Histology of Respiratory System

• Trachea


  Pseudostratified ciliated columnar epithelium with basal cells, goblet cells, brush cells, Clara cells and neuroendocrine cells
• Trachea


  • Loose connective tissue, elastic fiber layers, tracheal glands in lamina propria
  • Irregular dense connective tissue, glands, blood and lymphatic vessels in submucosa
  • Hyaline type cartilage and dense connective tissue in adventitia
• Typical Respiratory Epithelium (TRE)

  Pseudostratified and ciliated columnar epithelium
• As airways subdivide


  1. Become smaller in diameter

  2. Respiratory epithelium shorter with fewer goblet cells and more Clara cells

  3. Glands, connective tissue and cartilage become less

  4. Elastic and smooth muscle tissue increase
• Bronchus


  Air-conducting passage with hyaline cartilage reinforcement in its walls as plates<|>Smooth muscle always present<|>Cartilage declines and smooth muscle increases as diameter decreases
• Bronchiole


  Air-conducting passage with only smooth muscle in its walls and no cartilage<|>Plicated or rugated in cross section<|>Lined with TRE<|>Cartilage disposed as plates that overlap in large bronchi<|>Glands in the connective tissue below producing mucous/serous secretions<|>BALT may be present<|>Smooth muscle and elastic fibers present
• Bronchiole


  • Epithelium more cuboidal, sparse goblet cells, absent basal cells but Clara cells numerous
  • Lamina propria glandless connective tissues with smooth muscle present
  • Adventitia no cartilage, but extensive network of elastic fibers around the smooth muscle layers
• Respiratory bronchiole


  • Continuation of terminal bronchioles interrupted by thin-walled outpocketing (alveoli)
  • Epithelium, lamina propria and smooth muscle altered in the occurrence of alveoli
  • Epithelium simple squamous
  • Smooth muscle and smooth muscle move around each pocketing
• Alveolar ducts and sacs

  Lined only by alveoli, no epithelium and muscle<|>Alveolar sacs are areas where small clusters of alveoli form a blind outpocketing
• Alveoli


  Gas exchange unit<|>Semiglobar airspace lined by squamous epithelium<|>Closely associated with capillary network<|>Occur as small outpocketings of the respiratory bronchioles, alveolar duct and alveolar sacs
• Pneumocyte type I


  • Occupies >95% of the surface area of each alveolus
  • Intimately associated with underlying well-vascularized connective tissue
  • Flattened nucleus
  • Thin squamous, thin cytoplasm (<200nm)
  • Organelles: mitochondria, RER and polysome
  • Adjacent cells form tight junctions to prevent serum leakage into alveolar lumen
• Pneumocyte type II


  • Abundant at the septal portion: septal cells
  • More or less cuboidal in shape
  • Dome-like apical projection
  • Rounded, centrally located nucleus
  • Have secretory vesicles containing pulmonary surfactant (mixture of lipids and proteins to reduce surface tension)
  • May play a role in host defense
• Other cells in alveoli

  • Alveolar macrophages
  • Mast cells
• Interalveolar septum


  Area where 2 adjacent alveoli are joined<|>Very thin<|>Flattened capillary lined on both sides by type I pneumocytes<|>Thinnest part possesses capillaries involved in gaseous exchange, known as the blood-gas barrier<|>Alveolar pores allow the air spaces of the 2 alveoli to be shared
• Blood-gas barrier


  Comprises the thickness of the capillary endothelium + basal lamina, the thickness of intervening connective tissue, the thickness of type I pneumocyte + basal lamina, and the overlying surfactant<|>Actual gas exchange occurs at the site of red blood cells within the capillary