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Skin Anatomy

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Topic updated on 09/08/17 11:23am

 

skin

Anatomy
  •  Epidermis 
    • Function: protection, temperature regulation, moisture retention
    • Major cell types: keratinized squamous epithelial cells and melanocytes
    • Layers (top to bottom): 
      • stratum corneum
      • stratum lucidum
      • stratum granulosum
      • stratum spinosum
      • stratum basale (contains pluripotent stem cells)
      • "Californians Like Girls in String Bikinis" 
    • From basale to corneum, cells mature and lose their organelles and become keratinized
  • Dermis 
    • Function: Structure, sensation, wound healing.
    • Contains hair follicles, sebaceous glands, apocrine glands , specialized nerve endings, and vasculature
    • Mostly made up of extracellular matrix: collagen, glycosaminoglycans, proteoglycans, and glycoproteins
    • Major cell types: fibroblasts, langerhans cells, pluripotent stem cells (in hair follicles), endothelial cells
    • Layers (top to bottom)
      • papillary dermis
      • reticular dermis
  • Subcutaneous Tissues
    • Consist of fat, muscle, tendon, ligament, bone, etc. 
Normal Wound Healing
  • Phases: not discrete, but rather overlapping 
    • Hemostasis (1-24h)
      • Endothelial damage leads to exposure of the basement membrane, activation of intrinsic and extrinsic coagulation cascade, and ultimately deposition of fibrin with creation of a platelet plug. 
      • Platelets are integral to initiating wound healing.  They release cytokines that cause leukocyte migration and chemotaxis into the wound.
    • Inflammation (1-5d)
      • Mast Cells
        • Native cells Initiate the inflammatory phase
        • Secrete cytokines that cause vasodilation and increase vascular permeability
        • Allows influx of neutrophils and macrophages to the wound bed
      • Neutrophils
        • present early in inflammatory phase
        • clear intralesional pathogens
        • prepare the wound bed by removing damaged cells
        • secrete cytokines that stimulate infux of macrophages.
      • Macrophages 
        • Present late in inflammatory phase
        • Early on act in coordination with neutrophils to phagocytose bacteria and dead cells
        • Secrete cytokines and growth factors that drive fibroblast proliferation and angiogenesis
        • Also act to downregulate the initial infammatory response
    • Proliferation (3-7d)
      • Fibroblasts lay down type III collagen
      • Myofibroblasts (fibroblasts with contractile filaments) initiate wound contraction
      • Angiogenesis and vasculogenesis lay down new blood vessels
      • Granulation tissue (newly laid collagen with neovascularization) forms
      • Epithelialization occurs from surrounding basal keratinocytes and hair follicle basal cells
    • Maturation (Up to 1 year)
      • Type III collagen remodeled to type I collagen
      • Vessels mature, and excess vasculature involutes
      • Erythema and raised appearance of wound resolves
Types of Wound Healing
  • Primary Intention - Wound edges are approximated.  Grafts and flaps are considered primary closure. 
  • Secondary Intention - Wound edges are left open and allowed to fill in
  • Tertiary Intention - Wound edges are left open and allowed to granulate, and are approximated and closed at a later time.  Also known as delayed closure. 



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