The mysterious pulmonary brush cell: a cell in search of a function

Abstract

Brush cells, also termed tuft, caveolated, multivesicular, and fibrillovesicular cells, are part of the epithelial layer in the gastrointestinal and respiratory tracts. The cells are characterized by the presence of a tuft of blunt, squat microvilli (approximately 120-140/cell) on the cell surface. The microvilli contain filaments that stretch into the underlying cytoplasm. They have a distinctive pear shape with a wide base and a narrow microvillous apex. The function of the pulmonary brush cell is obscure. For this reason, a working group convened on August 23, 2004, in Bethesda, Maryland, to review the physiologic role of the brush (microvillous) cell in normal airways and alveoli and in respiratory diseases involving the alveolar region (e.g., emphysema and fibrosis) and airway disease characterized by either excessive or insufficient amounts of airway fluid (e.g., cystic fibrosis, chronic bronchitis, and exercise-induced asthma). The group formulated several suggestions for future investigation. For example, it would be useful to have a panel of specific markers for the brush cell and in this way separate these cells for culture and more direct examination of their function (e.g., microarray analysis and proteomics). Using quantitative analysis, it was suggested to examine the number and location of the cells in disease models. Understanding the function of these cells in alveoli and airways may provide clues to the pathogenesis of several disease states (e.g., cystic fibrosis and fibrosis) as well as a key for new therapeutic modalities.

Figure 1.

Electron micrograph of an alveolar brush cell from a rat exposed to hypoxia. The cell exhibits microvilli that project into the alveolar space. The microvilli contain filaments that extend into the supranuclear portion of the cell. The cell is covered along its alveolar surface by type I pneumonocytes (at arrows) to the base of the microvilli (B.M., unpublished observations).

Figure 2.

Part of an alveolar brush cell showing parallel arrays of smooth endoplasmic reticulum (arrow; B.M., unpublished observations).

Figure 3.

Distribution of brush cells in the rat lung. Data are mean ± SEM. Alv = alveolar; Dis = distal; Prox = proximal. (Modified by permission from Reference 17).