Overview of the Anatomy of the Lungs Large Airways and Small Airways
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Overview of the Histology of the Lungs from the Trachea to the Alveolus
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Histology of the terminal and respiratory bronchiole showing the cellular components of the mucosa including the ciliated cuboidal cells, cells, Clara cell and the neuroendocrine cells
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Histology of the Lung
Category Details Applied Anatomy (Imaging Findings)
Etymology
  • Histology: Greek histos (“web” or “tissue”) + logia.
  • Micro to Macro: While histology describes microscopic tissue, HRCT (High-Resolution CT) is essentially “Gross Pathology of the Living,” allowing us to infer histologic changes (e.g., fibrosis) non-invasively.
Definition
  • The microscopic study of the lung’s cellular architecture.
  • Divided into Conducting Zone (Air transport) and Respiratory Zone (Gas exchange).
  • Biopsy: The definitive diagnosis for many Interstitial Lung Diseases (ILD) requires a wedge biopsy to view this histology, though HRCT patterns (like UIP) are often specific enough to avoid surgery.
Epithelial Gradient
  • Proximal (Bronchi): Pseudostratified Ciliated Columnar (Respiratory Epithelium).
  • Middle (Bronchioles): Simple Cuboidal.
  • Distal (Alveoli): Simple Squamous (Type I).
  • Metaplasia: Chronic irritation (smoking) changes delicate ciliated epithelium into rugged squamous epithelium. This is the precursor to Squamous Cell Carcinoma (central lung masses).
Cells: Ciliated
  • Found in trachea to respiratory bronchioles.
  • Function: Mucociliary escalator (sweeps debris up).
  • Immotile Cilia Syndrome (Kartagener’s): Failure of cilia leads to mucus retention and chronic infection. Imaging shows Bronchiectasis and Situs Inversus.
Cells: Goblet
  • Mucus-secreting cells.
  • Numerous in bronchi; decrease distally; absent in terminal bronchioles.
  • Mucous Plugging: In Asthma/ABPA, goblet cell hyperplasia leads to thick mucus. On CT, this creates “Finger-in-Glove” opacities or “Tree-in-Bud” patterns.
Cells: Club (Clara)
  • Dome-shaped cells in bronchioles.
  • Secrete surfactant-like protein (CC16); detoxify xenobiotics.
  • Obliterative Bronchiolitis: Injury to Club cells leads to fibrosis of the bronchiole. On CT, the airway lumen vanishes, leaving Mosaic Perfusion (air trapping).
Cells: Type I Pneumocyte
  • Large, flat squamous cells covering 95% of alveolar surface.
  • Responsible for Gas Exchange.
  • Diffuse Alveolar Damage (DAD): Injury to Type I cells causes fluid leakage (ARDS). Imaging shows diffuse, bilateral consolidation or “White-out.”
Cells: Type II Pneumocyte
  • Cuboidal cells (5% of surface area, but 60% of cell number).
  • Produce Surfactant.
  • Stem cells for Type I repair.
  • Ground Glass Opacity (GGO): Hyperplasia of Type II cells (during repair/inflammation) creates a subtle increase in density on CT, seen as “hazy” lung.
Cells: Macrophage
  • “Dust Cells” patrolling the alveolar surface.
  • Phagocytose particles and bacteria.
  • Centrilobular Nodules: In smokers, macrophages filled with pigment accumulate in respiratory bronchioles (RB-ILD), creating faint, fuzzy nodules on CT.
Cells: Neuroendocrine
  • Kulchitsky cells (APUD system).
  • Found in clusters (Neuroepithelial bodies).
  • Carcinoid Tumor: These cells are the origin of Carcinoid tumors (highly vascular, central airway masses) and Small Cell Lung Cancer.
Connective Tissue: Elastin
  • Elastic fibers provide lung recoil (exhalation).
  • Found in alveolar walls and airway interstitium.
  • Emphysema: Destruction of elastin (by proteases/smoking) leads to loss of recoil and alveolar wall breakdown. Imaging shows cystic airspaces and hyperinflation.
Connective Tissue: Collagen
  • Provides structural strength.
  • Located in axial (bronchovascular) and peripheral (septal) interstitium.
  • Pulmonary Fibrosis (UIP): Excessive collagen deposition stiffens the lung. HRCT shows Reticulation (net-like lines), Traction Bronchiectasis, and Honeycombing.
The Interstitium
  • The potential space between the alveolar epithelium and capillary endothelium.
  • Normally very thin to allow diffusion.
  • Interstitial Lung Disease (ILD): Any pathology (edema, cells, fibrosis) expanding this space thickens the walls, visible as “Reticular Opacities” or “Septal Thickening” on CT.
Vascular Endothelium
  • Continuous, non-fenestrated capillaries.
  • Fused basement membrane with alveolus.
  • Vasculitis: Inflammation of these vessels (e.g., GPA/Wegener’s) can cause hemorrhage (GGO) or nodules with cavities.
BALT
  • Bronchus-Associated Lymphoid Tissue.
  • Immune nodules in bronchial walls.
  • MALToma: Low-grade lymphoma arising from this tissue. appears as consolidation or mass-like areas with air bronchograms.
Key Points & Pearls
  • Type I = Structure/Exchange.
  • Type II = Surfactant/Repair.
  • Elastin = Recoil.
  • Correlation:

    Alveolar filling = Consolidation/GGO.

    Interstitial thickening = Reticulation/Lines.

    Elastin loss = Emphysema/Cysts.

 

Histology of the respiratory bronchiole showing the cellular components of the mucosa including the ciliated cuboidal cells, cells, Clara cell and the neuroendocrine cells
Ashley Davidoff MD TheCommonVein.net lungs-0747
Histology of the terminal bronchiole showing the cellular components of the mucosa including the ciliated columnar cells, Clara cell and the neuroendocrine cells
Ashley Davidoff MD TheCommonVein.net lungs-0746
Cells of the Bronchioles
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Cells of the Bronchioles
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Histology of the Large and Medium Sized Airways
Airways are lined by a pseudostratified ciliated columnar epithelium interspersed with mucus secreting goblet cells
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As the medium sized airways progress to to the small airways they lose many of the goblet cells, become a simple epithelium and remain ciliated
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As the terminal bronchial transitions to the respiratory bronchial the mucosa becomes non ciliated and cuboidal
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At the level of the mebranous airways (respiratory bronchiole, alveolar duct, alveolar sac and alveoli, the mucosa becomes mostly a simple squamous epithelium
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Cells

Club Cell
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The Alveolus
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Cellular Makeup of the Normal Alveolus
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Macrophage
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Smokers Macrophage
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Alveolar Macrophages
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Simple cuboidal cell with reddish foamy and sometimes vacuolated cytoplasm
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Simple squamous epithelium with pale staining cytoplasm
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Diseases

Membranous airways (respiratory bronchiole, alveolar ducts, alveolar sacs)
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Pathophysiology of Cigarette Smoking on Medium Sized Airways
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Pathophysiology of Cigarette Smoking on Medium Sized Airways, Small Airways and Alveoli
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Smoking and the Alveolus –
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ARDS – Causes
The lung is injured either by direst causes most commonly pneumonia, aspiration or from inhalation of toxic substances. Severe systemic illnesses, most commonly sepsis with shock, and severe trauma are considered indirect causes.
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Early Events in the Pathophysiology of the ARDS
The initial injury results in an acute severe inflammatory response consisting hyperemia , edema with migration initially of neutrophils in the first 6-24 hours followed by monocytes (24-48hours). The intra -alveolar macrophages are activated.
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Result of Cellular Response
The cells of the immune system release cytokines, chemotactic agents and proteases. Immune cells , macrophages and fibroblasts are attracted to the interstitium. Some of proinflammatory agents are toxic to the cell lining causing damage to the surfactant, type 1 pneumocytes and the capillary endothelium. There is progressive edema.
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Hyaline Membrane
A hyaline membrane evolves covering the damaged surface of the alveolus. This impedes gas exchange
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Chronic Inflammation – The Basics
Ashley Davidoff MD TheCommonVein.net Chronic Inflammation – The Basics
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Basic Structure of Tubular Systems
key words art mucosa submucosa muscularis adventitia serosa histology tube colon small bowel lung bronchus bronchi esophagus stomach large bowel bile duct ureter tube principles        Ashley Davidoff TheCommonVein.net 32347
Mucosal Lesion
key words  mucosa submucosa muscularis adventitia serosa mucosal mass polyp neoplasm carcinoma acute angles with the lumen histopathology imaging diagnosis Ashley Davidoff TheCommonVein.net 32347d01
Submucosal Lesion
 key words mucosa submucosa muscularis adventitia serosa submucosal mass edema hemorrhage obtuse angles or right angle 90 degree ninety degree angle with the lumen histopathology imaging diagnosis 
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Submucosal Lesion
 key words mucosa submucosa muscularis adventitia serosa submucosal mass edema hemorrhage obtuse angles or right with the lumen histopathology imaging diagnosis Ashley Davidoff TheCommonVein.net 32347d03
Extrinsic Lesion
key words mucosa submucosa muscularis adventitia serosa submucosal mass edema hemorrhage neoplasm malignancy benign obtuse angles with the lumen histopathology imaging diagnosis
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Circumferential Lesion
key words mucosa submucosa muscularis adventitia serosa submucosal mass edema hemorrhage neoplasm malignancy benign obtuse angles with the lumen circumferential narrowing constriction obstruction histopathology imaging diagnosis
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