Bio 201 (chapter 5)

Chapter 5: The Integumentary System

Introduction

• The integumentary system consists of skin and its accessory structures.

• Functions of the Integumentary System:

  • Protects the body

  • Maintains body temperature

  • Provides sensory information about the environment

Structure of the Skin

• The skin has two primary layers:

  • Epidermis

  • Dermis

• Additionally, there is a Subcutaneous Layer (or hypodermis), which is not considered part of the skin.

Overview of Epidermis

• The epidermis is composed of:

  • Stratified squamous epithelium

  • It is avascular, meaning it contains no blood vessels.

  • Four Types of Cells within the epidermis (details in the next slide):

  • It consists of 4-5 distinct strata (layers) of cells.

Four Principal Cells of the Epidermis

1. Keratinocytes

   • Produce the protein keratin for protection.

2. Melanocytes

   • Produce melanin, contributing to skin color and absorbing damaging ultraviolet (UV) light.

3. Langerhans Cells

   • Participate in the immune response.

4. Merkel Cells

   • Associated with Merkel discs, these cells function in the sensation of touch.

• Most cells in the epidermis are keratinocytes.

• Langerhans cells are a type of dendritic cell of the immune system.

• Melanocytes are located in the stratum basale but possess long processes extending into the next layer.

Layers (Strata) of the Epidermis

1. Stratum Basale

2. Stratum Spinosum

3. Stratum Granulosum

4. Stratum Lucidum

5. Stratum Corneum

• The five layers play significant roles in skin function and structure.

Stratum Basale (Stratum Germinativum)

• Contains Merkel cells, melanocytes, keratinocytes, and stem cells that undergo repeated division.

• Cells are attached to one another through desmosomes and to the basement membrane via hemidesmosomes.

Stratum Spinosum

• This layer provides strength and flexibility to the skin.

  • During slide preparation, cells shrink, creating a spiny appearance.

• Melanin from melanocytes transfers to keratinocytes via vesicles.

Stratum Granulosum

• Acts as a transition zone between living and dead cells.

• Comprises 3-5 layers of flat cells, illustrating apoptosis.

• Contains lamellar granules that release lipids that repel water.

Stratum Lucidum

• Present only in the fingertips, palms of the hands, and soles of the feet.

• Characterized by three to five layers of clear, flat, dead cells.

Stratum Corneum

• Composed of 25 to 30 layers of flat dead cells that are filled with keratin and encased in lipids.

• These cells are continuously shed.

• The lipids surrounding the dead cells are secreted by keratinocytes in the stratum granulosum as lamellar granules.

Epidermal Ridges

• Formed during the third month of fetal development.

• Increase the surface area of the epidermis, enhancing grip by increasing friction.

• Larger surface area allows for more nutrient exchange from the dermis and strengthens resistance against shear forces.

Dermis

• The dermis includes:

  • Hair follicles

  • Glands

  • Nerves

  • Blood vessels

• It has two major regions:

  1. Papillary Region

  2. Reticular Region

Dermis - Papillary Region

• The top 20% of the dermis.

• Composed of areolar connective tissue.

• Contains finger-like projections called dermal papillae that:

  • Anchor the epidermis to the dermis

  • Contain capillaries that nourish the epidermis

  • House Meissner’s corpuscles for touch and free nerve endings for sensations of heat, cold, pain, tickle, and itch.

Dermis - Reticular Region

• Accounts for the remaining 80% of the dermis.

• Made up of dense irregular connective tissue.

• Contains interlacing collagen and elastic fibers.

• Packed with:

  • Oil glands

  • Sweat gland ducts

  • Fat and hair follicles

• Provides strength, extensibility, and elasticity to the skin.

• Fun fact: Stretch marks result from dermal tears due to extreme stretching.

Tattoos

• Tattooing involves the permanent coloration of the skin by injecting a foreign pigment into the dermis.

Skin Color Pigments

• Melanin is produced in the epidermis by melanocytes, which convert tyrosine to melanin.

• UV exposure from sunlight increases melanin production.

• The number of melanocytes is the same in all individuals; however, the amount of pigment produced varies.

Types of Melanin

• Melanin is derived from the amino acid tyrosine.

• The structure of true melanin (eumelanin) is critical to its function.

Light and Skin Penetration

• Ultraviolet (UV) light is categorized into different regions based on wavelength:

  • Near UV

  • Far UV

• Light's energy correlates with its wavelength: shorter wavelengths carry more energy, while longer ones penetrate deeper.

• UV-A penetrates more deeply, reaching the reticular region of the dermis.

• UV-B penetrates as deep as the epidermis, but most is absorbed by the atmosphere.

• UV-C is absorbed by the ozone layer.

Effects of UV Light

• UV light (A, B, and C) can break down folic acid (a B vitamin).

Skin Color Variation and UV Light

• Melanin protects folate from UV-A, which is crucial for:

  • Cell division (important for embryonic and sperm cells)

  • DNA repair

• Lack of melanin can lead to complications such as neural tube defects and miscarriage during pregnancy.

Vitamin D Production

• UVB is essential for the synthesis of Vitamin D3, which is necessary for calcium absorption.

• A deficiency of Vitamin D can lead to conditions like rickets.

Role of Pigments in Skin Color

• Skin color diversity arises from three main pigments:

  • Melanin

  • Carotene

  • Hemoglobin (present in blood within capillaries)

• In regions with high UV exposure, the benefits of increased melanin production outweigh the disadvantages of reduced Vitamin D production, and vice versa.

Accessory Structures of Skin

• Include:

  • Epidermal ridges

  • Dermal papillae

  • Capillary loops

  • Sweat pores

  • Sebaceous (oil) glands

  • Corpuscle of touch (Meissner corpuscle)

  • Arrector pili muscle

  • Hair follicles

  • Hair roots

  • Eccrine sweat glands

  • Apocrine sweat glands

  • Lamellated (Pacinian) corpuscles

  • Sensory nerves

  • Adipose tissue.

Structure of Hair

• Hair is composed of:

  1. Shaft (visible part), which includes:

  • Medulla

  • Cortex

  • Cuticle

  1. Root (below the surface), surrounded by the hair follicle.

Hair-Related Structures

• Arrector pili: Smooth muscle in the dermis that contracts in response to cold or fear.

• Hair root plexus: Detects hair movement.

• Sebaceous glands: Connected to hair follicles.

Types of Hair

• Lanugo: Fine, non-pigmented hair that covers the fetus.

• Vellus Hair: Short, fine hair that replaces lanugo and covers most of the body.

• Terminal Hair: Coarse, pigmented hair that appears in response to androgens.

Hair Growth Cycles

• Consists of:

  1. Growth Stage: Lasts 2 to 6 years.

  2. Resting Stage: Lasts for about 3 months during which matrix cells are inactive, and the follicle atrophies. Old hair falls out as the growth stage begins again.

• Normal hair loss averages 70 to 100 hairs per day.

Hair Structure

• Made of dead cells:

  1. Medulla: Innermost layer, filled with air.

  2. Cortex: Middle layer composed of keratin-containing cells.

  3. Cuticle: Outermost layer with overlapping cells acting like shingles to protect inner layers.

Hair Color

• Determined by the type and amount of melanin:

  • Dark hair contains eumelanin.

  • Blond and red hair contain lesser amounts and may have phaeomelanin.

  • Graying hair results from a decline in melanin production.

  • White hair has air bubbles in the medullary shaft.

Hair Body

• The shape of the hair follicle influences hair type:

  • Round: straight hair

  • Oval: wavy hair

  • Flattened: curly hair

Functions of Hair

• Functions include:

  • Preventing heat loss

  • Reducing sunburn

  • Eyelashes protect the eyes

  • Hair root plexus functions as a light touch sensory receptor.

Glands of the Skin

• Specialized exocrine glands in the dermis include:

  1. Sebaceous (oil) glands

  2. Sudiferous (sweat) glands

  3. Ceruminous (wax) glands

  4. Mammary (milk) glands

Sebaceous (Oil) Glands

• Connected to hair follicles and produce sebum, which contains:

  • Cholesterol

  • Proteins

  • Fats

  • Salts

• Functions of Sebum:

  • Moistens hairs.

  • Waterproofs and softens skin.

  • Inhibits growth of bacteria and fungi.

• Acne is caused by the bacterial inflammation of these glands, stimulated by hormones during puberty.

Exocrine Glands vs Endocrine Glands

• Exocrine Glands: Secrete sweat, ear wax, saliva, and digestive enzymes onto body surfaces or cavities via ducts.

• Endocrine Glands: Release hormones directly into the bloodstream for homeostasis regulation.

Functional Classification of Exocrine Glands

• Merocrine Glands: Produce and secrete substances without losing cellular material (e.g., sweat).

• Apocrine Glands: A portion of the cell is lost during secretion.

• Holocrine Glands: Entire cells rupture and discharge contents.

Types of Sweat Glands

• Eccrine Sweat Glands:

  • Distributed widely across the skin.

  • Ducts lead to skin surface.

  • Help eliminate waste such as urea.

• Apocrine Sweat Glands:

  • Limited to areas like the axilla and pubis.

  • Ducts open into hair follicles.

  • Secretions are thicker and may produce odor due to bacterial metabolism.

Functions of Eccrine Sweat Glands

• Main purposes include:

  • Thermoregulation: Evaporation of water cools the skin.

  • Humectant: Absorbs and traps moisture from the air.

  • Antimicrobial Properties: Due to small peptides.

  • Waste Removal: Excretes products like urea, which comes from metabolized proteins.

Purpose and Effects of Apocrine Sweat Glands

• The exact purpose of apocrine glands remains unclear, but they may have evolved from similar glands in other mammals.

Ceruminous Glands

• Modified apocrine sweat glands that produce waxy cerumen in the ear canal.

Structure of Nails

• Comprised of tightly packed keratinized cells.

• Components include:

  • Nail Body

  • Nail Root: The lunula appears white due to a thickened stratum basale.

  • Nail Matrix: Responsible for mitotic division.

  • Eponychium (cuticle): A layer of stratum corneum.

Other Functions of Skin

• Synthesis of Vitamin D: Activated precursor molecules in the skin by UV light are modified in the liver and kidneys to produce calcitriol, the active form of Vitamin D, which is essential for calcium absorption from the gastrointestinal tract.

Wound Healing

Epidermal Wound Healing

• Involves minor abrasions or burns:

  • Basal cells migrate across the wound.

  • Epidermal growth factors stimulate cell division.

Deep Wound Healing

• Healing occurs in four phases:

  1. Hemostasis: Blood clotting to stop bleeding.

  2. Inflammatory Phase: White blood cells clean the wound.

  3. Proliferative Phase: Fibroblasts produce collagen and new tissue is formed.

  4. Remodeling/Maturation Phase: The scab falls off, and tissue is remodeled for strength.

Hemostasis

• Blood clot forms to seal the wound and prevent bleeding.

• White blood cells arrive to clear debris and bacteria.

• Increased blood flow delivers O2 and nutrients, causing swelling and warmth in the area.

Remodeling of Tissue

• Fibroblasts produce collagen, forming a framework for new cells.

• As collagen is remodeled, tissue becomes stronger and more elastic.

Scar Formation

• Characteristics:

  • Scar tissue has fewer blood vessels, resulting in a lighter color.

  • May have reduced hair follicles.

  • Collagen structure differs from normal skin.

Aging and the Integumentary System

• Changes in the integumentary system are most pronounced in the dermis:

  • Development of wrinkles.

  • Slower hair and nail growth.

  • Dryness and cracking due to sebaceous gland atrophy.

  • Blood vessel walls thicken, decreasing nutrient supply and leading to thinner skin and loss of subcutaneous fat.

Skin Color Symptoms

• Jaundice: Yellowish skin due to bilirubin buildup from liver disease.

• Cyanosis: Bluish skin and nail beds indicating oxygen-depleted hemoglobin.

• Erythema: Redness caused by enlarged capillaries in dermis due to inflammation, infection, allergy, or burns.

Skin Color Pigments

• Freckles or liver spots: Patches of melanin.

• Albinism: Inherited absence of tyrosinase leading to lack of pigment.

• Vitiligo: Autoimmune condition causing the loss of melanocytes in skin regions, resulting in white patches.

Photodamage

• UV light (A and B) damages the skin and can lead to:

  • Sunburn from acute overexposure.

  • DNA damage in epidermal cells, potentially resulting in skin cancer (DNA absorbs UV light maximally at 260 nm).

  • UVA exposure leads to oxygen free radicals that damage collagen and elastic fibers, contributing to skin wrinkling.

Skin Cancer

• Prevalence: 1 million cases diagnosed annually.

• Three common forms:

  1. Basal Cell Carcinoma: Rarely metastasizes.

  2. Squamous Cell Carcinoma: May metastasize.

  3. Malignant Melanomas: Rapidly metastasize and are life-threatening; most common cancer in young women.

• Risk Factors:

  • Skin color

  • Sun exposure

  • Family history

  • Age

  • Immunological status.

ABCDE Guidelines for Skin Cancer Detection

• A: Asymmetry

• B: Border

• C: Color

• D: Diameter

• E: Evolving (changes over time).

Rickets

• Condition characterized by softening and weakening of bones in children, typically due to prolonged vitamin D deficiency.

Burns

• The systemic effects of severe burns pose a greater threat to life than local effects.

• If burn area exceeds 70%, over half of victims may die.

• Rule of Nines for assessing burn severity based on percentage of body surface area:

  • Anterior head and neck: 4.5%

  • Anterior shoulders, arms, forearms, and hands: 9% each

  • Anterior thighs, legs, and feet: 18% each

  • Anterior trunk: 18%

  • Perineum: 1%

Hypertrichosis and Hirsutism

• Hypertrichosis: Excessive hair growth in any region of the body, regardless of sex.

• Hirsutism: Refers specifically to excessive terminal hair growth in androgen-dependent areas in females.

Introduction to the Integumentary System

• The integumentary system is the largest organ system by surface area, approximately 2 \text{ m}^2 (22 \text{ square feet}) in adults.

• Primary Functions:

  • Protection: Physical, chemical, and biological barrier against the environment.

  • Thermoregulation: Via sweat production and adjusting blood flow in the dermis.

  • Excretion and Absorption: Elimination of salts and organic wastes; absorption of fat-soluble vitamins (A, D, E, K).

  • Synthesis of Vitamin D: Essential for calcium homeostasis.

  • Sensory Perception: Detection of touch, pressure, vibration, temperature, and pain.

The Epidermis: Cellular Dynamics

• Composed of keratinized stratified squamous epithelium.

• Four Principal Cell Types:

  1. Keratinocytes (90\%): Produce keratin and lamellar granules which release a water-repellent sealant.

  2. Melanocytes (8\%): Derived from the ectoderm; produce melanin (eumelanin and pheomelanin). Melanin granules aggregate over the nucleus to protect DNA from UV radiation.

  3. Langerhans Cells (Intraepidermal Macrophages): Arise from red bone marrow and migrate to the epidermis; they detect invading microbes.

  4. Merkel Cells: Located in the deepest layer; they contact the flattened process of a sensory neuron called a Merkel (tactile) disc.

Detailed Strata of the Epidermis

1. Stratum Basale: A single row of cuboidal or columnar keratinocytes. Contains stem cells that undergo continuous cell division. Also known as the stratum germinativum.

2. Stratum Spinosum: 8 to 10 layers of many-sided keratinocytes. Includes projections of melanocytes and Langerhans cells. The "spines" are formed by bundles of keratin intermediate filaments inserting into desmosomes.

3. Stratum Granulosum: 3 to 5 layers of flattened keratinocytes undergoing apoptosis. Contains dark-staining keratohyalin granules (convert filaments into keratin) and lamellar granules (lipid-rich secretion).

4. Stratum Lucidum: Found only in thick skin (palms, soles). Consists of 4 to 6 layers of flattened, clear, dead keratinocytes with large amounts of keratin.

5. Stratum Corneum: 25 to 30 layers of extremely flattened dead cells. The final product of keratinization; these cells are called corneocytes or squames.

The Dermis: Structural Support

• Composed primarily of dense irregular connective tissue containing collagen and elastic fibers.

1. Papillary Region (20\%):

   • Consists of areolar connective tissue.

   • Dermal papillae increase surface area for adhesion and nutrient diffusion.

   • Contains Meissner corpuscles (nerve endings sensitive to light touch).

2. Reticular Region (80\%):

   • Attached to the subcutaneous layer.

   • Contains bundles of thick collagen fibers, some elastic fibers, fibroblasts, and macrophages.

   • Structural orientation of collagen fibers forms lines of cleavage (Langer’s lines); incisions made parallel to these lines heal with less scarring.

Accessory Structures

• Hair (Pili):

  • Anatomy: The shaft projects above the surface; the root is deep to the surface. The hair bulb contains the germinal layer (stratum basale) known as the hair matrix.

  • Arrector Pili: Under physiological or emotional stress (cold/fright), autonomic nerve endings stimulate this smooth muscle to contract, pulling hair shafts perpendicular (goosebumps).

• Skin Glands:

  • Sebaceous Glands: Simple branched acinar glands. They use holocrine secretion, where the cell ruptures to release sebum. Sebum prevents hair from drying and inhibits some bacteria.

  • Sudoriferous (Sweat) Glands:

  • Eccrine: Simple coiled tubular glands. Function in thermoregulation (insensible and sensible perspiration).

  • Apocrine: Found in axilla and groin. Secretion via exocytosis (despite name). Stimulated during emotional stress and sexual excitement.

Wound Healing Mechanisms

1. Epidermal Wound Healing:

   • Basal cells break contact with basement membrane, enlarge, and migrate across the wound (contact inhibition stops migration).

   • Epidermal Growth Factor (EGF) stimulates basal stem cells to divide and replace migrated cells.

2. Deep Wound Healing:

   • Inflammatory Phase: Blood clot forms; neutrophils and monocytes phagocytize microbes.

   • Migratory Phase: Clot becomes a scab; fibroblasts migrate and begin synthesizing collagen and glycoproteins (granulation tissue).

   • Proliferative Phase: Extensive growth of epithelial cells beneath the scab.

   • Maturation Phase: Scab sloughs off; collagen fibers become more organized.

Homeostatic Imbalances and Clinical Conditions

• Skin Cancer:

  • Basal Cell Carcinoma (78\%): Arises from stratum basale; rarely metastasizes.

  • Squamous Cell Carcinoma (20\%): Arises from stratum spinosum; variable tendency to metastasize.

  • Malignant Melanoma (2\%): Arises from melanocytes; highly metastatic and resistant to chemotherapy.

• Burns:

  • First-degree: Only epidermis involved (mild pain/erythema).

  • Second-degree: Epidermis and part of dermis; characterized by blisters.

  • Third-degree (Full-thickness): Destroys epidermis, dermis, and subcutaneous layer; often lacks sensation due to destroyed nerve endings.