CHAPTER 11
Respiratory System
217
Figure 11-15A.
Alveolar macrophages, lung.
H&E,
3
725; inset
3
1,465
Alveolar macrophages
are also called
dust cells
; they can
be found on the surface of the alveoli and in the connective
tissue of the septa. They are derived from blood monocytes
and migrate out of the capillaries to enter alveoli. Alveolar
macrophages are irregular in shape and have round nuclei;
they often contain
phagocytized material
(
brown
in color)
in the cytoplasm of active cells. Their function is to remove
dust particles, debris, and bacteria on the surface of the
alveoli, and they may also play an important role in initiat-
ing and maintaining chronic infl ammatory processes and
regulating tissue repair and remodeling in the lung.
Phagocytized
Phagocytized
material
material
Alveolar
Alveolar
macrophage
macrophage
Alveolar
macrophage
Phagocytized
material
Alveolar
macrophage
Air space
(alveolus)
A
Pseudopodia
Pseudopodia
Pseudopodia
Lysosomes
Blood-air barrier
Collagen
Collagen
Erythrocyte
Erythrocyte
Collagen
Erythrocyte
Endothelium of capillary
Endothelium of capillary
Fused basal lamina
Fused basal lamina
Type I pneumocyte
Type I pneumocyte
Endothelium of capillary
Fused basal laminae
Type I pneumocyte
B
Figure 11-15B.
Alveolar macrophage.
EM,
3
12,000
The surfaces of alveoli are continually swept by
alveolar macrophages
. In common with macrophages elsewhere, these cells are derived
from
monocytes
that have left the circulation, in this case through the walls of pulmonary capillaries. These cells phagocytose any
particles that have escaped capture in the conducting portion of the respiratory system. They also have a role in turnover of
surfactant
produced by type II pneumocytes. As expected in a macrophage, the cytoplasm contains
lysosomes
, most of which appear to be primary
lysosomes in the cell shown here. Evidence of the cell’s motility is seen here as extensions of cytoplasmic processes (
pseudopodia
or
± lopodia
) extending from the surface of the cell that faces the surface of the alveolus. A view of the
blood-air barrier
can also be seen.
Clinically,
alveolar macrophages
may also be called
“heart failure cells.” During
heart failure
, the heart is
unable to pump blood at an adequate volume, and the
backup of blood causes increased pressure in the alveo-
lar capillaries. Red blood cells (erythrocytes) then leak
into the alveoli. Alveolar macrophages engulf these
erythrocytes. Pathologically, heart failure cells (alveolar
macrophages) are identiF
ed by a positive stain for iron
pigment (hemosiderin).
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