Heterogeneous localizations of Trk B among individual periodontal Ruffini endings in the rat incisor

The present immunocytochemical study examined the localization of Trk B, a high affinity neurotrophin receptor, in the neural elements of the periodontal ligament of the rat incisor. In light microscopy, the immunoreactivity was demonstrated in dendritic profiles in the alveolar half of the periodontal ligament. Their location and morphological features indicated that they were periodontal Ruffini endings. Occasional rounded cells associated with periodontal Ruffini endings, which had immunonegative kidney-shaped nuclei, were immunoreactive; these were judged to be terminal Schwann cells. Immunoelectron microscopy revealed the heterogeneous localization of Trk B among individual Ruffini endings. Some terminal Schwann cells contained immunoreactive products for Trk B in the cytoplasm, while others did not. Similarly, a part of the Schwann sheaths covering the axon terminals showed Trk B immunoreactivity. Most axon terminals associated with periodontal Ruffini endings were immunopositive for Trk B, though a few of them were immunonegative. The ordinary Schwann cells did not contain Trk B immunoreactive products. These findings imply that Trk B is required for the maintenance of periodontal Ruffini endings. The different expression pattern of Trk B suggests that neuronal and glial elements comprising individual periodontal Ruffini endings are subject to heterogeneous conditions with regard to the requirement of Trk B.     

Sensory receptors in the periodontal ligament of hamster incisors with special reference to the distribution, ultrastructure and three-dimensional reconstruction of Ruffini endings

The innervation of the periodontal ligament in hamster incisors was investigated by means of immunohistochemistry for nervous-specific proteins and electron microscopy. The lingual periodontal ligament was found to be exclusively innervated by Ruffini endings which appeared to be most developed in this species among rodents; the labial periodontal ligament lacked them. The Ruffini endings occupied the alveolar half of the periodontal ligament, being intertwined with transverse collagen fibers. In electron microscopy, the Ruffini endings displayed expanded axon terminals filled with large-sized mitochondria. Three-dimensional reconstructions of the Ruffini endings at the electron microscopic level revealed complicated shapes for the axon terminals and a characteristic relationship with the associated terminal Schwann cells. The axon terminals were plate- or knob-shaped, the former being predominant. Each axon terminal was covered by thick Schwann sheaths derived from more than two terminal Schwann cells whose cell bodies were located apart from the axon terminals and contained a developed Golgi apparatus and rough endoplasmic reticulum. On the other hand, each terminal Schwann cell simultaneously extended their cytoplasmic processes to several axon terminals just like astrocytes. The thick Schwann sheath, for the most part, was covered by a multiple layer of basal lamina. These findings have aided us in understanding the entire structure of the periodontal Ruffini endings.     

The periodontal Ruffini endings in brain derived neurotrophic factor (BDNF) deficient mice

Innervation and terminal morphology in the lingual periodontal ligament of the incisor were investigated in brain derived neurotrophic factor (BDNF) heterozygous mice and littermate wild-type mice (aged two months) using immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker. In addition, computer-assisted quantitative analysis was performed for a comparison of neuronal density in the periodontal ligament between heterozygous and wild-type mice. In wild-type mice, the periodontal ligament was found to be richly innervated by the mechanoreceptive Ruffini endings and nociceptive free nerve endings in the alveolus-related part of the periodontal ligament. The periodontal Ruffini endings in the wild-type mice incisor ligament were classified into two types: type I with ruffled outlines, and type II with a smooth outline. BDNF heterozygous mice showed malformations of the type I Ruffini endings which included fewer nerve fibers and fewer ramifications than those in wild-type mice as well as smooth outlines of the axon terminals. Quantitative analysis under a confocal microscope showed a roughly 18% reduction in neuronal density in the periodontal ligament of the heterozygous mice. These findings suggest that the development and maturation of the periodontal Ruffini endings require BDNF.     

Morphological and cytochemical characteristics of periodontal Ruffini ending under normal and regeneration processes

Current knowledge on the Ruffini endings, primary mechanoreceptors in the periodontal ligament is reviewed with special reference to their cytochemical features and regeneration process. Morphologically, they are characterized by extensive ramifications of expanded axonal terminals and an association with specialized Schwann cells, called lamellar or terminal Schwann cells, which are categorized, based on their histochemical properties, as non-myelin-forming Schwann cells. Following nerve injury, the periodontal Ruffini endings of the rat incisor ligament can regenerate more rapidly than Ruffini endings in other tissues. During regeneration, terminal Schwann cells associated with the periodontal Ruffini endings migrate into regions where they are never found under normal conditions. Also during regeneration, alterations in the expression level of various bioactive substances occur in both axonal and Schwann cell elements in the periodontal Ruffini endings. Neuropeptide Y, which is not detected in intact periodontal Ruffini endings, is transiently expressed in their regenerating axons. Growth-associated protein-43 (GAP-43) is expressed transiently in both axonal and Schwann cell elements during regeneration, while this protein is localized in the Schwann sheath of periodontal Ruffini endings under normal conditions. The expression of calbindin D28k and calretinin, both belonging to the buffering type of calcium-binding proteins, was delayed in periodontal Ruffini endings, compared to their morphological regeneration. As the importance of axon-Schwann cell interactions has been proposed, further investigations are needed to elucidate their molecular mechanism particularly the contribution of growth factors during the regeneration as well as development of the periodontal Ruffini endings.     

Time-related changes in periodontal mechanoreceptors in rat molars after the loss of occlusal stimuli

The effect of a loss of occlusal stimuli upon the distribution and structure of the periodontal mechanoreceptors of the rat mandibular molar was examined after extracting opposing molars. The hypofunctional periodontal ligament narrowed significantly two weeks after tooth extraction, associated with an altered morphology of the Ruffini endings that showed typical dendritic profiles in normal controls. At four weeks and later periods after extraction, the Ruffini endings-including those without light microscopic changes demonstrated unusual ultrastructural features such as the eccentric localization of mitochondria along the axonal membrane and loss of other cell organelles, unusual elongation of axonal microprojections, or a deep invagination of the Schwann sheath into the axoplasm. Immunoreactivity for the growth-associated protein-43 (GAP-43) in the Ruffini endings was restricted to the Schwann element in both the normal and hypofunctional periodontal ligament, but the reaction was weaker and even negligible in some cases in the latter ligament. The present results suggest that occlusal stimuli are essential for maintaining the structural integrity of the periodontal ligament, including that of periodontal mechanoreceptors. A decreased immunoreactivity for GAP-43 in the Schwann sheaths supports the notion of a possible functional alteration in the Ruffini endings that showed no structural abnormality.     

Innervation of the periodontal ligament in the dog with special reference to the morphology of Ruffini endings.

The distribution and terminal formation of nerves in the periodontal ligament of dog incisors and canines were investigated by immunohistochemistry for neurofilament protein (NFP) and by electron microscopy. The NFP-immunoreactive nerve fibers were found to be densely distributed in the apical third of the periodontal ligament, while they were sparse in the coronal two thirds. Most of the nerve endings in the periodontal ligament showed a tree-like appearance and resembled those nerve endings demonstrated in the periodontal ligament of human and monkey under the category of free nerve endings. Presumable axon terminals of these were slightly thicker than preterminal portions, running along periodontal collagen fibers and tapering within them. In light microscopic images, at least, they differed from the Ruffini endings which are commonly seen in rodents, displaying a glove-like configuration with extremely expanded tips. Under the electron microscope, however, the tree-like endings of the dog appeared similar to the Ruffini endings of rodents: their terminals were filled with mitochondria, covered with a cytoplasmic process of a Schwann cell, and surrounded by collagen fibers. These ultrastructural findings, combined with the results of previous physiological studies suggest that the nerve endings demonstrated in the present study can be identified as Ruffini endings. It is even stressed that the dog-type of Ruffini ending can be regarded as a representative of the sensory receptors in the mammalian periodontal ligament. In addition to these endings, knobbed endings, corpuscular (lamellated and glomerular) endings, and free nerve endings were rarely encountered in the periodontal ligament of incisors and canines of the dog.     

Postnatal expression of calretinin-immunoreactivity in periodontal Ruffini endings in the rat incisor: a comparison with protein gene product 9.5 (PGP 9.5)-immunoreactivity

The postnatal expression of immunoreactivity for calretinin, one of the calcium binding proteins, and for protein gene product 9.5 (PGP 9.5), a general neuronal marker, was investigated in mechanoreceptive Ruffini endings in the periodontal ligament of the rat incisor. Age-related changes in the expression of these two proteins in periodontal nerves were further quantified with a computerized image analysis. At 1 day after birth, a few PGP 9.5-immunoreactive nerve fibers and a still smaller number of calretinin-positive fibers were found in the periodontal ligament: they were thin and beaded in appearance and no specialized nerve terminals were recognized. Tree-like terminals, reminiscent of immature Ruffini endings, were recognizable in 4-day-old rats by PGP 9.5-immunohistochemistry, while calretinin-immunostaining failed to reveal these specialized endings. At postnatal 7-11 days when PGP 9.5-immunostaining could demonstrate typical Ruffini endings, calretinin-immunopositive nerve fibers merely tapered off without forming the Ruffini type endings. A small number of Ruffini endings showing calretinin-immunoreactivity began to occur in the periodontal ligament at 24-26 days after birth when the occlusion of the first molars had been established. At the functional occlusion stage (60-80 days after birth), the Ruffini endings showing calretinin-immunoreactivity drastically increased in number and density, but less so than those positive for PGP 9.5-immunoreaction. The delayed expression of calretinin suggests that the function of the periodontal Ruffini endings is established after the completion of terminal formation because Ca2+, which binds to calcium binding proteins including calretinin with high affinity, plays an important role in mechano-electric transduction.     

A comparative physiological and morphological study of periodontal ligament mechanoreceptors represented in the trigeminal ganglion and the mesencephalic nucleus of the cat

A correlative morphological study was carried out on two electrophysiologically identified and located periodontal ligament mechanoreceptors in anaesthetised cats. One mechanoreceptor had its cell body in the mesencephalic nucleus and the other had its cell body in the trigeminal ganglion. Physiological recordings were made from each of their cell bodies. The two receptors were located by punctate and electrical stimuli in the labial aspect of the periodontal ligament of the left mandibular canine tooth. Both receptors had similar positions relative to the tooth apex and fulcrum and were situated in the labial part of the ligament in each tooth. The receptor loci were marked, and these regions were studied in a series of semi-thin and ultra-thin sections. Only Ruffini nerve endings were observed under each ink mark. Both Ruffini nerve endings branched, were unencapsulated and were incompletely surrounded by terminal Schwann cells with extensions projecting towards collagen bundles. The results indicate that periodontal ligament mechanoreceptors with cell bodies in the mesencephalic nucleus and those with their cell bodies in the trigeminal ganglion can both be Ruffini nerve endings. Furthermore, there was no apparent morphological difference between the two periodontal ligament mechanoreceptors.     

Involvement of GDNF and its receptors in the maturation of the periodontal Ruffini endings

Our recent study revealed an intense immunoreaction for GDNF and its receptors in the Ruffini endings, primary mechanoreceptors in the periodontal ligament, of young rats. However, no information is available for the expression of GDNF and its receptors during their development. The present study aimed to reveal postnatal changes in the immuno-expression of GDNF, GFRalpha1 and RET in the periodontal Ruffini endings of the rat incisors by double immunofluorescent staining. At postnatal day 3 (PO 3d), no structure with GDNF-, GFRalpha1-, or RET-immunoreaction existed in the periodontal ligament. The PGP 9.5-positive nerve fibers without GDNF- and RET-immunoreaction displayed a dendritic fashion at PO 1w, with a GFRalpha1-reaction found around these nerves. At PO 2w, GDNF-positive terminal Schwann cells occurred near the thick and dendritic axons, a part of which showed a RET-reaction, with no reactive cells near the thin nerves. The terminal Schwann cells became positive for GFRalpha1, but lacked RET-immunoreaction. At PO 3w, when the formation of the periodontal Ruffini endings had proceeded, GDNF-positive terminal Schwann cells began to increase in number. This stage-specific immuno-expression pattern suggests that GDNF is a key molecule for the maturation and maintenance of the periodontal Ruffini endings.     

Vesicular Glutamate Transporter Immunoreactivity in the Periodontal Ligament of the Rat Incisor

The distribution of three vesicular glutamate transporter (VGluT) isoforms, VGluT1, VGluT2, and VGluT3, were investigated in the trigeminal ganglion of the periodontal ligament in the rat incisor—a receptive field of trigeminal ganglion neurons. In the trigeminal ganglion, mRNAs for all VGluT isoforms were detected and proteins were observed in the cytoplasm of trigeminal ganglion cells. VGluT1 immunoreactions were localized within the cytoplasm for all sizes of trigeminal neurons, although predominately in medium–large trigeminal neurons. Double‐labeling showed that most VGluT1 contained both VGluT2 and VGluT3. In the periodontal ligament of the incisor, the Ruffini endings, principal periodontal mechanoreceptors, displayed VGluT1 and VGluT2 immunoreactivities. However, lacked immunoreactions for VGluT3. At the electron microscopic level, VGluT1 immunoreactions were localized around the vesicle membranes at the axon terminal of Ruffini endings. The present results indicate that VGluT is expressed in the sensory nerve endings where apparent synapses are not present. Thus, glutamate in the sensory nerve endings is thought to be used in metabotropic functions. This is because glutamate is a general metabolic substrate, and/or acts as a neurotransmitter as proposed in muscle spindles. Anat Rec, 2012. © 2011 Wiley Periodicals, Inc.     

Morphological basis of periodontal nerve endings

The periodontal ligament, a dense collagenous tissue between tooth and alveolar bone, receives a rich sensory nerve supply, and contains two kinds of sensory receptors; nociceptor and mechanoreceptor. The mechanical stimuli for teeth can evoke various oral reflexes, which facilitate mastication via the periodontal mechanoreceptors. In spite of many reports on the periodontal sensory receptors, recent studies have revealed that the Ruffini endings, categorized as low-threshold slowly adapting type II (SA II) stretch receptors, are primary mechanoreceptors in the periodontal ligament. This paper summarized recent findings on the morphological features and developmental aspects of the periodontal Ruffini endings.     

Involvement of brain-derived neurotrophic factor (BDNF) in the development of periodontal Ruffini endings

The periodontal Ruffini ending has been reported to show immunoreactivity for tyrosine kinase B (trkB), the high-affinity receptor for brain-derived neurotrophic factor (BDNF), in the periodontal ligament of the rat incisor. Furthermore, adult heterozygous BDNF-mutant mice showed malformation and reduction of the periodontal Ruffini endings. To investigate further roles of BDNF in these structures, the development, distribution, and terminal morphology of Ruffini endings were examined in the incisor periodontal ligament of heterozygous and homozygous BDNF mutant mice, as well as in the wild-type littermate by immunohistochemistry for protein gene product (PGP) 9.5, a general neuronal marker. A similar distribution and terminal formation of PGP 9.5-immunoreactive nerve fibers was recognized in the periodontal ligament of all phenotypes at postnatal week (PW) 1. At this stage, the nerve fibers had a beaded appearance, but did not form the periodontal Ruffini endings. At PW2, the heterozygous and wild-type mice started to show ramified nerve fibers resembling the mature shape of periodontal Ruffini endings. At PW3, the Ruffini endings occurred in the periodontal ligament of the wild-type and heterozygous mice. While the Ruffini endings of the wild-type mice appeared either ruffled or smooth, as reported previously, most of these structures showed a smooth outline in the heterozygous mice. The homozygous mice lacked the typical Ruffini endings at PW3. In the quantitative analysis, homozygous mice had the smallest percentages of PGP 9.5-immunoreactive areas at the same postnatal periods, but there were no significant differences between wild-type and heterozygous mice during PW1-3. These findings suggest a possible involvement of BDNF during the postnatal development and, in particular, the maturation of periodontal Ruffini endings. Furthermore, other neurotrophins may play a role in the development and/or early maturation of the periodontal nerve fibers, as indicated by the presence of nerve fibers in the BDNF-homozygous mice.     

Delayed expression of calbindin D28k during regeneration of the periodontal Ruffini endings of the rat incisor following injury to the inferior alveolar nerve

Expression of calbindin D28k (CB)-like immunoreactivity (-LI) was compared with that of protein gene product 9.5 (PGP 9.5), a general neuronal marker, in the periodontal ligament of the rat lower incisor following resection of the inferior alveolar nerve (IAN). In normal animals, the periodontal nerve fibers showing PGP 9.5-LI formed either Ruffini endings with expanded arborization or thin free nerve endings in the alveolar half of the ligament. Thick CB-like immunoreactive (-IR) nerve fibers terminated in a dendritic fashion in the same region, but thin CB-IR nerve fibers were rarely detected. During the 3 days following resection of the IAN, most of the PGP 9.5-IR and all CB-IR nerve fibers disappeared. Regenerated PGP 9.5-IR nerve fibers appeared around 7 days after resection, in contrast to the very small number of regenerated CB-IR nerve fibers. Around 21-28 days following resection, the number and terminal morphology of regenerated PGP 9.5-IR nerve fibers were comparable to those observed in normal animals, but the number of regenerated CB-IR nerve fibers was still smaller. The terminal morphologies of these regenerated CB-IR nerve fibers showed less expansion compared with normal animals at these post-injured periods. The number of regenerated CB-IR nerve fibers increased gradually to return to normal by 56 days following injury. The delayed expression of CB in the regenerated periodontal Ruffini endings suggests that the functional recovery of periodontal Ruffini endings occurred after the regeneration of periodontal Ruffini endings had been completed.     

Neurotrophin-4/5-depletion induces a delay in maturation of the periodontal Ruffini endings in mice

Neurotrophin-4/5 (NT-4/5) - a member of the neurotrophic factors - is a ligand for TrkB, which has been reported to be expressed in the mechanoreceptive Ruffini endings of the periodontal ligament. The present study examined developmental changes in the terminal morphology and neural density in homozygous mice with a targeted disruption of the nt-4/5 gene and wild-type mice by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker, and by quantitative analysis using an image analyzer. Postnatal development of terminal Schwann cells was also investigated by enzymatic histochemistry for non-specific cholinesterase activity (ChE). Furthermore, the immuno-expression of TrkB and low affinity nerve growth factor receptor (p75-NGFR) was surveyed in the periodontal Ruffini endings as well as trigeminal ganglion. At postnatal 1 week, the lingual periodontal ligament of both types of mice contained PGP 9.5-positive nerve fibers showing a tree-like ramification with axonal swellings in their course. In both types of mice at 2 weeks of age, comparatively thick nerve fibers with a smooth outline increased in number, and frequently ramified to form nerve terminals with dendritic profiles. However, no typical Ruffini endings with irregular outlines observed in the adult wild-type mice were found in the periodontal ligament at this stage. At postnatal 3 weeks, typical Ruffini endings with irregular outlines were discernable in the periodontal ligament of the wild-type mice while the dendritic endings showing smooth outlines were restricted to the homozygous mice. After postnatal 8 weeks, both types of mice showed an increase in the number of Ruffini endings, but the morphology differed between the wild-type and NT-4/5 homozygous mice. In the wild-type mice, a major population of the Ruffini endings expanded their axonal branches and developed many microprojections, resulting in a reduction of endings with smooth outlines. In contrast, we failed to find such typical Ruffini endings in the periodontal ligament of the homozygous mice: A majority of the periodontal Ruffini endings continued to show smooth outlines at postnatal 12 weeks. Quantitative analysis on neural density demonstrated a reduction in the homozygous mice with a significant difference by postnatal 8 weeks. Enzymatic histochemistry for non-specific ChE did not exhibit a distinct difference in the distribution and density of terminal Schwann cells between wild-type and homozygous mice. Furthermore, TrkB and p75-NGFR mRNA and proteins did not differ in the trigeminal ganglion between the two types. The periodontal Ruffini endings also displayed immunoreactivities for TrkB and p75- NGFR in both phenotypes. These findings suggest that the nt-4/5 gene depletion caused a delay in the formation and maturation of the periodontal Ruffini endings in the mice by inhibiting the growth of the periodontal nerves at an early stage, and indicate that multiple neurotrophins such as NT- 4/5 and BDNF might play roles in the development and/or maturation of the periodontal Ruffini endings.     

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

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 Ruffini 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 as 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 an 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 Ruffini endings, presumably corresponding to the lamellar cells in the inner bulb of sensory corpuscles.     

Differential cell-specific location of Cav-1 and Ca2+-ATPase in terminal Schwann cells and mechanoreceptive Ruffini endings in the periodontal ligament of the rat incisor

Caveolae are involved in clathrin-independent endocytosis, transcytosis, signal transduction, and tumor suppression – all of which depend on their main constituent protein caveolin families. The periodontal Ruffini ending has been reported to develop a caveola-like structure on the cell membrane of both the axon terminals and Schwann sheaths, suggesting the existence of an axon–Schwann cell interaction in the periodontal Ruffini endings. However, little information is available concerning the functional significance of these caveolae. The present study was undertaken to examine the immunolocalization of caveolin-1, -3 (Cav-1, Cav-3) and Ca²⁺-ATPase in the periodontal Ruffini endings of the rat incisor. Decalcified sections of the upper jaws were processed for immunocytochemistry at the levels of light and electron microscopy. Some immunostained sections were treated with histochemistry for nonspecific cholinesterase (nChE) activity. Observations showed the periodontal Ruffini endings were immunopositive for Cav-1, but not Cav-3. Immunoreactive products for Cav-1 were confined to caveola-like structures in the cell membranes of the cytoplasmic extensions and cell bodies of the terminal Schwann cells associated with the periodontal Ruffini endings. However, the axonal membranes of the terminals did not express any Cav-1 immunoreaction. Double staining with Ca²⁺-ATPase and either protein gene product 9.5 (PGP 9.5) or S-100 protein disclosed the co-localization of immunoreactions in the axonal branches of the periodontal Ruffini endings, but not in the terminal Schwann cells. As Ca²⁺ plays an important role in mechanotransduction, these characteristic immunolocalizations show Cav-1/Ca²⁺-ATPase might be involved in the quick elimination of intracellular Ca²⁺ in mechanotransduction.     

Cholinesterase activity in terminal Schwann cells associated with Ruffini endings in the periodontal ligament of rat incisors

A nonspecific cholinesterase activity was demonstrated in terminal Schwann cells associated with Ruffini endings in the periodontal ligament of rat incisors at the light and electron microscopic levels. The terminal Schwann cells are ultrastructurally characterized by a well-developed Golgi apparatus and rough endoplasmic reticulum. The cells in this study were positive for nonspecific cholinesterase, whereas ordinary Schwann cells associated with more proximal nerve fibers reacted negatively. The reaction products were densely deposited in the cisternae of the rough endoplasmic reticulum and along the nuclear envelop. A moderately intense labeling was found in the cytoplasmic extensions, in which the reaction products gathered in caveolae and vesicles. These findings indicate that nonspecific cholinesterase is a useful marker to distinguish terminal Schwann cells from ordinary Schwann cells and that the enzyme may be synthesized in the rough endoplasmic reticulum and conveyed toward the axon terminals. Since this enzyme has been known to be shared by the inner bulb of Pacinian corpuscles and the lamellar cells of Meissner's corpuscles, its possible involvement in mechanoreceptive functions in these specialized Schwann cells deserves further investigation.     

Immunohistochemical localization of laminin in the periodontal Ruffini endings of rat incisors: a possible function of terminal Schwann cells

Ruffini endings in the periodontal ligament of rodents are ensheathed by a special type of terminal Schwann cell with a particularly developed rough endoplasmic reticulum and Golgi apparatus, and further enveloped by a characteristic multi-layered structure. In order to reveal the functional significance of the structures, localization of a laminin molecule in the periodontal Ruffini endings of rats was immunohistochemically investigated at the levels of light and electron microscopy. Immunostaining using an anti-laminin serum clearly demonstrated the profiles of the Ruffini endings as well as those of the blood vessels. Ultrastructurally, reaction products for laminin were deposited in the entire thickness of the multi-layered structure, supporting the idea that this structure is derived from the basal lamina. The basal lamina, immunoreacting with laminin antiserum, was penetrated by periodontal collagen fibers, possibly serving as an adhesive device between the Ruffini endings and surrounding collagen fibers. The laminin immunoreactive materials were also recognized in the vesicles and caveolae of the terminal Schwann cells which tended to gather at the interstitial surface of the cells. The terminal Schwann cells are therefore believed to be directly involved in the formation of the multilayered basal lamina through the active production of its materials.     

The involvement of brain-derived neurotrophic factor (BDNF) in the regeneration of periodontal Ruffini endings following transection of the inferior alveolar nerve

The present study employed immunohistochemistry for protein gene product 9.5 (PGP 9.5) to examine the regeneration process of Ruffini endings, the primary mechanoreceptor in the periodontal ligament, in heterozygous mice with targeted disruption of the brain-derived neurotrophic factor (BDNF) gene and their littermates, following transection of the inferior alveolar nerve. When immunostained for PGP 9.5, periodontal Ruffini endings appeared densely distributed in the periodontal ligament of the heterozygous mice, but the density of the positively stained nerve fibers in the ligament was 20% lower than that in the control littermates. At 3 days after surgery, the PGP 9.5-positive neural elements had disappeared; they began to appear in the periodontal ligament of both animals at 7 days. However, the recovery pattern of the PGP 9.5-positive nerves differed between heterozygous and wild type mice, typical periodontal Ruffini endings morphologically identical to those in the control group appeared in the wild-type mice at 7 days, whereas such Ruffini endings were detectable in the heterozygous mice at 28 days, though much smaller in number. On day 28, when PGP 9.5-positive nerves were largely regenerated in wild type mice, their distribution was much less dense in the ligament of the heterozygous mice than in the non-treated heterozygous mice. The density of PGP 9.5-positive nerve fibers was significantly lower in the heterozygous mice than in wild type mice at any stage examined. These data showing that a reduced expression of BDNF causes delayed regeneration of the periodontal Ruffini endings suggest the involvement of BDNF in the regeneration process of these mechanoreceptors.