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The ultrastructure of ruffini endings in the periodontal ligament of rat incisors with special reference to the terminal schwann cells (K-cells).

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THE ANATOMICAL RECORD 223:95-103 (1989)
The Ultrastructure of Ruffini Endings in the
Periodontal Ligament of Rat Incisors With Special
Reference to the Terminal Schwann Cells (K-Cells)
Department of Oral Anatomy, Niigata University School of Dentistry, Gakkocho (TM., O.S.,
S.K.); and Department ofAnatomy, Niigata University School of Medicine, Asahimachi
(TI., Ill?), Niigata 951, Japan
The Ruffini endings and associated cells in the periodontal ligament
of rat incisors were investigated by means of immunohistochemistry for glia-specific
S-100 protein and electron microscopy. Numerous Ruffini endings, which were immunoreactive for S-100 protein as well as for neurofilament protein, were distributed
in the alveolus-related part of the lingual periodontal ligament. In electron microscopy, the Ruffini endings displayed expanded axoplasmic spines filled with a large
number of mitochondria and neurofilaments; some of the spines directly contacted
the surrounding collagen fibers via fingerlike projections. The axoplasmic spines
and Schwann sheath, for the most part, were covered alternately by single or
multiple layers of the basal lamina.
Several rounded cells showing S-100 immunoreactivity occurred in the vicinity of
the Ruffini endings. The rounded cells associated with Ruffhi endings possessed a
kidney-shaped nucleus and enveloped the axoplasmic spines with their cytoplasmic
processes. From these morphological features, the cells in question were identified
a s the K-cells described by Everts et al. (1977). These K-cells developed Golgi
apparatus and rough endoplasmic reticulum, suggesting active synthesis of proteins.
Immunohistochemistry at the electron microscopic level revealed a n intense immunoreactivity for S-100 protein in the cytoplasm of the K-cell and led to a conclusion
that the K-cells were terminal Schwann cells associated with R e i n i endings,
presumably corresponding to the lamellar cells in the inner bulb of sensory
Physiological studies and clinical experience have
shown that the mechanical stimuli to the periodontal
ligament induce oral reflexes, including the jaw-opening
and closing reflex and also that these oral reflexes make
regular and smooth mastication possible (Matthews,
1975). Nerves distributed in the dental pulp are responsible for pain sensation, whereas those in the periodontal ligament transmit both this and the sense of touch,
pressure, and displacement in the teeth. The periodontal
ligament has been shown physiologically to contain two
types of mechanoreceptors: slowly and rapidly adapting
receptors (Hannam, 1982). It has been suggested that
the slowly adapting receptors respond to the stretch of
periodontal fibers (Cash and Linden, 1982).
By means of immunohistochemistry for neurofilament
protein (NFP), our research group recently succeeded in
demonstrating, in the periodontal ligament of rat molars and incisors, peculiar Ruffini-like nerve endings
believed to function as stretch receptors (Maeda et al.,
1987; Sat0 et al., 1988). In particular, a characteristic
distribution of NFP-positive Ruffini endings was revealed in the rat incisors by Sat0 et al. (1988): Numerous
Ruffini endings gathered in the midregion of the lingual
periodontal ligament, whereas the labial periodontal lig0 1989 ALAN R. LISS. INC
ament contained only free nerve endings. The Ruffini
endings were found to be closely related to transverse
periodontal fibers that directly connect the tooth with
the alveolar bone. Such a distribution of endings seemed
purposeful for the mechanoreception in the periodontal
ligament, because the lingual periodontal ligament in
rodent incisors is regularly stretched during mastication.
While observing Ruffini endings in the periodontal
ligament of rat incisors under the electron microscope,
peculiar cells were found, which were associated with
the nerve terminals. These cells appeared to correspond
to the cells originally described by Beertsen et al. (1974)
and more precisely studied by Everts et al. (1977). The
latter authors recognized rounded cells closely associated with nerve terminals in the periodontal ligament
of mice incisors and termed them K-cells because of
their "kidney-shaped" nucleus. However, the origin and
functional significance of these cells remains to be
Received March 21, 1988; accepted June 10, 1988.
Address reprint requests to Takeyasu Maeda, Department of Oral
Anatomy, Niigata University School of Dentistry, Gakkocho 2, Niigata 951, Japan.
Fig. 1. The lingual periodontal ligament of the rat incisor. PAP
staining with S 100-antiserum. S-100 immunopositive neural elements
ramify in a dendritic fashion and form Ruffni endings (arrows), restricted to the alveolus-related part (AR). TR:tooth-related part. x280.
Fig. 2. High magnification of Ruffhi endings immunostained for S100 protein. Three rounded cells associated with the endings are stained
positively. X1,500.
The purpose of the present study was to reveal the
ultrastructural features of Ruffini endings in the periodontal ligament of rat incisors, with special reference to
the K-cells. In order to reveal the cellular origin of the
K-cells, we performed immunohistochemistry a t the
light and electron microscopic levels for S-100 protein,
one of the established .
solution for 3 days a t 4"C,a s previously reported (Maeda
et al., 1986).After immersion in a 30% sucrose solution
overnight, the specimens were rapidly frozen in dry iceacetone. Frozen sections, 20-40 pm thick, were cut with
a freezing microtome.
Five male Wistar rats, weighing about 150-200 g,
were used. The animals were anesthetized with a n intraperitoneal injection of pentobarbiturate (0.4 mlkg)
and perfused with Bouin's fluid via the ascending aorta.
The maxillae were removed immediately and immersed
en bloc in the same fixative for 6 hours. Following fixation, the maxillae were decalcified with Plank-Rychlo's
alveolus-related part
axoplasmic spines
basal lamina
periodontal ligament
lmmunohistochemistry for 9100 Protein
Floating sections were processed for the peroxidaseantiperoxidase (PAP) method according to Sternberger
(1979) and incubated overnight a t 4°C with a n S-100
antiserum diluted 1:1,500. The S-100 antiserum was
obtained from a rabbit injected with a n S-100 antigen
purified from bovine cerebra. A detailed characterization of this antiserum has been reported previously (Masuda et al., 1983). To check the specificity of the
immunoreactions, a n absorption test was performed using the antiserum pretreated with the antigen (10 pg/ml
diluted antiserum). The antigen-absorbed antiserum did
not stain any cellular or intercellular elements.
Conventional and lmmunohistochemical Electron Microscopy
An additional six rats were perfused with either 2.5%
glutaraldehyde in 0.05 M cacodylate buffer (pH 7.4) or,
for immunohistochemistry, a mixture of 4% paraformaldehyde and 0.5% glutaraldehyde in 0.05 M phosphate
buffer (pH 7.4). The upper incisors then were removed
and immersed in the same fixative for 6 hours. After
lmmunohistochemistry for S-100 protein
fixation, the specimens were decalcified in 4.13% EDTA2Na (ethylene diamine tetra acetic acid, disodium salt)
solution for 3 weeks at room temperature. Decalcified
tissue blocks were rinsed thoroughly overnight in the
same buffer solution and cut sagittally at a 500 pm
thickness with a vibratome (Oxford Co. Ltd.). For conventional electron microscopy, the sections were dehydrated after postfixation in 1% osmium tetraoxide for 3
hours, through a graded series of ethanol and embedded
in Epon-812. Ultrathin sections were stained with both
uranyl acetate and lead citrate and examined with a
JEM-100 S X transmission electron microscope.
Materials fixed with a mixture of paraformaldehyde
and glutaraldehyde were used for immunohistochemistry. The vibratome sections were stained with the PAP
method using the S-100 antiserum and mounted on polyL-lysine-coated glass slides. They were postfixed in 1%
osmium tetroxide for 1hour and embedded in Epon-812.
Ultrathin sections were observed in a transmission electron microscope without electron staining.
The S-100 antiserum could demonstrate clearly the
courses of nerves in the periodontal ligament of rat
incisors, as previously reported (Sat0 et al., 1988),in the
sections fixed with both Bouin’s fluid and a mixture of
paraformaldehyde and glutaraldehyde. Dense concentrations of Ruffini endings showing S-100 immunoreactivity were found in the midregion of the lingual periodontal ligament. Ruffini endings were restricted to the
alveolar half of the periodontal ligament, which was
referred to a s the alveolus-related part (Fig. 1).Observation of the cross-sectioned endings indicated that the
Schwann sheath surrounding axon terminals was selectively immunostained with the S-100 antiserum; this
finding was confirmed by immunohistochemistry at the
electron microscopic levels.
Rounded cell bodies showing intense immunoreactivity for S-100 protein frequently occurred near the axo-
Fig. 3.Electron micrograph showing an axoplasmic spine (AS) and
(K) in the collagen-rich zone of the periodontal ligament
(PL). K-cells (terminal Schwann cells) possess developed Golgi appara-
tus (arrowheads) and broad endoplasmic reticulum (arrows). The cytoplasmic process of the K-cell envelopes the nerve terminals filled with
a large number of mitochondria. ~ 9 , 3 0 0 .
two K-cells
plasmic spines of the Ruffini endings. They were rich in
cytoplasm and possessed a rounded or ovoid nucleus,
frequently showing a marked indentation, thus taking
a kidney-like shape (Fig. 2).
Electron Microscopic Observation of Ruffini Endings
Ruffhi endings in the lingual periodontal ligament
were observed by electron microscope. They were easily
identified as expanded axoplasmic spines filled with
mitochondria. The axoplasmic spines were distributed
exclusively within the collagen-rich zone. The axoplasmic spines contained a large number of neurofilaments and microtubules as well as mitochondria,
but no synaptic vesicles or multivesicular bodies.
The peculiar cells with the kidney-shaped nucleus (Kcells) were frequently recognized in the vicinity of the
expanded axoplasmic spines (Fig. 3). Their rich cytoplasm contained well-developed Golgi apparatus and
rough endoplasmic reticulum. The latter was characterized by broad intracisternal spaces that were more
electron-dense than the cytoplasm elsewhere. The cytoplasmic process of the K-cells usually extended to axoplasmic spines to envelope them (Fig. 3). Furthermore,
it was not uncommon for the broad cytoplasmic process
of K-cells to surround circumferentially the axons filled
with mitochondria or for one K-cell to hold more than
two mitochondria-rich axons (Figs. 4,5). These findings
lead us to regard the K-cells as terminal Schwann cells.
The terminal Schwann cells developed numerous caveolae on the cell membrane facing the axoplasmic spines,
whereas only a few caveolae were present on the other
cell surface (Figs. 6-8). No desmosomal junctions were
observed between the terminal Schwann cells and axoplasmic spines of the Ruffini endings. The sheath of
terminal Schwann cells was not continuous, so that axoplasmic spines were partially exposed to the surrounding basal lamina or collagen fibers through the slits of
the Schwann sheath (Figs. 6-9). The exposed portions of
the axons frequently were equipped with fingerlike projections, with longer ones penetrating into collagen bundles a t right angles (Fig. 7b). The axoplasmic spines of
Ruffini endings, together with their sheaths of terminal
Schwann cells, were covered alternately by either single
layer or multiple layers of basal lamina; the multiple
layers of the basal lamina, which usually were circumferential, were not formed evenly between the Schwann
covering and collagen fibers (Figs. 6, 7a, 9). The axoplasmic spines partially lacked the basal lamina, especially in their tip equipped with fingerlike projections
(Fig. 7b). Moreover, surrounding fibroblasts devoted any
basal lamina covering to the periodontal ligament of rat
When we observed specimens immunostained for S100 by electron microscope, terminal Schwann cells pos-
Figs. 4, 5. Electron microaaphs of K-cells (K) in the periodontal organelles. They embrace the mitochondria-rich axons (arrowheads) in
ligament of a rat incisor. The K-cells (terminal Schwann cells) are their cytoplasmic extension. In Figure 5, the cytoplasmic process surcharacterized by their kidney-shaped nucleus and well-developed cell rounds circumferentially one axon. X 7,000.
Fig. 6.An electron micrograph of branched nerve terminal in the
periodontal ligament. Several fingerlike projections (arrows) extend
through the slits of the Schwann sheath. Caveolae of Schwann sheath
(arrowheads) are more numerous on the side facing the axoplasmic
spine than on the other surface. ~ 6 , 5 0 0 .
vitz and Shore, 19781, and in the molars of rats (Byers,
1985), although some researchers did not consider the
endings they observed to be Ruffhi endings. Byers (1985)
described in detail the morphology of the Ruffini-like
endings in the periodontal ligament of rat molars with
the use of autoradiography and electron microscopy. The
ultrastructural features of the Ruffini endings observed
in the present study are virtually the same as those
described by Byers (1985). It is a common feature that
the axoplasmic spines sometimes extend fingerlike projections into the collagen zone, and these were completely free, with no Schwann cell or basal lamina
covering. Byers (1985)reported that in rat molars there
is symmetric multiple layers of basal lamina constantly
The ultrastructure of the Ruffini endings in the perio- forming between the Schwann sheath and collagen, aldontal ligament has been observed in the incisors of though this coverage of the basal lamina was incomplete
mice (Evzrts et al., 1977), in the incisors of rats (Berko- in the periodontal ligament of incisors.
sessing kidney-shaped nucleus were easily detected in
the periodontal ligament because of their electron-dense
reaction product (Fig. lob). The immunoreactive product
for S-100 protein was distributed throughout the cytoplasmic matrix, but no reaction-or a weak reaction
product-was present in the karyoplasm. Their cytoplasm contained well-developed, rough endoplasmic reticulum and mitochondria that were free from the S-100like immunoreactivity. On the other hand, the fibroblasts distributed in the periodontal ligament were negative in S-100 immunoreaction.
Fig. 7. Two axoplasmic spines (AS) in the collagen-rich zone of the
periodontal ligament (PL) covered by the cellular sheath, possibly
derived from that of the K-cell (terminal Schwann cell) and partially
by several layers of basal lamina (EL). At the tip of the axoplasmic
spine (rectangle, a), fingerlike projections directly contact the collagen
fibers at right angles (arrowheads, b). a, ~ 5 , 0 0 0b,
: ~16,000.
K-cells could be recognized in association with the
Ruffini endings in the periodontal ligament of rat incisors. Some researchers have reported these peculiar cells
in the periodontal ligament of the rat (Beertsen et al.,
1974; Berkovitz and Shore, 1978) and mouse incisors
(Everts et al., 1977). These authors pointed out that the
K-cells were closely associated with the terminal region
of nerve fibers. Berkovitz and Shore (1978) also noticed
the existence of nontypical Schwann cells surrounding
the mitochondria-rich axons in the rat incisors. The
present study confirmed the close relation of the K-cells
to the Ruffini endings; the cytoplasmic process of K-cells
directly enveloped the axoplasmic spines.
These findings have led us to propose that the K-cells
represent terminal Schwann cells. This idea is strongly
supported by the immunohistochemical staining in
which these cells were selectively immunoreactive for
glia-specific S-100protein. Terminal Schwann cells were
found to have numerous caveolae on the cell membrane,
this being a morphological feature in common with the
lamellar cells of Meissner’s and Pacinian corpuscles,
which were immunoreactive for S-100 protein (Iwanaga
et al., 1982). In the present study, we observed some
terminal Schwann cells circumferentially embracing the
mitochondria-rich asoplasmic spines with their cell body
or cell process. This figure may represent a transitional
form into the lamellated corpuscles such as Pacinian
and Meissner’s corpuscles. Berkovitz et al. (1983) found
one lamellated nerve corpuscle in the periodontal ligament of rat incisors, although other researchers (Sato et
al., 1988) including the present study, were unable to
observe such lamellated corpuscle in the same tissue.
It is notewnrthy that terminal Schwann cells in the
periodontal ligament contained well-developed cell organella, especially the Golgi apparatus and rough endoplasmic reticulum. These cytological features are
Fig. 8. An electron micrograph of a Ruffini ending, in the lingual
periodontal ligament (PL) of the rat incisor. An axoplasmic spine (AS)
containing a large number of mitochondria is surrounded by the sheath
of K-cell (terminal Schwann cell). This cellular sheath is not continuous, so that the axoplasmic spine frequently is exposed to the surrounding collagen fibers (arrows). x 12,000.
generally related to a n active synthesis of proteins. Highenzyme activities, including nonspecific cholinesterase,
were demonstrated to be localized a t the Golgi apparatus and rough endoplasmic reticulum in the lamellar
cells of Pacinian corpuscles (Ide and Saito, 1980) and cat
Ruffini endings (Toyoma, 1985). In our preliminary
study, terminal Schwann cells in the periodontal ligaments of rat incisors were shown to contain intense
cholinesterase activity in the endoplasmic reticulum
(authors’ unpublished observation). A clear enzyme activity for cholinesterase also was localized in the caveolae of the terminal Schwann cells, which predominantly
were found on the cell membrane facing the axoplasmic
spines in Ruffini endings. These findings suggest that
the caveolae in the terminal Schwann cells might be
involved in conveying these enzymes and other substances. The possibility arises that these terminal
Schwann cells may differentiate to lamellar cells of the
sensory corpuscles.
Fig. 9. A cross-sectioned axoplasmic spine (AS) of a Ruffini ending.
The axoplasmic spine lacks both the glial sheath and multiple layer of
basal lamina (BL) on the lateral side and directly contacts the collagen
fibers with fingerlike projections (arrows). ~ 9 , 0 0 0 .
Fig. 10. Immunohistochemistry for 5-100 protein at electron microscopic level. The Schwann cell and its sheath is) are specificab immunostained with S-100 antiserum. a: The axon and myelin sheath
(AX) are negative in reaction. ~ 6 , 0 0 0b:
. The cytoplasm of the K-cells
(K) is filled with electron-dense immunoreacted materials. On the
other hand, the adjacent fibroblast (F)is free from the S-100 immuno-
Little information is available on the functional significance of terminal Schwann cells in the periodontal
Ruffini endings. The existence of cells resembling periodontal terminal Schwann cells has not been reported in
the Ruffini endings of other tissues. Moreover, Byers
(1985) failed to observe the terminal Schwann cells associated with the Ruffini-like endings in the periodontal
ligament of rat molars. The functions of the terminal
Schwann cells possibly may be related to some condition
characteristic of the rodent incisor. As the incisors of
rodents grow continuously throughout life, active remodeling of the periodontal fibers should take place
there. These terminal Schwann cells may have a special
function in extending support and nutrition to nerve
terminals under such severe conditions.
This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science
and Culture, Japan (No. 63440069 and 63790412).
The authors are grateful to Professor Y. Takahashi,
Department of Neuropharmacology, Brain Research Institute, Niigata University, for his generous gift of the
antiserum and antigen against S-100 protein. We also
thank Mr. M. Hoshino for his photographic assistance.
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ultrastructure, periodontal, schwann, ending, terminal, ligament, references, rat, incisors, special, cells, ruffini
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