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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 Dermis under epidermis Stratified squamous Keratinizing 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 Skin Controls internal body temperature: 1. 1. 2. 2. 3. Heat loss: activates sweat glands and allows blood to flush into skin capillary beds so heat can radiate from skin surface Heat retention: not allowing blood to flush to skin capillary beds Aids in excretion of urea and uric acid: perspiration by sweat glands 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 regions 1. 2. 3. Epidermis—superficial region Dermis—middle region 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 (a) Dermis Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Stratum granulosum Three to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Stratum basale Deepest 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. Figure 5.2a 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 Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Stratum granulosum Three to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Stratum basale Deepest 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 Melanin granule Melanocyte (b) Keratinocytes Dermis Sensory nerve ending Epidermal Tactile dendritic cell (Merkel) cell Figure 5.2b Layers of the Epidermis: Stratum Granulosum (Granular Layer) Thin; three to five cell layers in which the cells flatten Keratohyaline and lamellated granules accumulate (a) Dermis Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Stratum granulosum Three to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Stratum basale Deepest 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. Figure 5.2a 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 Stratum corneum Most superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. Stratum granulosum Three to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Stratum basale Deepest 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 Melanin granule Melanocyte (b) Keratinocytes Dermis Sensory nerve ending Epidermal Tactile dendritic cell (Merkel) cell Figure 5.2b Dermis Strong, flexible connective tissue Cells include fibroblasts, macrophages, and occasionally mast cells and white blood cells Two layers: Papillary Reticular Hair shaft Epidermis Papillary layer Dermis Reticular layer Hypodermis (superficial fascia) Nervous structures • Sensory nerve fiber • Pacinian corpuscle • Hair follicle receptor (root hair plexus) Dermal papillae Subpapillary vascular plexus Pore Appendages of skin • Eccrine sweat gland • Arrector pili muscle • Sebaceous (oil) gland • Hair follicle • Hair root Cutaneous vascular plexus Adipose tissue Figure 5.1 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 Friction ridges Openings of sweat gland ducts (a) Figure 5.4a (b) Figure 5.4b 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 2. Carotene 3. Produced in melanocytes; migrates to keratinocytes where it forms “pigment shields” for nuclei Freckles and pigmented moles Local accumulations of melanin Yellow to orange, most obvious in the palms and soles 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 Jaundice Yellow 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 marks Bruising 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) Tubular secretory structure Simple duct structure Compound duct structure (duct does not branch) (duct branches) Simple tubular Simple branched tubular Example Example Compound tubular Intestinal glands Stomach (gastric) glands Duodenal glands of small intestine Example Alveolar secretory structure Simple alveolar Simple branched alveolar Compound alveolar Example Example Example No important example in humans Sebaceous (oil) glands Mammary glands Surface epithelium Duct Compound tubuloalveolar Example Salivary glands Secretory epithelium Figure 4.5 Modes of Secretion Merocrine Products are secreted by exocytosis (e.g., pancreas, sweat and salivary glands) Holocrine Products 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 Sweat pore Eccrine gland Sebaceous gland Duct Dermal connective tissue Secretory cells (b) Photomicrograph of a sectioned eccrine gland (220x) Figure 5.5b 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 Sweat pore Dermal connective tissue Sebaceous gland Sebaceous gland duct Eccrine gland Hair in hair follicle Secretory cells (a) Photomicrograph of a sectioned sebaceous gland (220x) Figure 5.5a 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 What is Acne? 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 Follicle wall Hair shaft Arrector pili Sebaceous gland Hair root • Connective tissue root sheath • Glassy membrane • External epithelial root sheath • Internal epithelial root sheath Hair • Cuticle • Cortex • Medulla (a) Diagram of a cross section of a hair within its follicle What is the role of hair conditioners? Hair bulb Figure 5.6a Follicle wall • Connective tissue root sheath • Glassy membrane • External epithelial root sheath • Internal epithelial root sheath Hair Hair shaft Arrector pili Sebaceous gland Hair root • Cuticle • Cortex • Medulla (b) Photomicrograph of a cross section of a hair and hair follicle (250x) 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” Hair shaft Arrector pili Sebaceous gland Hair root Hair bulb Follicle wall • Connective tissue root sheath • Glassy membrane • External epithelial root sheath • Internal epithelial root sheath Hair root • Cuticle • Cortex • Medulla Hair matrix Hair papilla Melanocyte Subcutaneous adipose tissue (c) Diagram of a longitudinal view of the expanded hair bulb of the follicle, which encloses the matrix Figure 5.6c Follicle wall • Connective tissue root sheath • Glassy membrane • External epithelial root sheath • Internal epithelial root sheath Hair root • Cuticle • Cortex • Medulla Hair matrix Hair papilla Subcutaneous adipose tissue Hair shaft Arrector pili Sebaceous gland Hair root Hair bulb (d) Photomicrograph of longitudinal view of the hair bulb in the follicle (160x) 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) Lateral nail fold Lunule (a) Free edge Body of nail of nail Eponychium (cuticle) Nail bed Proximal nail fold Root of nail Nail matrix (b) Hyponychium Phalanx (bone of fingertip) Figure 5.7 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 watersoluble 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 Body temperature regulation 2. ~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 Cutaneous sensations 3. Temperature, touch, and pain Functions of the Integumentary System Metabolic functions 4. 5. 6. Synthesis of vitamin D precursor and collagenase Chemical conversion of carcinogens and some hormones Blood reservoir—up to 5% of body’s blood volume 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 & Allergies Skin 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 (graywhite) 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 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. Ultraviolet Light 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.