Integumentary System Lecture Notes

Overview

• Lecture focused on the Integumentary System (Chapter 3 of the textbook).

• Instructor promised end-of-class discussion on quizzes; not content-relevant, but remember a mid-term will test these details.

• Audience: ~24 students on the call.

Major Components of the Integumentary System

• Skin (largest organ of the body)

• Hair

• Nails

• Cutaneous glands
  • Sweat glands (eccrine + apocrine)
  • Sebaceous (oil) glands
  • Ceruminous (ear-wax) glands

Six Core Functions (memorize for mid-term)

1. Protection
   • First physical & microbial barrier.
   • Shields deeper tissues from mechanical injury.

2. Sensation / Communication
   • Houses receptors for touch, pressure, pain, heat, cold.

3. Prevention of Desiccation
   • Keratinized surface limits water loss, keeps internal “salt solution” bath stable.

4. Thermoregulation
   • Capillary networks + sweat glands adjust heat gain/loss.
   • Vasodilation, vasoconstriction, evaporative cooling.

5. Excretion
   • Eliminates excess water, salts, small amounts of urea via sweat.

6. Vitamin D Synthesis
   • UV converts a cholesterol-based compound into cholecalciferol (vit D₃).

Skin: Three Structural Levels

1. Epidermis (outer; “epi =” upon)
   • Avascular, stratified squamous epithelium.
   • Main layers:
   – Stratum corneum: dead, keratin-filled cells that slough.

   • – Stratum lucidum: present only in thick skin; provides an additional layer of protection.
     – Stratum granulosum: where keratinization begins and cells start to die.
     – Stratum spinosum: contains keratinocytes and Langerhans cells, providing immune defense.
     – Stratum basale(germinativum): the deepest layer with mitotically active cells, including melanocytes and Merkel cells.
     
     • Average replacement cycle ≈ 1 month; humans shed >1 pound \,(\approx 0.5\,\text{kg}) of skin/year.

2. Dermis
   • Dense irregular connective tissue rich in collagen → toughness.
   • Contains: blood vessels, nerves, sensory receptors, hair follicles, sweat & sebaceous glands.
   • Sub-layer: dermal papillae (capillary loops) interdigitate with epidermis.

3. Subcutaneous / Hypodermis / Superficial Fascia
   • Loose connective tissue + adipose.
   • Functions: energy storage, insulation, shock absorption, anchors skin to fascia.

Thickness Variations & Examples

• Eyelid skin = ultra-thin; minimal adipose → easier blinking.

• Cheek skin = thin but thicker than eyelid for facial expression.

• Back/calf = thicker epidermis & dermis; greater adipose; less mobility required.

• Evolutionary note: ability to store calories as adipose freed hominins from constant foraging, supporting larger brain development ("berry-patch" example).

Epidermal Cell Journey

• Basal cells divide → push upward → accumulate keratin → lose nucleus/cytoplasm → die → form protective stratum corneum → flake into household dust.

• Keratin: tough fibrous protein replacing cytoplasm; also main constituent of nails & hair shaft.

Hair Structure & Function

• Shaft: visible portion; dead keratinized cells (⇒ painless haircuts).

• Root: embedded in dermis; surrounded by hair follicle (epidermal + connective tissue sheath).

• Bulb: enlargement at base; contains living matrix.
  • Nourished by papilla (vascular connective tissue).

• Arrector pili muscle: smooth muscle attaching follicle → dermis.
  • Contracts with cold/fear → hair stands ("goosebumps").

• No hair on palms, soles, lips.

Cutaneous Glands

Sebaceous (Oil) Glands

• Holocrine glands; usually open into hair follicle.

• Secrete sebum → lubricates skin/hair, reduces water loss, antibacterial.

• Blockage/enlargement:
  • Whitehead (closed comedo)
  • Blackhead (open comedo – oxidized sebum).

Sweat (Sudoriferous) Glands

• Eccrine (Merocrine)
  • Widely distributed; coiled tubular glands in dermis/hypodermis.
  • Secrete dilute saltwater → thermoregulation.

• Apocrine
  • Axillae, groin, areolae; activate at puberty.
  • Thick, milky secretion; odorless until metabolized by skin bacteria → body odor.

Ceruminous Glands

• Modified apocrine glands in external ear canal.

• Produce cerumen (ear wax) → traps debris & insects.

Nails

• Plates of hard keratin.

• Lunula: white crescent; active growth region.

• Pink appearance from underlying capillaries (diagnostic clues: cyanosis, clubbing, etc.).

Pigmentation & Skin Colour

• Melanin (produced by melanocytes in stratum basale) absorbs UV → protects DNA.

• Quantity/activity genetically determined; darker skin = more active melanocytes, not more numerous.

• Carotene: yellow-orange pigment from diet; can tint skin.

• Albino / Albinism: melanocytes present but cannot synthesize melanin → very light skin/hair.

• Myth debunked: darker-skinned people can sunburn; still need UV protection.

Sun Exposure, DNA Damage & Vitamin D

• UV → more melanin synthesis (tanning) = protective feedback.

• Excess UV penetrates epidermis → damages nuclear DNA in basal cells → mutations → potential skin cancers (basal-cell, squamous-cell, melanoma).

• Skin contains 7-dehydrocholesterol → UVB converts to pre-vitamin D₃ → liver & kidney hydroxylate to calcitriol (\text{1,25-(OH)}2\text{D}3).
  • High latitudes (e.g., Dawson Creek, Fort St John) ⇒ vitamin D deficiency risk; supplementation common.

Clinical & Practical Connections

• Vitamin D insufficiency: seasonal affective disorder, bone demineralisation.

• Jaundice in newborns: bilirubin accumulation; skin appears yellow (easier to notice on light skin).

• Fascial planes: hypodermis overlies deep fascia; knowledge crucial in surgery.

• Body-temperature nursing: capillary refill, sweat rate, goosebumps as quick assessments.

Fun Facts & Numerical Nuggets

• Average epidermis turns over every \sim30 days.

• Human sheds >1\;\text{pound} (0.5\,\text{kg}) of skin yearly → major component of household dust.

• Hank Green’s supplemental videos ≈ 10 min each (optional study aid).

Study/Exam Tips

• Be able to list all six functions verbatim.

• Diagram a cross-section of skin; label epidermis layers, dermis structures, hypodermis.

• Compare eccrine vs apocrine glands (location, secretion, function).

• Use mnemonics:
  • "Please Send Dollars To Exotic Venues" → Protection, Sensation, Desiccation, Thermoregulation, Excretion, Vitamin D.

• Connect histology to function: thick collagen → tensile strength; adipose → insulation.

• Real-world link: frostbite destroys dermal circulation; burns graded by depth (epidermal = 1°, dermal = 2°, hypodermal = 3°).

Ethical / Evolutionary / Philosophical Notes

• Adipose tissue’s energy storage pivotal in human brain evolution; challenges cultural bias against “fat.”

• Equity in health messaging: sunscreen necessary for all skin tones; debunks historical myths.

End of chapter 3 content. Next lecture: Skeletal System (heavier detail load—start pre-reading now).