Overview
This lecture introduces the structure and functions of the skin, the main organ of the integumentary system, and discusses skin layers, cell types, pigmentation, common medical conditions, burns, and skin cancer.
The Integumentary System Overview
- The integumentary system includes the skin, hair, nails, and cutaneous glands.
- Skin is the largest and heaviest organ, making up about 15% of body weight.
- Dermatology is the study and treatment of the integumentary system.
Functions of the Skin
- Protects against trauma, infection, dehydration, UV radiation, and chemicals.
- Acts as a sensory organ for touch, pressure, pain, temperature, and vibration.
- Regulates body temperature via blood vessels and sweat glands.
- Initiates vitamin D synthesis for calcium absorption and skeletal health.
- Enables non-verbal communication and facial expressions.
- Provides waterproofing and repairs itself after minor damage.
Structure and Layers of Skin
- Skin thickness varies from 0.5 mm (eyelids) to 6 mm (shoulders).
- Two main layers: epidermis (outer, keratinized stratified squamous epithelium) and dermis (underlying connective tissue).
- Below the dermis is the hypodermis (subcutaneous tissue), mostly adipose tissue.
Epidermis: Structure and Cell Types
- Four primary cell types: keratinocytes (produce tough keratin protein), stem cells (produce new keratinocytes), melanocytes (produce melanin pigment), tactile (Merkel) cells (detect touch), and dendritic cells (immune defense).
- Main cell junctions: desmosomes (anchoring) and tight junctions (barrier).
Epidermal Layers (from deep to superficial)
- Stratum basale: single layer with stem cells, melanocytes, and tactile cells.
- Stratum spinosum: several layers of keratinocytes; contains dendritic cells.
- Stratum granulosum: 3-5 layers, contains keratin granules and releases lipids for waterproofing.
- Stratum lucidum: only in thick skin (palms, soles), translucent and dead keratinocytes.
- Stratum corneum: up to 30 layers of dead, keratinized cells for abrasion resistance.
Dermis and Hypodermis
- Dermis: mainly collagen fibers, contains blood vessels, nerves, glands, hair follicles, and papillae (increase surface area and attach to epidermis).
- Papillary layer: thin, areolar tissue, rich in blood vessels and nerves.
- Reticular layer: thick, dense irregular connective tissue, provides strength and elasticity.
- Hypodermis: mostly adipose tissue; stores energy, insulates, and serves as site for hypodermic injections.
Skin Color and Variations
- Melanocytes produce melanin (eumelanin: brown/black; pheomelanin: red/yellow).
- All humans have similar numbers of melanocytes; skin color depends on melanin amount and breakdown rate.
- Skin color is polygenetically inherited.
- Variations and conditions: vitiligo (loss of melanocytes), albinism (no melanin), pallor (pale), hematoma (bruise), jaundice (yellowing), erythema (redness), cyanosis (blue hue), carotene accumulation (orange hue), hemangiomas (birthmarks).
Skin Cancer
- Main types: basal cell carcinoma (least dangerous, from stratum basale), squamous cell carcinoma (from stratum spinosum, may metastasize), and malignant melanoma (from melanocytes, most deadly).
- ABCDE rule for identifying melanoma: Asymmetry, Border, Color, Diameter, Evolution/Elevation.
- UV exposure increases cancer risk; both UVA ("tanning") and UVB ("burning") rays are harmful.
Burns
- Classified by depth: first-degree (epidermis), second-degree (epidermis and part of dermis), third-degree (all skin layers, may affect deeper tissues).
- Rule of nines estimates body area affected by burns.
- Burns are critical if >25% of body is second-degree, >10% is third-degree, or if face/hands/feet are affected.
Key Terms & Definitions
- Keratinocyte — main skin cell producing keratin protein.
- Melanocyte — pigment cell producing melanin for UV protection.
- Desmosome — cell junction providing mechanical strength.
- Tight junction — cell junction preventing substance passage between cells.
- Hypodermis — subcutaneous layer below dermis, mainly fat storage.
- Stratum corneum — outermost epidermal layer of dead, keratinized cells.
- ABCDE rule — criteria for melanoma identification (Asymmetry, Border, Color, Diameter, Evolution/Elevation).
- Rule of nines — method to estimate total body surface area burned.
Action Items / Next Steps
- Review Chapter 6 in your textbook, especially the summary tables.
- Create your own table of epidermal cell types and skin layers for study.
- Prepare for the next lecture on accessory organs of the integumentary system.