CHAPTER 11
Respiratory System
215
Figure 11-13A.
Type I pneumocytes, lung.
H&E,
3
725; inset,
3
1,145
Type I pneumocytes
are squamous cells and make up
95% to 97% of the alveolar wall. A small percentage
of the alveolar wall is covered by
type II pneumocytes
(Fig. 11-14A,B). Each type I pneumocyte has a fl
at, dark
oval nucleus and very thin cytoplasm. These cells form
the
blood-air barrier
together with the endothelial cells
of the capillaries. They connect with each other by
tight
junctions
to prevent leakage of fl uid into the airspace.
Type I pneumocytes are not able to divide; if they are
damaged, type II pneumocytes will differentiate to replace
the damaged type I cells. There are delicate connective
tissues (including ±
broblasts, elastic, and reticular ±
bers)
and capillaries between the alveoli, forming the
alveolar
septa
(Fig. 11-12A). Alveolar septa contain a blood-air
barrier where gas exchange occurs. It is not easy to distin-
guish between type I pneumocytes and endothelial cells,
because they are both squamous cells.
Figure 11-13B.
Type I pneumocytes and other constituents of alveoli.
EM,
3
5,250
The open spaces in this view are a mixture of air-± lled spaces (
alveoli
) and the lumens of capillaries that were emptied of blood in
the preparation of the specimen. The distinction between the air spaces and the blood spaces is not obvious because of the similar-
ity in ultrastructural appearance of the
endothelial cells
lining the capillaries and the
type I pneumocytes
lining most of the alveolar
surfaces. Both cell types are extremely fl
attened to produce thin sheets of cytoplasm. The type I pneumocyte (squamous alveolar
cell) provides the covering of most (about 97%) of the surface of alveoli.
Type II cells
cover the remaining small fraction, and the
single type II cell provides an important clue in distinguishing the capillaries from the alveoli here. Adjacent to the type II cell is
the nucleus of a type I cell. This is the only part of the type I cell that is not extremely fl
attened. The various cells and structures in
the ± eld provide some context for appreciating the thinness of the
blood-air barrier
.
Air space
(alveolus)
Type I
pneumocytes
Interalveolar
septa
Type I
pneumocyte
Type I
pneumocytes
A
Capillaries
Type I
pneumocyte
Type II
pneumocyte
Leukocyte in
capillary
Capillary
Capillary
Air
Air
Blood-air barrier
Capillary
Air
Air
Air
Air
B
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