CHAPTER 3
Epithelium and Glands
39
Three Apical Specializations of Epithelium
A
B
C
Cilia
Microvilli
Basal
bodies
Terminal
web
Stereocilia
Tight junction
Tight junction
(zonula occludens)
(zonula occludens)
Tight junction
(zonula occludens)
Tight junction
Tight junction
Tight junction
Cilia (motile)
Microvilli (nonmotile)
Stereocilia (nonmotile)
Figure 3-11A.
Cilia, basal body, and
junctional complex.
EM,
3
9,500; inset
color photomicrograph
3
724
Cilia
are 0.2
μ
m in diameter and 5 to 10
μ
m long, so they can be seen as individual
structures with the light microscope. The
core
(axoneme)
of each cilium is com-
posed of
microtubules
and associated pro-
teins, most notably the molecular motor
dynein
. The microtubules are arranged as
nine peripheral doublets with two central
singlets. Each cilium extends from a
basal
body
just beneath the apical surface of
the epithelial cell. Basal bodies also have
microtubules as a major component. These
form an orderly array of nine peripheral
triplets with no central microtubules, an
arrangement seen also in
centrioles
.
Figure 3-11B.
Microvilli,
terminal web,
and junctional complex.
EM,
3
9,500;
inset color photomicrograph
3
723
Microvilli
of the intestinal epithelium
are about 0.08
μ
m in diameter and 1
μ
m
long, so they cannot be distinguished as
individual structures with the light micro-
scope, but the row of tightly packed
microvilli can be seen as a
brush border
.
The core of each microvillus contains a
bundle of 6-nm
actin ±
laments
, which
extend from the actin F
laments that form
the
terminal web
just beneath the apical
surface of the cell.
Figure 3-11C.
Stereocilia and junc-
tional complex.
EM,
3
9,500; inset color
photomicrograph
3
565
Stereocilia
are extremely long
microvilli
.
Like ordinary microvilli, stereocilia are
less than 0.1
μ
m in diameter, but they
can attain lengths of 10
μ
m or more.
Stereocilia are characteristic of the pseu-
dostratiF
ed columnar epithelium of the
ductus epididymis, which is the site of
absorption of the large volumes of tes-
ticular fl
uid produced by the
seminiferous
tubules
. The greatly expanded surface
area afforded by the stereocilia probably
contributes to this function.
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