216
UNIT 3
Organ Systems
Figure 11-14B.
Type II pneumocyte.
EM,
3
10,900
Type II pneumocytes
are easy to identify in transmission electron micrographs, owing to the presence in their cytoplasm of
lamellar
bodies
with their distinctive concentric lamellae. Unlike type I cells, the type II cells are compact in shape, either oval or cuboidal.
They have two main functions: They are precursors of type I cells and they secrete
surfactant
, an essential complex of lipids and
proteins that serves to reduce surface tension, thereby preventing collapse of the alveoli. Type II pneumocytes are connected to their
neighboring type I pneumocytes by junctional complexes. Part of an
alveolar macrophage
is visible in this F
eld.
Alveolar
macrophage
Type II
pneumocyte
Lamellar
bodies of type II
pneumocyte
Cytoplasm of
type I pneumocyte
Collagen
fibrils
Erythrocyte in
Erythrocyte in
capillary lumen
capillary lumen
Erythrocyte in
capillary lumen
B
Figure 11-14A.
Type II pneumocytes, lung.
H&E,
3
725; inset,
3
1,253
Type II pneumocytes
are also called
septal cells
or
type II
alveolar cells
. These are large polygonal cells (or cuboi-
dal cells) with a large round nucleus. They bulge into the
air space, often sit at the corner of the alveoli (
alveolar
septa
), and make up 3% to 5% of the alveolar wall. Type
II pneumocytes have
microvilli
in their apical surfaces and
contain
lamellar bodies
in the cytoplasm (±ig. 11-14B).
Type II pneumocytes can divide and also regenerate both
type I and II pneumocytes. They produce a
pulmonary
surfactant
(
phospholipids
and
proteins
), which is impor-
tant in reducing the surface tension of the alveoli, thereby
preventing lung collapse.
Type I pneumocyte
Air space
(alveolus)
Type II pneumocyte
Type II pneumocyte
A
An example of lack of surfactant is
respiratory
distress
syndrome
(
RDS
) in premature infants. These infants’
lungs are not sufF
ciently well developed to produce
adequate surfactant. They have difF
culty breathing and
apnea
(pauses in breathing). Treatment with surfactant
and the use of a mechanical respirator are necessary to
assure their survival.
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