the skin and associated accessory structures

• Epidermis


  Outer, thinner layer of the skin, consists of epithelial tissue

  
• Dermis

  Inner, thicker layer of the skin

  
• Subcutaneous (subQ) layer


  Also called hypodermis, attaches the skin to the underlying tissues and organs

  
• Cells of the epidermis

  • Keratinocytes
  • Langerhans cells
  • Melanocytes
  • Merkel cells

  
• Keratinocytes


  Produce keratin, provide a barrier against environmental damage, undergo keratinization and cornification

  
• Melanocytes


  Melanin-producing cells, located in the stratum basale, protect against UV radiation

  
• Langerhans cells


  Dendritic cells involved in immune responses, present in all layers of the epidermis

  
• Merkel cells


  Found in stratum basale, have contacts with somatic sensory nerves, associated with light pressure and tactile discs

  
• Layers of the epidermis

  • Stratum basale
  • Stratum spinosum
  • Stratum granulosum
  • Stratum lucidum
  • Stratum corneum

  
• Stratum basale


  Highly mitotic (dividing often)
• Stratum lucidum


  Present only in thick skin (fingertips, palms, soles)

  
• Stratum corneum


  Composed of many sublayers of flat, dead keratinocytes called corneocytes, continuously shed, can form calluses
• Layers of the dermis

  • Papillary region
  • Reticular region

  
• Papillary region


  Consists of areolar connective tissue, dermal papillae, corpuscles of touch, free nerve endings
• Reticular region


  Consists of dense irregular connective tissue, fibroblasts, macrophages, hair follicles, nerves, sebaceous and sudoriferous glands
• Fibroblasts


  Found in the dermis, important in the production of the extracellular matrix (collagen and elastin)

  
• Epidermal ridges


  Reflect contours of the underlying dermal papillae, form the basis for fingerprints, increase grip by increasing friction

  
• Dermal papillae


  Increase surface area for exchange of materials, interconnection with epidermal ridges resists shearing forces

  
• Sensory receptors in human skin

  • Merkel cells
  • Free nerve endings
  • Meissner's corpuscles
  • Ruffini corpuscles
  • Pacinian's corpuscles

  
• Merkel cells


  Detect light pressure
• Free nerve endings


  Detect thermoreception (warm, cold) and nociception (pain, hot, cold, pinch, chemicals)
• Meissner's corpuscles


  Detect discriminative touch and low-frequency vibration
• Ruffini corpuscles


  Detect heat/deep pressure and stretch
• Pacinian's corpuscles


  Detect deep pressure, stretch, and high-frequency vibration
• Subcutaneous layer/Hypodermis


  Not part of the skin, contains areolar and adipose tissue, attaches skin to underlying tissues and organs, contains lamellated (pacinian) corpuscles

  
• Accessory structures of the skin

  • Hair
  • Nails
  • Sebaceous glands
  • Ceruminous glands
  • Eccrine sweat glands
  • Apocrine sweat glands
• Hair


  Composed of dead, keratinized epidermal cells, consists of shaft, root, and hair follicle
• Hair follicle

  Epithelial root sheath (external and internal) and dermal root sheath, houses the hair bulb with papilla and matrix
• Sebaceous glands


  Secrete sebum to prevent dehydration of hair and skin, inhibit bacterial growth, located in dermis and open into hair follicles
• Eccrine sweat glands


  Numerous, located in deep dermis, secrete sweat to cool the body by evaporation
• Apocrine sweat glands


  Located mainly in axilla, groin, areolae, and bearded facial regions, secrete milky or yellowish sweat during emotional stress and sexual excitement
• Ceruminous glands


  Modified sweat glands located in the ear canal, produce cerumen (earwax) to provide a sticky barrier
• Nails


  Composed of hard, keratinized epidermal cells, located on the dorsal surfaces of fingers and toes, have a free edge, nail body, lunula, and nail root
• The skin (cutaneous membrane) covers the body and is the largest organ of the body by surface area and weight

  
• Thin skin has 4 major layers of the epidermis, thick skin has 5 major layers