Skin (Integument) — Structure, Appendages and Clinical Aspects

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The skin is the largest organ of the body and a frequent exam topic due to its accessibility and clinical relevance. INI CET questions often test knowledge of skin layers, appendages, and pathological conditions. Focus on the epidermis layers, appendage distribution, and how burns affect body surface area.

High-Yield Facts for INI CET:

• Epidermis: Keratinised stratified squamous epithelium; 5 layers (stratum basale → spinosum → granulosum → lucidum → corneum); avascular
• Dermis: Two layers — papillary (areolar CT + dermal papillae + capillary loops) and reticular (dense irregular CT + collagen bundles)
• Hypodermis: Subcutaneous tissue — not part of skin proper; contains fat (panniculus adiposus), blood vessels, nerves
• Skin appendages: Hair follicle, sebaceous gland, sweat gland (eccrine/apocrine), nail

⚡ Exam tip: The Rule of Nines is frequently used in burn assessment — head/neck 9%, each upper limb 9%, each lower limb 18%, anterior trunk 18%, posterior trunk 18%, perineum 1%. Also know that melanocytes are found in the stratum basale and are the origin of melanoma.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Structure of the Skin:

Epidermis (Epithelial Component)

• Keratinised stratified squamous epithelium; derived from ectoderm
• Avascular — nutrients diffuse from dermis
• Contains: Keratinocytes (90% — produce keratin), melanocytes (produce melanin pigment), Langerhans cells (antigen-presenting — immune function), Merkel cells (sensory — light touch)
• Layers (deep to superficial):

Stratum Basale (Stratum Germinativum):

• Single layer of columnar cells; attached to basement membrane by hemidesmosomes
• Stem cells that continuously divide → push older cells outward
• Contains melanocytes (5-10% of cells in basal layer); also contains Merkel cells
• S100 markers positive in melanocytes

Stratum Spinosum (Prickle Cell Layer):

• Multiple layers of polyhedral cells
• Cells have desmosomes connecting them (give “spiny” appearance under microscope)
• Langerhans cells are prominent here (immune surveillance)
• Cytoplasm contains tonofilaments (keratin intermediate filaments)

Stratum Granulosum (Granular Layer):

• 3-5 layers of flattened cells
• Cells contain basophilic keratohyalin granules (precursor to keratin) and lamellar granules (lipids — waterproofing)
• Cells are undergoing apoptosis — nuclei and organelles being degraded
• Keratinisation process begins here

Stratum Lucidum (Clear Layer):

• Thin, transparent layer — only in thick skin (palms, soles)
• Cells have no nuclei or organelles — packed with eleidin (transformation of keratohyalin)
• Provides mechanical protection

Stratum Corneum (Horny Layer):

• Outermost layer; 15-20 layers of dead, keratinised squamous cells
• Cells are anucleate, filled with keratin
• Continuously desquamate (shed) — replaced every 2-4 weeks
• Waterproof barrier

Thick vs Thin Skin:

• Thick skin: Palms and soles; all 5 layers; no hair follicles; more layers in stratum corneum; 0.5-0.6mm
• ** Thin skin:** Most of body; 4 layers (no stratum lucidum); has hair follicles; only 0.1-0.15mm

Dermis (Connective Tissue Component)

• Derived from mesoderm
• Two layers:

Papillary Dermis:

• Superficial layer; loose areolar CT
• Dermal papillae (projections) interdigitate with epidermal rete ridges → mechanical anchoring
• Contains: Capillary loops (nutrient supply to epidermis), Meissner’s corpuscles (light touch — fingertips), Pacinian corpuscles (deep pressure — palm, sole), free nerve endings (pain, temperature)
• Reticular ridge pattern creates unique fingerprints (genetically determined, established in utero)

Reticular Dermis:

• Deep layer; dense irregular CT
• Thick bundles of Type I collagen (arranged in parallel to skin surface)
• Elastic fibers provide elasticity (elastin damaged by UV → wrinkles)
• Contains: Hair follicles, sebaceous glands, sweat glands, Pacinian corpuscles, Ruffini endings (stretch)
• Sweat gland ducts open onto skin surface (eccrine) or into hair follicle (apocrine — puberty onwards)

Hypodermis (Subcutaneous Tissue)

• Not part of skin proper; connects skin to underlying structures (fascia, muscle, bone)
• Adipose tissue (panniculus adiposus) — energy storage, insulation
• Contains: Blood vessels, nerves, lymphatics, sometimes superficial lymph nodes
• Areas with little fat: scalp, eyelid, external ear

Skin Appendages

Hair Follicle:

• Root sheath (outer + inner) + hair shaft
• Hair bulb at base contains papilla (vascular connective tissue) + matrix cells (rapidly dividing — produce hair)
• Arrector pili muscle: Smooth muscle connecting hair follicle to dermis; contracts → “goosebumps” → hair stands up (vestigial in humans)
• Hair types: Lanugo (fine, fetal), Vellus (fine, body), Terminal (coarse, scalp, eyebrows, pubic area)
• Hair growth cycle: Anagen (growth) → Catagen (regression) → Telogen (resting) → hair sheds → anagen again

Sebaceous Glands:

• Holocrine secretion (whole cell breaks down to release sebum)
• Associated with hair follicle (pilosebaceous unit); open into hair canal
• Sebum: oils skin, prevents desiccation; contains triglycerides, wax esters, squalene, cholesterol
• Largest and most active on face, chest, back (acne-prone areas)
• Meibomian glands in eyelids are modified sebaceous glands (sebaceous → oily tear film prevent evaporation)

Eccrine Sweat Glands:

• Most numerous sweat glands; distributed throughout body (most on palms, soles)
• Coiled tubular glands in dermis; duct opens directly onto skin surface
• Clear, watery secretion (99% water + NaCl + waste products)
• Thermoregulation — regulated by sympathetic cholinergic innervation
• Apocrine glands open into hair follicle; produce viscous secretion; activated at puberty; found in axillae, areola, perineum; associated with body odour (bacterial decomposition)

Nails:

• Hard keratin plate (alpha-keratin — same protein as hair, hooves, horns)
• Nail bed: Stratified squamous epithelium under nail plate; contributes to nail growth
• Nail matrix: Germinal epithelium at proximal end; produces nail plate; lunula (white crescent) = visible part of matrix
• Cuticle (eponychium): Fold of stratum corneum over proximal nail
• Lunula: White crescent at base of nail = visible nail matrix
• Nails grow at ~0.5mm/week (fingernails faster than toenails)

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Clinical and Pathological Aspects of Skin:

Burns — Classification and Assessment

Burn Depth (Traditional classification):

First-degree (superficial): Epidermis only; erythema, pain, no blisters; heals in 3-7 days; no scarring

• Example: Sunburn

Second-degree (partial thickness):

• Superficial partial thickness: Epidermis + superficial dermis; blisters, wet, very painful; heals in 7-21 days; minimal scarring
• Deep partial thickness: Epidermis + deep dermis; mottled, less painful, no blisters; heals in 3-8 weeks; significant scarring; may need surgery

Third-degree (full thickness): Epidermis + dermis destroyed; white/charred; leathery; painless (nerve endings destroyed); cannot heal without surgery (needs skin grafting)

• Example: Scald from boiling water

Fourth-degree: Extends into muscle, tendon, bone; carbonisation; life-threatening

Rule of Nines (Adult):

• Head and neck: 9%
• Each upper limb: 9%
• Each lower limb: 18%
• Anterior trunk: 18%
• Posterior trunk: 18%
• Perineum: 1%

Parkland Formula for fluid resuscitation:

• Total fluid required (first 24h) = 4 mL × body weight (kg) × %TBSA burned
• Give half in first 8 hours; second half over next 16 hours
• Use Hartmann’s solution or Ringer’s lactate

Inhalation injury: Suspect when burns occur in enclosed space, singed nasal hairs, carbonaceous sputum, hoarseness → requires early airway management

Skin Cancer

Basal Cell Carcinoma (BCC):

• Most common skin cancer (70%); slow-growing, rarely metastasises
• Pearly/nodular lesion with telangiectasia; rolled, translucent border; rodent ulcer
• Risk factor: UV light (especially UVB)
• Location: Head and neck (face — “mask area” around eyes, nose, ears)
• Histology: Palisading nuclei at tumour margins, cleft spaces (retraction artefact)
• BCC arises from basal layer of epidermis or hair follicle

Squamous Cell Carcinoma (SCC):

• Second most common; can metastasise (2-5% of cases)
• Risk factors: UV light, chemical carcinogens (tar, arsenic), chronic wounds/scars (Marjolin ulcer), immunosuppression
• Presents as: Scaly, crusted, ulcerated lesion; fast-growing
• Location: Sun-exposed areas (lower lip, ears, hands)
• Histology: Keratin pearls, intercellular bridges, atypical keratinocytes
• Actinic keratosis: Premalignant SCC in situ

Malignant Melanoma:

• Arises from melanocytes (stratum basale)
• Most lethal skin cancer; metastasises early via lymphatics and blood
• Risk factors: UV exposure (especially UVA), fair skin, dysplastic naevus syndrome, family history, CDKN2A mutation
• ABCD warning signs: Asymmetry, Border irregularity, Colour variation (multiple shades), Diameter >6mm
• Types: Superficial spreading (most common, ~70%), Nodular, Lentigo maligna, Acral lentiginous (palms/soles, worst prognosis)
• Breslow depth (thickness) is most important prognostic factor
• Sentinel lymph node biopsy is done for lesions >1mm Breslow depth

Inflammatory Skin Conditions

Eczematous Dermatitis:

• Atopic dermatitis (eczema): Chronic, pruritic, relapsing; flexural surfaces (antecubital, popliteal fossae); associated with “atopic triad” — eczema, asthma, allergic rhinitis; caused by Filaggrin gene mutations → impaired skin barrier
• Contact dermatitis: Allergic (type IV hypersensitivity — poison ivy, nickel) or irritant (detergents)
• Seborrhoeic dermatitis: Scaly, greasy plaques on scalp (dandruff), eyebrows, nasolabial folds; associated with Malassezia yeast

Psoriasis:

• Chronic, remitting, scaly plaques; well-demarcated; silvery-white scales on erythematous base
• Auspitz sign: Pinpoint bleeding when scales scraped off
• Histology: Parakeratosis, regular acanthosis (elongation of rete ridges), Munro microabscesses (neutrophils in stratum corneum), dilated capillaries in dermal papillae
• Associated with psoriatic arthritis, metabolic syndrome, cardiovascular disease

Bullous Disorders:

• Pemphigus vulgaris: Autoantibodies against desmoglein 3 (cell adhesion molecule); intraepidermal blisters; flaccid blisters that rupture easily; positive Nikolsky sign; oral mucosa commonly affected first; serum shows intercellular IgG (fishnet pattern on immunofluorescence)
• Bullous pemphigoid: Autoantibodies against hemidesmosomes (BP180, BP230); subepidermal blisters; tense bullae; Nikolsky negative; less mucosal involvement; linear basement membrane IgG on immunofluorescence
• Dermatitis herpetiformis: Autoantibodies against tTG (tissue transglutaminase); associated with coeliac disease; grouped vesicles on extensor surfaces (elbows, knees, buttocks); intensely pruritic; granular IgA deposits at dermal papillae

Skin Appendage Disorders

• Acne vulgaris: Pilosebaceous unit inflammation; Propionibacterium acnes; comedones (blackheads/whiteheads), papules, pustules, nodules, cysts; androgen-driven sebum overproduction
• Alopecia areata: Autoimmune destruction of hair follicle; non-scarring; round patches of hair loss; “exclamation point” hairs at margins
• Androgenetic alopecia: Male/female pattern hair loss; DHT-mediated follicular miniaturisation; finasteride (5-alpha reductase inhibitor) for men