SKIN AND ITS APPENDAGES
1. ANATOMY OF THE SKIN
1.1 General Features
✔ Largest organ of the body (~15% of body weight, ~1.5-2 m² surface area).
✔ Major functions:
- Barrier Protection → Against mechanical, chemical, microbial, UV damage.
- Thermoregulation → Through sweat glands & blood flow modulation.
- Sensory Perception → Touch, pain, temperature, vibration.
- Immune Defense → Langerhans cells & skin microbiome.
1. ANATOMY OF THE SKIN (CONTINUED)
1.2 Layers of the Skin
The skin consists of three primary layers, each with distinct components and functions.
1.2.1 Epidermis (Superficial, Avascular Layer)
✔ Derived from ectoderm.
✔ Stratified squamous keratinized epithelium.
✔ Main cell types:
- Keratinocytes → Major structural cells, produce keratin.
- Melanocytes → Located in stratum basale, produce melanin (skin pigmentation).
- Langerhans Cells → Antigen-presenting cells (APCs), initiate immune responses.
- Merkel Cells → Mechanoreceptors for fine touch.
✔ Layers of the Epidermis (Mnemonic: "Come Let's Get Sun Burned"):
- Stratum Corneum → Outer dead keratinized cells, protective barrier.
- Stratum Lucidum → Transparent layer, present only in palms & soles.
- Stratum Granulosum → Keratohyalin granules, waterproofing function.
- Stratum Spinosum → "Prickle cell layer," desmosomes provide cohesion.
- Stratum Basale (Germinativum) → Contains melanocytes & stem cells, site of cell division.
✔ Epidermal Turnover Rate → ~28 days (faster in psoriasis).
1.2.2 Dermis (Middle, Vascular Layer)
✔ Derived from mesoderm.
✔ Composed of dense irregular connective tissue with collagen (Type I) & elastin.
✔ Divided into:
- Papillary Dermis → Loose connective tissue, capillaries, Meissner’s corpuscles.
- Reticular Dermis → Dense collagen bundles, sweat glands, hair follicles, Pacinian corpuscles.
✔ Contains: - Blood vessels → Thermoregulation.
- Nerves → Sensory perception.
- Lymphatics → Immune defense.
1.2.3 Hypodermis (Subcutaneous Tissue, Fat Layer)
✔ Composed of adipose tissue → Provides insulation, energy storage, and shock absorption.
✔ Contains larger blood vessels & nerves.
✔ Connects skin to underlying muscles & bones.
1.3 Skin Appendages
1.3.1 Hair Follicles
✔ Structure: Hair shaft, root, bulb, and dermal papilla.
✔ Functions: Protection, sensory perception, thermoregulation.
✔ Hair Growth Cycle:
- Anagen (Active Growth Phase) → 2-6 years (~85% hairs).
- Catagen (Transitional Phase) → 2-3 weeks (~1% hairs).
- Telogen (Resting Phase) → 3 months (~14% hairs).
✔ Conditions: - Androgenic alopecia → Genetic hair loss, influenced by DHT.
- Alopecia areata → Autoimmune hair loss.
1.3.2 Sebaceous Glands
✔ Holocrine glands, secrete sebum (lipid-rich).
✔ Stimulated by androgens.
✔ Found everywhere except palms & soles.
✔ Overactivity → Acne, seborrhea.
1.3.3 Sweat Glands
✔ Eccrine Glands → Distributed all over, important for thermoregulation.
✔ Apocrine Glands → Located in axilla, groin, perianal region; activated at puberty (pheromones).
1.3.4 Nails
✔ Keratinized plates from stratum basale.
✔ Growth: 0.1 mm/day (fingernails faster than toenails).
✔ Clinical signs:
- Clubbing → Hypoxia (COPD, cyanotic heart disease).
- Koilonychia → Iron deficiency anemia.
- Beau’s lines → Systemic illness.
2. PHYSIOLOGY OF THE SKIN
2.1 Barrier Function
✔ Prevents dehydration, protects against microbial & chemical insults.
✔ Stratum corneum lipid barrier maintains water homeostasis.
2.2 Thermoregulation
✔ Vasodilation (Heat loss) → Increased blood flow to skin.
✔ Vasoconstriction (Heat retention) → Reduced skin blood flow.
✔ Sweat Glands → Evaporative cooling.
2.3 Sensory Function
✔ Mechanoreceptors:
- Meissner’s corpuscles → Light touch (fingertips).
- Merkel cells → Fine pressure, texture discrimination.
✔ Pain & Temperature: - Free nerve endings → Detect pain & temperature changes.
✔ Deep Pressure & Vibration: - Pacinian corpuscles → Deep pressure, vibration.
- Ruffini endings → Skin stretch.
2.4 Pigmentation & Melanin Production
✔ Melanocytes synthesize melanin via Tyrosinase enzyme.
✔ UV exposure increases melanin → Tanning.
3. PATHOLOGY OF THE SKIN
3.1 Inflammatory Skin Diseases
✔ Acne Vulgaris → Hyperkeratinization, Propionibacterium acnes, sebum excess.
✔ Psoriasis → Autoimmune, rapid keratinocyte turnover, Auspitz sign.
✔ Atopic Dermatitis (Eczema) → Type I hypersensitivity, itch-scratch cycle.
✔ Lichen Planus → Purple, Pruritic, Polygonal Papules, Wickham’s striae.
3.2 Infectious Skin Diseases
✔ Bacterial → Impetigo, Cellulitis, Necrotizing Fasciitis.
✔ Viral → Herpes simplex, Varicella-Zoster, HPV (warts).
✔ Fungal → Tinea infections, Candidiasis.
3.3 Autoimmune Skin Diseases
✔ Pemphigus Vulgaris → Anti-desmoglein antibodies, Nikolsky’s sign (+).
✔ Bullous Pemphigoid → Anti-hemidesmosome antibodies, Nikolsky’s sign (-).
✔ Vitiligo → Autoimmune destruction of melanocytes.
3.4 Skin Cancers
✔ Basal Cell Carcinoma (BCC) → Most common, pearly nodules, sun-exposed areas.
✔ Squamous Cell Carcinoma (SCC) → Ulcerative, invasive, sun-exposed sites.
✔ Melanoma → Most aggressive, ABCDE criteria.
4. CLINICAL EXAMINATION OF SKIN
4.1 Inspection
✔ Color → Pallor (anemia), Jaundice (liver disease), Cyanosis (hypoxia).
✔ Lesions → Macules, papules, nodules, vesicles, bullae, pustules.
4.2 Palpation
✔ Temperature → Increased in infections/inflammation.
✔ Texture → Smooth (normal) vs. thickened (lichenification).
✔ Skin Turgor → Decreased in dehydration.
4.3 Special Tests
✔ Dermoscopy → Magnified skin lesion examination.
✔ Nikolsky’s Sign → Positive in Pemphigus vulgaris.
✔ Dimple Sign → Seen in dermatofibroma.
5. HIGH-YIELD PG EXAM PEARLS
✔ Psoriasis → T-cell mediated, Munro microabscesses.
✔ Pemphigus Vulgaris vs. Bullous Pemphigoid → Nikolsky’s sign differentiates.
✔ Melanoma → BRAF mutations, aggressive behavior.
✔ Basal Cell Carcinoma → Pearly, slow-growing, local invasion only.
No comments:
Post a Comment