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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.
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