club cell

**Location and histology:** Club cells are found in the epithelium of small airways, most abundantly in terminal and respiratory bronchioles. They are non-ciliated, columnar to cuboidal, with a dome-shaped apical surface projecting into the lumen. They have abundant smooth endoplasmic reticulum (SER) and secretory granules. Bronchioles lack cartilage and goblet cells, and club cells replace goblet cells as the main secretory cell type in this region.

**Functions (high-yield):** 1. **Secretion:** They secrete a low-protein, surfactant-like, mucus-free fluid. It includes club cell secretory protein (CC16/CCSP, an anti-inflammatory and immunomodulatory protein), surfactant apoproteins, and lysozyme-like and other antimicrobial components. This protects the bronchiolar lining and helps keep small airways from collapsing. 2. **Detoxification:** The abundant SER contains the cytochrome P450 monooxygenase system, which metabolizes inhaled toxins and xenobiotics. Bioactivation of some compounds (e.g., naphthalene) can cause club cell injury. 3. **Progenitor/stem-like function:** They divide and differentiate into ciliated cells and, in some conditions, goblet cells. This is how bronchiolar epithelium regenerates after injury. They can also give rise to type II pneumocytes in some injury models. 4. **Mucociliary clearance:** Their secretions help the mucociliary escalator and degrade mucus. Their products also help modulate the local immune response.

**Clinical correlations:** - **Serum CC16** is a biomarker of lung epithelial injury or permeability. It is leaked into serum when the alveolar-capillary barrier is damaged (e.g., ARDS, smoke exposure). Low levels are associated with chronic smoking and COPD, reflecting club cell loss. - **COPD and smoking:** Chronic smoking depletes club cells and reduces CC16. Small airway disease and bronchiolitis are associated with club cell loss or dysfunction. - **Asthma:** Reduced CC16 is linked to more airway inflammation and remodeling. Goblet cell metaplasia replaces club cells in chronic inflammation. - **Lung adenocarcinoma:** Club cells and type II pneumocytes are considered cells of origin for some adenocarcinomas, such as the former bronchioloalveolar type (now adenocarcinoma in situ and lepidic-predominant). These are usually peripheral tumors, and some are associated with EGFR or KRAS mutations. - **Neonatal lung:** Club cells help maintain surfactant-like function in small airways, but type II pneumocytes produce most alveolar surfactant (this is a common point of confusion).

**Board tips:** - Do not confuse them with **goblet cells** (mucus-secreting, in larger airways), **type II pneumocytes** (alveolar surfactant production and repair of the alveolar epithelium), or **ciliated cells**. - Bronchioles are distinguished by: no cartilage, no glands, no goblet cells, and a simple cuboidal epithelium with club cells. - Formerly "Clara cells." The name was changed to club cells because of the historical association of Max Clara with Nazi-era research (the new name also reflects their club-shaped apical projections).