Effects of ageing on proliferative ability, and the expressions of secreted protein, acidic and rich in cysteine (SPARC) and osteoprotegerin (osteoclastogenesis inhibitory factor) in cultures of human periodontal ligament cells

Hypertrophic Ligamentum Flavum in Lumbar Spine Stenosis Is Associated With the Increased Expression of Secreted Protein Acidic and Rich in Cysteine

STUDY DESIGN

Basic research.

OBJECTIVES

Secreted protein acidic and rich in cysteine (SPARC) is a critical pro-fibrotic mediator. This study aims to characterize the role of SPARC in hypertrophic ligamentum flavum (LF) and fibrosis.

METHODS

Hypertrophic LF samples were obtained from 8 patients with L4/5 lumbar spinal stenosis (LSS) during the decompressive laminectomy. Non-hypertrophic LF from age- and sex-matched 8 patients with L4/5 lumbar disc herniation was selected as control. An in vitro model of fibrosis in human LF cells was established by interleukin 6 (IL-6) to assess SPARC expression.

RESULTS

Hypertrophic LF samples had higher fibrosis scores than control samples by Masson's trichrome staining (3.6 vs. 1.3, P < .001). Hypertrophic LF samples had significantly more positive staining for collagen and SPARC. Collagen III (Col3), α smooth muscle actin (α-SMA), and SPARC mRNA expression levels were significantly higher in hypertrophic LF samples than in control samples by qPCR. SPARC expression and fibrotic and inflammatory makers (collagen I, Col3, IL-6, interleukin 1β) were significantly upregulated in IL-6 stimulation of normal LF in vitro.

CONCLUSION

SPARC was detected in human LF and significantly upregulated in the clinical samples of hypertrophic LF compared to their normal counterparts. We also demonstrated an increased level of SPARC in an in vitro fibrosis model of LF. Thus, SPARC could be a crucial biomarker for the pathogenesis of hypertrophic LF and a therapeutic target for LSS.

Porphyromonas gingivalis lipopolysaccharide enhances the proliferation of human periodontal ligament cells via upregulation of cyclin D1, cyclin A and cyclin B1

Human periodontal ligament cells (hPDLCs) play a notable role in periodontal tissue homeostasis and regeneration. However, the effect of Porphyromonas gingivalis lipopolysaccharide (Pg-LPS) on the proliferation of hPDLCs remains unclear. The present study investigated the effects of Pg-LPS on the proliferation profile of hPDLCs, and the involvement of cyclins and cyclin-dependent kinases in the process. hPDLCs were treated with Pg-LPS, and cell proliferation and cycle were detected using Cell Counting Kit-8 assays and flow cytometry. The mRNA expression levels of the cyclins and cyclin-dependent kinases (CDKs), including cyclins A, B1, D1 and D2 and CDK1, 2 and 4, were detected using reverse transcription-quantitative PCR. The protein expression levels of cyclins A, B1 and D1 were analysed using western blotting. The proliferation of hPDLCs was significantly increased after treatment with Pg-LPS at the concentrations of 0.001, 0.01, 0.1, 1 and 10 µg/ml for 24, 36 and 48 h compared with the cells cultured without LPS (P<0.01). The proliferation index of hPDLCs was significantly enhanced after treatment with Pg-LPS (0.0001, 0.001, 0.01, 0.1, 1 and 10 µg/ml) for 24 h (P<0.01). However, the S-phase fraction (SPF) only significantly increased after treatment with Pg-LPS at 0.01 µg/ml for 24 h (P<0.05), while the G₂/M-phase fraction increased (P<0.01) and the G₀/G₁-phase fraction decreased (P<0.01) compared with the controls. The proliferation index and SPF increased, peaked at 24 h and then decreased at 48 h in both Pg-LPS-stimulated and control groups. Notably, Pg-LPS significantly upregulated the expression levels of cyclins D1, A and B1 after 24 h compared with those in the controls. Overall, the present study indicated that Pg-LPS may enhance the proliferation of hPDLCs, potentially through upregulation of cyclins D1, A and B1.

Sedentary time, physical activity, and serum SPARC in a middle-aged population

The influence of habitual physical activity (PA) on the circulating levels of secreted protein acidic and rich in cysteine (SPARC) remains unclear. The purpose of the current study was to clarify the effects of sedentary time, light-intensity PA (LPA), and moderate- to vigorous-intensity PA (MVPA) on the serum SPARC in a general middle-aged population. The current study is a cross-sectional study of 4,000 men and 6,040 women (40-69 years). Sedentary time, LPA, and MVPA were objectively measured by an accelerometer. The serum SPARC concentration was measured by enzyme-linked immunosorbent assay. Using an isotemporal substitution model, cross-sectional associations of replacing sedentary time with either LPA or MVPA on serum SPARC levels were analysed according to sex. Interactions with subject characteristics, such as the body mass index (BMI), smoking, and alcohol consumption, were also examined. In men, replacing 60 min of sedentary time with 60 min of MVPA was significantly associated with 23 ng/mL lower serum SPARC levels (confidence interval: -43, -2) after adjusting for confounders, including the BMI (P = 0.028). A significant interaction between replacing sedentary behaviour with LPA and the BMI on SPARC was detected in women (P = 0.029), although the stratified associations for each BMI level (<25 or ≥25 kg/m²) did not reach significance. The current study suggests that replacing sedentary time with MVPA is associated with reduced serum SPARC levels in middle-aged men, but not in women. In addition, a potential interaction between LPA and the BMI on SPARC was also found in women.

Lifelong aerobic exercise protects against inflammaging and cancer

Biological aging is associated with progressive damage accumulation, loss of organ reserves, and systemic inflammation ('inflammaging'), which predispose for a wide spectrum of chronic diseases, including several types of cancer. In contrast, aerobic exercise training (AET) reduces inflammation, lowers all-cause mortality, and enhances both health and lifespan. In this study, we examined the benefits of early-onset, lifelong AET on predictors of health, inflammation, and cancer incidence in a naturally aging mouse model (C57BL/J6). Lifelong, voluntary wheel-running (O-AET; 26-month-old) prevented age-related declines in aerobic fitness and motor coordination vs. age-matched, sedentary controls (O-SED). AET also provided partial protection against sarcopenia, dynapenia, testicular atrophy, and overall organ pathology, hence augmenting the 'physiologic reserve' of lifelong runners. Systemic inflammation, as evidenced by a chronic elevation in 17 of 18 pro- and anti-inflammatory cytokines and chemokines (P < 0.05 O-SED vs. 2-month-old Y-CON), was potently mitigated by lifelong AET (P < 0.05 O-AET vs. O-SED), including master regulators of the cytokine cascade and cancer progression (IL-1β, TNF-α, and IL-6). In addition, circulating SPARC, previously known to be upregulated in metabolic disease, was elevated in old, sedentary mice, but was normalized to young control levels in lifelong runners. Remarkably, malignant tumours were also completely absent in the O-AET group, whereas they were present in the brain (pituitary), liver, spleen, and intestines of sedentary mice. Collectively, our results indicate that early-onset, lifelong running dampens inflammaging, protects against multiple cancer types, and extends healthspan of naturally-aged mice.

Regenerative effect of hOPG gene-modified autologous PDLs in combination with cell transplantation on periodontal defection in beagle dogs

This study evaluated the ability of human osteoprotegerin gene-modified autologous periodontal ligament cells (PDLCs) in combination with cell transplantation to promote periodontal regeneration in beagle dogs. Adenovirus Ad5-hOPG-EGFP-transfected PDLCs and BME-10X collagen membranes were fabricated and used for periodontal repair. Buccal periodontal defects (mesiodistal width × depth: 5 × 5 mm) were created on the second, third, and fourth mandibular premolars in six normal beagle dogs, and the defects were histologically and histomorphometrically assessed for periodontal regeneration in the following four groups: (1) hOPG-PDLCs + BME-10X, (2) mock-PDLCs + BME-10X, (3) PDLCs + BME-10X, and (4) BME-10X. The radiographic and histological results suggested that hOPG-PDLCs significantly promoted periodontal defect repair. This study demonstrates the potential of hOPG-modified PDLCs for periodontal tissue regeneration.

Role of periodontal ligament fibroblasts in osteoclastogenesis: a review

During the last decade it has become clear that periodontal ligament fibroblasts may contribute to the in vitro differentiation of osteoclasts. We surveyed the current findings regarding their osteoclastogenesis potential. Periodontal ligament fibroblasts have the capacity to select and attract osteoclast precursors and subsequently to retract and enable migration of osteoclast precursors to the bone surface. There, fusion of precursors takes place, giving rise to osteoclasts. The RANKL-RANK-osteoprotegerin (OPG) axis is considered crucial in this process. Periodontal ligament fibroblasts produce primarily OPG, an osteoclastogenesis-inhibitory molecule. However, they may be influenced in vivo by direct or indirect interactions with bacteria or by mechanical loading. Incubation of periodontal ligament fibroblasts with bacteria or bacterial components causes an increased expression of RANKL and other osteoclastogenesis-stimulating molecules, such as tumor necrosis factor-α and macrophage-colony stimulating factor. Similar results are observed after the application of mechanical loading to these fibroblasts. Periodontal ligament fibroblasts may be considered to play an important role in the remodelling of alveolar bone. In vitro experiments have demonstrated that periodontal ligament fibroblasts adapt to bacterial and mechanical stimuli by synthesizing higher levels of osteoclastogenesis-stimulating molecules. Therefore, they probably contribute to the enhanced osteoclast formation observed during periodontitis and to orthodontic tooth movement.

Tooth wear and dentoalveolar remodeling are key factors of morphological variation in the Dmanisi mandibles

The Plio-Pleistocene hominin sample from Dmanisi (Georgia), dated to 1.77 million years ago, is unique in offering detailed insights into patterns of morphological variation within a paleodeme of early Homo. Cranial and dentoalveolar morphologies exhibit a high degree of diversity, but the causes of variation are still relatively unexplored. Here we show that wear-related dentoalveolar remodeling is one of the principal mechanisms causing mandibular shape variation in fossil Homo and in modern human hunter-gatherer populations. We identify a consistent pattern of mandibular morphological alteration, suggesting that dental wear and compensatory remodeling mechanisms remained fairly constant throughout the evolution of the genus Homo. With increasing occlusal and interproximal tooth wear, the teeth continue to erupt, the posterior dentition tends to drift in a mesial direction, and the front teeth become more upright. The resulting changes in dentognathic size and shape are substantial and need to be taken into account in comparative taxonomic analyses of isolated hominin mandibles. Our data further show that excessive tooth wear eventually leads to a breakdown of the normal remodeling mechanisms, resulting in dentognathic pathologies, tooth loss, and loss of masticatory function. Complete breakdown of dentognathic homeostasis, however, is unlikely to have limited the life span of early Homo because this effect was likely mediated by the preparation of soft foods.

Prospective potency of TGF-β1 on maintenance and regeneration of periodontal tissue

Periodontal ligament (PDL) tissue, central in the periodontium, plays crucial roles in sustaining tooth in the bone socket. Irreparable damages of this tissue provoke tooth loss, causing a decreased quality of life. The question arises as to how PDL tissue is maintained or how the lost PDL tissue can be regenerated. Stem cells included in PDL tissue (PDLSCs) are widely accepted to have the potential to maintain or regenerate the periodontium, but PDLSCs are very few in number. In recent studies, undifferentiated clonal human PDL cell lines were developed to elucidate the applicable potentials of PDLSCs for the periodontal regenerative medicine based on cell-based tissue engineering. In addition, it has been suggested that transforming growth factor-beta 1 is an eligible factor for the maintenance and regeneration of PDL tissue.

Evaluation of senescence in mesenchymal stem cells isolated from equine bone marrow, adipose tissue, and umbilical cord tissue

Mesenchymal stem cells (MSCs) from adult and neonatal tissues are intensively investigated for their use in regenerative medicine. The purpose of this study was to compare the onset of replicative senescence in MSCs isolated from equine bone marrow (BMSC), adipose tissue (ASC), and umbilical cord tissue (UCMSC). MSC proliferation (cell doubling), senescence-associated β-galactosidase staining, telomere length, Sox-2, and lineage-specific marker expression were assessed for MSCs harvested from tissues of 4 different donors. The results show that before senescence ensued, all cell types proliferated at ∼1 day/cell doubling. BMSCs significantly increased population doubling rate by passage 10 and ceased proliferation after a little >30 total population doublings, whereas UCMSCs and ASCs achieved about 60 to 80 total population doublings. UCMSC and ASCs showed marked β-galactosidase staining after ∼70 population doublings, whereas BMSCs stained positive by ∼30 population doublings. The onset of senescence was associated with a significant reduction in telomere length averaging 10.2 kbp at passage 3 and 4.5 kbp in senescent cultures. MSCs stained intensively for osteonectin at senescence compared with earlier passages, whereas vimentin and low levels of smooth muscle actin were consistently expressed. Sox-2 gene expression was consistently noted in all 3 MSC types. In conclusion, equine BMSCs appear to senesce much earlier than ASCs and UCMSCs. These results demonstrate the limited passage numbers of subcultured BMSCs available for use in research and tissue engineering and suggest that adipose tissue and umbilical cord tissue may be preferable for tissue banking purposes.

SPARC/osteonectin functions to maintain homeostasis of the collagenous extracellular matrix in the periodontal ligament

Expression of secreted protein acidic and rich in cysteine (SPARC)/osteonectin, a collagen-binding matricellular protein, is frequently associated with tissues with high rates of collagen turnover, such as bone. In the oral cavity, expression of SPARC/osteonectin has been localized to the periodontal ligament (PDL), a collagen-rich tissue with high rates of collagen turnover. The PDL is critical for tooth position within the alveolar bone and for absorbing forces generated by chewing. To characterize the function of SPARC/osteonectin in PDL, SPARC/osteonectin expression in murine PDL was evaluated by immunochemistry at 1, 4, 6, and >18 months. Highest levels of SPARC/osteonectin were detected at 1 and >18 months, with decreased levels associated with adult (4-6 months) PDL. To determine whether the absence of SPARC/osteonectin expression influenced cellular and fibrillar collagen content in PDL, PDL of SPARC-null mice was evaluated using histological stains and compared with that of wild-type (WT). Our results demonstrated decreased numbers of nuclei in PDL of SPARC-null mice at 1 month. In addition, decreased collagen volume fractions were found at 1 and >18 months and decreases in thick collagen fiber volume fraction were detected at 4, 6, and >18 months in SPARC-null PDL. The greatest differences in cell number and in collagen content between SPARC-null and WT PDL coincided with ages at which levels of SPARC/osteonectin expression were highest in WT PDL, at 1 and >18 months. These results support the hypothesis that SPARC/osteonectin is critical in the control of tissue collagen content and indicate that SPARC/osteonectin is necessary for PDL homeostasis.

Loss of proliferation and differentiation capacity of aged human periodontal ligament stem cells and rejuvenation by exposure to the young extrinsic environment

The application of periodontal ligament stem cells (PDLSCs) may be effective for periodontal regenerative therapy. As tissue regenerative potential may be negatively regulated by aging, whether aging and its microenvironment modify human PDLSCs remains a question. In this study, we compared the proliferation and differentiation capacity of PDLSCs obtained from young and aged donors. Then, we exposed aged PDLSCs to young periodontal ligament cell-conditioned medium (PLC-CM), and young PDLSCs were exposed to aged PLC-CM. Morphological appearance, colony-forming assay, cell cycle analysis, osteogenic and adipogenic induction media, gene expression of cementoblast phenotype, and in vivo differentiation capacities of PDLSCs were evaluated. PDLSCs obtained from aged donors exhibited decreased proliferation and differentiation capacity when compared with those from young donors. Young PLC-CM enhanced the proliferation and differentiation capacity of PDLSCs from aged donors. Aged PDLSCs induced by young PLC-CM showed enhanced tissue-regenerative capacity to produce cementum/periodontal ligament-like structures, whereas young PDLSCs induced by aged PLC-CM transplants mainly formed connective tissues. To our knowledge, this is the first study to mimic the developmental microenvironment of PDLSCs in vitro, and our data suggest that age influences the proliferation and differentiation potential of human PDLSCs, and that the activity of human PDLSCs can be modulated by the extrinsic microenvironment.

Age-Related Changes of Periodontal Ligament Surface Areas during Force Application

Objective: To investigate the age-dependent morphology of the periodontal ligament (PDL) tissue and changes in its surface area (SA) during force application provided with a standardized orthodontic setup for a period of 12 weeks in young and adult rats.

Methods: Two groups of 30 rats, age 6 weeks and 9 to 12 months, were used. Orthodontic appliances were placed to move the maxillary molars mesially with the contralateral sides used as controls. At 1, 2, 4, 8, and 12 weeks, groups of animals were killed. The PDL SA and the PDL SA ratio between pressure and tension regions were determined.

Results: An age-related decrease in the PDL SA was noted at control sides. Significant changes during the experimental period occurred only at experimental sides: The PDL SA was smaller at pressure than at tension regions only at week 1 in young rats; in adult rats, the difference between the two regions was significant at week 8. These changes were confirmed by the morphologic disorganization of the PDL and alterations in the PDL SA ratio.

Conclusions: During force application, the PDL at the pressure regions became disorganized and subsequently was reorganized, as is shown by the histologic changes and SA of the PDL over time. This process occurred earlier and was more prominent in young rats; it occurred later and was more prolonged in adult animals.

Influence of aging on biological properties of periodontal ligament cells

The majority of patients eligible for periodontal regenerative therapies are aged subjects. Since periodontal ligament cells (PDLC) are essential for periodontal regeneration, the aim of the present study was to determine the effect of cellular aging on PDLC, including genes associated with extracellular matrix metabolism and growth-associated factors. PDLC cultures were obtained from subjects aged 15 to 20 years and subjects aged more than 60 years. Proliferation, cell viability, mineralization assays, and mRNA levels were assessed for type I and III collagen, platelet-derived growth factor (PDGF)-1, basic fibroblast growth factor (bFGF), metalloproteinase (MMP)-2 and-8, and tissue inhibitor of metalloproteinases (TIMP)-1 and-2. Data analysis demonstrated that aging negatively influenced cell proliferation and mineral nodule formation (p < 0.05). Gene expression analysis further showed that mRNA levels for bFGF, PDGF-1, and TIMP-2 were not affected by aging (p > 0.05). In addition, mRNA levels for type I and III collagen were significantly lower in aged cells (p < 0.05), whereas MMP-2 and-8 and TIMP-1 mRNA levels were higher (p < 0.05). Within the limits of the present study, data analysis suggests that aging modulates important biological properties of periodontal ligament cells, diminishes the potential for mineral nodule formation, and favors extracellular matrix degradation.

Periodontal healing may be affected by aging: a histologic study in rats

BACKGROUND AND OBJECTIVE

Although wound healing has been reported to be impaired with aging, very little is known about its effect on periodontal tissues. Therefore, the aim of this study was to evaluate, histologically in rats, the influence of aging on a spontaneous periodontal healing model.

MATERIAL AND METHODS

Twenty-four male Wistar rats were used and assigned to the following groups: control (n = 12; 2 mo old) and aged (n = 12; 18 mo old). Fenestration defects (4 x 3 x 1 mm) were created bilaterally at the buccal aspect of the distal root of the first mandibular molars, and the mandibulae were retrieved 3 and 6 wk postoperatively. The percentage of bone fill and density of newly formed bone, new cementum formation (NC), and the extension of the remaining defect (ERD) were histometrically obtained.

RESULTS

Intragroup analysis demonstrated that, except for cementum, all histological parameters significantly improved over time (p < 0.05). Intergroup analysis additionally showed that the defects were initially similar in size, and that at 3 wk aging negatively influenced newly formed bone (86.38 +/- 2.99% and 73.06 +/- 3.21%, p < 0.001, for groups control and aged, respectively), BF (75.84 +/- 16.53% and 57.70 +/- 22.28%, p = 0.014) and ERD (0.41 +/- 0.20 mm and 1.17 +/- 0.37 mm, p < 0.001). At 6 wk, aging negatively influenced newly formed bone (88.12 +/- 2.90% and 78.19 +/- 5.35%, p < 0.001, for groups control and aged, respectively) and ERD (0.01 +/- 0.006 mm and 0.34 +/- 0.18 mm, p = 0.003), but not BF (98.15 +/- 2.43% and 87.87 +/- 11.63%, p > 0.05). No new cementum was formed along the root surface in the above groups.

CONCLUSION

Within the limits of the present study, data analysis suggests that aging may impair, but not prevent, periodontal healing.

Cloning of differentially expressed genes in skin fibroblasts from centenarians

Normal human fibroblasts undergoing serial passaging have been extensively used to identify genes linked with aging. Most of the isolated genes relate to growth retardation signals and the failure of homeostasis that accompanies aging and senescence. In contrast, there is still limited knowledge regarding the nature of the genes that influence positively the rate of aging and longevity. Healthy centenarians represent the best example of successful aging and longevity. Studies using samples from these individuals have proved very valuable for identifying a variety of factors that contribute to successful aging. The aim of the current work was to take advantage of skin fibroblast cultures established from healthy donors including centenarians in order to clone differentially expressed genes in centenarians. First, we demonstrate that centenarian derived cultures follow the typical Hayflick curve and they enter senescence after serial passaging. Application of differential screening techniques in minimally passaged cultures of four control donors of different ages (18-80 years old) and four centenarians has resulted in the cloning of six differentially expressed genes in centenarians. Four of the cloned genes, namely adlican, KBL, EST 38 and EST 39, were over-expressed in centenarians, while VDUP1 and OCIF were down-regulated in the same samples. We have also compared the expression levels of two representative cloned genes in cultures of human embryonic and adult fibroblasts to establish potential links with replicative senescence. Interestingly, VDUP1 was found over-expressed in late passage cells, while EST 39 was down-regulated in the same cultures. Thus our work demonstrates that a combination of the use of both biopsies derived cells and classical in vitro cells passaging will facilitate the better understanding of the biology of aging and longevity.

Neurotrophins in cultured cells from periodontal tissues

We review the basic functions of neurotrophins and their receptors and discuss the expression and functions of neurotrophins and their specific receptors based on recent data using cultured cells from human periodontal tissues. Neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) play crucial roles in the differentiation and survival of neural cells. Neurotrophins activate 2 different receptor classes: the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases (TrkA, TrkB, and TrkC) and the p75 receptor, a member of the tumor necrosis factor receptor superfamily. Neurotrophins regulate both cell death and cell survival through activations of Trk receptors and/or p75 neurotrophin receptor. It has been reported that neurotrophins are also produced from non-neuronal cells, such as leukocytes, osteoblasts, or fibroblasts, and act in many other ways on non-neuronal cells. Neurotrophin expression during bone fracture healing is especially interesting, and neurotrophins are now implicated in hard tissue regeneration. It is well known that neurotrophins and their receptors are expressed in tooth development. Recent studies have found that neurotrophins and Trk receptors are expressed in mouse osteoblastic cell lines. Human periodontal ligament cells, human gingival fibroblasts, and human gingival keratinocytes expressed mRNA for NGF and TrkA. The secretion of bioactive NGF peptides from human periodontal ligament cells and human gingival keratinocytes was confirmed by bioassay using PC12 cells (rat adrenal pheochromocytoma cells). The expression of NGF and TrkA.mRNA was regulated by interleukin (IL)-1beta. NGF increased DNA synthesis and expressions of mRNA for bone-related proteins, alkaline phosphatase, and osteopontin in human periodontal ligament cells. Neurotrophins and Trk receptors expressed in human periodontal tissue may contribute to regeneration as well as innervation of periodontal tissue through local autocrine and paracrine pathways. Recent data suggest that some functions of neurotrophins and Trk receptors relate to periodontal disease and periodontal tissue regeneration. However, in vivo studies will be required to clarify the roles of neurotrophins and their receptors, including p75, in periodontal disease and periodontal tissue regeneration.

Proliferative ability and alkaline phosphatase activity with in vivo cellular aging in human pulp cells

Little is known about the effect of aging on characteristic functions of pulp cells. When damaged pulp is recovered and mineralized tissue is formed to protect remaining pulp tissue, the general responses of pulp tissue after adequate stimuli (pulp cell proliferation and activation of alkaline phosphatase [ALPase]) are thought to be essential. In this study, we compared proliferative ability and ALPase activity between cultures of human pulp (HP) cells obtained from young and aged donors. The in vitro proliferative lifespan of HP cells from young donors was longer than HP cells from aged donors. Growth rates and ALPase activity of HP cells decreased with increasing donor age. These findings suggest that impaired repair of pulp and dentin in aged patients is partly due to a decrease in the proliferative ability and ALPase activity in aged pulp cells.

Mediators of periodontal osseous destruction and remodeling: principles and implications for diagnosis and therapy

Osteoclastic bone resorption is a prominent feature of periodontal disease. Bone resorption via osteoclasts and bone formation via osteoblasts are coupled, and their dysregulation is associated with numerous diseases of the skeletal system. Recent developments in the area of mediators of osteoclastic differentiation have expanded our knowledge of the process of resorption and set the stage for new diagnostic and therapeutic modalities to treat situations of localized bone loss as in periodontal disease. This review describes the current state of knowledge of osteoclast differentiation and activity, mediators, and biochemical markers of bone resorption and their use and potential use in clinical periodontics. Finally, therapeutic strategies based on knowledge gained in the treatment of metabolic bone diseases and in periodontal clinical trials are discussed, and the potential for future strategies is proposed relative to their biologic basis. The intent is to update the field of periodontics on the current state of pathophysiology of the osteoclastic lesion and outline diagnostic and therapeutic strategies with a rational basis in the underlying biology.