420
UNIT 3
Organ Systems
A
Cupula
Ampulla
Crista ampullaris
Hair cells
B
Hair cells
Basal
lamina
Hair cells
Cupula
Support
cells
Planum
semilunatum
Remnant
of cupula
Connective
tissue
Dark
cells
Figure 21-9A.
Ampulla of the semicircular canal.
Each of the three semicircular ducts has an enlargement called the
ampulla
near one of the points at which the duct joins the utricle.
There is a ridge, the
crista ampullaris
, on the fl
oor of each ampulla.
Partially surrounding the ridge and extending to the ceiling of the
ampulla is a wall, the
cupula
, which completely blocks the duct.
The cupula consists of a F
rm gel of proteins and polysaccharides.
This structure is normally dissolved during the tissue preparation
process and only remnants are typically seen in histological sec-
tions. When the head rotates, the
endolymph
within the semicircu-
lar ducts moves (
red arrows
) and exerts pressure on the cristae and
their respective cupulae, causing them to defl
ect slightly. This defl ec-
tion bends the hair cells in the cristae (±ig. 21-11B) and modulates
the frequency of action potentials that are going to the brainstem
vestibular centers, thereby producing the sensation of motion.
Figure 21-9B.
Crista ampullaris.
H&E,
3
166
The
crista ampullaris
(also known as the
ampullary crest
) is a pro-
jection of connective tissue covered with epithelium within the
ampulla. The epithelium consists of
hair cells
,
support cells
, and
dark cells
. The cilia of the
hair
cells
are embedded in the gelatinous
material of the
cupula
. The hair cells are cradled by
supporting
cells
that rest on the
basal lamina
of the epithelium. There are two dis-
tinct types of hair cells in the cristae, termed
type I
and
type II hair
cells
. These will be described in greater detail in ±igure 21-11A,B.
The
planum semilunatum
is a region of endothelium composed of
a single layer of cells called “dark cells,” because they stain more
intensely than other epithelial cells in the internal ear.
Dark cells
dis-
play cytological characteristics of cells with high metabolic activity
and are believed to be important in controlling the ionic composi-
tion of the endolymph. They are found in several other locations
within the labyrinthine ducts, including the stria vascularis.
CLINICAL CORRELATION
Figure 21-9C.
Ménière Disease.
Ménière disease
is a disorder of the labyrinth of the
inner ear, characterized by intermittent episodes of
hearing loss
,
tinnitus
,
aural pressure
, and
vertigo
. Its
causes are uncertain but may include autoimmune dis-
orders, viral infections, genetic predisposition, aller-
gies, and head trauma. Disorders of secretory cells
in the membranous labyrinth and endolymphatic sac
may produce ionic imbalance between endolymph and
perilymph, resulting in
endolymphatic hydrops
(swell-
ing of the membranous labyrinth) and producing
many of the above symptoms. Diagnosis is based on
history, clinical symptoms, audiometry, and vestibu-
lar testing. Postmortem histopathologic F
ndings may
include perisaccular F
brosis, atrophy of the endo-
lymphatic sac, and other membranous changes. Treat-
ments include reduction of caffeine and salt intake,
diuretics, antinausea medications, glucocorticoid ther-
apy, intratympanic gentamicin injection, surgical laby-
rinthectomy, and vestibular nerve section.
C
Normal membranous
labyrinth
Dilated membranous labyrinth
in Ménière’s disease
(endolymphatic hydrops)
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