Introduction to Dermatology

Dermatology

• It is the study and treatment of the integumentary system. Comprising the skin (integument), hair, nails, and cutaneous glands.
• Skin is the body's largest organ.
• The skin has two main layers and a nearby layer.
  • Epidermis consists of epithelium.
  • Dermis is made up of connective tissue.
  • Hypodermis is connective tissue just underneath the skin.

Skin (Integument)

• Skin regions include:
  • Epidermis which is the superficial region and consists of epithelial tissue.
  • Dermis which underlies the epidermis and is mostly fibrous connective tissue.
  • Hypodermis consist of superficial fascia that is a subcutaneous layer deep to skin.
    • Functions although not part of the skin.
    • Adipose tissue is present, absorbing shock and providing insulation.
    • Anchors skin to underlying structures, particularly muscles.

Cells of the Epidermis

• Keratinocytes:
  • Produce the fibrous protein keratin.
  • Most of the epidermis' cells are keratinocytes.
  • Tightly connected by desmosomes.
• Melanocytes:
  • This is about 10-25% of the cells in the deepest epidermis.
  • They produce melanin which is packaged into melanosomes.
  • It protects the apical surface of the keratinocyte nucleus from UV damage.
• Dendritic (Langerhans) Cells:
  • Macrophages are key activators of the immune system.
• Tactile (Merkel) Cells:
  • Sensory touch receptors

Layers of the Epidermis: Stratum Basale (Basal Layer)

• This is the deepest epidermal layer.
• Also called stratum germinativum.
• Firmly attached to the dermis.
• Single row of stem cells that are actively mitotic.
• Produces two daughter cells.
  • One cell journeys from basal layer to surface in 25–45 days, dying as it moves.
  • One cell remains in stratum basale as a stem cell.
• Melanocytes compose 10–25% of this layer.

Layers of the Epidermis: Stratum Spinosum (Prickly Layer)

• Several layers thick.
• Cells have a web-like system of intermediate prekeratin filaments attached to desmosomes.
• Abundant melanosomes and dendritic cells.

Layers of the Epidermis: Stratum Granulosum (Granular Layer)

• Thin, approximately four to six cell layers.
• Cell appearance changes.
  • Cells flatten.
  • Nuclei and organelles disintegrate.
  • Keratinization begins.
  • Accumulate keratohyaline granules helping form keratin in the upper layers.
  • Accumulate lamellar granules, water-resistant glycolipid slows water loss.
• Cells above this layer die, as they are too far from dermal capillaries.

Layers of the Epidermis: Stratum Lucidum (Clear Layer)

• Only in thick skin.
• Located as a thin, translucent band superficial to the stratum granulosum.
• Composed of a rows of flat, dead keratinocytes.

Layers of the Epidermis: Stratum Corneum (Horny Layer)

• 20–30 rows of dead, flat, anucleate keratinized membranous sacs.
• Three-quarters of epidermal thickness.
• Cells are dead, but they have functions to include:
  • Protecting deeper cells from the environment and water loss.
  • Protection from abrasion and penetration.
  • Barrier against biological, chemical, and physical assaults.

Dermis

• Composed of strong, flexible connective tissue.
• Cells includes fibroblasts, macrophages, and occasionally mast cells and white blood cells.
• Fibers in the matrix bind the body together, often used to make leather.
• Contains nerve fibers, blood and lymphatic vessels.
• Contains epidermal hair follicles, oil and sweat glands.
• Has two layers:
  • Papillary
  • Reticular

The Dermis

• Rich in collagen fibers.
• Has Large blood supply.
• Contains cutaneous glands, hair follicles, piloerector muscles, and nail roots.

Layers of the Dermis: Papillary Layer

• Composed of areolar connective tissue with collagen and elastic fibers and blood vessels.
• Loose tissue where phagocytes can patrol for microorganisms.
• Contains dermal papillae that are superficial peglike projections.
• Collectively these ridges are called friction ridges that:
  • Enhance gripping ability.
  • Contribute to the sense of touch.
  • Pattern is fingerprints.

Layers of the Dermis: Reticular Layer

• Makes up ~80% of dermal thickness.
• Consists of dense fibrous connective tissue.
• Elastic fibers provide stretch-recoil properties.
• Collagen fibers:
  • Provide strength and resiliency.
  • Bind water.
• Cleavage lines due to most collagen fibers parallel to skin surface:
  • Externally invisible.
  • Important to surgeons because Incisions parallel to cleavage lines gap less and heal more readily.

Skin Markings

• Flexure Lines occur at or near joints.
• Dermal folds exist at flexible areas.
• Dermis tightly secured to deeper structures, where skin cannot slide easily for joint movement.
• Visible on hands, wrists, fingers, soles, toes.

Other Skin Markings

• Striae occur commonly and are silvery-white scars or stretch marks.
  • Occurs when extreme stretching causes dermal tears.
• Blisters are pockets with fluid that arise from acute, short-term trauma.
  • Fluid separates epidermal and dermal layers.

Skin Color

• Three pigments contribute to skin color:
  • Melanin is a yellow to reddish-brown to black pigment, responsible for dark skin colors.
    • Produced by melanocytes in stratum basale via freckles and pigmented moles.
  • Carotene is a yellow to orange pigment, most obvious in the palms and soles of the feet which is the stratum corneum.
  • Hemoglobin is a reddish pigment responsible for the pinkish hue of the skin.
    • Is more apparent during blushing/exercise.

Skin Color in Diagnosis

• Cyanosis indicates blue skin color due to low oxygenation of hemoglobin.
• Erythema indicates redness due to fever, hypertension, inflammation, or allergy.
• Pallor indicates blanching due to anemia, low blood pressure, fear, or anger.
• Jaundice shows yellow cast, indicating liver disorder.
• Bronzing shows inadequate steroid hormones in Addison's disease.
• Bruises happen because of clotted blood beneath the skin.

Appendages of the Skin

• All are derivatives of the epidermis.
• Includes:
  • Hairs and hair follicles
  • Nails
  • Sweat glands
  • Sebaceous (oil) glands

Hair

• Dead keratinized cells of hard keratin
  • More durable than soft keratin of skin
• Not in palms, soles, lips, nipples, portions of external genitalia
• Functions include:
  • Warn of insects on skin
  • Physical trauma
  • Heat loss
  • Sunlight
• Hair pigments
  • Melanins (yellow, rust, brown, or black).
  • Trichosiderin in red hair.
  • Gray/white hair occurs when there is a decreased melanin production, and increased air bubbles in shaft

Cross Sectional Structure of a Hair

• Features 3 layers:
  • Medulla is an internal layer and has loosely arranged cells and air spaces.
  • Cortex is the bulk of hair and contains several layers of long cells.
  • Cuticle is Outer layer consisting of overlapping scaly cells.

Hair Follicles

• Extend from epidermal surface to dermis.
• They are two-layered wall - part dermis, part epidermis.
• Hair bulb features:
  • Expanded deep end
  • Hair follicle receptor is (root hair plexus) which are sensory nerve endings that are touch receptors.
  • Hair matrix exists as an actively dividing area.
• Arrector pili consist of:
  • Smooth muscle attached to follicle.
  • Responsible for "goose bumps".
• Hair papilla features dermal tissue and blood supply.

Types and Growth of Hair

• Vellus hair which is Pale, fine body hair of children and adult females.
• Terminal hair which is coarse, long hair of eyebrows and scalp.
  • At puberty :
    • Appears in axillary and pubic regions of both sexes.
    • Face and neck of males.
• Nutrition and hormones affect hair growth.
• Follicles cycle between active and regressive phases.
• The average is 2.25 mm growth per week.
• Lose approximately 90 scalp hairs a day.

Hair Thinning and Baldness

• Alopecia occurs as hair thinning in both sexes after age 40.
• True (frank) baldness is genetically determined and sex-influenced condition.
  • Male pattern baldness caused by follicular response to DHT (dihydrotestosterone) is treated with:
    • Minoxidil (rogaine)
    • Finasteride (propecia)

Nails

• Derivatives of stratum corneum.
• Composed of very thin, dead, scaly cells with parallel rows of keratin.
• Primates have flat nails, as opposed to claws, for easier manipulation.
• Nail plate
• Free edge
• Nail body
• Nail root
• Nail fold
• Nail groove
• Nail bed
• Hyponychium
• Nail matrix
• Lunule
• Eponychium (cuticle)

Sweat Glands

• Also called sudoriferous glands.
• All skin surfaces except nipples.
• ~3 million per person.
• Two main types:
  • Eccrine (merocrine) sweat glands.
  • Apocrine sweat glands.

Eccrine Sweat Glands

• The Most numerous sweat glands
• Abundant on palms, soles, and forehead.
• Ducts connect to pores.
• Responsible for thermoregulation.
• Regulated by sympathetic nervous system.
• Their secretion is sweat:
  • 99% water, salts, vitamin c, antibodies, dermcidin (microbe-killing peptide), and metabolic wastes.

Apocrine Sweat Glands

• Confined to axillary and anogenital areas.
• Secretion of sweat + fatty substances + proteins:
  • Viscous milky or yellowish.
  • Odorless until bacterial interaction which causes body odor.
• Larger than eccrine sweat glands.
• Ducts empty into hair follicles, beginning functioning at puberty.
• Function is unknown however it may act as sexual scent gland.
• Modified apocrine glands contains:
  • Ceruminous glands located at the lining of external ear canal that secrete cerumen (earwax).
  • Mammary glands which produce and secrete milk.

Sebaceous (Oil) Glands

• Widely distributed.
• Not in thick skin of palms and soles.
• Most develop from hair follicles and secrete into hair follicles.
• Relatively inactive until puberty.
• Stimulated by hormones, especially androgens.
• Secrete sebum:
  • Oily holocrine secretion.
  • Bactericidal
  • Softens hair and skin.

Functions of the Integumentary System

• Protection is a chemical, physical, and mechanical barrier.
• Body temperature regulation is accomplished by:
  • Dilation (cooling) and constriction (warming) of dermal vessels.
  • Increasing sweat gland secretions to cool the body.
• Cutaneous sensation – exoreceptors sense touch and pain.
• Metabolic functions – synthesis of vitamin D in dermal blood vessels.
• Blood reservoir – skin blood vessels store up to 5% of the body's blood volume.
• Excretion – limited amounts of nitrogenous wastes are eliminated from the body in sweat.

Biological Barriers

• Include dendritic cells of epidermis:
  • Present foreign antigens to white blood cells.
• Macrophages of dermis:
  • Present foreign antigens to white blood cells.
• DNA:
  • Its electrons absorb UV radiation, converting radiation to heat.

Skin Cancer

• Most skin tumors are benign (not cancerous) and do not metastasize (spread).
• Risk factors for cancers:
  • Overexposure to UV radiation.
  • Frequent irritation of skin.
• Some skin lotions contain enzymes that can repair damaged DNA.
• Three major types:
  • Basal cell carcinoma
  • Squamous cell carcinoma
  • Melanoma.

Skin Cancer: Basal Cell Carcinoma

• The most common skin cancer.
• Least dangerous.
• Hardly metastasizes.
• Arises in stratum basale and invades dermis.

Skin Cancer: Squamous Cell Carcinoma

• Arises from keratinocytes of stratum spinosum.
• Can metastasize to lymph nodes.
• Can be lethal.

Skin Cancer: Melanoma

• The most deadly skin cancer
• Arises from melanocyte of an existing mole.
• Metastasizes quickly and is often fatal if not treated quickly.

Burns

• Tissue damage caused by heat, electricity, radiation and certain chemicals.
  • Denatures proteins.
  • Kills cells.

First-Degree Burn (Partial-Thickness Burn)

• Only epidermis is damaged.
• Usually localized.
• Redness, slight edema, and pain occur.
• Ex: Sunburn.

Second-Degree Burn (Partial-Thickness Burn)

• Epidermis and part of dermis are damaged.
• Red, tan, or white.
• Blistered and painful.
• May damage hair follicles, nerve endings, and cutaneous glands.
• Ex: Severe sunburns and scalds.

Third-Degree Burn (Full-Thickness Burn)

• Epidermis and dermis are completely destroyed.
• Contracture and disfigurement can result.

Developmental Aspects of the Integumentary System

• Aging changes the skin.
  • Epidermal replacement slows; skin becomes thin, dry, and itchy (decreased sebaceous gland activity).
  • Subcutaneous fat and elasticity decrease, leading to cold intolerance and wrinkles.
  • Increased risk of cancer due to decreased numbers of melanocytes and dendritic cells.
  • Hair thinning.
• Ways to delay aging:
  • UV protection, good nutrition, lots of fluids, good hygiene