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Made available in DSpace on 2022-04-29T08:31:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-10-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Objectives: The present study aimed to compare two different models of orthodontic tooth movement (OTM) in rats by evaluating tooth movement efficiency and periodontal tissues remodelling. Design: Fifteen animals were randomly distributed into 3 groups: control group (untreated); ligature appliance (LA) as experimental OTM using a closed coil spring fixed around maxillary first molar by steel ligature; occlusal appliance (OA) as experimental OTM using a closed coil spring attached on the occlusal surface of the maxillary first molar. After 15 days, all animals were euthanized, and the maxilla of each animal was collected for qPCR, micro-computed tomography, and histological analyses. Results: Interleukin-1 beta, interleukin-6, and tumor necrosis factor-alpha gene expressions were significantly upregulated in the animals of the LA group as compared to the other groups. No significant difference was observed in tooth displacement between both methods. The LA group presented higher linear bone loss and lower values of bone volume fraction, bone mineral density, trabecular number and increased values of trabecular separation compared to the other groups. The birefringent collagen content in the tension side of the periodontal ligament contained significantly lower collagen content in the LA group than in the control group. Furthermore, on the pressure side, the collagen content was significantly lower in the LA and OA groups than in the control group. Conclusions: The OA group presented little or no deleterious effect on periodontal tissues compared to the LA group, suggesting its use may be more reliable for OTM induction in rats for 15 days. Department of Diagnosis and Surgery School of Dentistry at Araraquara São Paulo State University - UNESP Department of Periodontology and Operative Dentistry University Medical Center of the Johannes Gutenberg University Department of Morphology Genetics Orthodontics and Pediatric Dentistry School of Dentistry at Araraquara São Paulo State University - UNESP Department of Diagnosis and Surgery School of Dentistry at Araraquara São Paulo State University - UNESP Department of Morphology Genetics Orthodontics and Pediatric Dentistry School of Dentistry at Araraquara São Paulo State University - UNESP FAPESP: 2017/07137-2

Objective This study aimed to conduct a systematic review of the use of a cell sheet formed by mesenchymal stem cells derived from dental tissues (ddMSCs) for periodontal tissue regeneration in animal models in comparison with any other type of regenerative treatment. Design PubMed and Scopus databases were searched for relevant studies up to December 2020. The review was conducted based on the Preferred Reporting Items for Systematic Reviews and Meta-analysis guidelines. Results Of the 1542 potentially relevant articles initially identified, 33 fulfilled the eligibility criteria and were considered for this review. Even with a wide variety of selected study methods, the periodontal tissue was always regenerated; this indicates the potential for the use of these cell sheets in the future of periodontics. However, this regeneration process is not always complete. Conclusion Despite the implantation, ddMSCs sheets have a great potential to be used in the regeneration of periodontal tissue. More in vivo studies should be conducted using standardized techniques for cell sheet implantation to obtain more robust evidence of the relevance of using this modality of cell therapy for periodontal tissue regeneration.

Objective Oral biofilms burden host responses by induction of inflammatory mediators, exacerbating periodontal inflammation. Photobiomodulation Therapy (PBMT) has been shown to decrease levels of pro-inflammatory cytokines and chemokines. However, optimal wavelengths and exposure doses have not been established. This study investigated the effects of PBMT on human periodontal ligament fibroblasts (hPDLFs) stimulated with inflammatory mediators (LPS, TNF-α, and IL-1β). Methods Cytotoxic effects of laser wavelengths 660 nm and 810 nm were assessed by measuring their effects on cellular dehydrogenase activity. The study was expanded to include 980 nm, 660 nm + 810 nm, and 810 nm + 980 nm. P.g. LPS, TNF-α, and/or IL-1β were added one hour before irradiation, then exposed to laser irradiation to determine the most appropriate stimulus. The levels of INF-γ, IL-6, IL-8, IL-17A/F, and MCP-1 production in stimulated hPDLFs were measured and analyzed. Results P.g. LPS was a poor stimulus for hPDLFs, while TNF-α and IL-1β significantly elevated the analytes. The 660 nm laser treatment induced pro-inflammatory cytokines when stimulated, while 810 nm exhibited significant suppression. IL-1β was the stimulus of choice and the 810 nm wavelength alone exhibited anti-inflammatory effects for all analytes except IL-8, while the 810 nm in combination with 660 nm and/or 980 nm exhibited effects similar to 810 nm alone. Conclusions The downregulation of inflammatory mediators by the combination or individual treatment with 810 nm wavelength shows promise for the management of periodontal inflammation. PBMT may lead to the development of a novel approach in the management of periodontal disease.

Made available in DSpace on 2020-12-12T02:08:49Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-08-01 Objective: The aim of this study was to compare the periodontal tissue changes resulting from different methods of orthodontic tooth extrusion in dogs. Materials and methods: Notches were surgically prepared in the root surface at the bone crest level of the first premolars of mongrel dogs. After 37 days, extrusion of the first lower and upper premolars was randomly performed by 3 different methods: conventional orthodontic extrusion (OE); open flap debridement performed immediately before orthodontic extrusion (OF); and orthodontic extrusion associated with weekly fiberotomy and scaling (FS). For all groups, extrusion was performed for 21 days followed by one-month retention and sacrifice. Periodontal parameters, descriptive histology, and histomorphometric analyses were performed at the end of the experimental period. Results: The median extrusion was 2.25 in the fiberotomy group, 2.0 mm in the open flap group and 1.0 mm in the orthodontic extrusion group with no significant differences between groups. The highest distance between reference notch and bone crest was observed in the fiberotomy group (p < 0.05). Histologically, radicular resorption repaired with cellular cementum was detected in all groups. Conclusions: Tooth extrusion was successfully achieved with all of the different methods of orthodontic tooth extrusion with no statistical significance between techniques. The fiberotomy approach was effective in avoiding coronal displacement of periodontal tissues. Fiberotomy associated with scaling should be indicated if the objective of the treatment is extrusion without periodontal tissue displacement. Department of Dentistry II School of Dentistry at São Luís Maranhão Federal University – UFMA Department of Diagnostic and Surgery School of Dentistry at Araraquara Universidade Estadual Paulista (UNESP) Private Practice of Orthodontics Educational Foundation of Barretos – UNIFEB Section of Periodontology Department of Dentistry and Oral Health Aarhus University Department of Physiology and Pathology School of Dentistry at Araraquara Universidade Estadual Paulista (UNESP) Department of Diagnostic and Surgery School of Dentistry at Araraquara Universidade Estadual Paulista (UNESP) Department of Physiology and Pathology School of Dentistry at Araraquara Universidade Estadual Paulista (UNESP)

Objective This controlled split-mouth study aimed to estimate the effect of caries and related treatment on concentrations of interleukin (IL)-2, interferon (IFN)-γ, IL-12, IL-17A, IL-13, IL-10, IL-6, IL-5, IL-4, IL-22, tumor necrosis factor (TNF)-α, and IL1-β in gingival crevicular fluid (GCF) of caries affected teeth before (B), 7 (7D) and 30 (30D) days post-treatment and to compare them with concentrations from healthy teeth. Design Study population included 81 systemically and periodontally healthy non-smokers exhibiting at least one shallow occlusal/ inter-proximal caries and one healthy tooth from the same morphologic group at the contralateral position. Following clinical exam, the GCF samples were collected baseline as well as 7D and 30D, while the biomarker measurement was performed using multiplex flowcytometry. Results Caries affected teeth exhibited significantly higher levels of IFN-γ, IL-1β, IL-2, IL-4 and IL-6 when compared to healthy teeth. Post-treatment cytokines levels showed general trend of increase when compared to baseline, that was significant for IL-22 and IL-17 at 7D, while IFN-γ was significantly increased at 7D compared to the healthy teeth. At 30D, IFN-γ, TNF-α, IL-17 and IL-4 levels were significantly increased when compared to healthy teeth, while IL-2 levels were significantly higher than baseline levels. Conclusion Considering significantly increased periodontal levels of inflammatory markers in caries affected teeth and in response to performed treatment, it seems that dental caries and related restorative treatment might contribute to periodontal inflammation via additive effects already in early-stage caries.

a b s t r a c t Objective: Orthodontic tooth movement (OTM) is achieved by alveolar bone remodelling induced by mechanical loading. Whilst interleukin-1 (IL-1) is directly involved in OTM, the role of interleukin-1 receptor antagonist (IL-1Ra), a naturally occurring IL-1 antagonist, is not completely defined. Therefore, the aim of this study was to investigate the effects of IL-1Ra on OTM. Methods: An orthodontic appliance was placed in C57BL6 mice treated with vehicle or IL-1Ra (10 mg/kg/day). OTM and TRAP-positive osteoclasts were evaluated after 12 days of mechanical loading and the levels of cytokines on periodontal tissues were analysed by ELISA after 12 and 72 h. Results: Mice treated with IL-1Ra showed diminished OTM and decreased numbers of TRAPpositive osteoclasts. In line with this, lower levels of IL-1b and TNF-a, and higher levels of IL10, were observed on periodontal tissues of IL-1Ra-treated mice in relation to the vehicletreated group. Conclusion: The present study suggests that IL-1Ra downregulates OTM, probably by its antiinflammatory actions.

Matrix proteoglycans define matrix structure, mineralization, and resulting biomechanics of tissues and their attachment sites.We therefore investigated physical and (bio)chemical differences in enamel and periodontal tissues/attachment sites from mice that lack a specific nanoscale small leucine-rich proteoglycan (SLRPs) named biglycan (BGN).Experimental groups consisted of N=4, biglycan knockout (BGNKO) and N=5 wildtype (WT) 8-week-old, male C3H mice. Morphology, histochemical and mechanical analyses were performed through micro X-ray computed tomography (Micro XCT™), immunohistochemistry, and microindentation. Unless mentioned otherwise, all differences between BGNKO and WT were demonstrated to be statistically significant through Student's t-tests with a 95% confidence interval (P≤0.05).Histomorphometry performed by using Micro XCT™ images indicated significantly higher BGNKO-enamel (0.46 ± 0.03mm(3)) and BGNKO-root (1.81 ± 0.10mm(3)) volumes compared to WT-enamel (0.37 ± 0.02mm(3)) and WT-root (1.65 ± 0.07mm(3)). BGNKO tooth size was relatively larger than WT mice, with no significant difference between skull sizes. Immunohistochemistry indicated BGN expression in the periodontal ligament (PDL), alveolar bone (AB), at the bone-PDL and cementum-PDL attachment sites in WT mice. Deeper AB resorption pits within interdental region of BGNKO specimens compared to WT resulting in significant differences in PDL-space of BGNKO (93 ± 13μm) and WT (74 ± 11μm) were observed. Microhardness of BGNKO-enamel (2.46 ± 0.60GPa) and BGNKO-AB (0.52 ± 0.10GPa) was significantly lower than WT-enamel (2.67 ± 0.60GPa) and WT-AB (0.54 ± 0.10GPa).Results indicate that BGNKO-mice exhibit significant differences in tissue properties compared to WT-mice.

Oral epidemiologic investigations in China western territory have showed that the immigrants in the plateau have a higher morbidity with periodontitis. To find the possible relationship between the pathogenesis of periodontitis and altitude hypoxia, we designed a periodontitis rat model via housed in low pressure oxygen chamber and investigated the bacterial diversity in the gingival crevicular fluid (GCF).Eighty Sprague-Dawley rats were divided into CON-normal, CON-hypoxia, EXP-normal and EXP-hypoxia group, without or with periodontal induce, breeding in normal environment or altitude hypoxia environment. Periodontal parameters (including gingival index, GI, and tooth mobility, TM) were measured after 2, 4, 6 and 8 weeks; periodontal samples were collected for histological analysis after rats were sacrificed at the 8th week. The 16S rDNA of bacteria in GCF was amplified by PCR at the 8th week, and separated by the denaturing gradient gel electrophoresis (DGGE) approach.EXP-hypoxia group's GI and TM showed later and more severe periodontal tissue damage than EXP-normal (p0.05 or 0.01). The histologic analyses did not find any pathologic difference between EXP-hypoxia and EXP-normal groups except for a slight difference on the lesion degree. By the DGGE analyses, the bacteria of five samples in the same group showed high concordance, but the bacteria in the different groups showed a great diversity.The course of periodontitis in altitude hypoxia environment is later than normal, but the degree of periodontal lesion was more severe and microbial community in GCF can be affected by the altitude hypoxia environment.

Objective MUC1 is primarily involved in the protection of epithelial surfaces. Decreases in oral mucosal defence can be a predisposing factor for the development of oral mucosal diseases in the elderly. The aim of this study was to compare MUC1 expression level in oral mucosal epithelial cells of the elderly with that of young adults. Design Thirty elderly (mean age, 71.1 ± 4.6 years) and thirty young (mean age, 26.4 ± 2.4 years) adults (15 men and 15 women in each group) were included. Oral examination, including tooth, periodontal, and oral mucosal status, was performed and whole saliva samples were collected along with flow rate measurements. Precipitates of stimulated whole saliva were used for the evaluation of MUC1 expression using real-time PCR. Clarified supernatants were used for the measurement of pro-inflammatory cytokines, IL-1β, IL-6, and TNF-α. Results There was a significant decrease in the amounts of MUC1 transcripts in elderly subjects compared with those of young subjects, a result seen in both men (0.589-fold) and women (0.547-fold). The MUC1 expression level was not correlated with salivary cytokine level but did show a significant positive correlation with the level of periodontal inflammation (rs = 0.505, P Conclusions Oral mucosal defence provided by MUC1 was decreased in the elderly; this decrease may play a role in the development of oral mucosal diseases in the aged population.

Objectives Prostaglandin E 2 (PGE 2 ) is an important factor in the pathogenesis of periodontal disease because of bone resorbing activity and association with attachment loss. PGE 2 and PGE receptor subtypes (EPs) play an important role in modulating bone metabolism via osteoblasts. However, little is known about the effects of PGE 2 on cementoblasts. The aims of this study were to determine the expression of EPs in mature cementoblasts, and to examine the effect of PGE 2 and EPs on their cellular function. Design Expression of EPs in immortalized mouse cementoblasts (OCCM-30 cells), which were characterized as mature cementoblasts, was determined using reverse transcriptase polymerase chain reaction (RT-PCR). The effects of PGE 2 and EP agonists on mineralization were examined by studying nodule formation with alizarin red S staining. Alkaline phosphatase (ALP) activity with PGE 2 and EP4 agonist was examined using the Bessey–Lowry enzymologic method. Effects of the PGE 2 -EP4 pathway on expression levels of osteocalcin (OCN) and matrix metalloproteinase-13 (MMP-13) mRNA were examined using real-time RT-PCR. Results OCCM-30 cells expressed EP1, EP2, EP3 and EP4 mRNA. PGE 2 and EP4 agonist led to downregulation of mineralized nodule formation and ALP activity in OCCM-30 cells. OCN mRNA expression was suppressed and MMP-13 mRNA expression was stimulated via the PGE 2 -EP4 pathway in OCCM-30 cells. Conclusions Cementoblasts may downregulate their mineralization ability and upregulate MMP-13 production through the PGE 2 -EP4 pathway, and may contribute to destruction of connective tissue attachment under inflammatory conditions.

The purpose was to examine the effect of brief intrusive forces on human pulpal blood flow (PBF). Laser Doppler flowmetric measurements were made from 17 vital upper left central incisors of 17 participants who had clinically healthy tooth crowns and periodontal tissues. Brief intrusive forces (0.5,1,5 N; duration 20 s) were applied to the incisal edges of the examined teeth, and apical displacement of the teeth and the PBF were measured simultaneously. Recordings were made with and without an opaque rubber dam applied to the examined teeth. Intrusive force significantly reduced PBF flux both with and without the dam (P0.05, Friedman analysis). The results indicate that transient apical displacement can reduce PBF.

In 5-week-old Brookhaven National Laboratory short-lived mice, [3H]-galactose was utilized in all the oral tissues studied. Uptake and turnover of the tracer assessed by autoradiography revealed three uptake peaks. Synchronous fluctuation of total grain counts was repeatedly observed in different periodontal tissues. Acid glycosaminoglycans were strongly labelled and the neutral glycosaminoglycan fraction of the tissues was labelled to a lesser degree. The radiotracer became incorporated during their synthesis. The complex plots derived may represent several metabolic events occurring simultaneously. The uptake of radiotracer in fibrogenic, osteogenic and cementogenic cells was low. Accumulation of matrical output, however, was significant. The matrical output of the cementogenic cells and the osteogenic layer mesial to the alveolar bone was the highest of all the oral tissues studied throughout the 30-day period.

Lymphocytes isolated from chronically-inflamed tissues were assessed for their ability to produce lymphokines without further stimulation in vitro. Cells were extracted from tissue obtained from 42 patients undergoing periodontal surgery. Cultures were set up in triplicate and supernatants collected after 48 h were assayed for interleukin-2 (IL-2) or bone-resorptive activity. IL-2 was assayed in the cultured supernatants from 20 patients using a previously-standardized T-cell growth assay, with maximally-stimulated peripheral blood-lymphocytes as a positive control. Bone-resorptive activity (BRA) was assessed in culture supernatants from another 22 patients using an in-vitro mouse-calvaria culture-system in which calcium release was measured with a calcium analyser. IL-2 was detected in 12 out of the 20 unstimulated cultures; BRA was detected in 14 of the 22 unstimulated cultures. There appeared to be no relationship between IL-2 production and BRA and the severity of the disease as assessed by loss of attachment. Nevertheless, it seems that most of the cells extracted from chronically-inflamed tissue were producing lymphokines which may indicate stimulation in vivo prior to cell extraction.

The metabolic activity of collagen in rat molar periodontal tissues was measured by following the incorporation of [ 3 H]-proline into hydroxyproline. The sensitivity of the assay method allowed the rates of collagen to be determined in tissue samples taken from individual animals. The rate of synthesis of collagen in periodontal ligament was twice as fast as attached gingiva, four times as fast as skin corium, used as a reference tissue, and six times as fast as alveolar bone. Incorporation of proline label into mature collagen was highest in periodontal ligament, which had a rate five times faster than attached gingiva, six times faster than the bone and 15 times faster than skin. A comparison between the rates of incorporation of label into newly synthesized collagen and into mature collagen in each tissue indicated that newly synthesized collagen in periodontal ligament and bone was quantitatively converted into insoluble collagen, but in attached gingiva and in skin a conversion efficiency of 50 and 33 per cent respectively was found. The half-lives of newly synthesized collagen were calculated as 24 min for alveolar bone, 40 min for periodontal ligament, 80 min for gingiva and 360 min for skin. The half-lives of the turnover of the mature collagen were 1 day in periodontal ligament, 5 days in gingiva, 6 days in alveolar bone and 15 days in skin corium. The mature collagen half-lives were generally shorter than published values and suggest that recycling of labelled amino acids may affect results using conventional methods. The incorporation of [ 3 H]-proline into non-collagenous proteins soluble in 0.45 M sodium chloride showed an overall low metabolic activity for these proteins compared to collagen. However, there is some evidence of highly active non-collagenous proline-containing components insoluble in 0.45 M sodium chloride salt and 0.5 M acetic acid.

Fibroblasts and epithelial cells from periodontal tissues were grown in monolayer culture and the culture media analysed for collagenolytic enzyme activity and inhibitors of these activities. Collagenase was detected only in gingival fibroblast cultures whereas nonspecific and specific gelatinase activities were present in both ligament and gingival fibroblast cultures. The collagenase and non-specific gelatinase enzymes were present in the culture media in latent forms that could be activated by organo-mercurials or trypsinization. Proteins capable of inhibiting these enzymes were synthesized by gingival fibroblasts and ligament epithelial cells. The inhibitory activity was blocked by organo-mercurials or the trypsin treatment used to activate the latent enzymes, suggesting that the latent enzymes are enzyme-inhibitor complexes. High mol. wt inhibitory activity against the same enzymes was also produced by the ligament fibroblasts. This activity was not affected by organo-mercurials or trypsinization. This type of inhibition may explain the inability to detect latent collagenase activity in some fibroblast cultures.

The enzymes of the glycolytic cycle: phosphohexoisomerase, aldolase, phosphoglycerokinase and glucose-6-phosphatase were determined in the gingiva, periosteum and periodontium of the mandibular incisor region in the guinea pig and compared with their activities in the liver, kidneys, adrenals, pituitary and testes of the same animal. The enzymatic activities of the periodontal tissues were generally high. The phosphohexoisomerase and the aldolase activity in periosteum and periodontium was as high as in the other tissues. The phosphoglycerokinase activity of periosteum and gingiva were higher than in the other tissues. The glucose-6-phosphatase activity in periosteum and periodontium was about half as high as the liver activity.

The response of periodontal mechanoreceptor units was observed following changes in the rate of application of force to the teeth of anaesthetized dogs. In addition, short duration mechanical pulses were applied to the teeth in order to compare the thresholds of the units to mechanical stimulation. The behaviour of 60 units were studied, and all proved sensitive to changes in the rate of application of the stimulus. The rapidly adapting units responded only when the rate exceeded a critical value, and the latency of their response decreased when the rate of the stimulus was increased. There was considerable variation in the dynamic sensitivities of the slowly adapting units. However, of this group, those showing spontaneous activity were the least sensitive to dynamic forces. The thresholds to mechanical stimulation of the rapidly adapting units were consistently high, and those of the slowly adapting units showing spontaneous activity were consistently low. It is suggested that the variation in the nature of periodontal mechanoreceptor activity may be due simply to the spatial location of the receptors within the periodontal tissues.

Histochemical study of oxidative enzymes in tooth supporting tissues during tooth movement was made. Periodontal membrane adjacent to cementum in orthodontic conditions showed the same activity of oxidative enzymes as in normal periodontal membrane, but calcifying periodontal membrane in bone formation phase showed a fairly intense activity of lactate, malate, isocitrate and glucose-6-phosphate dehydrogenases. Osteoblasts associated with bone formation revealed a fairly intense activity of lactate, malate, isocitrate and glucose-6-phosphate dehydrogenases and a moderate activity of succinic dehydrogenase. Osteoclasts showed a strong activity of succinate, lactate, malate dehydrogenases, and a moderate activity of isocitrate and glucose-6-phosphate dehydrogenases. A bone formation-destruction pattern during tooth movement was characteristically detected in the present study.

The purpose of this study was to investigate the correlation of unstimulated parotid salivary IgA concentration and rate of secretion with degree of periodontal inflammation. Radial immunodiffusion was utilized to estimate the IgA levels in unstimulated parotid saliva from 54 systemically healthy adults. Twelve subjects were edentulous and free of any apparent oral inflammation. The other 42 subjects had at least 24 permanent teeth and received a clinical oral evaluation for degree of periodontal inflammation. Results indicated that unstimulated parotid salivary IgA concentration and secretion rate are independent of degree of periodontal inflammation and tobacco consumption.

The pattern of mobility of the mandibular incisor with intrusive loads was consistent with the view that the periodontal tissues are viscoelastic. A significant time-dependency of the response was observed. The pattern of mobility was similar to that reported for axially directed extrusive loads and for horizontal loads.

Axially-directed extrusive loads of 0.01-0.1N were applied to the lathyritic mandibular incisors of 12 rabbits. Tooth position was monitored continuously with a variable capacitance displacement transducer. Compared with pair-fed controls, lathyritic teeth showed markedly reduced resistance to extrusive loading, indicating that the periodontal collagen has a role in supporting the tooth. The pattern of tooth mobility, however, was little affected. The results may have implications for the eruptive mechanism. That eruption rates can appear unchanged by lathyrogens might not preclude an effect of the drug upon the eruptive force, assuming that there is a concomitant decrease in the resistance of the periodontal tissues to eruption. The teeth of rabbits in the pair-fed control group showed increased resistance to extrusive loading compared with animals fed ad libitum , indicating that changes in food and water intake may also influence the mechanical properties of the periodontium.

Fluid collected from non-inflamed sites contained only hyaluronic acid, whereas fluid collected from severely inflamed sites possessed additional components corresponding in electrophoretic mobility to dermatan sulphate and chondroitin-4-sulphate. The results imply that the source of gingival fluid glycosaminoglycans is the associated periodontal tissue and that the fluid may well be a vehicle for the transport and subsequent detection of glycosaminoglycans in the tooth integuments.

Summary Quantitative ultrastructural studies were conducted on eight rats (four aged 21 days and fouraged 8 weeks) using the periodontal tissues around the mesial root of the mandibular first molar. The periodontal ligament of the erupting tooth contained significantly more capillary fenestrations than the erupted tooth, both in terms of the number per unit area of endothelium (2.2/μm 2 ) and in terms of total number per cubic millimetre of tissue (30.5×10 6 /mm 3 ). Differences were also discerned with respect to the percentage of capillaries in the tissue. Thus, the periodontal vasculature demonstrates marked morphological changes which may be related to the eruptive phase of the tooth.

The behaviour of 60 spontaneously active dental mechanosensitive units was observed in dogs anaesthetized with pentobarbitone. The units could be stimulated by applying forces to the teeth and showed a post-stimulus depression, or silent period, in the discharge of nerve impulses when these forces were removed. The duration of the silent period was found to be increased by increases in the magnitude, rate of application, rate of removal and duration of force applied to the teeth, and it was inversely related to the rate of spontaneous discharge. Spontaneously active units were simulated by subjecting non-spontaneously active units to low-grade mechanical stimulation prior to test-loading the teeth, and the simulated units were shown to have the same characteristics as the true spontaneously active units. It is suggested that the silent period may be due to hyperpolarization of the receptors following removal of the stimulus, and that the cause of spontaneous activity may be simply low-grade mechanical deformation of the receptors within the periodontal tissues.