•  Epidermis 
    • function
      • protection, temperature regulation, moisture retention
    • major cell types
      • keratinized squamous epithelial cells, 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, lose their organelles, become keratinized
  • Dermis 
    • function
      • structure, sensation, wound healing
    • contains hair follicles, sebaceous glands, specialized nerve endings, vasculature
      • compression of vasculature can lead to pressure ulcers 
    • mostly made up of extracellular matrix
      • collagen, glycosaminoglycans, proteoglycans, 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. 
  • Exocrine glands
    • glands that retain ducts to body surfaces
    • 3 types, based on the manner in which they secrete substances
      • merocrine 
        • cells form membrane-bound secretory vesicles internal to the cell and move to the apical surface of the cell to release contents
          • most glands
      • apocrine
        • cells in which the apical portions of cells are pinched off and lost during the secretory process, resulting in secretory products that contain a variety of molecular components including those of the membrane
          • i.e., mammary glands
      • holocrine:
        • involves death of the cell
        • secretory cell is released, and as it breaks apart, the contents of the cell become the secretory product. 
          • i.esebaceous glands, sweat glands located in the axillae, pubic areas, around the areoli of the breasts
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
        • 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 
      • aka delayed closure


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