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- Conducting airways transition from ciliated pseudostratified columnar with goblet cells to simple cuboidal, then to simple squamous at alveoli.
- Cartilage and submucosal glands taper off distally; smooth muscle and elastic fibers remain to terminal bronchioles.
- Alveolar walls are delicate: Type I pneumocytes are attenuated; Type II are cuboidal with lamellar bodies.
- Capillaries hug alveolar epithelium—be cautious not to overcall capillary prominence as inflammation.
- Ciliated pseudostratified columnar with goblet cells; thick basement membrane; seromucous glands and cartilage plates.
- Smooth muscle bridges gaps in cartilage; elastic fibers in lamina propria.
- Progressive loss of cartilage and glands; epithelium shifts to ciliated simple columnar → cuboidal.
- Club (Clara) cells appear; goblet cells sparse in distal bronchioles.
- Terminal bronchioles lack goblet cells and glands; respiratory bronchioles have occasional alveoli outpouching.
- Type I pneumocytes flat, cover most surface; Type II cuboidal, make surfactant.
- Alveolar macrophages common in lumens; thin interstitium with capillaries, elastic fibers.
- Pores of Kohn connect alveoli; keep in mind when assessing focal emphysema or exudate spread.
- Type II pneumocytes/club cells: TTF-1+, Napsin A+, surfactant proteins; club cells may express SCGB1A1.
- Basal cells (proximal airways): p63+, CK5/6+.
- Ciliated cells: FOXJ1+; goblet cells MUC5AC+.
- Endothelium: CD31, ERG, vWF line capillaries adjacent to Type I pneumocytes.
