Integumentary System

• Integumentary System


  Composed of skin and epidermal derivatives (epithelial skin appendages)

  
• Skin


  • External covering of the body
  • Largest organ, 15-20% of total body mass
  • Acts as barrier for protection
  • Provides immunologic information
  • Produces homeostasis
  • Regulates body temperature and water loss
  • Conveys sensory information
  • Converts precursor molecules into hormonally active molecules (vitamin D)
  • Has excretory function (sweat, sebum)
  • Delivery route for therapeutic agents

  
• Epidermis


  Composed of keratinized squamous epithelium, derived from ectoderm

  
• Dermis


  Dense connective tissue that provides mechanical support, strength and thickness, derived from mesoderm

  
• Layers of Epidermis


  • Stratum Germinativum (Stratum Basale)
  • Stratum Spinosum
  • Stratum Granulosum
  • Stratum Lucidum
  • Stratum Corneum

  
• Stratum Germinativum (Stratum Basale)


  • Single layer of tall cuboidal keratinocytes resting on basement membrane
  • Keratinocytes have large nucleus and basophilic cytoplasm
  • Serve as stem cells, mitotically active for epidermal renewal
  • Contain melanin pigment from melanocytes
  • Cells bound by desmosomes and hemidesmosomes, undergo upward migration
• Stratum Spinosum


  • Several layers of large polyhedral keratinocytes with cytoplasmic processes/spines
  • Cells less basophilic than stratum germinativum
  • Deeper layer cells have limited mitotic capacity
  • Desmosomes appear as intercellular bridges, nodes of Bizzozero
• Stratum Granulosum


  • 3-5 layers of flattened keratinocytes with pyknotic nuclei
  • Cannot undergo mitosis
  • Presence of keratohyaline granules containing profilaggrin
  • Profilaggrin becomes filaggrin in stratum corneum, aggregates keratin filaments
  • Lamellar granules secrete lipids into intercellular spaces
• Stratum Lucidum


  • 4-6 layers of flat, dead, anucleated keratinocytes
  • Forms a discontinuous, light-staining translucent layer
  • Keratinocytes devoid of organelles, keratin filaments form thick bundles
  • Cells bound by desmosomes, subdivision of stratum corneum

  
• Stratum Corneum

  • 15-20 rows of flat, desiccated, anucleated keratinocytes with keratin filaments
  • Deeper cells attached by desmosomes, superficial cells desquamate
  • Keratinocytes have thick plasma membrane coated with extracellular lipids forming water barrier

  
• Types of Skin


  • Thick Skin (palms, soles)
  • Thin Skin (rest of body)

  
• Thick Skin


  • Hairless
  • Thick epidermal layer with all 5 layers present
  • Stratum lucidum present
  • Longer dermal papillae, more numerous
  • Contains sweat glands but no hair follicles or sebaceous glands
• Thin Skin


  • Covers whole body except palms and soles
  • With hair
  • Thinner stratum spinosum
  • Poorly developed or absent stratum granulosum
  • Absent stratum lucidum
  • Thin stratum corneum
  • Short dermal papillae
  • Contains sweat glands, sebaceous glands and hair follicles

  
• Types of Cells in Epidermis

  • Keratinocytes
  • Melanocytes
• Keratinocytes


  • Predominant cell type
  • Participate in epidermal water barrier formation
  • Immature keratinocytes highly basophilic due to free ribosomes engaged in keratin synthesis
  • Keratin assembled into filaments, grouped into tonofibrils
  • Keratohyaline granules contain profilaggrin and trichohyalin, promote keratin aggregation
  • Keratinization involves nucleus/organelle breakdown, plasma membrane thickening, pH change
  • Desquamation by proteolytic degradation of desmosomes by kallikrein-related peptidases in acidic pH, inhibited by LEKTI in neutral pH
• Epidermal Water Barrier


  Established by deposition of insoluble proteins on inner plasma membrane (cell envelope) and lipid layer on outer plasma membrane (lipid envelope)
• Cell Envelope


  • 15nm thick layer of cross-linked proteins like loricrin, providing mechanical strength