Ls i & 4 3 . an .. a . dtsae py Lettned Tad tL ct te ateNA Bronchiolar and Large Alveolar Cell in Pulmonary Phospholipid Metabolism Abstract. The nonciliated bronchio- lar cells (Clara cells) lining the terminal airways actively secrete a_ phospholipid. In contrast, the large alveolar epithelial cells (type H, granular pneumonocyte) are active phagocytic cells. It is proposed that the Clara cell is the main source of pulmonary phospholipid production (presumably sur- factant) while the large alveolar cell is responsible for its subsequent breakdown. chiolar cells which line the terminal and respiratory bronchioles, and which are rich in mitochondria and agranular cretory cells. Secretory droplets to form in the Golgi area, to at the apex of the cell, and to truded into the bronchiolar endoplasmic reticulum, are active se- (Fig. 3). These cells demonstrated no In contrast, the nonciliated bron- appear collect be exJumen Fig. 1 (left). Large alveolar cell from a mouse exposed to 1 percent carbon monoxide for 12 minutes. Cell appears to be engulfing osmiophilic material (OM) with beginning indentation of the cell wall (arrows). However, the distinction between phagocytosis (material entering the cell} and secretion (material leaving the cell) cannot be made from such observations. AS, alveolar space. The lung has a large metabolic capacity for producing phospholipids (7). Presumably these phospholipids line the smaller airways and alveoli with a film capable of markedly reducing surface activity, thus promoting alveolar stability (2). As a result of this unique prop- erty to reduce surface tension, the material extracted from lungs has been named pulmonary surfactant (3). It is widely accepted that the large alveolar cell is the source of pulmonary surfactant. This fact is based on indirect evidence (4) which fails to dis- tinguish between the production of and breakdown of phospholipids. The present study supports the concept that the nonciliated bronchiolar cell [Clara cell (5)] is the source of surfactant and that the large alveolar cell is a phagocytic cell responsible for its subsequent clearance from the lung. The large alveolar cell has been con- sidered a secretory cell with the ob- servation that lipid granules were extruded from the cell (6). However, in fixed tissue, it may be difficult to distinguish between the process of secretion and phagocytosis (Fig. 1). This problem was investigated by exposing unanesthetized mice to aerosolized carbon. The large sessile alveolar cells actively phagocytized carbon particles and lipid material from the alveolar space (Fig. 2). At a later stage, carbon particles were observed within osmiophilic lamellar bodies (Fig. 2), clear evidence that the lamellar bodies resulted from the ingestion of lipid material by, and were not a secretory product of, the cell. Acid phosphatase (7) was demonstrated at the membrane that lines many of the lamellar bodies, which indicates that these intracellular particles were lysosomes, that is, phagocytic vacuoles containing acid hydrolysates (8). X DFCFMRER 1947 Fig. 2. Large alveolar cell after unanesthetized mouse was exposed to aerosolized carbon for 30 seconds every 5 minutes over a 30-minute period. (A) Carbon particles (arrows) within membrane-limited osmiophilic granules. (B) Attenuated epithelial extension (arrows) enveloping osmiophilic material and carbon particle (arrow). Note normal lamellar bodies (LB) and attachment of cell to basement membrane (8), (C, inset) Lamellar body with carbon particles inside (arrow). AS, alveolar space. 1323