Integumentary System – Detailed Anatomical & Functional Notes (Gray’s Anatomy Style)

1. Definition & Components

The integumentary system forms the body’s outer covering and comprises:

• Skin (Cutis)

• Epidermis (stratified epithelium)

• Dermis (connective tissue)

• Subcutaneous Tissue (Hypodermis/Panniculus)

• Accessory Structures

• Hair and hair follicles

• Nails

• Sebaceous, eccrine, and apocrine glands

• Cutaneous sensory receptors

2. Gross Anatomy of the Skin

• Surface Area: ~1.5–2 m² in adults

• Thickness:

• Thin skin: 0.07–0.12 mm (most of body)

• Thick skin: 0.6–4.5 mm (palms, soles)

• Regions & Landmarks:

• Flexural vs. extensor surfaces

• Specialized areas: areola, perigenital skin, scalp

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3. Epidermis

• Origin: Surface ectoderm

• Layers (deep → superficial):

1. Stratum basale (germinativum)

• Single layer of cuboidal–columnar keratinocytes on basement membrane

• Contains melanocytes and Merkel cells

2. Stratum spinosum

• Polyhedral cells with desmosomal “spines”

• Langerhans (dendritic) cells present

3. Stratum granulosum

• Flattened cells with keratohyalin granules (profilaggrin) and lamellar bodies (lipid secretion)

4. Stratum lucidum (only in thick skin)

• Thin, translucent layer of dead, anucleate keratinocytes

5. Stratum corneum

• Multiple layers of anucleate, keratinized squames; final barrier to environment

• Cell Turnover: ~28‑day migration from basale → corneum

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4. Dermis

• Origin: Dermal mesenchyme (mesodermal)

• Divisions:

• Papillary layer (superficial 20%): loose connective tissue, dermal papillae interdigitating with epidermal rete ridges

• Reticular layer (deep 80%): dense irregular collagenous tissue, elastic fibers, adipocytes in deeper zones

• Cell Types: fibroblasts, mast cells, macrophages, plasma cells

• Structures within Dermis:

• Vascular plexuses: subpapillary and deep plexus

• Nerve endings: free nerve endings (pain, temperature), Meissner corpuscles (light touch), Pacinian corpuscles (vibration/pressure), Ruffini endings (stretch)

• Hair follicles, sebaceous and sweat glands embedded

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5. Hypodermis (Subcutaneous Tissue)

• Loose connective tissue with variable adipose content

• Attaches skin to underlying fascia and muscle

• Functions as insulation, energy reserve, and shock absorber

• Contains larger blood vessels and cutaneous nerves

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6. Skin Appendages

6.1 Hair & Hair Follicles

• Structure: follicle (infundibulum, isthmus, bulb), hair shaft (medulla, cortex, cuticle), arrector pili muscle

• Types: lanugo, vellus, terminal hairs

• Growth cycle: anagen (growth), catagen (regression), telogen (resting)

6.2 Nails

• Anatomy: nail plate over nail bed; proximal nail matrix (growth center); eponychium (cuticle)

• Function: protection of distal phalanges, enhancement of fine touch

6.3 Glands

• Sebaceous glands

• Holocrine secretion of sebum into hair follicles (absent on palms/soles)

• Eccrine sweat glands

• Merocrine secretion; thermoregulatory; distributed over entire body

• Apocrine sweat glands

• Found in axillae, areola, perineum; viscous secretion into hair follicles; odor formation by bacterial action

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7. Vascular Supply & Lymphatics

• Arterial supply: cutaneous branches from subdermal plexus (dermal capillary loops nourish epidermis)

• Venous drainage: networks accompany arterial plexuses; communicates with deep venous system

• Lymphatic drainage: superficial lymphatic plexus beneath dermis → deep lymphatics; patterns mirror dermatome distributions

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8. Innervation

• Somatic sensory

• Free nerve endings: pain, temperature

• Encapsulated receptors: Meissner, Pacinian, Ruffini, Merkel discs

• Autonomic (sympathetic) fibers

• Innervate blood vessels, erector pili muscles, sweat glands

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9. Embryology

• Epidermis: from surface ectoderm; periderm evolves into stratum corneum

• Dermis: from mesenchyme—dermatome of somites in trunk, lateral plate mesoderm in limbs

• Appendages:

• Hair, glands develop from downgrowths of epidermis into dermis

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10. Functions

1. Barrier: against mechanical injury, pathogens, UV radiation, water loss

2. Thermoregulation: via vasomotor control and sweat production

3. Sensory reception: tactile discrimination, nociception, temperature

4. Metabolic: vitamin D synthesis (7‑dehydrocholesterol → cholecalciferol)

5. Immune defense: Langerhans cells, dermal macrophages

6. Endocrine: conversion of precursors, local hormone metabolism (e.g., cortisol activation by 11β‑HSD1)

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11. Clinical Correlations

• Wound healing: hemostasis → inflammation → proliferation → remodeling

• Burns: depth classification (I–III) determines management and prognosis

• Disorders:

• Inflammatory: atopic dermatitis, psoriasis

• Infectious: impetigo, dermatophytosis

• Neoplastic: basal cell carcinoma, squamous cell carcinoma, melanoma

• Degenerative/Aging: loss of collagen/elastic fibers → wrinkles, thinning.

Embryology

• Epidermal origin: Surface ectoderm → periderm → definitive epidermis

• Dermal origin: Dermatomal (somitic) mesenchyme in trunk; lateral plate mesoderm in limbs

• Appendage development: Epidermal placodes for hair, glands, nails

2. Skin Barrier & Physiology

• Stratum corneum “brick‑and‑mortar” model: corneocytes + lipid matrix

• Acid mantle: pH ~5.5, role in antimicrobial defense

• Transepidermal water loss (TEWL): regulation by ceramides and natural moisturizing factors

3. Skin Immunology

• Langerhans cells: antigen presentation in stratum spinosum

• Dermal dendritic cells and macrophages

• Cutaneous lymphoid tissues: role in delayed‑type hypersensitivity.

Skin Microbiome

4.1. Composition

• Bacteria (dominant):

• Staphylococcus epidermidis and other coagulase‑negative staphylococci

• Cutibacterium acnes (formerly Propionibacterium acnes)

• Corynebacterium spp.

• Fungi:

• Malassezia spp. (lipophilic yeasts)

• Viruses:

• Bacteriophages targeting resident bacteria

• Human papillomaviruses (commensal strains)

• Archaea & Mites:

• Methanobrevibacter spp. in moist sites

• Demodex mites in pilosebaceous units

4.2. Site‑Specific Niches

• Sebaceous (oily) areas: face, chest, back

• High Cutibacterium density; Malassezia yeasts

• Moist areas: axillae, groin, toe webs

• Enriched in Corynebacterium and certain staphylococci

• Dry areas: forearms, legs

• Greatest diversity; mix of staphylococci, Corynebacterium, Gram‑negative rods

4.3. Functions & Host Interactions

• Barrier reinforcement: compete with pathogens for nutrients and attachment sites

• Immune modulation:

• Induce antimicrobial peptide (e.g., β‑defensin) production by keratinocytes

• Educate cutaneous T cells and Langerhans cells to tolerate commensals

• Metabolic contributions:

• Breakdown of sebum triglycerides to free fatty acids (acid mantle maintenance)

• Vitamin metabolism (e.g., biotin synthesis)

4.4. Regulation of the Microbiome

• Host factors:

• Sebum production, sweat composition, skin pH (~5.5)

• Innate immunity (AMPs, sebum lipids)

• Environmental factors:

• Temperature, humidity, UV exposure

• Hygiene practices and topical products (soaps, antibiotics, emollients)

• Microbial interactions:

• Quorum sensing among bacteria

• Bacteriocin production to inhibit competitors

4.5. Dysbiosis & Clinical Implications

• Acne vulgaris: overgrowth of C. acnes and inflammation in pilosebaceous units

• Atopic dermatitis: reduced microbial diversity; Staphylococcus aureus colonization correlates with flares

• Psoriasis: altered ratios of Corynebacterium and staphylococci in lesions

• Seborrheic dermatitis: proliferation of Malassezia in sebaceous‐rich regions

• Therapeutic modulation:

• Probiotics/prebiotics (oral and topical) to restore balance

• Bacteriophage therapy targeting specific pathogens

• Microbiome‑friendly cleansers to preserve commensal populations.