Primary and Secondary Lesions

• Primary lesions

  Direct result of the underlying disease process, most important diagnostically
• Secondary lesions

  Evolve from primary lesions (via self trauma, altered keratinization, etc), less important diagnostically
• Primary lesions

  • Macule
  • Patch
  • Papule
  • Plaque
  • Nodule
  • Tumor
  • Cyst
  • Vesicle
  • Bulla
  • Pustule
  • Abscess
  • Wheal
• Lesions that can be primary or secondary

  • Scale
  • Crust
  • Comedo
• Abnormalities of hair

  • Alopecia
  • Hypotrichosis/atrichia
  • Effluvium/defluxion
  • Hypertrichosis (hirsutism)
• Abnormal pigmentation/coloration

  • Red (erythema)
  • Black (hypermelanosis, melanotrichia)
  • White (hypomelanosis, leukotrichia, albinism)
  • Hemorrhage (purpura, petechia, ecchymoses)
  • Yellow (icterus)
  • Blue (cyanosis)
• Epidermal collarette


  Circular rim of keratin flakes following loss of the "roof" of vesicle, pustule
• Erosion

  Shallow epidermal defect that doesn't penetrate the basement membrane
• Ulcer


  Break in the epidermis with exposure of the underlying dermis
• Excoriation


  Erosions or ulcers caused by scratching, biting, or rubbing
• Scar


  Area of fibrous tissue that has replaced the damaged dermis and/or subcutis
• Fissure


  Linear cleavage of usually thickened, inelastic skin
• Lichenification


  Thickening and hardening of the skin with exaggeration of the superficial markings
• Callus

  Thickened, rough, hyperkeratotic, alopecic, often lichenified plaque that develops on the skin
• Skin is the largest organ of the body
• Skin is in direct contact with the external and internal environments, exposed to many injurious stimuli
• Skin has a limited range of responses
• Exogenous and endogenous factors influencing the skin

  • Light/sun, photochemical reactions
  • Hypersensitivity
  • Autoimmune
• In non-haired areas (nose & footpads) of dogs and cats the epidermis is much thicker with a prominent granular layer
• On light microscopy the basement membrane (ie basal lamina + reticular lamina on EM) can be seen more readily with a PAS stain

  
• The basal layer of keratinocytes is firmly attached to the basal lamina by cell adhesion molecules in hemidesmosomes

  
• The basal lamina is then firmly attached to the underlying reticular lamina / dermis by anchoring fibrils (mostly type VII collagen)

  
• Desmosomes


  A type of tight junctions which function in cell-to-cell adhesion, providing resistance to shearing forces, found in stratified squamous epithelium (eg epidermis)
• Cells in the epidermis

  • Keratinocytes
  • Melanocytes
  • Langerhans cells
  • Merkel's cells

  
• Melanocytes


  Produce melanin
• Merkel cells

  Mechanoreceptor and/or neuroendocrine
• Langerhans cells

  Dendritic cell (antigen processing cells)
• Adnexal glands

  • Sebaceous glands
  • Apocrine glands
• Components of the dermis

  • Fibers (collagen, elastin)
  • Ground substance (glycosaminoglycans, proteoglycans)
  • Cells (fibroblasts, mast cells, histiocytes, smooth muscle)
  • Vessels (blood, lymph)
  • Nerves

  
• Innervation of the skin

  • Unmyelinated cutaneous nerve endings (touch, pressure, temperature, pain, itch)
  • Motor fibers (autonomic nervous system - adrenergic, cholinergic)

  
• Layers of the skin

  • Stratum corneum
  • Granular layer
  • Spinous layer
  • Basal layer
• Functions of the skin

  • Enclosing barrier
  • Protection
  • Sensory
  • Temperature & blood pressure regulation
  • Storage
  • Adnexa production
  • Pigmentation
  • Immunoregulation / antimicrobial
• The average human head has 100,000 hairs
• Approximately 90% of head hair is in the anagen (growing) stage, which lengthens by ~1 cm/month for 2-6 years
• Approximately 10% of hair is in the catagen/telogen (transition/resting) stage for 2-6 months, randomly distributed
• In dogs, the anagen phase can be 3-4 months in short-coated breeds and 18 months in long-coated breeds
• In dogs, the telogen phase can range from months to years in various breeds
• In dogs (and many other mammals) the hair cycle can be somewhat synchronized (ie shedding) which is influenced by genetics, photoperiod, temperature, nutrition/health status & hormones
• The subcutis (hypodermis) contains lipocytes, collagen, elastin, vessels, and nerves

  
• The subcutis is the innermost layer of the skin and contains the panniculus adiposus (fat) and muscle layer