CHAPTER 11
Respiratory System
213
Figure 11-11A.
Clara cells, terminal bronchioles.
H&E,
3
284; inset
3
1,181
Clara cells
are secretory cells that are scattered among
ciliated cells and often project into the lumen of the bron-
chioles. They are
dome-shaped cells
without cilia and con-
tain apical granules (visible only with a special stain); they
are more abundant in terminal bronchioles. The substances
(including
glycosaminoglycans
and
secretory proteins
) pro-
duced by Clara cells help to form the lining of the bron-
chiole. Clara cells play a role in
immunomodulatory
and
anti-infl ammatory
activities, thereby helping to protect the
bronchiolar epithelium. They also function as progenitor
cells that can differentiate into other epithelial cell types,
especially in epithelial repair after airway injury.
Clara cells
Clara cells
Clara cells
Clara cells
Clara cells
Clara cells
Alveolus
Alveolus
Alveolus
Alveolus
Alveolus
Alveolus
Lumen of a
Lumen of a
terminal bronchiole
terminal bronchiole
Lumen of a
terminal bronchiole
A
Fig.11-11C
Fig.11-11C
inset
inset
S
m
o
o
th
t
h
m
m
u
s
c
le
le
Smooth muscle
Respiratory
Respiratory
bronchiole
bronchiole
Respiratory
bronchiole
Terminal
Terminal
bronchiole
bronchiole
Terminal
bronchiole
Fig.11-11C
inset
B
Figure 11-11B.
Terminal bronchiole, lung.
H&E,
3
70;
inset
3
179
The
terminal bronchioles
are the last segment (most
distal)
of the conducting portion of the respiratory system. They
are lined by simple cuboidal cells consisting mainly of
Clara cells, with some ciliated cells and a few basal cells.
Gradually, as the bronchioles proceed distally in the lung,
the epithelium changes from columnar to cuboidal cells.
The terminal bronchioles contain large amounts of
smooth
muscle
in the airway wall. This smooth muscle is con-
trolled by the sympathetic and parasympathetic nervous
systems. At this point, cartilage and submucosal glands
are absent from all bronchioles.
Alveolar sac
Alveolar sac
Alveoli
Alveoli
Terminal
Terminal
bronchiole
bronchiole
Respiratory
Respiratory
bronchiole
bronchiole
Alveolar
Alveolar
duct
duct
Alveolar
Alveolar
duct
duct
Terminal
bronchiole
Respiratory
bronchiole
Alveolar sac
Alveoli
Alveolar
duct
Alveolar
duct
C
Figure 11-11C.
Respiratory bronchioles, lung.
H&E,
3
71; inset
3
179
The
terminal bronchioles
give rise to
respiratory
bronchioles
. Respiratory bronchioles are the F
rst airways
that function in gas exchange. They are lined by cuboidal
cells and have gradually increasing numbers of alveoli.
Respiratory bronchioles connect to alveolar ducts.
Alveo-
lar ducts
are lined by squamous alveolar cells (type I pneu-
mocytes) and knobs of cuboidal epithelium lying on the
smooth muscle cells. Each alveolar duct functions struc-
turally as a corridor, which connects to several alveoli
(±ig. 11-9). Each
alveolar sac
is composed of two or more
alveoli that share a common opening. The arrows indicate
the direction of the airfl
ow, from the terminal bronchiole
to the respiratory bronchiole, then to the alveolar duct
and, eventually, into the alveolar sac.
Research has shown that in heavy
smokers
,
Clara cells
are greatly decreased and
goblet
(
mucus-producing
)
cells
are greatly increased in the epithelium of the bronchi-
oles. These changes are caused by directly inhaled irri-
tants and chronic exposure to harmful substances.
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