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Ultrastructural evidence of cell communication between epithelial dark cells and melanocytes in vestibular organs of the human inner ear.

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THE ANATOMICAL RECORD 242:267-277 (1995)
Ultrastructural Evidence of Cell Communication Between
Epithelial Dark Cells and Melanocytes in Vestibular Organs
of the Human Inner Ear
MASAZUMI MASUDA, KAZUTO YAMAZAKI, JIN KANZAKI, AND
YASUHIRO HOSODA
Departments of Otorhinolaryngology (M.M., J.K.) and Pathology (K.Y., Y.H.),
Keio University School of Medicine, Tokyo, Japan
ABSTRACT
Background: The possibility of interaction between epithelial dark cells and melanocytes in the mammalian inner ear has been
pointed out because of their morphological and biochemical characteristics, although very few studies have dealt directly with communication
between these two types of cells. We investigated the dark cell area of
human vestibular organs in order to clarify the ultrastructual evidence for
cell interaction between epithelial dark cells and melanocytes.
Methods: All of the material was obtained from vestibular schwannoma
operations. Paraffin sections were stained with hematoxylin and eosin
(H&E) and by the Fontana-Massontechnique. Other paraffin sections were
also stained immunohistochemically for S-100 protein. Glutaraldehyde
fixed specimens were investigated by scanning electron microscopy (SEM)
and transmission electron microscopy (TEM).
Results: Light microscopy revealed melanin pigment granules in the cytoplasm of epithelial dark cells. Melanocytes in the subepithelial layer
stained positively for 5-100 protein. The presence of intraepithelial melanocytes was confirmed by the presence of cell profiles with a large number of
melanin pigment granules and S-100protein in the cytoplasm. SEM showed
that the dark cells had a pentagonal surface with microvilli on the apical
surface edge. They had complicated structures at the basal portion of their
cytoplasm. Melanocytes extending cytoplasmic processes to adjacent areas
were observed under the dark cells. TEM showed that the dark cells were
tightly linked by junctional complexes in the upper lateral portion of their
cytoplasmic membrane and interdigitated by lateral infoldings. Compound
melanosomes (phagosomes or secondary lysosomes) found in the cytoplasm of the dark cells contained poorly pigmented melanosomes with a
periodic internal structure. Gap junctions were clearly showed between
adjacent melanocytes in the subepithelial layer.
Conclusions: The characteristic substructures of dark cells and melanocytes suggested the presence of intimate cell interaction between these two
types of cells in the vestibular organs of the human inner ear, although it is
not clear at this stage whether such cell interaction is specific only for
patients with vestibular schwannoma. Dark cells and melanocytes form a
cell community that serves to maintain homeostasis in vestibular organs
through communication in which cell information obtained by both dark
cells and melanocytes serves to facilitate the system. o 1995 Wiley-Liss, Inc.
Key words: Human inner ear, Dark cell, Melanocyte, Eumelanosome,
Pheomelanosome, Gap junction, Membranous apposition,
5-100 protein
A number Of studies have been conducted On the dark
and melanocytes Of the inner ear using such experimental mammals as guinea pigs, gerbils, and mice
(Quevedo and Fleischmann, 1980; Yoshihara et al.,
0
1995 WILEY-LISS, INC.
Received October 14, 1994; accepted January 18, 1995.
Address reprint requests to Masazumi Masuda, M.D., Department
of Otorhinolaryngology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160, Japan.
268
M. MASUDA ET AL.
TABLE 1. Clinical details of patients
Age
Case no.
and ID
1 M. H.
2 T. 0.
3 K . 0.
4 I. 0.
5 F. I.
6M.T.
7 T. 0.
8M.U.
9 T. K.
10 T. I.
llM.N.
12 K.A.
13 C. S.
(years)
and sex
46,F
43, M
55,F
48; M
52. F
47;F
65, F
31,M
59, M
57, M
42.F
57;F
36, M
Side and size'
of neurinoma
(0.
cm)
Right (1.0)
Right (1.5)
Left (2.5)
Right (3.0)
Left (3.0)
Left (2.0)
Left (1.0)
Left (1.5)
Right (1.0)
Right (2.0)
Left (1.5)
Left (1.2)
Left (1.5)
Chief comdaint
Rt. hearing loss
Rt. hearing loss
Lt. hearing loss
Rt. hearing loss, dizziness
Lt. hearing loss, facial palsy
Lt. hearing loss
Lt. hearing loss
Lt. hearing loss
Rt. hearing loss
Rt. tinnitus
Lt. hearing loss. dizziness
Lt. hearing loss'
Lt. hearing loss, tinnitus
'All cases were neurinoma in pathological investigation.
'EMCF = extended middle cranial fossa approach.
3LSC = lateral semicircular canal, SSC = superior semicircular canal, PSC
41HC = immunohistochemically stained with S-100 protein.
1987; Carlisle et al., 1990; Cable and Steel, 1991), but
there have been few studies in humans due to the difficulty of obtaining human material (Ishida et al.,
1986; Igarashi et al., 1989, Masuda et al., 1994). The
results of the studies in experimental mammals have
suggested that dark cells play some role in the absorption and secretion of endolymph and perilymph, because they possess well-developed cytoplasmic infoldings that are quite similar to those of renal tubule cells,
choroid plexus cells, and the marginal cells of the stria
vascularis (Rhodin, 1958; Hinojosa and Rodriguez,
19661, and because of the high concentrations of Na-KATPase (Yoshihara and Igarashi, 19871, Ca-ATPase
(Yoshihara et al., 1987), and carbonic anhydrase (Watanabe and Ogawa, 1984) in cytoplasmic infoldings
that contain a large number of mitochondria.
Several reasons suggest that melanocytes have some
essential role in the metabolism of the inner ear: (1)
melanosomes themselves have several characteristic
biochemical functions, such as being a biological reservoir for divalent ions, including M 2 + and Ca2+,and
for other chemical substances essential to enzymatic
activation or cell membrane regulation (Sealy et al.,
1980; Meyer zum Gottesburge-Orsulakova and Kaufmann, 1986); (2) melanosomes are scavengers and producers of toxic-free radicals (Barr, 1983; Schllreuter
and Wood, 1989), and (3) they possess semiconductive
properties for various stimuli, phonic, acoustic, and
electrical (Mcginnes et al., 1974; Barr, 1983). Intimate
interaction between these two types of cells, i.e., epithelial dark cells and melanocytes, can be assumed because of the characteristic physiological function and
close location of these cells in the inner ear. In the latest
studies, several researchers have pointed out that both
melanocytes and their dendritic processes enter the epithelial dark cell layer across the basal lamina under
certain conditions, such as experimental endolymphatic
hydrops (Meyer zum Gottesburge-Orsulakova, 1988)
and exposure to high intensity noise (Kawaguchi,
1992). In the white spotting mouse mutant (Steel and
Barkway, 1989; Carlisle et al., 1990),the essential role
of melanocytes for the normal development of stria vascularis, especially for normal process of interdigitation
=
0peration
EMCF~
1 or 2
2
1
1
2
2
1
1
2
1
2
1
1
1
Surgical
materials3
LSC
LSC
ssc, LSC
LSC
PSC
ssc, LSC
Ut, LSC, PSC, ssc
LSC, PSC, ssc
LSC, PSC, ssc
LSC, PSC, ssc
PSC. ssc
Ut, Lsc
PSC, ssc
posterior semicircular canal, Ut
=
Methods
TEM
TEM
TEM
TEM
TEM
TEM
TEM
TEM
SEM
TEM
IHC~
TEM
IHC~
utricle.
between marginal and basal cell processes results in
generation of the endocochlear potential.
The purpose of the present study is to clarify the
ultrastructural evidence of cell interaction between
dark cells and melanocytes using human material. According to former studies (Ishida et al., 1986; Igarashi
et al., 19891, some variations may exist in the morphology of melanosomes in the human inner ear. Melanosomes with internal structure resembling that of eumelanosome have been found in the human fetus
(Igarashi et al., 19891, but the melanocytes in the adult
human inner ear possess melanosomes with internal
structure resembling that of pheomelanosome (Ishida
et al., 1986).Therefore, another purpose of this study is
not only to identify the cellular phenotype of the melanocytes of the human inner ear by immunohistochemical techniques, but to determine the detailed morphology of the melanosomes in the human inner ear,
MATERIALS AND METHODS
Cases
Thirteen surgical specimens obtained from patients
with vestibular schwannoma were studied, all taken
from Japanese adults (see Table 1for detailed clinical
information). Six of the 13 patients were male, and
seven were female. The age of the patients ranged from
31 to 65 years, and their average age was 49 years.
Five specimens were from the right ear, and the other
eight were from the left ear. Surgery consisted of an
extended middle cranial fossa approach (EMCF)type 1
or type 2 (Kanzaki et al., 1991). Material was obtained
from the lateral semicircular canal (10 cases), superior
semicircular canal (8 cases), posterior semicircular canal (7 cases), and utricle (2 cases). It was immediately
processed for light and electron microscopic examination.
Light Microscopy
The excised specimens were immediately fixed in
20% formalin and processed according to a conventional histologic technique for subsequent hematoxylin
and eosin (H&E) staining. We confirmed that there
was no marked invasion of the schwannoma and no
CELL INTERACTION I N HUMAN INNER EAR
other pathological changes. To determine the exact distribution of both melanin pigment granules and melanin pigment containing cells, specimens adjacent to
those submitted to light microscopy were stained by
the Fontana-Masson technique.
Light Microscopic lmmunohistochemistry
To make clear phenotypes of cells, especially cells in
the epithelial layer, we applied immunohistochemical
staining method using S-100 protein for the excised
specimen adjacent to those stained with haematoxylin
and eosin, and by the Fontana-Masson technique. To
localize the immunohistochemical marker S-100 protein, melanin pigment bleeching was performed before
the immunohistochemical procedure. Deparaffinized
tissue sections obtained from two patients were incubated in a 10% solution of H,Oz in 0.05M Tris-HC1
buffer (pH 7.6) for 5 hours. The immunohistochemical
staining reaction was performed after it was confirmed
that the color of melanin had completely disappeared
through the light microscope. An indirect immunostaining method was applied to these bleeched sections
to demonstrate S-100 protein. In brief, sections were
exposed to a X 200 dilution of primary antibody for 3
hours a t room temperature. Polyclonal rabbit antihuman S-100protein was purchased from DAKO Co. (CA).
Scanning Electron Microscopic Examination (SEM)
Specimens were fixed in 2.5% glutaraldehyde for 2
hours at 4°C. The material was washed three times in
phosphate buffer, postfixed for 2 hours in 1%similarly
buffered OsO4, and dehydrated in graded alcohol. The
alcohol that the specimens contained was then replaced
by acetaticisoamyl for 10 minutes following a critical
point drying procedure. The dried specimens were
coated with 8 nm thick platinum and examined with a
Hitachi S-4000 scanning electron microscope.
Transmission Electron Microscopy (TEM)
The material was immediately fixed in 2.5% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4) for 4
hours, washed three times in phosphate buffer, postfixed for 2 hours in 1%similarly buffered OsO4, dehydrated in graded alcohol and acetone, and embedded in
Epoxy resin. Thick sections, in the 1 pm range, were
stained with toluidine blue. Suitable blocks were thinsectioned using an LKB ultratome with a diamond
knife. Sections in the gray to silver range were collected on 150-mesh grids, stained with uranyl acetate
and lead citrate, and examined with a Joel-1200EX
electron microscope.
Observations
So far, no apparent differences could be seen between
materials taken from different parts of the vestibular
organ in a patient through light microscopic and electron microscopic observation. There have been no apparent differences between patients neither.
Light Microscopic Investigation
Dark cells comprised part of the epithelial lining of
vestibular organs. Brown pigment granules were
present in both epithelial dark cell layer and subepithelial layer (Fig. la). We employed the Fontana-Masson technique to make clear the localization of these
269
pigment granules. In addition to the dark cells, which
had pigment granules stained positively by the Fontana-Masson technique in their cytoplasm, there was
another kind of cell with a number of pigment granules
in the intraepithelial layer (Fig. lb). With this technique, it is possible that those pigment granules were
in fact in the cytoplasmic processes of subepithelial
melanocytes interdigitating into the epithelial layer
and not in the cytoplasm of dark cells or another kind
of intraepithelial cells. Melanocytes that had well-developed dendritic processes, fibroblasts with spindleshape cell bodies, and small blood vessels could be discerned in the subepithelial layer (Fig. la,b).
Light Microscopic lmmunohistochemistry
After applying the bleeching technique, we could distinguish that the melanocytes in the subepithelial
layer stained immunohistochemically for S-100 protein. It was also suggested that the existence of two
types of cells in the epithelial dark cell layer, i.e., cells
that were positively stained for S-100 protein (intraepithelial melanocytes) and cells that were not positively
stained for S-100 protein (epithelial dark cells) (Fig.
lc).
Scanning Electron Microscopic Investigation
Viewed from the endolymphatic lumen, the dark
cells usually appeared pentagonal or polygonal (Fig.
2a). There was a specialized surface structure of rich
microvilli along the border between the adjacent dark
cells. The diameter of these dark cells was 2-4 pm. We
could not see any melanocytes on the luminal surface of
these dark cells. Viewed from the fractured side, we
could see the border between adjacent epithelial dark
cells; however, there was no space between neighboring cells (Fig. 2b). These dark cells were 7 pm tall and
3 pm wide in the fractured plane. There were complicated structures at the basal portion of the dark cells.
Melanocytes were present immediately below the epithelial dark cells and extended cytoplasmic processes
from their cell bodies.
Transmission Electron Microscopic lnvestigation
Dark cells. Dark cells had irregularly shaped nuclei
and cuboidal cell bodies with relatively high electrondense cytoplasm (Fig. 3a). These dark cells ranged from
9 to 12 pm high and from 4 t o 6 pm wide. There were
short microvilli in the apical portion of their cytoplasm
and well-developed cytoplasmic infoldings in the basolateral portion of their cytoplasm. We observed rough
endoplasmic reticulum and numerous mitochondria in
their cytoplasm. These dark cells were tightly linked
with each other through a junctional complex consisting of tight junctions, zonulae adherentes, and desmosomes in the apical portion of their lateral surface (Fig.
3b). They also had interdigitated lateral cytoplasmic
infoldings, and there were no obvious wide spaces
around the basolateral cytoplasmic infoldings in relatively well fixed specimens (Fig. 3c).
Phagocytosed compound melanosomes (phagosomes
or secondary lysosomes) were frequently (1compound
melanosomes per every 10-13 dark cells) found in the
cytoplasm of dark cells (Fig. 4a). These compound melanosomes enclosed by an unit-limiting- membrane con-
270
M. MASUDA ET AL.
Fig. 1. Light micrographs of posterior semicircular canal form case 11.
(a) H&E stain. Epithelial dark cells (E), melanocytes (M), fibroblasts(F),
and intraepithelial melanocytes (arrowheads) with a number of melanin
granules. x 330 (b) Fontana-Masson stain. Epithelial dark cells (E) with
melanin granules. Im: intraepithelial melanocytes. M: melanocytes in the
subepithelial layer. 13:blood vessel. x 330 (c) lmmunohistochemicalstain
with S-100 protein. Some of the intraepithelial cells (arrowheads) as well
as melanocytes (M) in the subepithelial layer are positively stained for
S-100protein. x 330
CELL INTERACTION IN HUMAN INNER EAR
Fig. 2. SEM of superior semicircular canal from case 9. (a) Endolymphatic luminal surface view of
epithelial dark cells. Dark cells (E) exhibiting a pentagonal or polygonal surface. Microvilli (arrowheads). ~ 4 0 0 0(b) Fractured side view of the vestibular dark cell area. Lateral cell borders (small
arrowheads) between two adjacent dark cells. Complicated structures (large arrowheads) a t the basal
portion of the dark cells. Subepithelial dendritic melanocyte (M). x 4000
271
Fig. 3.
CELL INTERACTION IN HUMAN INNER EAR
273
Fig. 4. TEM of lateral semicircular canal from case 10(a), and case 3(b,c). (a) Liberated melanin
granules (P) in the endolymphatic lumen. Compound melanosomes (C) in the cytoplasm of dark cells (E).
Melanosomes (arrowheads) in the dendritic processes of subepithelial melanocytes. x 9000 (b,c) High
magnification views of poorly pigmented melanosomes in the compound melanosomes. An unit-limiting
membrane (arrowheads) enclosing melanosomes could be seen. x 115000
Fig. 3.TEM of lateral semicircular canal from case 10(a,c)and case
l(b,d). (a)Melanin granules (P) in the endolymphatic lumen. Epithelial dark cells (E) with microvilli on their luminal surface. Melanosomes (arrows) in the cytoplasm of dark cells. Melanosomes (arrowheads) in both intraepithelial and subepithelial melanocytes. Mi: a
number of mitochondria. Im: intraepithelial melanocytes. M: subepithelial melanocytes. x 4300 (b)Junctional complex of epithelial dark
cells. Tight junction (arrowheads), zonula adherens (A), and desmosome (D). X 52000 ( c ) Lateral infoldings of epithelial dark cells. E:
epithelial dark cells. X 20000 (d)High-magnification view of melanosomes in the cytoplasm of subepithelial melanocytes. Poorly pigmented melanosomes (Stage I1 or I11 pheomelanosomes)that have the
spherical granular form of internal structure range from 200 to 400
nm in diameter. x 55000
274
M. MASUDA ET AL.
tained several poorly pigmented melanosomes. They
had the ellipsoidal lamellar form of internal structure
compatible with that of eumelanosome (Jimbow et al.,
1979; Quevedo and Fleischmann 1980; Jimbow e t al.,
1983) (Fig. 4b,c). These melanosomes ranged from 200
to 500 nm long. We also occasionally found melanin
granulelike material in the endolymphatic lumen
(Figs. 3a, 4a).
lntraepithelial rnelanocytes. In the present study we
frequently (one melanocyte per every 7-9 dark cells)
found melanocytes within the epitheleial dark cell
layer. They had ellipsoidal nuclei with fairly evenly
dispersed chromatin and relatively clear cytoplasm
(Fig. 3a). The shape and size of these intraepithelial
melanocyte cell bodies varied slightly, but their mean
size was 6 x 10 pm. In the cytoplasm there were spherical melanosomes and cell organelles, such as mitochondria and rough endoplasmic reticulum. Poorly pigmented melanosomes revealed the spherical granular
form of internal structure. The morphological characteristics of these melanosomes were compatible with
those containing pheomelanin (Stage I1 or I11 pheomelanosomes) (Jimbow et al., 1979; Quevedo and Fleischmann 1980; Jimbow et al., 1983) and their average size
was 300 x 400 nm (Fig. 3a). There were no marked
differences between the shape and size of the melanosomes in the epithelial melanocytes and in the subepithelial melanocytes (Fig. 3d). We also observed two
parallel opposing membranes between intraepithelial
melanocytes and the dendritic processes of other melanocytes (Fig. 5a). The gap between the outer leaflets
was 24 nm wide and the gap between inner leaflets was
6 nm wide (Fig. 5b).
Gap junctions. In the subepithelial layer, gap junctions were clearly recognized between the plasma
membranes of adjacent melanocytes as well as between
melanocyte cell bodies and their own cytoplasmic processes (Fig. 6a,b) (Staehelin, 1974). The gaps between
the two outer leaflets of the plasma membranes of
these gap junctions were 12 nm, and gaps between the
inner leaflets of these gap junctions were 2 nm (Fig.
6b). Periodic substructures with relatively constant repeating lattices were seen in the interspace between
the opposed plasma membranes of gap junctions. We
were able to observe a t most 3-4 gap junctions around
a single subepithelial melanocyte.
DISCUSSION
lntraepithelial Melanocytes
In the present study, examination of H&E, FontanaMasson, and immunohistochemically stained specimens suggested the existence of intraepithelial melanocytes. Detailed TEM observations clearly showed
the presence of both intraepithelial melanocytes and
subepithelial melanocytes. At the same time, these
melanocytes were positively stained for S-100 protein,
demonstrating that they originated in neural crest during development (Clemmensen and Fenger, 1991).
Cells that do not originate in neural crest can possess
melanin pigment granules. This has been clearly
shown in such spotting mouse mutants as the S1/Sld
mouse and the W/W” mouse (Mayer and Green, 1968).
Fig. 5. TEM of posterior semicircular canal from case 7. (a) Membranous appositions with narrow gaps (small arrowheads) between
the intraepithelial melanocyte and another melanocyte dendrite.
Small vesicle (large arrowhead) in the dendritic process of a melanocyte. E: nucleus of a n epithelial dark cell. N: nucleus of a n intraepithelial melanocyte. x 18000 (b) High-magnification view of the same
membranous appositions(arrowhead). Small vesicle (V) with a round
configuration and short spikelike projections. x 37000
There are no melanocytes in inner ear in these animals, and they have black eyes whose melanin pigment
is produced by retinal epithelial cells embryonically
derived from the optic cup. In the present study the
melanocytes in the vestibular dark cell area were
found to have the same origin as those in other parts of
the human body, e.g., in the skin, mucous membranes,
lentigo, pia mater, brain, and choroid membrane (Jimbow e t al., 1976).
Melanosomes in Human Inner Ears
In the present study we occasionally found compound
melanosomes (phagosomes or secondary lysosomes) in
CELL INTERACTION I N HUMAN INNER EAR
275
Fig. 6. TEM of lateral semicircular canal from case 1. (a)Gap junction (arrowhead) between a subepithelial melanocyte cell body and a dendritic process. N nucli of subepithelial melanocytes. M: melanosomes. Mi: mitochondria. F: fine filamentous material around the melanocyte and its dendritic processes.
x 18000 (b) High-magnification view of the same gap junction. G: gap junction. x 100000
the cytoplasm of dark cells. Some of them contained
poorly pigmented melanosomes. They had the ellipsoidal lamellar form of internal structure compatible with
that of eumelanosome (Jimbow et al., 1979; Quevedo
and Fleischmann 1980; Jimbow et al., 1983). Igarashi
et al. (1989) showed that melanosomes in the inner ear
display an internal structure resembling that of eumelanosome using human fetal material obtained when
the inner ear structure had attained developmental
maturity (Igarashi and Ishii, 1980).Ishida et al. (19861,
in contrast, said melanosomes exhibit an internal
structure resembling that of pheomelanosome in the
adult human inner ear and that these melanosomes
have the biochemical characteristics of eumelanin, although there is a discrepancy in the form of their internal structure.
The melanosomes in our studies of human adults,
mainly displayed an internal structure resembling
that of pheomelanosome. Although there is evidence
that many natural melanins are copolymers of eumelanin and pheomelanin and that hardly any pure eu-
melanin or pheomelanin exist in mammals (Prota,
1980), it is generally accepted that the morphological
characteristics of melanosomes, i.e., ellipsoidal lamellar eumelanosome and spherical granular pheomelanosome correspond well to the biochemical characteristics
of each melanin content, i.e., eumelanin and pheomelanin according to the studies on melanogenesis in normal skin and hair follicles (Jimbow et al., 1983). Thus
based on the fact that there are differences between the
shape of human fetal and adult melanosomes and that
a few melanosomes with an internal structure resembling that of eumelanosome are actually present in
specimens from human adults, we can speculate that
the switch from eumelanogenesis to pheomelanogenesis occurs around the time of birth. In a study on human red hair, two different populations of melanocytes,
generating eumelanin and pheomelanin, respectively,
were found to be present in the same hair follicle (Jimbow et al., 1983). In agouti mice, in contrast, eumelanosomes and pheomelanosomes coexist in the same
melanocytes because of a switch from eumelanogenesis
276
M. MASUDA ET AL.
to pheomelanogenesis during the normal growth of
hair follicles (Sakurai et al., 1975).
It is said that generally the genetic factors of not only
melanocytes but of the tissue around them in various
organs have a powerful effect on the type of melanogenesis in melanocytes (Quevedo and Fleischmann,
1980). Now we suspect that these changes in eumelanogenesis and pheomelanogenesis in the human inner
ear are caused by differences in the environment before
and after the birth, such as sound stimuli being directly delivered to the ear, not via the maternal body,
and exposure to ultraviolet rays that are capable of
stimulating melanocytes in other parts of the body
(Jimbow and Fitzpatrick, 19751, or by the effects of
aging alone. Cable et al. (1991) suggested in their
study of melanocytes, using the inbred CBNCa strain
and normal littermates of the viable dominant spotting
mouse mutant, that aging could be one of the causes of
the morphological changes in the melanocytes of the
inner ear. To the best of our knowledge, no biochemical
analysis of melanogenesis in the human inner ear has
been performed; however, if this kind of change in melanogenesis really does occur in the human inner ear
and is characteristic only of the melanocytes of the human inner ear, it will support the concept of melanocytes playing an important role in inner ear metabolism.
Melanin Transmission
Our light microscopic and TEM observations frequently showed that melanosomes are actually present
in the cytoplasm of epithelial dark cells (Masuda et al.,
1994). This suggests the possibility of melanosome
transmission from both intra- and subepithelial melanocytes to dark cells. Earlier researchers have noted
the disappearance of the plasma membranes of both
the basal infoldings of dark cells and melanocyte dendrites in the subepithelial layer. Fusion between the
cytoplasmic processes of these two cells has been described (Meyer zum Gottesburge-Orsulakova, 1988;
Igarashi et al., 19891,but this disappearance of plasma
membranes is caused by the cutting angle of the specimen, and phagocytosis seems to be a rational explanation when we consider the process of melanosome
transmission between dark cells and melanocytes.
Moreover, we sometimes found that melanosomes were
present in the endolymphatic lumen just beyond epithelial dark cells containing compound melanosomes.
We hypothesize two possibilities based on this findings.
One is that this only represents a waste disposal process, i.e., that melanocytes that died during the normal
cell cycle were being transferred into the epithelial
layer and then into the endolymphatic lumen. The
other is that it represents a process of melanosome secretion from epithelial dark cells into the endolymph,
i.e., that when melanocytes are in an activated metabolic state, the melanocytes in the subepithelial layer
synthesize a number of melanosomes (Masuda et al.,
1994) and that these melanosomes are transferred into
the epithelial dark cells for secretion into the endolymph. The possibility of phagocytic uptake of melanin pigment granules by epithelial dark cells from the
endolymphatic lumen still remains, although how and
why the granules occur in the lumen pose a problem.
Gap Junctions
In the previous report we demonstrated the presence
of gap junctions in the melanocytes of the human inner
ear by means of detailed TEM examination (Masuda et
al., 1994). Thus far, gap junctions have been found between the plasma membranes of adjacent melanocytes
and between melanocyte cell bodies and their own dendritic processes in the subepithelial layer, but no gap
junctions have been detected connecting the apposed
plasma membranes of melanocytes and other kinds of
cells. Currently the most general and important function of gap junctions is believed to be the transmission
of regulatory and signaling substances that might play
an inductive or repressive role in controlling the rate of
division and differentiation or response to hormones in
both developing and adult tissues. Moreover, gap junctions can provide cells with a way to obtain cellular
information in a cell community (Staehelin, 1974; Alberts et al., 1983). In the vestibular organs of the human inner ear, melanocytes can construct a cell community by transmitting cell information or other kinds
of signals through these gap junctions.
Cell Communication in Human Vestibular Organs
In our earlier studies we frequently observed isolated
cilia in subepithelial melanocytes (Masuda et al.,
1994). Although there is still controversy about the
physiological functions of isolated cilia at this stage
(Masuda et al., 19941,the following hypothesis is based
on the physiological functions of appropriate melanocyte substructures, such as isolated cilia and gap junctions in the dark cell area of human vestibular organs.
Melanocytes in the subepithelial layer may monitor
environmental changes around them via isolated cilia.
This information is then transmitted from one melanocyte to another at a suitable speed and integrated via
gap junctions. Based on this cellular information, stimulated melanocytes try actively to generate melanosomes (Masuda et al., 1994). In earlier studies it was
speculated that exposure to noise activates the metabolic state in the tissues of the organ of Corti and stria
vascularis (Ryan et al., 1982) and that this results in
increased levels of toxic free radicals and unpaired
electrons. Melanosomes have characteristic functions
as powerful scavengers of toxic free radicals (Ban-,
1983; Schllreuter and Wood, 1989) and ability to bind
these compounds (Sealy et al., 1980; Meyer zum Gottesburge-Orsulakova and Kaufmann, 1986). Melanocytes
monitoring various environmental changes in the tissues of the subepithelial layer may produce melanosomes to achieve homeostasis in the vestibular organs.
Exposure of the inner ear to noise also leads to increased levels of Ca2+ in the endolymph (Ikeda et al.
1988). Characteristic dark cell enzymes, such as Ca2+'
ATPase, would be of help in maintaining Ca2+concentrations in the endolymph (Yoshihara et al., 1987).
Moreover, it appears that dark cells utilize melanosomes and may excrete melanosomes into the endolymph, because melanosomes have the ability to
serve as divalent ion reservoirs (Sealy et al., 1980;
Meyer zum Gottesburge-Orsulakova and Kaufmann,
1986). The observation that melanocytes in the vestibular organs of guinea pigs are activated to produce
melanosomes because of imbalanced calcium homeo-
CELL INTERACTION I1X HUMAN INNER EAR
stasis in experimental endolymphatic hydrops corroborates this suspicion (Meyer zum Gottesburge-Orsulakova, 1988). Based on transcellular information about
environmental changes around them, some subepithelial melanocytes transfer their dendritic processes into
the epithelial layer for effective transmission of melanosomes to dark cells. For the same purpose, melanocyte cell bodies themselves sometimes move in between
epithelial dark cells and turn into intraepithelial melanocytes, although some intraepithelial melanocytes
may be present between epithelial dark cell layers
from the outset.
Based on changes in dark cell metabolism, those melanosome generation and transmission may be regulated by signal transfer between dark cells and melanocytes via junctional specializations such as gap
junctions. If dark cells themselves really have some
perceptive role through their apical membrane specializations, e.g., microvilli, dark cells could monitor the
various changes in endolymph directly, more effectively, and more rapidly than monitoring by melanocytes in the subepithelial layer. This would suggest a
communication system facilitated by transcellular signals resulting from the perceptive function of both epithelial dark cells and melanocytes.
Based on the ultrastructural morphological evidence
in the present study, which suggests the possibility of
cell communication, we hypothesize that the cell community, which mainly consists of dark cells and melanocytes, works as a whole to maintain the homeostasis of vestibular organs by transmission of intercellular
signals or information via gap junctions and transmission of essential materials by melanosome transmission, although we cannot completely deny the possibility that such relatively active cell communication
between melanocytes and dark cells is specific only for
the patients with vestibular scwannoma.
ACKNOWLEDGMENTS
We express gratitude t o Dr. Y. Inoue, Dr. Y. Satoh,
Dr. S. Yoshihara, and Dr.T. Matsunaga for kindly providing specimens, and to Dr. Y. Toyama (Dept. of Anatomy, Chiba Univ.) for valuable discussion. The technical assistence of Y. Enomoto, N. Yagi, K. Takeichi, and
T. Fujiwara is gratefully acknowledged.
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