The varus alignment and morphologic alterations of proximal tibia affect the onset of medial knee osteoarthritis in rural Japanese women: Case control study from the longitudinal evaluation of Matsudai Knee Osteoarthritis Survey.

BACKGROUND: Relationship malalignment of the knee and the onset of knee OA are unclear. Moreover, it has not been clarified whether malalignment of the knee affects the morphological change of the proximal tibia. The purpose of current study was to investigate whether varus knee alignment and morphological change of the proximal medial tibia influence the onset of medial knee OA and to clarify the relationship between them. METHODS: The subjects comprised 736 knees from 390 women that went under both the first survey in 1979 and the fourth survey in 2000 in the Matsudai district in Niigata Prefecture in Japan. None of the subjects demonstrated radiographic knee OA at the first survey. OA was graded according to the Kellgren-Lawrence classification and the subjects were divided according to the OA grades in the forth survey into the 3 groups (non-OA, early OA, and advanced OA). Age, BMI, Femorotibial angle (FTA), the tibial plateau angle and medial tibial cortical bone thickness (MTCBT) were measured. We performed multivariate logistic regression analysis using the stepwise method to identified the risk factors regarding onset of knee OA and calculated the odds ratio (OR) and 95% confidence intervals (95% CI). RESULTS: Advanced OA had a significantly higher OR compared to non-OA in 1979 with the tibial plateau angle (OR: 1.15) and MTCBT (OR: 2.11). And also advanced OA had a significantly higher OR compared to early-OA in 1979 with the tibial plateau angle (OR: 1.17) and MTCBT (OR: 1.62). CONCLUSIONS: In the advanced OA, varus alignment of the proximal tibia had already existed before the onset of knee OA. In addition, we thought that varus alignment due to the proximal tibia had influenced the morphologic alterations of proximal medial tibia.

Usefulness of urinary CTX-II and NTX-I in evaluating radiological knee osteoarthritis : the Matsudai knee osteoarthritis survey.

BACKGROUND: To assess the usefulness of the urinary crosslinked C-telopeptide of type II collagen (uCTX-II) or crosslinked N-telopeptide of type I collagen (uNTX-I) for evaluating radiological knee osteoarthritis (OA), a cross-sectional study was conducted in the cohorts of the Matsudai knee osteoarthritis survey performed in Niigata, Japan. METHODS: Urine specimens and standing knee AP X-rays were obtained from 1040 subjects who provided informed consent. The relationship between these markers and gender, age (patients aged 40-59 or 60-79 years), use of bisphosphonates, and OA grades (K-L classification) were analyzed. The diagnostic ability of uCTX-II to detect radiological knee OA was confirmed in the over 60-year-old subjects using a ROC curve. RESULTS: The over 60-year-old men with OA grade 3,4 group had significantly higher uCTX-II levels than the other OA grade groups. In the over 60-year-old women, the uCTX-II levels significantly increased according to the progression of the knee OA grade. No significant difference was observed between the uNTX-I levels in the different OA grade groups. From the standpoint of biomarkers, the higher quartiles of the uCTX-II and uNTX-I levels gradually included higher numbers of grade ≥2 OA subjects in the over 60 year-old women. The area under the curve (AUC) in ROC analysis of uCTX-II exhibited a significant association with the diagnosis of knee OA in women (AUC 0.63), although the accuracy was evaluated to be low in the single measurement of our health checkup-based analysis. CONCLUSIONS: This population-based study indicates that the uCTX-II level is strongly correlated with the knee OA grade in women over age 60. A further analysis is needed to clarify its predictive accuracy.

Inhibition of cathepsin K reduces cartilage degeneration in the anterior cruciate ligament transection rabbit and murine models of osteoarthritis.

OBJECTIVE: To investigate the disease modifying effects of cathepsin K (CatK) inhibitor L-006235 compared to alendronate (ALN) in two preclinical models of osteoarthritis (OA). METHODS: Skeletally mature rabbits underwent sham or anterior cruciate ligament transection (ACLT)-surgery and were treated with L-006235 (L-235, 10 mg/kg or 50 mg/kg, p.o., daily) or ALN (0.6 mg/kg, s.c., weekly) for 8-weeks. ACLT joint instability was also induced in CatK(-/-) versus wild type (wt) mice and treated for 16-weeks. Changes in cartilage degeneration, subchondral bone volume and osteophyte area were determined by histology and µ-CT. Collagen type I helical peptide (HP-I), a bone resorption marker and collagen type II C-telopeptide (CTX-II), a cartilage degradation marker were measured. RESULTS: L-235 (50 mg/kg) and ALN treatment resulted in significant chondroprotective effects, reducing CTX-II by 60% and the histological Mankin score for cartilage damage by 46% in the ACLT-rabbits. Both doses of L-235 were more potent than ALN in protecting against focal subchondral bone loss, and reducing HP-I by 70% compared to vehicle. L-235 (50 mg/kg) and ALN significantly reduced osteophyte formation in histomorphometric analysis by 55%. The Mankin score in ACLT-CatK(-/-) mice was ~2.5-fold lower than the ACLT-wt mice and was not different from sham-CatK(-/-). Osteophyte development was not different among the groups. CONCLUSION: Inhibition of CatK provides significant benefits in ACLT-model of OA, including: 1) protection of subchondral bone integrity, 2) protection against cartilage degradation and 3) reduced osteophytosis. Preclinical evidence supports the role of CatK as a potential therapeutic target for the treatment of OA.

Molecular changes in articular cartilage and subchondral bone in the rat anterior cruciate ligament transection and meniscectomized models of osteoarthritis.

BACKGROUND: Osteoarthritis (OA) is a debilitating, progressive joint disease. METHODS: Similar to the disease progression in humans, sequential events of early cartilage degradation, subchondral osteopenia followed by sclerosis, and late osteophyte formation were demonstrated in the anterior cruciate ligament transection (ACLT) or ACLT with partial medial meniscectomy (ACLT + MMx) rat OA models. We describe a reliable and consistent method to examine the time dependent changes in the gene expression profiles in articular cartilage and subchondral bone. RESULTS: Local regulation of matrix degradation markers was demonstrated by a significant increase in mRNA levels of aggrecanase-1 and MMP-13 as early as the first week post-surgery, and expression remained elevated throughout the 10 week study. Immunohistochemistry confirmed MMP-13 expression in differentiated chondrocytes and synovial fibroblasts at week-2 and cells within osteophytes at week-10 in the surgically-modified-joints. Concomitant increases in chondrocyte differentiation markers, Col IIA and Sox 9, and vascular invasion markers, VEGF and CD31, peaked around week-2 to -4, and returned to Sham levels at later time points in both models. Indeed, VEGF-positive cells were found in the deep articular chondrocytes adjacent to subchondral bone. Osteoclastic bone resorption markers, cathepsin K and TRAP, were also elevated at week-2. Confirming bone resorption is an early local event in OA progression, cathepsin K positive osteoclasts were found invading the articular cartilage from the subchondral region at week 2. This was followed by late disease events, including subchondral sclerosis and osteophyte formation, as demonstrated by the upregulation of the osteoanabolic markers runx2 and osterix, toward week-4 to 6 post-surgery. CONCLUSIONS: In summary, this study demonstrated the temporal and cohesive gene expression changes in articular cartilage and subchondral bone using known markers of OA progression. The findings here support genome-wide profiling efforts to elucidate the sequential and complex regulation of the disease.

Chondromodulin-1 directly suppresses growth of human cancer cells.

BACKGROUND: Chondromodulin-1 (ChM1), an endogenous anti-angiogenic factor expressed in cartilage, has been suggested to inhibit invasion of endothelial cells into cartilage. In addition, the ectopic administration of ChM1 has been reported to suppress tumorigenesis in vivo. However, it is unclear whether the anti-tumor effect is due to not only the anti-vascularization effect of ChM1, but also its direct action against oncocytes. In the present study, we sought to determine whether ChM1 has a direct action on tumor cells. METHODS: BrdU incorporation assay was performed on human umbilical vein endothelial cells (HUVECs), normal human dermal fibroblasts (NHDFs), HepG2 cells and HeLa cells in the presence or absence of recombinant human ChM1 (rhChM1). An adenovirus that expresses ChM1, Ad-ChM1, was established and applied to the tumor xenografted in vivo, and to in vitro tumor cells cultured on plates or in soft agar. Cell cycle-related proteins and the phosphorylation of Erk, Akt, and GSK3beta, the downstream molecules of the extracellular matrix-integrin signaling pathways, in HepG2 cells treated with or without Ad-ChM1 were detected by western blot analysis. Luciferase reporter assays of STAT, GAS, and ISRE, which participate in another cytokine signaling pathway, ware performed in HepG2, HeLa, and HUVEC cells. RESULTS: ChM1 suppressed BrdU incorporation in HUVECs and in HepG2 cells dose-dependently, but did not suppress BrdU incorporation in NHDFs and HeLa cells cultured on plates. In soft agar, however, ChM1 suppressed the growth of HeLa cells, as well as HepG2 cells. Western blot analyses demonstrated that ChM1 decreased the levels of cyclin D1, cyclin D3, and cdk6 and increased those of p21cip1 without affecting the phosphorylation levels of Erk, Akt, and GSK3beta in HepG2 cells. The luciferase reporter assay demonstrated that ChM1 suppressed the transcriptional activities of STAT and GAS but not of ISRE. CONCLUSION: ChM1 directly suppressed the proliferation of tumor cells in an anchorage-independent manner. However, ChM1 did not alter the phosphorylation of downstream molecules, at which the signaling pathways through growth factor and cytokine receptors converge with the anchorage-dependent pathway. Our results show that ChM1 has a direct anti-tumor effect; moreover, this effect occurs by inhibiting the STAT signaling pathway.

Relationship between radiological knee osteoarthritis and biochemical markers of cartilage and bone degradation (urine CTX-II and NTX-I): the Matsudai Knee Osteoarthritis Survey.

Biochemical markers of cartilage and bone degradation are becoming increasingly important in the evaluation of knee osteoarthritis (OA). To clarify the correlation between radiological knee OA and urine CTX-II (C-terminal crosslinking telopeptide of collagen type II) or urine NTX-I (N-terminal crosslinking telopeptide of type I collagen), we conducted a cross-sectional study in the cohorts of the epidemiological knee survey at the Matsudai district in Niigata Prefecture, Japan. Urine specimens were collected from 296 subjects, and CTX-II and NTX-I were measured using ELISA. Standing knee AP X-rays were obtained and graded according to the Kellgren-Lawrence classification. The subjects were then divided by gender, age (40- to 59-year-old group and 60- to 79-year-old group), and the X-ray grade (Grade 0, 1, Grade 2, and Grade 3, 4). In non-OA (Grade 0, 1) subjects, the 60- to 79-year-old group had significantly higher CTX-II values than the younger group only in females. The subjects of both genders aged over 60 years of age with OA Grade 3, 4 had significantly higher CTX-II values than the Grade 0, 1 group or the Grade 2 group. For NTX-I, there were no significant differences between each OA grade although the Grade 3, 4 group females from 60 to 79 years of age had higher values than the Grade 2 group. In addition, in the 60- to 79-year-old subjects of both genders, a positive correlation was observed between the urine CTX-II and urine NTX-I. For the subjects ranging from 60 to 79 years of age in both genders, the urine CTX-II values indicate the progression of OA. In addition, the weak but positive correlation between urine CTX-II and urine NTX-I in the subjects ranging from 60 to 79 years of age in both genders suggests that bone resorption and cartilage degradation appear to develop in parallel.

[Osteoarthritis of the knee joint as a cause of musculoskeletal ambulation disability symptom complex (MADS)].

Osteoarthritis is considered as a chronic disease of the whole joint, including articular cartilage, meniscus, ligament, and peri-articular muscle that may result from multiple pathophysiological mechanisms. Osteoarthritis is a painful and disabling disease that affects millions of patients. Its aetiology is largely unknown, but is most likely multi-factorial. No proven disease-modifying therapy exists for osteoarthritis and current treatment options for chronic osteoarthritic pain are insufficient, but new pharmacotherapeutic options are emerging. Surgical treatment, such as high tibial osteotomy and total knee arthroplasty are chosen for the patient who have severe pain and disabled patient. In recent 10 years, operation cases are increasing in number, but these procedure still have many issues to resolve. Osteoarthritis of the knee joint is one of a major cause of Musculoskeletal Ambulation Disability Symptom Complex (MADS) , and it is important to improve conservative and surgical treatment.

Inverse relationship between matrix remodeling and lipid metabolism during osteoarthritis progression in the STR/Ort mouse.

OBJECTIVE: The biologic changes associated with osteoarthritis (OA) are incompletely understood. The aim of this study was to elucidate the molecular mechanisms underlying OA progression in an STR/Ort murine model of spontaneous disease. METHODS: Global patterns of gene expression were assessed using microarray analysis of articular cartilage/subchondral bone from the tibial plateaus of STR/Ort mice at 3, 9, and 12 months of age. The age-dependent severity of osteophyte formation and extent of cartilage damage were determined in the corresponding femurs using microfocal computed tomography and the Mankin histologic scoring system. Pathway analysis was used to identify the functions of genes associated with OA progression, and changes in gene expression were confirmed using immunohistochemistry. RESULTS: Six hundred twenty-one genes were associated with both osteophyte formation and cartilage damage in the STR/Ort joints. Genes involved in the development/function of connective tissue and in lipid metabolism were most significantly enriched and regulated during disease progression. Genes directly interacting with peroxisome proliferator-activated receptor alpha (PPARalpha)/PPARgamma were down-regulated, whereas those genes involved with connective tissue remodeling were up-regulated during disease progression. Associations of down-regulation of myotubularin-related phosphatase 1 (a phosphoinositide 3-phosphatase involved in lipid signaling) and up-regulation of biglycan (a member of the small leucine-rich protein family known to modulate osteoblast differentiation and matrix mineralization) with OA progression were confirmed by immunohistochemistry. CONCLUSION: Since adipogenesis and osteogenesis are inversely related in the developing skeletal tissue, these results suggest that a shift in the differentiation of mesenchymal cells from adipogenesis toward osteogenesis is a component of the OA pathophysiologic processes occurring in the tibial plateau joints of STR/Ort mice.

[Up-date of cartilage repair technique].

There are two major conditions which require cartilage repair, one is partial cartilage defect due to trauma or osteochondritis dissecanse and the other is cartilage degeneration and loss induced by osteoarthritis. In this section, we introduce several surgical techniques which are available in clinical cases. In patient with relatively small traumatic cartilage defect, microfracture technique, autologous osteochondral graft transplantation, or autologous cultured chondrocytes transplantation are performed. In patients with osteoarthritis, which usually larger region is affected than traumatic cartilage defect, cell based technique and osteochondral transplantation are not sufficient to cover the region, therefore abrasion arthroplasty is performed accompanied with osteotomy to correct mechanical property in the joint.

Characterization of articular cartilage and subchondral bone changes in the rat anterior cruciate ligament transection and meniscectomized models of osteoarthritis.

Osteoarthritis (OA) is a chronic joint disease characterized by cartilage destruction, subchondral bone sclerosis, and osteophyte formation. Subchondral bone stiffness has been proposed to initiate and/or contribute to cartilage deterioration in OA. The purpose of this study was to characterize subchondral bone remodeling, cartilage damage, and osteophytosis during the disease progression in two models of surgically induced OA. Rat knee joints were subjected either to anterior cruciate ligament transection (ACLT) alone or in combination with resection of medial menisci (ACLT + MMx). Histopathological changes in the surgical joints were compared with sham at 1, 2, 4, 6, and 10 weeks post-surgery. Using a modified Mankin scoring system, we demonstrate that articular cartilage damage occurs within 2 weeks post-surgery in both surgical models. Detectable cartilage surface damage and proteoglycan loss were observed as early as 1 week post-surgery. These were followed by the increases in vascular invasion into cartilage, in loss of chondrocyte number and in cell clustering. Histomorphometric analysis revealed subchondral bone loss in both models within 2 weeks post-surgery followed by significant increases in subchondral bone volume relative to sham up to 10 weeks post-surgery. Incidence of osteophyte formation was optimally observed in ACLT joints at 10 weeks and in ACLT + MMx joints at 6 weeks post-surgery. In summary, the two surgically induced rat OA models share many characteristics seen in human and other animal models of OA, including progressive articular cartilage degradation, subchondral bone sclerosis, and osteophyte formation. Moreover, increased subchondral bone resorption is associated with early development of cartilage lesions, which precedes significant cartilage thinning and subchondral bone sclerosis. Together, these findings support a role for bone remodeling in OA pathogenesis and suggest that these rat models are suitable for evaluating bone resorption inhibitors as potential disease-modifying pharmaco-therapies.

The LATS2/KPM tumor suppressor is a negative regulator of the androgen receptor.

The androgen receptor (AR) is a member of the steroid receptor superfamily that plays critical roles in the development and maintenance of the male reproductive system and in prostate cancer. Actions of AR are controlled by interaction with several classes of coregulators. In this study, we have identified LATS2/KPM as a novel AR-interacting protein. Human LATS1 and LATS2 are tumor suppressors that are homologs of Drosophila warts/lats. The interaction surface of LATS2 is mapped to the central region of the protein, whereas the AR ligand binding domain is sufficient for this interaction. LATS2 functions as a modulator of AR by inhibiting androgen-regulated gene expression. The mechanism of LATS2-mediated repression of AR activity appears to involve the inhibition of AR NH2- and COOH-terminal interaction. Chromatin immunoprecipitation assays in human prostate carcinoma cells reveal that LATS2 and AR are present in the protein complex that binds at the promoter and enhancer regions of prostate-specific antigen, and overexpression of LATS2 results in a reduction in androgen-induced expression of endogenous prostate-specific antigen mRNA. Immunohistochemistry shows that LATS2 and AR are localized within the prostate epithelium and that LATS2 expression is lower in human prostate tumor samples than in normal prostate. The results suggest that LATS2 may play a role in AR-mediated transcription and contribute to the development of prostate cancer.

The role of subchondral bone remodeling in osteoarthritis: reduction of cartilage degeneration and prevention of osteophyte formation by alendronate in the rat anterior cruciate ligament transection model.

OBJECTIVE: It has been suggested that subchondral bone remodeling plays a role in the progression of osteoarthritis (OA). To test this hypothesis, we characterized the changes in the rat anterior cruciate ligament transection (ACLT) model of OA and evaluated the effects of alendronate (ALN), a potent inhibitor of bone resorption, on cartilage degradation and on osteophyte formation. METHODS: Male Sprague-Dawley rats underwent ACLT or sham operation of the right knee. Animals were then treated with ALN (0.03 and 0.24 microg/kg/week subcutaneously) and necropsied at 2 or 10 weeks postsurgery. OA changes were evaluated. Subchondral bone volume and osteophyte area were measured by histomorphometric analysis. Coimmunostaining for transforming growth factor beta (TGF beta), matrix metalloproteinase 9 (MMP-9), and MMP-13 was performed to investigate the effect of ALN on local activation of TGF beta. RESULTS: ALN was chondroprotective at both dosages, as determined by histologic criteria and collagen degradation markers. ALN suppressed subchondral bone resorption, which was markedly increased 2 weeks postsurgery, and prevented the subsequent increase in bone formation 10 weeks postsurgery, in the untreated tibial plateau of ACLT joints. Furthermore, ALN reduced the incidence and area of osteophytes in a dose-dependent manner. ALN also inhibited vascular invasion into the calcified cartilage in rats with OA and blocked osteoclast recruitment to subchondral bone and osteophytes. ALN treatment reduced the local release of active TGF beta, possibly via inhibition of MMP-13 expression in articular cartilage and MMP-9 expression in subchondral bone. CONCLUSION: Subchondral bone remodeling plays an important role in the pathogenesis of OA. ALN or other inhibitors of bone resorption could potentially be used as disease-modifying agents in the treatment of OA.

Expression of the cartilage derived anti-angiogenic factor chondromodulin-I decreases in the early stage of experimental osteoarthritis.

OBJECTIVE: Chondromodulin-I (ChM-I), a cartilage derived anti-angiogenic factor, has been shown to regulate the vascular invasion during endochondral bone formation. We evaluated the expression and localization of ChM-I in articular cartilage during the progression of osteoarthritis (OA) in the rat, and correlated ChM-I expression with the increase in vascular invasion into OA articular cartilage. METHODS: Expression of ChM-I, type II collagen, basic fibroblast growth factor, vascular endothelial growth factor (VEGF), and matrix metalloproteinases MMP-9 and MMP-13 were examined in articular cartilage of intact growing and adult rats and in the surgically induced OA model using in situ hybridization, Western blot analysis, and immunohistochemistry. Co-immunostaining for ChM-I and CD-31 was performed to localize ChM-I and neovascularization in articular cartilage at advanced stage of OA. RESULTS: Abundant expression of ChM-I protein was detected in avascular regions of the developing and adult healthy articular cartilage. In early OA, ChM-I expression decreased in the superficial zone of articular cartilage, while levels of proteoglycan and type II collagen were comparable to control. In advanced OA, ChM-I expression was reduced in all zones of articular cartilage, and the number of VEGF-expressing cells was increased. Immunohistochemical studies showed that vascular invasion occurred in proximity to chondrocytes with high expression of pro-angiogenic markers, and decreased expression of ChM-I. CONCLUSION: High expression of ChM-I was detected in articular cartilage of growing and normal adult joints, implicating its role in the maintenance of avascularity of intact articular cartilage. Expression of ChM-I decreased, while expression of VEGF and other pro-angiogenic factors increased, in OA cartilage. These findings suggest the loss of ChM-I from articular cartilage might be responsible in part for promoting blood vessel invasion into the cartilage during progression of OA.

Chondromodulin-I expression in rat articular cartilage.

The localization and expression of chondromodulin-I (ChM-I), an angiogenesis inhibitor, in the rat articular cartilage during maturation from 2 to 10 weeks of age were examined by immunohistochemistry, Western blot analysis and ribonuclease protection assay, and the results were compared with those in the epiphyseal cartilage. ChM-I was found to be diffusely immunostained in the inter-territorial space of the cartilage matrix from the intermediate to the deep layers at the immature stage. As the articular cartilage matured, the immunoreactivity was localized around the hypertrophic chondrocytes in the deep layer and the immunoreactivity became weak after maturation. In contrast, the ChM-I immunoreactivity was intense in the epiphyseal cartilage at all ages examined. ChM-I was detected by Western blotting as a broad band or occasionally as a cluster of multiple bands (approximately 25 kDa) in both the articular and the epiphyseal cartilage. The intensity of the bands decreased gradually with age in the articular cartilage, but was unchanged in the epiphyseal cartilage at all ages. Ribonuclease protection assay revealed that ChM-I mRNA also decreased gradually with age in the articular cartilage in parallel with the maturation of the articular cartilage, while no decrease in ChM-I mRNA was found in the epiphyseal cartilage. The expression of ChM-I mRNA in the articular cartilage was less than that in the epiphyseal cartilage at all ages. The decrease in amount of ChM-I in the mature articular cartilage suggests that ChM-I plays a more important role in the maintenance of avascularity in the immature articular cartilage than in the mature one. The avascular condition may be preserved by angiogenic inhibitors or mechanisms other than ChM-I in the mature articular cartilage.

Effect of dexamethasone on growth inhibition and chondrogenic maturation of human chondrosarcoma.

The effect of dexamethasone, a synthetic glucocorticoid, on in vitro and in vivo growth and differentiation of the human chondrosarcoma cell line (OUMS-27) was studied. Cells were treated with various doses of dexamethasone, and increasing doses produced an inhibitory effect on OUMS-27 tumor cell proliferation and induced maturation. Cell counts for OUMS-27 on day 9 ranged from 59% of the control at 10(-8) M to 45% of the control at 10(-5) M dexamethasone. Northern blot analysis revealed that the type II collagen mRNA level in cells given dexamethasone was lower than that in the controls, and the type X collagen mRNA level was higher than that in the controls. Phase-contrast microscopy revealed that cells grown in control medium formed monolayers consisting of small, polygonal cells, whereas dexamethasone-treated cells became larger and more irregular in shape. In the in vivo study the growth rate of masses in nude mice induced by inoculating OUMS-27 cells was also reduced in a dose-dependent manner with dexamethasone administration. These results suggest that dexamethasone caused growth inhibition and induced chondrogenic maturation of human chondrosarcoma cells.

Human PTH (1-34) induces longitudinal bone growth in rats.

The growth plate is a specialized structure that is responsible for longitudinal bone growth (LGR). Growth plate organization is altered with loading in rats. Parathyroid hormone (PTH) is known to induce mitogenic effect on chondrocytes in vitro. Type I PTH/PTH related peptide (rP) receptor is expressed in growth plate cartilage in rats. We therefore investigated the effect of PTH administration on the organization and longitudinal growth rate of the growth plate in rats. We also investigated the effect of PTH on the changes induced by unloading in the organization and growth of the growth plate. Thirty 6-week-old and 30 15-week-old male Sprague-Dawley rats were randomly assigned to five groups (n = 6 per group), i.e., basal controls, control (i.e., normally loaded), PTH-treated control (i.e., PTH-treated under normal loading), unloaded, and PTH-treated under unloading. PTH-treated animals received human PTH (1-34) at a dose of 80 microg/kg per day five times per week for 3 weeks, for the duration of unloading. In young loaded rats treated with the systemic administration of PTH, growth plate thickness, chondrocyte number, and LGR were increased in the proximal tibiae compared with findings in young loaded rats without PTH administration. Hindlimb unloading induced a reduction in growth plate thickness, chondrocyte number, and LGR. In young rats, systemic administration of PTH partly prevented these changes induced by unloading. These preventive effects of PTH were observed only in young rats; not in adult rats. These results show that the systemic administration of PTH stimulates longitudinal bone growth, and diminishes the reduction in growth plate growth induced by unloading in young rats.