The Skin – Structure, Functions & Healing

Page 1 – Lesson Learning Objectives

• Describe the structure of the skin.
• Explain the principal functions of the skin.
• Describe the phases the skin passes through to heal an injury (primary healing).

Page 2 – Why Is Skin Important? (Brainstorm)

1. Sensory reception – detects touch, pressure, temperature, and pain.

2. Protection – guards against impacts, chemicals, and pathogens.

3. Thermoregulation – conserves or dissipates heat.

4. Synthesis & storage of nutrients – produces Vitamin $D_3$ and stores lipids.

5. Excretion & secretion – removes salts, water, wastes, and certain chemicals.

Significance: Together these roles maintain homeostasis and protect overall health; any failure can compromise multiple body systems.

Page 3 – Structure of the Skin (Macro-View)

The integument consists of three continuous layers:
• Epidermis (outermost)
• Dermis (middle)
• Hypodermis / subcutaneous (deepest)

Crash Course reference videos reinforce the anatomy and physiology of each layer.

Page 4 – Accessory Structures Embedded in Skin

• Sebaceous (oil) glands – secrete bactericidal, water-proofing sebum.
• Arrector pili muscles – smooth-muscle fibers that raise hairs for insulation.
• Pacinian corpuscles – mechanoreceptors for deep pressure.
• Sweat (sudoriferous) glands – produce sweat to cool the body.
• Extensive blood-vessel & nerve networks – supply/innervate all layers.

Page 5 – Epidermis (Detailed Focus)

• Stratum basale (deepest epidermal layer)
– Possesses epidermal ridges that interlock with dermal papillae, enlarging surface area for nutrient diffusion.
– Houses basal (stem) cells that constantly divide.
• Stratum corneum (superficial layer)
– Composed of $4$–$5$ dead, keratin-filled keratinocyte strata (~$0.08\,\text{mm}$ thick).
– Avascular; depends on diffusion from deeper layers.

Physiological importance: Tight dermal–epidermal junction prevents shear injury, and keratinization forms a durable barrier.

Page 6 – Fingerprints

• Alternating epidermal ridges and dermal papillae create individual friction-ridge patterns (fingerprints).
• Ridges improve grip by increasing skin-surface friction.
• Forensics uses the uniqueness for identification.

Page 7 – Complete Epidermal Stratification

Ordered deep → superficial:

1. Stratum basale – basal, Merkel, melanocyte cells.

2. Stratum spinosum – keratinocytes, Langerhans cells.

3. Stratum granulosum – keratohyalin/lamellar granules.

4. Stratum lucidum – clear layer (only in thick skin).

5. Stratum corneum – dead, flattened keratinocytes.

Diagrammatic cues: tactile discs, sensory neurons illustrate neuro-epidermal interaction.

Page 8 – Key Epidermal Cell Types

1. Keratinocytes – synthesize the fibrous protein keratin for structural integrity.

2. Langerhans (dendritic) cells – bone-marrow–derived macrophages (~$800/\text{mm}^2$) that trigger immune responses.

3. Melanocytes – produce melanin pigment; protect DNA against UV radiation.

4. Merkel cells – mechanoreceptors associated with tactile discs; detect delicate touch.

Page 9 – Epidermal Disorder: Psoriasis

• Pathophysiology: Basal cells divide almost daily vs. the normal $\approx20\,\text{day}$ cycle.
• Outcome: Hyperkeratosis ⇒ excess flaky plaques (esp. elbows, palms, scalp).
• Complication: Pus formation from massive leukocyte infiltration.
• Prevalence: Affects $\sim5\%$ of the population.

Page 10 – Epidermis & Vitamin $D_3$

• UV exposure stimulates epidermal conversion of 7-dehydrocholesterol → Vitamin $D<em>3$ (cholecalciferol). • Liver converts $D</em>3$ to calcitriol, a hormone enabling intestinal absorption of calcium & phosphorus.
• Insufficient $D_3$ → weak bones (osteoporosis) + other metabolic disorders.
• Required sunlight dose varies with skin pigmentation; darker skin needs longer exposure.

Ethical/Practical Note: Balancing sunlight for $D_3$ synthesis with UV-induced skin-cancer risk is a public-health concern.

Page 11 – Dermis (Middle Layer)

• Papillary layer (superficial dermis)
– Areolar connective tissue; provides vascular & neural support to the epidermis.
• Reticular layer (deep dermis)
– Dense irregular connective tissue rich in collagen & elastin ⇒ tensile strength and elasticity.
– Dominant cell: fibroblast.
– Houses hair follicles, glands, vessels, and nerves.

Page 12 – Dermal Micro-Anatomy Illustration

• Shows papillary & reticular layers, vascular & neural networks, eccrine/apocrine sweat glands, arrector pili, hair follicles, and sebaceous glands.
• Highlights vertical organization from epidermis → dermis → hypodermis.

Page 13 – Dermal Disorder: Dermatitis

• Definition: Inflammation of skin, largely within papillary dermis.
• Variants & triggers:
– Contact dermatitis – allergen/irritant contact.
– Eczema – chronic inflammatory hypersensitivity.
– Urticaria (hives) – histamine-mediated wheals.
• Characteristic immune hallmarks: redness, heat, swelling, pain.

Page 14 – Hair Formation & Growth Cycle

• Hair follicle contains a hair matrix of rapidly dividing epithelial stem cells.
• As cells push upward, they keratinize and die.
• Growth phase: $2$–$5\,\text{years}$ at $0.3\,\text{mm day}^{-1}$.
• Resting phase: $3$–$5\,\text{years}$; new hair later replaces old (shedding).
• Hair papilla houses melanocytes → determine hair color.
• Arrector pili contracts to elevate hair (piloerection) for insulation/emotional cues.

Page 15 – Sebaceous (Oil) Glands

• Secrete sebum: antibacterial, conditioning, lubricates hair and epidermis.
• Holocrine mode – whole cells rupture to release lipid contents.
• Opens into hair follicles or directly to skin surface (e.g., lips).

Page 16 – Sebaceous Disorder: Acne Vulgaris

• Puberty: Sex-hormone surge → hypersecretion of sebum.
• Blocked duct traps sebum; Propionibacterium bacteria proliferate.
• Immune response → inflamed papules/pustules (red, hot, swollen).

Page 17 – Sweat (Sudoriferous) Glands

• Density: $\approx500$ glands per $\text{cm}^2$.
• Sweat composition: $99\%$ water + salts, nutrients, urea.
• Primary role: Thermoregulation via evaporative cooling.
• Secondary roles: Dilutes harmful chemicals; washes away microbes.
• Body odour: Bacterial metabolism of sweat components.

Page 18 – Hypodermis (Subcutaneous Layer)

• Functions
– Anchors skin to underlying tissues while allowing independent movement.
– Areolar tissue with abundant adipocytes → insulation, energy reserve, shock absorption.
• Fat distribution varies with age, sex hormones, diet.
• Clinical note: Few vital organs; ideal site for subcutaneous injections (hypodermic needle).

Page 19 – TED-Ed Reference: “The Science of Skin”

• Supplementary video deepens understanding of skin’s biomechanical and biochemical properties.

Page 20 – Learning Objectives Revisited

• Reinforces mastery of structure, function, and wound-healing phases (hemostasis, inflammation, proliferation, remodeling).

Page 21 – Activity: How the Skin Heals (TED Video)

• Video visually tracks primary healing:

1. Hemostasis – vasoconstriction & clot formation.

2. Inflammation – phagocytic cleanup; cytokine release.

3. Proliferation – fibroblast activity, collagen deposition, angiogenesis.

4. Remodeling – collagen realignment; scar maturation.
   • Importance: Demonstrates orchestrated cellular & molecular events restoring integrity.

Page 22 – Study Skills: Modeling the Skin

• Hands-on model creation of skin layers & accessories enhances spatial understanding.
• Advised: Label all structures; anticipate quiz on terminology & function.

Page 23 – Quiz (Kahoot)

• Immediate formative assessment to reinforce retention of lesson content.