis: Stratum Basale (Basal Layer)

Deepest epidermal layer firmly attached to the dermis

Single row of stem cells Also called stratum germinativum: cells

undergo rapid division Journey from basal layer to surface

Takes 25–45 days

Figure 5.2a

Dermis

Stratum corneumMost superficial layer; 20–30 layers of deadcells represented only by flat membranoussacs filled with keratin. Glycolipids inextracellular space.

Stratum granulosumThree to five layers of flattened cells,organelles deteriorating; cytoplasm full oflamellated granules (release lipids) andkeratohyaline granules.

Stratum spinosumSeveral layers of keratinocytes unified bydesmosomes. Cells contain thick bundles ofintermediate filaments made of pre-keratin.

Stratum basaleDeepest epidermal layer; one row of activelymitotic stem cells; some newly formed cellsbecome part of the more superficial layers.See occasional melanocytes and epidermaldendritic cells.(a)

Layers of the Epidermis: Stratum Spinosum (Prickly Layer)

Cells contain a weblike system of intermediate prekeratin filaments attached to desmosomes

Abundant melanin granules and dendritic cells

Figure 5.2b

MelanocyteMelanin granule

Tactile(Merkel) cell

Sensorynerve ending Epidermal

dendritic cell

Dermis

KeratinocytesStratum corneumMost superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space.Stratum granulosumThree to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated granules (release lipids) and keratohyaline granules.

Stratum spinosumSeveral layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin.

Stratum basaleDeepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and epidermal dendritic cells.

Desmosomes

(b)

Layers of the Epidermis: Stratum Granulosum (Granular Layer)

Thin; three to five cell layers in which the cells flatten

Keratohyaline and lamellated granules accumulate

Figure 5.2a

Dermis

Stratum corneumMost superficial layer; 20–30 layers of deadcells represented only by flat membranoussacs filled with keratin. Glycolipids inextracellular space.

Stratum granulosumThree to five layers of flattened cells,organelles deteriorating; cytoplasm full oflamellated granules (release lipids) andkeratohyaline granules.

Stratum spinosumSeveral layers of keratinocytes unified bydesmosomes. Cells contain thick bundles ofintermediate filaments made of pre-keratin.

Stratum basaleDeepest epidermal layer; one row of activelymitotic stem cells; some newly formed cellsbecome part of the more superficial layers.See occasional melanocytes and epidermaldendritic cells.(a)

Layers of the Epidermis: Stratum Lucidum (Clear Layer)

In thick skin Thin, transparent band superficial to the

stratum granulosum A few rows of flat, dead keratinocytes

Layers of the Epidermis: Stratum Corneum (Horny Layer) 20–30 rows of dead, flat, keratinized

membranous sacs Three-quarters of the epidermal thickness Functions

Protects from abrasion and penetration Waterproofs Barrier against biological, chemical, and physical

assaults

Figure 5.2b

MelanocyteMelanin granule

Tactile(Merkel) cell

Sensorynerve ending Epidermal

dendritic cell

Dermis

KeratinocytesStratum corneumMost superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space.Stratum granulosumThree to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated granules (release lipids) and keratohyaline granules.

Stratum spinosumSeveral layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin.

Stratum basaleDeepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and epidermal dendritic cells.

Desmosomes

(b)

Dermis

Strong, flexible connective tissue Cells include fibroblasts, macrophages, and

occasionally mast cells and white blood cells Two layers:

Papillary Reticular

Figure 5.1

Epidermis

Hair shaft

Dermis Reticularlayer

Papillarylayer

Hypodermis(superficial fascia)

Dermal papillae

Pore

Subpapillaryvascular plexus

Appendagesof skin • Eccrine sweat gland• Arrector pili muscle• Sebaceous (oil) gland• Hair follicle• Hair rootNervous structures

• Sensory nerve fiber• Pacinian corpuscle• Hair follicle receptor (root hair plexus)

Cutaneous vascularplexus

Adipose tissue

Layers of the Dermis: Papillary Layer

Papillary layer Areolar connective tissue with collagen and

elastic fibers and blood vessels Dermal papillae contain:

Capillary loops Meissner’s corpuscles Free nerve endings Epidermal ridges lie atop deeper dermal papillary

ridges to form friction ridges of fingerprints

Figure 5.4a

Friction ridges

(a)

Openings ofsweat gland ducts

Figure 5.4b

(b)

Layers of the Dermis: Reticular Layer

Reticular layer ~80% of the thickness of dermis Collagen fibers provide strength and resiliency Elastic fibers provide stretch-recoil properties

Collagen fibers arranged in bundles form externally invisible cleavage (tension) lines

Incisions made parallel to cleavage lines heal more readily

Skin Color

Three pigments contribute to skin color:1. Melanin

Yellow to reddish-brown to black, responsible for dark skin colors

Produced in melanocytes; migrates to keratinocytes where it forms “pigment shields” for nuclei

Freckles and pigmented moles Local accumulations of melanin

2. Carotene Yellow to orange, most obvious in the palms and soles

3. Hemoglobin Responsible for the pinkish hue of skin

Erythema

Redness of the skin caused by

embarrassment, fever, hypertension, inflammation, allergy…or even massage, acne medicine, waxing, lyme disease

30-50% erythema of unknown cause

Pallor or Blanching

Caused by fear, anger, certain emotional stress

Often a symptom of anemia or low blood pressure

Raynaud’s syndrome

JaundiceYellow cast

Can be a result of a liver disorder in which yellow bile pigments, bilirubin, accumulate in the body

Also could be a result of eating a lot of carotene…carotenemia

Albinism

Absence of pigment in the skin, hair and eyes.

Melanocytes are present but melanin not produced because of missing or disabled enzyme

Black and Blue marksBruising

Bruise can also be called a contusion

Where blood has escaped from the circulation and clotted beneath the skin

Mild Hematoma

Cyanosis

When hemoglobin is poorly oxygenated both the blood and often the skin appear blue

Skin become cyanotic during heart failure and severe respiratory disorders

Not as evident in darker skinned people

Blue Fugates of Troublesome Creek, Kentucky

Genetic Disorder that was amplified in a small Appalachain community from a French descendant

Form of hemoglobin, methemoglobin cannot be recycled back into hemoglobin because of an enzyme deficiency

Part 2 Skin Appendages Compare the structure and locations of sweat

and oil glands and their secretions. Compare and contrast eccrine and apocrine

glands. List the parts of a hair follicle. Describe

functional relationship of arrector pili muscles to the hair follicle.

Name the regions of a hair and explain the basis of hair color.

Describe the structure of nails.

Sweat Glands

Two main types of sweat (sudoriferous) glands

1. Eccrine (merocrine) sweat glands—abundant on palms, soles, and forehead

Sweat: 99% water, NaCl, vitamin C, antibodies, dermcidin (microbe killing peptide), metabolic wastes (urea, uric acid and ammonia)

pH from 4-6 Ducts connect to pores Function in thermoregulation

Multicellular Exocrine Glands

Multicellular exocrine glands are composed of a duct and a secretory unit

Classified according to:Duct type (simple or compound) Structure of their secretory units

(tubular, alveolar, or tubuloalveolar)

Figure 4.5

Compound duct structure(duct branches)

Simple tubular

ExampleIntestinal glands

Simple branchedtubular

ExampleStomach (gastric)glands

Compound tubular

ExampleDuodenal glands of small intestine

Compound alveolar

ExampleMammary glands

Simplealveolar

ExampleNo importantexample in humans

Simple branchedalveolar

ExampleSebaceous (oil)glands

Compoundtubuloalveolar

ExampleSalivary glands

Tubularsecretorystructure

Alveolarsecretorystructure

Surface epithelium Duct Secretory epithelium

Simple duct structure(duct does not branch)

Modes of SecretionMerocrine

Products are secreted by exocytosis (e.g., pancreas, sweat and salivary glands)

HolocrineProducts are secreted by rupture

of gland cells (e.g., sebaceous glands)

What type of Secretion is each picture?

Eccrine Sweat Glands are also called merocrine sweat glands

Sweating is regulated by the sympathetic division of the autonomic nervous system

Major role is to prevent overheating of body Heat induced sweating begins on forehead

and spreads inferiorly over body Emotionally induced sweating begins on

palms, soles and axillae then spreads to other areas of body

Figure 5.5b

(b) Photomicrograph of a sectioned eccrine gland (220x)

Secretory cells

Dermal connectivetissue

Duct

Sebaceousgland

Sweat pore

Eccrinegland

Sweat Glands (cont.)2. Apocrine sweat glands—confined to axillary and

anogenital areas Sebum: sweat + fatty substances and proteins so

secretion is yellow or whitish Ducts connect to hair follicles Functional from puberty onward (as sexual scent

glands?)/ basis of body odor Also merocrine glands as opposed to other apocrine

glands (3rd type of gland not seen in humans) Specialized apocrine glands

Ceruminous glands—in external ear canal; secrete cerumen…ear wax!

Mammary glands

Sebaceous (Oil) Glands

Widely distributed, large on face, neck and upper chest

Most develop from hair follicles Become active at puberty Sebum

Oily holocrine secretion Bactericidal Softens hair and skin

Figure 5.5a

(a) Photomicrograph of a sectioned sebaceous gland (220x)

Sebaceousgland duct

Hair inhair follicle

Secretory cells

Dermalconnectivetissue

Sebaceousgland

Sweatpore

Eccrinegland

What is Acne?

Whitehead: When a sebaceous gland duct is blocked by accumulated sebum.

If sebum oxidezes and dries: blackhead

Cradle Cap (seborrhea) is from overactive sebaceous glands

“pores” on face are external outlet of hair follicles, where sebaceous glands empty

Acne is active inflammation of sebaceous lands accompanied by “pimples” which are pustules or cysts on skin

Acne is usually caused by bacterial infection, often staphylococcus.

Hair

Functions in Humans Alerting the body to presence of insects on the skin Guarding the scalp against physical trauma, heat

loss, and sunlight As opposed to other mammal hair functions like

trapping body heat Distribution

Entire surface except palms, soles, lips, nipples, and portions of external genitalia

Hair Consists of dead keratinized cells Contains hard keratin; more durable than soft

keratin of skin (and doesn’t flake off) Three layers: medulla, cortex and cuticle Hair pigments: melanins (yellow, rust brown,

black) produced by melanocyctes at base of hair follicle amd transferred to cortical cells Gray/white hair: decreased melanin production,

increased air bubbles in shaft Red hair due to iron containing pigment

Figure 5.6a

Hair shaft

ArrectorpiliSebaceousglandHair root

Hair bulb

(a) Diagram of a cross section of a hair within its follicle

• Connective tissue root sheath• Glassy membrane• External epithelial root sheath• Internal epithelial root sheath

Follicle wall

• Cuticle• Cortex• Medulla

Hair

What is the role of hair conditioners?

(b) Photomicrograph of a cross section of a hair and hair follicle (250x)

• Connective tissue root sheath

Follicle wall

• Cuticle

• Glassy membrane

• Cortex• Medulla

• Internal epithelial root sheath

• External epithelial root sheath

Hair

Hair shaft

ArrectorpiliSebaceousglandHair root

Hair bulb

Figure 5.6b

Hair Follicle

Extends from the epidermal surface into dermis

Two-layered wall: outer connective tissue root sheath, inner epithelial root sheath

Hair bulb: expanded deep end Hair papilla of dermal tissue protrudes into the hair

bulb Contains a knot of capillaries that provide nutrients

to growing hair

Hair Follicle

Hair follicle receptor (root hair plexus) Sensory nerve endings around each hair bulb

Stimulated by bending a hair

Arrector pili Smooth muscle attached to follicle Responsible for “goose bumps”

Figure 5.6c

Hair shaft

ArrectorpiliSebaceousglandHair root Hair bulb

(c) Diagram of a longitudinal view of the expanded hairbulb of the follicle, which encloses the matrix

• Internal epithelial root sheath• External epithelial root sheath

• Connective tissue root sheathFollicle wall

Hair matrix

MelanocyteHair papilla

Subcutaneous adipose tissue

• Medulla• Cortex• Cuticle

• Glassy membrane

Hair root

(d) Photomicrograph of longitudinal view of the hair bulb in the follicle (160x)

Follicle wall

Hair matrix

Hair papilla

Subcutaneousadipose tissue

Hair root

• Connective tissue root sheath• Glassy membrane• External epithelial root sheath• Internal epithelial root sheath

• Cuticle• Cortex• Medulla

Hair shaft

ArrectorpiliSebaceousglandHair root

Hair bulb

Figure 5.6d

Types of Hair

Vellus—pale, fine body hair of children and adult females

Terminal—coarse, long hair of eyebrows, scalp, axillary, and pubic regions (and face and neck of males)

Hair growth and density influenced by nutrition, hormones and local blood flow (that can be increased by physical irritation)

Hirsutism: excessive hairiness (particularly in women)

May result in an adrenal gland or ovarian tumor that secretes abnormally large amounts of androgens.

Types of Hair

Hair Growth Growth rate averages 2.5 mm per week Each follicle goes through growth cycles:

Growth phase (weeks to years) followed by regressive stage and resting phase (1–3 months)

Growth phase varies (6–10 years in scalp, 3–4 months in eyebrows)

During regressive stage, hair falls out. After resting phase, cycling starts again and new hair is formed to replace one that fell out

Loose an average of 90 scalp hairs daily

Hair Thinning and Baldness Alopecia—hair thinning in both sexes after age

40 True (frank) baldness

Genetically determined and sex-influenced condition Male pattern baldness is caused by follicular response

to DHT Until recently, the only cure for male pattern baldness

was to inhibit testosterone production…problems with this?

By accident, minoxidil to reduce HBP, also stimulates hair regrowth

Structure of a Nail

Scalelike modification of the epidermis on the distal, dorsal surface of fingers and toes

Made of hard keratin Each nail has free edge, body and proximal root Nail matrix at proximal root; responsible for nail

growth Normally appear pink/ region over thick nail

matrix looks like half moon (lunule)

Figure 5.7

Lateralnail fold

Lunule

Nailmatrix

Root of nail

Proximalnail fold

Hyponychium

Nail bed

Phalanx (bone of fingertip)

Eponychium(cuticle)

Bodyof nail

Free edgeof nail

(a)

(b)

Nail Basics Proximal and lateral

borders of nail are overlapped by skin folds called nail folds

Proximal nail fold is the eponychium (aka: cuticle)

Region beneath the free edge of the nail where dirt and debris accumulate is the hyponychium (quick)

Functions of the Integumentary System

1. Protection—three types of barriers Chemical

Low pH secretions (acid mantle) and defensins retard bacterial activity

Physical and Mechanical Keratin and glycolipids block most water and water-

soluble substances Limited penetration of skin by lipid-soluble substances,

plant oleoresins (e.g., poison ivy), organic solvents, salts of heavy metals, some drugs

Biological barriers Dendritic cells, macrophages, and DNA

Functions of the Integumentary System

2. Body temperature regulation ~500 ml/day of routine insensible perspiration (at

normal body temperature) At elevated temperature, dilation of dermal

vessels and increased sweat gland activity (sensible perspirations) cool the body

3. Cutaneous sensations Temperature, touch, and pain

Functions of the Integumentary System

4. Metabolic functions Synthesis of vitamin D precursor and

collagenase Chemical conversion of carcinogens and some

hormones

5. Blood reservoir—up to 5% of body’s blood volume

6. Excretion—nitrogenous wastes and salt in sweat

Part Three: Homeostatic Imbalances of Skin

Summarize the characteristics of three major skin cancers

Explain why serious burns are life threatening. Describe how to determine the extent of a burn and differentiate first, second and third degree burns.

Discuss various common homeostatic imbalances from acne to psoriasis.

ID effects of tattoo on skin

Homeostatic Imbalances of Skin: Infections & AllergiesSkin can develop more than 1000 different

conditions and ailments.Objectives:1. Describe cause of several common skin

disorders .2. Summarize the characteristics of the three

major types of skin cancers.3. Explain why serious burns are life

threatening. Describe how to determine the extent of a burn and differentiate first, second and third-degree burns.

Burns

When skin is burned, 2 life threatening problems result: body loses supply of fluids containing proteins

and electrolytes. Dehydration and electrolyte imbalance can lead to kidney shut down and circulatory shock (inadequate blood flow to body)

After 24 hours, infection is important threat…leading cause of death in burn victims.

Rule of Nines: way to determine volume of fluid lost by burns

Rule of Nines

Divides the body into 11 areas each representing 9% of total body area, with genitals accounting for remaining 1%

This is obviously only an approximation

Classification of Burns First Degree

Only the epidermis is damaged

Red and swollen Heals in 2-3 days Sunburn

Second Degree Injury to epidermis and

upper region of dermis Red, painful, blisters Care to protect from

infection

Third Degree Destroys entire thickness

of skin Full thickness burn Appears blanched (gray-

white) or blackened Nerve endings are

destroyed/ not painful Regeneration not

possible/ skin grafting necessary

Skin Cancer The single most common type of cancer in

humans 1 in 5 Americans will develop skin cancer Most important risk factor: overexposure

ultraviolet radiation in sunlight Damages DNA bases (pyrimidines: C and T) UV light disables tumor suppressor gene: p53

Most skin neoplasms are benign and do not metastasize. For example: a wart

There are three types of malignant skin neoplasms

Basal Cell Carcinoma Least malignant Most common skin

cancer/ 80% Skin cells no longer

honor boundary between epidermis and dermis/ cannot form keratin

Shiny domed shaped nodules

Slow growing Surgically removed, full

cure in 99% cases

Squamous Cell Carcinoma Lesion is scaly, reddened,

small and rounded that gradually forms an ulcer

Arises from keratinocytes Most often occurs on

scalp, ears, dorsum of hands, lower lip

Grows rapidly Sun induced Chance of cure good if

caught & removed early

Malignant Melanoma Cancer of melanocytes Metastasizes rapidly (50%

survival rate if metastasized)

Resistant to chemotherapy Forms wherever there is

pigment, sometimes moles ABCD(E) rule:

Asymmetry/ Border irregular/ Colors/ Diameter larger than 6mm/ elevation above skin

Your epidermal cells scream for sunscreen!

Sunscreens are currently rated for their ability to prevent sunburn but not for their ability to protect against DNA damage.

SPF: Sun Protection Factor

Research = levels of radiation not strong enough to burn still affect DNA

Ultraviolet Light

UVA (ultraviolet-A): long- wave solar rays of 320-400 nanometers (billionths of a meter). Although less likely than UVB to cause sunburn, UVA penetrates the skin more deeply, and is considered the chief culprit behind wrinkling, leathering, and other aspects of "photoaging." The latest studies show that UVA not only increases UVB 's cancer-causing effects, but may directly cause some skin cancers, including melanomas.

UVB (ultraviolet-B): short-wave solar rays of 290-320 nanometers. More potent than UVA in producing sunburn, these rays are considered the main cause of basal and squamous cell carcinomas as well as a significant cause of melanoma.

Suncreens Sunblocks and sunscreens: Sunscreens chemically absorb UV rays,

sunblocks physically deflect them. Sunscreen has long blocked UVB effectively, but until recently provided less UVA protection. New ingredients such as octylcrylene and the benzophenones have improved sunscreen's defenses against shorter UVA rays, and the revolutionary chemical avobenzone (Parsol 1789) works against all UVA wavelengths.

Sunblocks have also markedly improved. New preparations such as micronized titanium dioxide are less conspicuous on the skin and offer substantial protection against both UVA and UVB.

SPF (sun protection factor): measures the length of time a product protects against skin reddening from UVB, compared to how long the skin takes to redden without protection. If it takes 20 minutes without protection to begin reddening, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer -- about 5 hours. (Actually, it may take up to 24 hours after sun exposure for redness to become visible.) To maintain the SPF, reapply sunscreen every two hours and right after swimming.

Look for new DNA protecting Sunscreen Contains enzymes in

liposomes that initiate repair of DNA, particularly at the pyrimidines that have fused together.