Osteonal and hemi-osteonal remodeling: the spatial and temporal framework for signal traffic in adult human bone

Identification of osteoblast/osteocyte factor 45 (OF45), a bone-specific cDNA encoding an RGD-containing protein that is highly expressed in osteoblasts and osteocytes

We describe the cloning and characterization of a novel bone-specific cDNA predicted to encode an extracellular matrix protein. This cDNA was identified by subtractive hybridization based upon its high expression in bone marrow-derived osteoblasts. By Northern blot analysis, we detected a single 2-kilobase mRNA transcript in bone, whereas no expression was detected in other tissues. Immunohistochemistry revealed that the protein was expressed highly in osteocytes within trabecular and cortical bone. RNA and protein expression analysis using in vivo marrow ablation as a model of bone remodeling demonstrated that this gene was expressed only in cells that were embedded within bone matrix in contrast to the earlier expression of known osteoblast markers. The cDNA was predicted to encode a serine/glycine-rich secreted peptide containing numerous potential phosphorylation sites and one RGD sequence motif. The interaction of RGD domain-containing peptides with integrins has been shown previously to regulate bone remodeling by promoting recruitment, attachment, and differentiation of osteoblasts and osteoclasts. Secretion of this RGD-containing protein from osteocytes has the potential to regulate cellular activities within the bone environment and thereby may impact bone homeostasis. We propose the name OF45 (osteoblast/osteocyte factor of 45 kDa) for this novel cDNA.

Osteoclasts differentiate from resident precursors in an in vivo model of synchronized resorption: a temporal and spatial study in rats

Osteoclasts differentiate from mononucleated precursors expressing monocyte markers, which gradually evolve to preosteoclasts expressing the osteoclast phenotype. Although the role of osteogenic cells in these changes has been well documented in vitro, their contribution in vivo has not been established. In this study, a synchronized wave of resorption was activated along the mandibular periosteum. The periosteum adjacent to the bone surface studied was separated by a computer-assisted technique into an osteogenic alkaline phosphatase-positive compartment and an outer nonosteogenic compartment. Specific markers (nonspecific esterase [NSE], tartrate-resistant acid phosphatase [TRAP], and ED1 antibody, a marker of the monocyte-macrophage lineage) were used to follow osteoclast differentiation quantitatively as a function of time after activation of resorption, from day 0 to day 4 (peak of resorption in this model). Local cell proliferation was assessed in parallel. Between day 0 and day 3, the thickness of the osteogenic compartment decreased by 50% (p < 0.0002). In the osteogenic compartment, proliferating cell numbers fell by 80% at 12 day, NSE(+) cells (located farthest from the bone surface) increased 3. 9-fold on day 4 vs. day 0 (p < 0.005), ED1(+) cells decreased between day 0 and day 2 (p < 0.02) before returning to their initial value, and TRAP(+) cells increased 2.7-fold between day 1 and day 3 (p < 0.0005). Resorption was absent in the site studied on day 0, but on day 4 there were 20.5 osteoclast nuclei per millimeter of bone surface. The cell ratio changed from 30.3 NSE(+) and ED1(+) (some of which were also TRAP(+)) cells per millimeter on day 0 to 37.6 mononucleated cells plus 20.5 osteoclast nuclei on day 4. In the nonosteogenic compartment, an entry of ED1(+)/NSE(-) was observed on 12 day (+23 cells, p < 0.02 vs. day 0). This was followed by a return of ED1(+) cell numbers to the control level on day 1, and a transient increase in NSE(+) cells (+47% on day 2 vs. day 1, p < 0.02). TRAP(+) cells were never seen in this compartment. Proliferating cell numbers did not change throughout the study. Our results strongly suggest that the osteoclasts present on day 4 differentiated from the pool of TRAP(+), ED1(+), and NSE(+) cells present at the site on day 0. The osteogenic compartment was gradually replenished by cells migrating from the nonosteogenic compartment, which was supplemented by ED1(+) cells recruited from the circulation early after activation. Moreover, osteogenic cells appeared to be as crucial in vivo for the acquisition of the TRAP phenotype as previously shown in vitro.

Structural and cellular changes during bone growth in healthy children

Normal postnatal bone growth is essential for the health of adults as well as children but has never been studied histologically in human subjects. Accordingly, we analyzed iliac bone histomorphometric data from 58 healthy white subjects, aged 1.5-23 years, 33 females and 25 males, of whom 48 had undergone double tetracycline labeling. The results were compared with similar data from 109 healthy white women, aged 20-76 years, including both young adult reference ranges and regressions on age. There was a significant increase with age in core width, with corresponding increases in both cortical width and cancellous width. In cancellous bone there were increases in bone volume and trabecular thickness, but not trabecular number, wall thickness, interstitial thickness, and inferred erosion depth. Mineral apposition rates declined on the periosteal envelope and on all subdivisions of the endosteal envelope. Because of the concomitant increase in wall thickness, active osteoblast lifespan increased substantially. Bone formation rate was almost eight times higher on the outer than on the inner periosteum, and more than four times higher on the inner than on the outer endocortical surface. On the cancellous surface, bone formation rate and activation frequency declined in accordance with a fifth order polynomial that matched previously published biochemical indices of bone turnover. The analysis suggested the following conclusions: (1) Between 2 and 20 years the ilium grows in width by periosteal apposition (3.8 mm) and endocortical resorption (3.2 mm) on the outer cortex, and net periosteal resorption (0.4 mm) and net endocortical formation (1.0 mm) on the inner cortex. (2) Cortical width increases from 0.52 mm at age 2 years to 1.14 mm by age 20 years. To attain adult values there must be further endocortical apposition of 0.25 mm by age 30 years, at a time when cancellous bone mass is declining. (3) Lateral modeling drift of the outer cortex enlarges the marrow cavity; the new trabeculae filling this space arise from unresorbed cortical bone and represent cortical cancelization; (4) Lateral modeling drift of the inner cortex encroaches on the marrow cavity; some trabeculae are incorporated into the expanding cortex by compaction. (5) The net addition of 37 microm of new bone on each side of a trabecular plate results from a <5% difference between wall thickness and erosion depth and between bone formation and bone resorption rates; these small differences on the same surface are characteristic of bone remodeling. (6) Because the amount of bone added by each cycle of remodeling is so small, the rate of bone remodeling during growth must be high to accomplish the necessary trabecular hypertrophy.

Options for tissue engineering to address challenges of the aging skeleton

There will be more than 52 million Americans over the age of 65 by the year 2020 (U.S. Census Bureau). Regenerating form and function to bone defects in an elderly, osteoporotic population of this magnitude will be a daunting challenge. Tissue engineering options must be considered to answer this challenge. Options can include gene transfer technology, stem cell therapy, and recombinant signaling molecules. An additional component will be a carrier that localizes, protects, predictably releases cues and cells, as well as establishes an environment for restoring osseous form and function. The purposes of this article are to present an overview of the bone regenerating decrement affecting osteoporotic, elderly patients and to highlight some tissue engineering options that could offset this decrement.

Modulation of bone remodeling by active vitamin D: its role in the treatment of osteoporosis

Active vitamin D drugs are used for the treatment of osteoporosis in a number of countries, including Japan. However, their use is controversial. Here, we briefly discuss two issues that are important for understanding the role of active vitamin D (rather than plain vitamin D) in the treatment of osteoporosis: (1) whether or not its skeletal effects are mediated solely through its effects on intestinal calcium absorption in calcium- and vitamin D-replete states, and (2) how it modulates the bone remodeling process.

Unbiased determination of cytokine localization in bone: colocalization of interleukin-6 with osteoblasts in serial sections from monkey vertebrae

Few data are available describing the in vivo localization of cytokines in bone. The objective of this study was to describe the histological localization of interleukin-6 (IL-6) relative to osteoblasts (alkaline phosphate [ALP]-positive cells) and osteoclasts (tartrate-resistant acid phosphate [TRAP]-positive cells) in midsagittal, paraffin-embedded serial sections of thoracic 13 (T-13) vertebrae from 49 female cynomolgus monkeys. Serial sections 1 and 4 were immunostained for IL-6, section 2 was histochemically stained for TRAP, and section 3 was immunostained for ALP. Sixteen centrally located fields were measured in the cancellous compartment and grid alignment among sections was verified using image analysis. Using a Merz grid, IL-6 localized to 6% of the bone surface on sections 1 and 4, whereas TRAP localized to 8.5% and ALP to 12% of the bone surface. Colocalization was defined as positive staining within an 80 x 80 microm block in the first serial section that "overlapped" staining in either the corresponding block or its eight surrounding blocks within the second serial section. For each section, 1600 blocks were analyzed. Using Monte Carlo simulations, random colocalization was calculated to determine the statistical significance of experimental colocalizations. Colocalization of approximately 90% between the two IL-6 sections verified staining reproducibility and proper grid alignment among sections. Colocalization of TRAP and ALP was not statistically different from random (p 0.3). As identified using ALP- or TRAP-positive surfaces, there was significant IL-6 colocalization with osteoblasts (p < 0.003), but not with osteoclasts (p 0.3). These in vivo colocalization data support the hypothesis that osteoblasts produce and respond to IL-6.

Pathogenesis of bone erosions in rheumatoid arthritis

Patients with rheumatoid arthritis are at risk for the development of a generalized form of bone loss affecting the axial and appendicular skeleton. In addition, juxta-articular osteopenia and focal erosion of marginal and subchondral bone are commonly seen. The pathogenesis of focal bone erosions is an area of active investigation. Studies of tissue sections from sites of bone erosion in rheumatoid arthritis and in animal models of inflammatory arthritis have identified multinucleated cells with the phenotype of osteoclasts in bone resorption lacunae in these sites, suggesting that osteoclasts mediate a component of this pathologic bone loss. Numerous soluble and cell-membrane factors produced by rheumatoid synovial tissues are likely to play a role in the initiation and progression of bone erosions. In addition, recent studies suggest a role for T lymphocytes and their products in osteoclast-mediated bone loss. This paper reviews the cellular mechanisms and factors implicated in bone erosions in rheumatoid arthritis, and discusses the possible therapeutic strategies suggested by these findings.

Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis

The adult skeleton regenerates by temporary cellular structures that comprise teams of juxtaposed osteoclasts and osteoblasts and replace periodically old bone with new. A considerable body of evidence accumulated during the last decade has shown that the rate of genesis of these two highly specialized cell types, as well as the prevalence of their apoptosis, is essential for the maintenance of bone homeostasis; and that common metabolic bone disorders such as osteoporosis result largely from a derangement in the birth or death of these cells. The purpose of this article is 3-fold: 1) to review the role and the molecular mechanism of action of regulatory molecules, such as cytokines and hormones, in osteoclast and osteoblast birth and apoptosis; 2) to review the evidence for the contribution of changes in bone cell birth or death to the pathogenesis of the most common forms of osteoporosis; and 3) to highlight the implications of bone cell birth and death for a better understanding of the mechanism of action and efficacy of present and future pharmacotherapeutic agents for osteoporosis.

Essential requirement of BMPs-2/4 for both osteoblast and osteoclast formation in murine bone marrow cultures from adult mice: antagonism by noggin

Bone morphogenetic proteins (BMPs) have been heretofore implicated in the induction of osteoblast differentiation from uncommitted progenitors during embryonic skeletogenesis and fracture healing. We have tested the hypothesis that BMPs are also involved in the osteoblastogenesis that takes place in the bone marrow in postnatal life. To do this, we took advantage of the properties of noggin, a recently discovered protein that binds BMP-2 and -4 and blocks their action. Addition of human recombinant noggin to bone marrow cell cultures from normal adult mice inhibited both osteoblast and osteoclast formation; these effects were reversed by exogenous BMP-2. Consistent with these findings, BMP-2 and -4 and BMP-2/4 receptor transcripts and proteins were detected in these primary cultures, in a bone marrow-derived stromal/osteoblastic cell line, as well as in murine adult whole bone; noggin expression was also documented in all these preparations. Moreover, addition of antinoggin antibody caused an increase in osteoblast progenitor formation. These findings suggest that BMP-2 and -4 are expressed in the bone marrow in postnatal life and serve to maintain the continuous supply of osteoblasts and osteoclasts; and that, in fact, BMP-2/4-induced commitment to the osteoblastic lineage is a prerequisite for osteoclast development. Hence, BMPs, perhaps in balance with noggin and possibly other antagonists, may provide the tonic baseline control of the rate of bone remodeling on which other inputs (e.g., hormonal, biomechanical, etc.) operate.

VEGF expression in an osteoblast-like cell line is regulated by a hypoxia response mechanism

Angiogenesis is essential for the increased delivery of oxygen and nutrients required for the reparative processes of bone healing. Vascular endothelial growth factor (VEGF), a potent angiogenic growth factor, has been implicated in this process. We have previously shown that hypoxia specifically and potently regulates the expression of VEGF by osteoblasts. However, the molecular mechanisms governing this interaction remain unknown. In this study, we hypothesized that the hypoxic regulation of VEGF expression by osteoblasts occurs via an oxygen-sensing mechanism similar to the regulation of the erythropoietin gene (EPO). To test this hypothesis, we examined the kinetics of oxygen concentration on osteoblast VEGF expression. In addition, we analyzed the effects of nickel and cobalt on the expression of VEGF in osteoblastic cells because these metallic ions mimic hypoxia by binding to the heme portion of oxygen-sensing molecules. Our results indicated that hypoxia potently stimulates VEGF mRNA expression. In addition, we found that nickel and cobalt both stimulate VEGF gene expression in a similar time- and dose-dependent manner, suggesting the presence of a hemelike oxygen-sensing mechanism similar to that of the EPO gene. Moreover, actinomycin D, cycloheximide, dexamethasone, and mRNA stabilization studies collectively established that this regulation is predominantly transcriptional, does not require de novo protein synthesis, and is not likely mediated by the transcriptional activator AP-1. These studies demonstrate that hypoxia, nickel, and cobalt regulate VEGF expression in osteoblasts via a similar mechanism, implicating the involvement of a heme-containing oxygen-sensing molecule. This may represent an important mechanism of VEGF regulation leading to increased angiogenesis in the hypoxic microenvironment of healing bone.

Apoptosis and osteoporosis

During normal bone remodeling, the rate of supply of new osteoblasts and osteoclasts and the timing of the death of osteoclasts, osteoblasts, and osteocytes by apoptosis are critical determinants of the initiation of new BMUs and the extension or reduction of the lifetime of existing ones. Disruption of the fine balance among these processes may be an important mechanism behind the deranged bone turnover found in most metabolic disorders of the adult skeleton. Like most armies, the amount 5 of work done by bone cells is far more dependent on numbers than vigor. Therapeutic agents that alter the prevalence of apoptosis of osteoblasts and osteoclasts can correct the imbalance in cell numbers that is the basis of the diminished bone mass and increased risk of fractures in osteoporosis.

Mechanisms of bone loss in inflammatory arthritis: diagnosis and therapeutic implications

Rheumatoid arthritis represents an excellent model in which to gain insights into the local and systemic effects of joint inflammation on skeletal tissues. Three forms of bone disease have been described in rheumatoid arthritis. These include: focal bone loss affecting the immediate subchondral bone and bone at the joint margins; periarticular osteopenia adjacent to inflamed joints; and generalized osteoporosis involving the axial and appendicular skeleton. Although these three forms of bone loss have several features in common, careful histomorphometric and histopathological analysis of bone tissues from different skeletal sites, as well as the use of urinary and serum biochemical markers of bone remodeling, provide compelling evidence that different mechanisms are involved in their pathogenesis. An understanding of these distinct pathological forms of bone loss has relevance not only with respect to gaining insights into the different pathological mechanisms, but also for developing specific and effective strategies for preventing the different forms of bone loss in rheumatoid arthritis.

Computer simulation of trabecular remodeling using a simplified structural model

A simplified three-dimensional simulation of trabecular bone remodeling has been developed. The model utilizes 441 planar structural units to represent approximately 50 mm3 of initial bone volume with 199 basic multicellular units (BMUs). The simulation takes into account trabecular perforation in the structural model. The cases of male bone remodeling with no menopause and female bone remodeling with menopause are examined from the period of simulated age 25-80 years. Menopause is arbitrarily started at age 45 and extends for 7.5 years. Zero-, first-, and second-order BMU activation responses are employed to examine how the bone would be affected by the method of increase of BMU activation during menopause. At age 80, the female bone remodeling simulation produced a bone volume loss of approximately 49% for all three activation responses. This compared to a 38% bone volume loss for the case of no menopause. For the menopause simulations, an average of about 40% of the total bone loss was due to perforation.

Osteoporose induzida por glicocorticóide

Os glicocorticóides (GC) são substâncias amplamente usadas por sua alta potência anti-inflamatória, porém seu uso é acompanhado de efeitos colaterais sistêmicos importantes. A osteoporose é um dos efeitos deletérios mais previsíveis dos GC e está associada a um alto índice de incapacitação devido a fraturas. Pode ocorrer em 30 a 50% dos pacientes em uso crônico de GC e principalmente em áreas ricas em osso trabecular como a coluna lombar. Na patogênese da osteoporose induzida por GC estão envolvidos vários efeitos sistêmicos e locais sobre o osso e metabolismo mineral. Apesar de muitas vezes a osteoporose ser colocada em segundo plano devido a gravidade da doença de base, o tratamento profilático deve ser instituído sempre, para impedirou diminuir os danos dos GC sobre o tecido ósseo.

The relationship between basic multicellular unit activation and origination in cancellous bone

Activation frequency is often used as a measure of basic multicellular unit (BMU) activity in cancellous bone. However, activation frequency expresses the rate of BMU appearance in a histologic slide and not the rate of origination, which is a more physiologic indicator of remodeling activity and is necessary for the development of BMU-level bone remodeling simulations. Using identical assumptions to those for calculating the activation frequency, it is shown that the origination frequency in cancellous bone is equal to the activation frequency divided by the total distance traveled by the BMU and its width.

On the trabecular "thickness"-number problem

The decrease in trabecular number with aging and in osteoporosis seems to be naturally irreversible. In one view the resulting loss of bone strength would be irreversible too, but in another view the remaining trabeculae could increase their strength, chiefly by thickening. Whether that can occur has become important for osteoporosis research and management as well as for some biomechanical issues. This article reviews evidence and ideas that concern that matter. It suggests why the second view could be correct although previous studies did not detect it. However, that view still needs the kind of proof the general skeletal science community could accept.

Chemical sympathectomy impairs bone resorption in rats: a role for the sympathetic system on bone metabolism

The possibility that the nervous system may control bone metabolism has been raised, as neuromediators physiologically conveyed by sympathetic fibers (eg, vasoactive intestinal peptide) influence bone resorption in vitro. In this study, the sympathetic system was inactivated by treating rats with guanethidine (40 mg/kg/day), a sympathetic neurotoxic, for 21 days, after which a wave of osteoclastic resorption was induced along the mandibular buccal cortex. The effects of denervation were assessed 4 days later (corresponding to the peak of resorption in this model). The rats exhibited ptosis soon after starting guanethidine, proving the success of the sympathectomy. This was associated with a significant increase in calcitonin gene-related peptide- (+54%, p < 0.02) and substance P-immunoreactive sensory fibers (+29%,p < 0.02), a known effect of sympathectomy. For the quantitation of the bone parameters, the study zone was divided into a juxta-osseous alkaline phosphatase-positive osteogenic compartment and a nonosteogenic compartment. In the osteogenic compartment, the resorption surface was reduced by 56% (p < 0.001) in the treated animals, together with a fall in the number of osteoclasts (-25%,p < 0.05) and impaired osteoclast access to the bone surface. Tartrate-resistant acid phosphatase-positive (TRAP+) mononuclear preosteoclasts were found only in this compartment; they were reduced by 43% (p < 0.05) by the sympathectomy. No change in non-specific esterase (NSE)+ osteoclast precursors was found. In the nonosteogenic compartment, vasodilation was the only effect of sympathectomy (+80%,p < 0.05); in particular, the number of NSE+ cells was not modified. Our results indicate that: (1) interactions of NSE+ precursors with osteogenic cells are required for their differentiation into TRAP+ preosteoclasts; (2) the sympathetic nervous system is not involved in osteoclast precursor recruitment; but (3) has a significant effect on resorption by inhibiting preosteoclast differentiation and disturbing osteoclast activation. These data suggest that depletion of sympathetic mediators may disturb osteogenic cell-mediated osteoclast differentiation.

Isoforms of bone alkaline phosphatase: characterization and origin in human trabecular and cortical bone

Alkaline phosphatase (ALP) is a glycoprotein and functions as an ectoenzyme attached to the cell membrane by a hydrophobic glycosyl-phosphatidylinositol (GPI) anchor. Three bone ALP (BALP) isoforms in human serum were separated and quantitated by high-performance liquid chromatography. B/I, a minor fraction, is composed on average of bone (70%) and intestinal (30%) ALP, and two major isoforms, B1 and B2. Treatment with GPI-specific phospholipase C (GPI-PLC) did not influence the activities or retention times for B1 and B2, indicating that the biochemical differences between B1 and B2 are likely to be due to different glycosylation patterns. The B/I fraction in serum, on average 4% of total ALP, was found to be composed of B1 and B2 isoforms, each with an intact hydrophobic GPI cell membrane anchor. We investigated the origin of these three BALP isoforms and osteocalcin in human femora from five healthy individuals (four males), mean age 51 years, obtained from a tissue bank. Bone was sampled from three sites: cortical bone, trabecular bone from the diaphysis, and trabecular bone from the greater trochanter. Trabecular bone, from both sites, had higher BALP activities compared with cortical bone. Conversely, the osteocalcin content of cortical bone was more than 3-fold greater than that of trabecular bone. Cortical bone had approximately 2-fold higher activity of B1 compared with B2, whereas trabecular bone had approximately 2-fold higher activity of B2 compared with B1. We observed a previously undescribed BALP isoform (B1x) in all bone samples. B1x was also observed in sera from some patients (60%) with severe renal insufficiency and on chronic dialysis therapy (n = 20). The isoforms of BALP may provide information relating to bone metabolism within specific bone compartments.

Inhibition of TGF-beta receptor signaling in osteoblasts leads to decreased bone remodeling and increased trabecular bone mass

Transforming growth factor-beta (TGF-beta) is abundant in bone matrix and has been shown to regulate the activity of osteoblasts and osteoclasts in vitro. To explore the role of endogenous TGF-(beta) in osteoblast function in vivo, we have inhibited osteoblastic responsiveness to TGF-beta in transgenic mice by expressing a cytoplasmically truncated type II TGF-beta receptor from the osteocalcin promoter. These transgenic mice develop an age-dependent increase in trabecular bone mass, which progresses up to the age of 6 months, due to an imbalance between bone formation and resorption during bone remodeling. Since the rate of osteoblastic bone formation was not altered, their increased trabecular bone mass is likely due to decreased bone resorption by osteoclasts. Accordingly, direct evidence of reduced osteoclast activity was found in transgenic mouse skulls, which had less cavitation and fewer mature osteoclasts relative to skulls of wild-type mice. These bone remodeling defects resulted in altered biomechanical properties. The femurs of transgenic mice were tougher, and their vertebral bodies were stiffer and stronger than those of wild-type mice. Lastly, osteocyte density was decreased in transgenic mice, suggesting that TGF-beta signaling in osteoblasts is required for normal osteoblast differentiation in vivo. Our results demonstrate that endogenous TGF-beta acts directly on osteoblasts to regulate bone remodeling, structure and biomechanical properties.

Effects of high-impact exercise on ultrasonic and biochemical indices of skeletal status: A prospective study in young male gymnasts

Physical activity has been proposed as one strategy to enhance bone mineral acquisition during growth. The aim of this study was to determine whether frequent impact loading associated with gymnastics training confers a skeletal benefit on pre- and peripubertal male gymnasts. We measured broadband ultrasonic attenuation (BUA, dB/MHz) at the calcaneus (CBUA); ultrasound velocity (m/s) at the calcaneus (CVOS), distal radius (RVOS) and phalanx (PVOS); serum osteocalcin (OC); total alkaline phosphatase (ALP) and insulin-like growth factor-I (IGF-I) every 3-4 months over an 18-month period in elite male gymnasts and matched normoactive controls (pubertal stage </=2). Ground reaction forces of common gymnastics maneuvers were determined using a force platform and loading histories of the upper and lower extremities approximated from video recordings. Ultrasound results were expressed as a standardized score (Z score) adjusted for age, height, and weight. At baseline, no differences were detected between the gymnasts (n = 31) and controls (n = 50) for CBUA, although ultrasound velocity at each site was higher in the gymnasts (0.6-1.5 SD) than the predicted mean in controls (p </= 0. 001). Over 18 months, CBUA Z scores increased significantly in the gymnasts from baseline (0.3 vs. 1.0, p < 0.05, n = 18). In contrast, ultrasound velocity did not increase in either group, although CVOS and RVOS remained significantly higher in gymnasts compared with controls (range p < 0.01 and < 0.001). No differences between groups were found for OC, ALP, or IGF-I at any time. Gymnastics training was associated with on average 102 and 217 impacts per session on the upper and lower extremities, respectively, with peak magnitudes of 3.6 and 10.4 times body weight. These results suggest that frequent high-impact, weight-bearing exercise during the pre and peripubertal period may enhance the mechanical competence of the skeleton, perhaps offering an important strategy for osteoporosis prevention if the benefits are maintained.

Influence of biphasic calcium phosphate granulometry on bone ingrowth, ceramic resorption, and inflammatory reactions: preliminary in vitro and in vivo study

Calcium-phosphate ceramics used in surgery, as bone-bonding materials, are currently available in different forms (blocks, granules, etc.). However, progress in noninvasive surgery has favored the development of injectable composite materials associating a polymeric and a dusty mineral phase. The purpose of this study was the in vivo evaluation of biphasic calcium phosphate of various grains sizes, to elucidate the role of granulometries in ceramic degradation/resorption, bone ingrowth, and inflammatory reactions. Three particle sizes were compared: 10-20, 80-100, and 200-400 microm. The 10-20-microm powders provided the best bone ingrowth, with a higher resorption/degradation rate in conjunction with stronger early inflammatory reactions. The 200-400-microm powders showed higher bone ingrowth than 80-100-microm ones, indicating that properties of cell recruitment for osseous apposition and mechanical support for bone bonding may both play a role in both ingrowth mechanisms. Our results suggest that the strong inflammatory reaction in 10-20-microm granulated powders was due to a faster reversal of the resorption/apposition sequence in bone. This may have resulted from massive release of bone ingrowth factors, which implies that the brief inflammatory process observed in the early stages of implantation was favorable to the osteoconduction process.

Selective attachment of osteoprogenitors to laminin

During endochondral ossification and bone remodeling, osteoprogenitors (OP) attach to the matrix and differentiate into osteoblasts. To identify matrix proteins binding specifically these precursors, fetal rat calvaria (RC) cells were plated for 5-20 min in serum-free medium, on wells coated with various proteins and saturated with bovine serum albumin (BSA) to block nonspecific binding sites. Adherent cells were either counted or grown to assess bone colony (nodule) formation. As each nodule originates from the clonal division of one OP, the ratio (nodules/100 cells attached) measures the proportion of OP among adherent cells. Of numerous purified matrix proteins tested, laminin-1 and tenascin inhibited cell attachment, whereas fibronectin, bone sialoprotein, and type I collagen increased cell attachment and others had no effect. Only laminin-1 and, to a lesser extent, tenascin, enriched the cell population in OP. Laminin-1 acted time- and dose-dependently. In experiments in which cell attachment to laminin-coated but unsaturated wells was ensured by plating for 24 h in 10% fetal calf serum, laminin-1 had no effect on cell attachment nor on OP differentiation. In contrast, repeated plating of RC cells on laminin-1-coated/saturated wells depleted the population in OP, confirming that OP selection was a cell-attachment effect. When RC cell populations isolated by successive collagenase extractions were compared, the highest rate of OP enrichment on laminin-1 was obtained with the earliest populations, which were the most responsive to dexamethasone, a marker of early OP stages. In conclusion, laminin-1 recruits in vitro, through a cell-attachment effect, OP present in early RC cell populations, of which laminins are abundant extracellular matrix components.

Pamidronate reduces skeletal morbidity in women with advanced breast cancer and lytic bone lesions: a randomized, placebo-controlled trial. Protocol 18 Aredia Breast Cancer Study Group

PURPOSE

To assess whether pamidronate can reduce the frequency of skeletal morbidity in women with lytic bone metastases from breast cancer treated with hormone therapy.

PATIENTS AND METHODS

Three hundred seventy-two women with breast cancer who had at least one lytic bone lesion and who were receiving hormonal therapy were randomized to receive 90 mg of pamidronate or placebo as a 2-hour intravenous infusion given in double-blind fashion every 4 weeks for 24 cycles. Patients were evaluated for skeletal complications: pathologic fractures, spinal cord compression, irradiation of or surgery on bone, or hypercalcemia. The skeletal morbidity rate (the ratio of the number of skeletal complications to the time on trial) was the primary efficacy variable. Bone pain, use of analgesics, quality of life, performance status, bone tumor response, and biochemical parameters were also evaluated.

RESULTS

One hundred eighty-two patients who received pamidronate and 189 who received placebo were assessable. The skeletal morbidity rate was significantly reduced at 12, 18, and 24 cycles in patients treated with 90 mg of pamidronate (P = .028, .023, and .008, respectively). At 24 cycles, the proportion of patients having had any skeletal complication was 56% in the pamidronate group and 67% in the placebo group (P = .027). The time to the first skeletal complication was longer for patients receiving pamidronate than for those given placebo (P = .049). There was no statistical difference in survival or in objective bone response rate. Pamidronate was well tolerated.

CONCLUSION

Treatment with 90 mg of pamidronate as a 2-hour intravenous infusion every 4 weeks in addition to hormonal therapy significantly reduces skeletal morbidity from osteolytic metastases.

A comparison of radial peripheral quantitative computed tomography, calcaneal ultrasound, and axial dual energy X-ray absorptiometry measurements in women aged 45-55 yr

Perimenopausal bone loss is considered to affect trabecular bone preferentially. Peripheral quantitative computed tomography (pQCT) quantifies trabecular bone mineral density (BMD) independently at the ultradistal radius. This article examines differences in pQCT BMD between late premenopausal and early postmenopausal women, comparing the differences with calcaneal ultrasound and axial dual energy X-ray absorptiometry measurements. One hundred nineteen normal perimenopausal women aged 45-55 yr who attended a randomized osteoporosis screening program were stratified by menopausal status into premenopausal (PRE: n = 79) and postmenopausal (POST: n = 40) groups. All measurements were lower in the postmenopausal group with the exception of ultrasonic velocity (PRE vs POST: 1397 +/- 53.8 vs 1421 +/- 58.5 m/s, p = 0.037). Total (391.8 +/- 52.9 vs 366.3 +/- 68.6 g/cm(3), p = 0.013) and subcortical (533.6 +/- 59.4 vs 504.3 +/- 79.8 g/cm(3) p = 0.018), but not trabecular (187.5 +/- 38.8 vs 173.2 +/- 46.6 g/cm(3), p = 0. 098) or cortical (561 +/- 53.4 vs 551.2 +/- 66 g/cm(3), p = 0.174), pQCT BMD measurements were significantly lower in the POST group, as were ultrasonic attenuation (79.4 +/- 16 vs 72.3 +/- 18.0 dB/Mz, p = 0.034), DXA spine (1.032 +/-16 vs 0.959 +/- 0.2 g/cm(2), p = 0.003), and all hip (p </= 0.001) measurements. Although body mass index (BMI) was positively and menopausal status and age negatively correlated with most bone mass measurements, adjusting for BMI did not alter the relative deficits in postmenopausal compared with premenopausal women. This study suggests that early postmenopausal bone loss at the radius preferentially affects subcortical, rather than trabecular, bone in the appendicular skeleton, which suggests preferential trabecular bone loss in the axial skeleton.

Differences of bone alkaline phosphatase isoforms in metastatic bone disease and discrepant effects of clodronate on different skeletal sites indicated by the location of pain

We compared clodronate with placebo administration in 42 primarily or secondarily hormone-refractory prostate cancer patients with skeletal metastases and persisting pain. Serum total alkaline phosphatase (ALP), bone ALP isoforms, osteocalcin, cross-linked carboxy-terminal telopeptide of type I collagen, and prostate-specific antigen were analyzed before and after 1 month of treatment. Six ALP isoforms were quantified by HPLC: one bone/intestinal, two bone (B1, B2), and three liver ALP isoforms. The most apparent difference compared with healthy males was observed for the bone ALP isoform B2. Patients and healthy males had a B2 activity corresponding to 75% and 35% of the total ALP activity, respectively (P &lt;0.0001). We propose that the different bone ALP isoforms reflect different stages of osteoblast differentiation during the extracellular matrix maturation phase of osteogenesis. All bone markers except osteocalcin increased after 1 month of clodronate administration. These increases were associated with pain only in the upper part of the body. We suggest that the uptake of clodronate by the skeleton was not uniform during our treatment period.

Bisphosphonates and osteoporosis treatment in Italy

A treatment against osteoporosis can be considered efficacious only when its beneficial effects on bone remodeling, bone mass, and osteoporotic fracture incidence are proven. As for any treatment, proven efficacy must be combined with proven safety. This review analyzes published data on efficacy and safety of alendronate, clodronate and etidronate, the bisphosphonates currently marketed in Italy for osteoporosis treatment. Different studies have shown that alendronate, clodronate and etidronate reduce bone turnover, and increase bone mineral density in postmenopausal osteoporotic patients. Prospective, double blind, multicenter studies reported a reduction in osteoporotic fracture incidence for alendronate and etidronate. Fracture incidence reduction by clodronate, on the other hand, was shown only in an open label study. Finally, a reduction in fracture incidence by etidronate was shown in a large retrospective postmarketing study. Postmarketing surveillance evidenced that osteomalacia, a suspected side effect of etidronate treatment, does not occur at the currently used dose regimens. Postmarketing surveillance of alendronate has recently raised some concern regarding possible severe esophageal damage during alendronate treatment, especially when the drug is not taken according to the manufacturer's instructions.

Inhibition of osteoblastogenesis and promotion of apoptosis of osteoblasts and osteocytes by glucocorticoids. Potential mechanisms of their deleterious effects on bone

Glucocorticoid-induced bone disease is characterized by decreased bone formation and in situ death of isolated segments of bone (osteonecrosis) suggesting that glucocorticoid excess, the third most common cause of osteoporosis, may affect the birth or death rate of bone cells, thus reducing their numbers. To test this hypothesis, we administered prednisolone to 7-mo-old mice for 27 d and found decreased bone density, serum osteocalcin, and cancellous bone area along with trabecular narrowing. These changes were accompanied by diminished bone formation and turnover, as determined by histomorphometric analysis of tetracycline-labeled vertebrae, and impaired osteoblastogenesis and osteoclastogenesis, as determined by ex vivo bone marrow cell cultures. In addition, the mice exhibited a threefold increase in osteoblast apoptosis in vertebrae and showed apoptosis in 28% of the osteocytes in metaphyseal cortical bone. As in mice, an increase in osteoblast and osteocyte apoptosis was documented in patients with glucocorticoid-induced osteoporosis. Decreased production of osteoclasts explains the reduction in bone turnover, whereas decreased production and apoptosis of osteoblasts would account for the decline in bone formation and trabecular width. Furthermore, accumulation of apoptotic osteocytes may contribute to osteonecrosis. These findings provide evidence that glucocorticoid-induced bone disease arises from changes in the numbers of bone cells.

Osteoblastic responses to TGF-beta during bone remodeling

Bone remodeling depends on the spatial and temporal coupling of bone formation by osteoblasts and bone resorption by osteoclasts; however, the molecular basis of these inductive interactions is unknown. We have previously shown that osteoblastic overexpression of TGF-beta2 in transgenic mice deregulates bone remodeling and leads to an age-dependent loss of bone mass that resembles high-turnover osteoporosis in humans. This phenotype implicates TGF-beta2 as a physiological regulator of bone remodeling and raises the question of how this single secreted factor regulates the functions of osteoblasts and osteoclasts and coordinates their opposing activities in vivo. To gain insight into the physiological role of TGF-beta in bone remodeling, we have now characterized the responses of osteoblasts to TGF-beta in these transgenic mice. We took advantage of the ability of alendronate to specifically inhibit bone resorption, the lack of osteoclast activity in c-fos-/- mice, and a new transgenic mouse line that expresses a dominant-negative form of the type II TGF-beta receptor in osteoblasts. Our results show that TGF-beta directly increases the steady-state rate of osteoblastic differentiation from osteoprogenitor cell to terminally differentiated osteocyte and thereby increases the final density of osteocytes embedded within bone matrix. Mice overexpressing TGF-beta2 also have increased rates of bone matrix formation; however, this activity does not result from a direct effect of TGF-beta on osteoblasts, but is more likely a homeostatic response to the increase in bone resorption caused by TGF-beta. Lastly, we find that osteoclastic activity contributes to the TGF-beta-induced increase in osteoblast differentiation at sites of bone resorption. These results suggest that TGF-beta is a physiological regulator of osteoblast differentiation and acts as a central component of the coupling of bone formation to resorption during bone remodeling.

The role of parathyroid hormone in the pathogenesis, prevention and treatment of postmenopausal osteoporosis

Parathyroid hormone (PTH) is the principal regulator of bone remodeling in the adult skeleton. The acute in vivo effect of PTH is to increase bone resorption, although sustained increases in its circulating levels accelerate both formation and resorption. These divergent effects have focused attention on PTH as a factor contributing to bone loss in some postmenopausal women, as well as interest in its role as therapy for the disease. Sustained increases in PTH are classically seen in primary hyperparathyroidism. While still controversial, increasing evidence suggests that primary hyperparathyroidism is associated with increased rates of bone loss, particularly from cortical sites in the skeleton. It is clear that the remodeling space is increased in primary hyperparathyroidism, and that surgical correction of the disease leads to substantial increases in bone mass in patients with osteoporosis. Recently, secondary hyperparathyroidism has emerged as an important contributor to increased rates of bone turnover and bone loss in postmenopausal women. The etiology of secondary hyperparathyroidism in postmenopausal women is complex, and is probably related to alterations in vitamin D metabolism and tissue responsiveness to 1,25(OH)2vitamin D. PTH has emerged at the forefront of anabolic therapies for the treatment of postmenopausal osteoporosis. When given as a single agent, intermittent daily subcutaneous administration of PTH induces consistent gains in trabecular bone mass with more varying effects on the cortical envelope. However, recent therapeutic trials employing a second agent, most notably estrogen, give hope that this approach may provide the first truly efficacious anabolic therapy for this devastating disease.

Cellular and molecular mechanisms of osteoporosis

Osteoblasts and osteoclasts are derived from progenitors originating in the bone marrow, and the process of bone remodeling is controlled by growth factors and cytokines which regulate the birth and death of these cells. An overproduction of osteoclasts relative to the need for remodeling, and an undersupply of osteoblasts relative to the need for cavity repair, represent the fundamental pathophysiologic changes in postmenopausal and age-related osteopenia, respectively. As in these two forms of the disease, the osteoporosis induced by glucocorticoid excess is also caused by changes in the birth and death of bone cells, and in particular a decrease in osteoblastogenesis in the bone marrow, and an increased rate of osteoblast and osteocyte apoptosis.

Osteoblast programmed cell death (apoptosis): modulation by growth factors and cytokines

Once osteoblasts have completed their bone-forming function, they are either entrapped in bone matrix and become osteocytes or remain on the surface as lining cells. Nonetheless, 50-70% of the osteoblasts initially present at the remodeling site cannot be accounted for after enumeration of lining cells and osteocytes. We hypothesized that the missing osteoblasts die by apoptosis and that growth factors and cytokines produced in the bone microenvironment influence this process. We report that murine osteoblastic MC3T3-E1 cells underwent apoptosis following removal of serum, or addition of tumor necrosis factor (TNF), as indicated by terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling and DNA fragmentation studies. Transforming growth factor-beta and interleukin-6 (IL-6)-type cytokines had antiapoptotic effects because they were able to counteract the effect of serum starvation or TNF. In addition, anti-Fas antibody stimulated apoptosis of human osteoblastic MG-63 cells and IL-6-type cytokines prevented these changes. The induction of apoptosis in MG-63 cells was associated with an increase in the ratio of the proapoptotic protein bax to the antiapoptotic protein bcl-2, and oncostatin M prevented this change. Examination of undecalcified sections of murine cancellous bone revealed the presence of apoptotic cells, identified as osteoblasts by their proximity to osteoid seams and their juxtaposition to cuboidal osteoblasts. Assuming an osteoblast life span of 300 h and a prevalence of apoptosis of 0.6%, we calculated that the fraction that undergo this process in vivo can indeed account for the missing osteoblasts. These findings establish that osteoblasts undergo apoptosis and strongly suggest that the process can be modulated by growth factors and cytokines produced in the bone microenvironment.

Effect of estrogen suppression on the mineralization density of iliac crest biopsies in young women as assessed by backscattered electron imaging

The effects of estrogen suppression on bone mineralization in young women were studied by quantitative backscattered electron (BSE) imaging of transiliac biopsies taken before and after treatment for endometriosis. Treatment (6 months) was with analogs of gonadotrophin releasing hormone (GnRH) given either alone (six paired biopsies), which resulted in a marked reduction in the levels of circulating estrogen, or in conjunction with tibolone, a synthetic steroid with estrogenic, progestrogenic, and androgenic properties (four paired biopsies). Estrogen withdrawal increased (p < 0.01) and concomitant tibolone treatment decreased (p < 0.05) the overall mean bone density. Estrogen withdrawal increased the fraction of bone with a high mineralization density [pretreatment: 0.236+/-0.007; GnRH: 0.279+/-0.009, mean +/- standard error of the mean (SEM); p < 0.01]. The concomitant addition of tibolone reversed these effects and increased the proportion of bone with a low mineralization density (pretreatment: 0.198 +/- 0.005; tibolone: 0.230 +/-0.008, p < 0.01). Using previously published data, the mean bone density was inversely correlated with mean wall thickness in cancellous bone (p = 0.030) and with the percentage of active osteons (p = 0.023) in cortical bone. Although treatment had similar effects on the mean bone mineralization density of cortical and cancellous bone, there were different distributions of mineralization between the two sites, with cancellous bone having more skewed and kurtotic distributions both before and after estrogen withdrawal. This study indicates that a short-term estrogen suppression results in the accumulation of bone with a higher mineralization density. As bone with a high mineral content has a decreased impact resistance, this might increase fracture risk. Understanding the cellular and biochemical mechanisms responsible for the local distribution of bone mineral when estrogen is withdrawn may allow the development of new strategies for maintaining bone quality after menopause.

Activation of the Janus kinase/STAT (signal transducer and activator of transcription) signal transduction pathway by interleukin-6-type cytokines promotes osteoblast differentiation

We have previously established that stromal/osteoblastic cells collectively express receptors for all members of the cytokine subfamily that share the gp130 signal transducer and that different receptor repertoires may be expressed at different stages of differentiation of this lineage. We have now used human (MG-63) and murine (MC3T3-E1) osteoblastic cell lines as well as primary murine calvaria cells to test the hypothesis that these receptors mediate effects of the cytokines on the biology of osteoblasts. We report that as in other cell types, all of the osteoblastic cell models responded to interleukin-6 (IL-6)-type cytokines with activation of both the JAK/STAT (Janus kinase/signal transducer and activator of transcription) and the mitogen-activated protein kinase (MAPK) pathways. In addition, IL-6-type cytokines stimulated alkaline phosphatase activity and osteocalcin expression and inhibited (MG-63), stimulated (MC3T3-E1), or had no effect (calvaria cells) on the rate of cell proliferation. The ability of a given cell type to respond to a particular member of this family of cytokines was strictly dependent on the presence of the corresponding ligand-binding subunit (alpha) of the cytokine receptor, and the magnitude of all the effects was closely correlated with the concentration of this subunit. The relative contribution of the JAK/STAT and MAPK pathways to the biological effects of the cytokines was evaluated using kinase inhibitors. Cytokine-mediated modulation of cell proliferation as well as stimulation of alkaline phosphatase activity were abrogated by tyrosine kinase inhibitors as well as a threonine/serine kinase inhibitor, but were only minimally affected by a specific inhibitor of MAPK phosphorylation. These results demonstrate that IL-6-type cytokines, besides their osteoclastogenic properties, promote differentiation of committed osteoblastic cells toward a more mature phenotype and that this action is mediated primarily via the activation of the JAK/STAT pathway.

The functional matrix hypothesis revisited. 2. The role of an osseous connected cellular network

Intercellular gap junctions permit bone cells to intercellularly transmit, and subsequently process, periosteal functional matrix information, after its initial intracellular mechanotransduction. In addition, gap junctions, as electrical synapses, underlie the organization of bone tissue as a connected cellular network, and the fact that all bone adaptation processes are multicellular. The structural and operational characteristics of such biologic networks are outlined and their specific bone cell attributes described. Specifically, bone is "tuned" to the precise frequencies of skeletal muscle activity. The inclusion of the concepts and databases that are related to the intracellular and intercellular bone cell mechanisms and processes of mechanotransduction and the organization of bone as a biologic connected cellular network permit revision of the functional matrix hypothesis, which offers an explanatory chain, extending from the epigenetic event of muscle contraction hierarchically downward to the regulation of the bone cell genome.

Cortical remodeling following suppression of endogenous estrogen with analogs of gonadotrophin releasing hormone

The effects of estrogen suppression on osteonal remodeling in young women was investigated using transiliac biopsies (eight paired biopsies + four single pre; three single post biopsies) taken before and after treatment for endometriosis (6 months) with analogs of gonadotrophin releasing hormone (GnRH). Estrogen withdrawal increased the proportion of Haversian canals with an eroded surface (106%, p = 0.047), a double label (238%, p = 0.004), osteoid (71%, p = 0.002), and alkaline phosphatase (ALP) 116%, p = 0.043) but not those showing tartrate-resistant acid phosphatase (TRAP) activity (p = 0.25) or a single label (p = 0.30). Estrogen withdrawal increased TRAP activity in individual osteoclasts in canals with diameters greater than 50 microns (p = 0.0089) and also the number of osteons with diameters over 250 microns (p = 0.049). ALP activity in individual osteoblasts was increased but not significantly following treatment (p = 0.051). Wall thickness was significantly correlated with osteon diameter (p < 0.001). In a separate group of patients (four pairs + one post biopsy) on concurrent treatment with tibolone, there was no significant increase in the osteon density, cortical porosity, median canal diameter, or the markers of bone formation and resorption. Enzyme activities and numbers of active canals were also not increased with the concurrent treatment, but there was still an increase in the osteon diameter. As previously shown for cancellous bone, estrogen withdrawal increased cortical bone turnover. We have now shown that resorption depth within Haversian systems was also increased with treatment. The enhanced TRAP activity in individual osteoclasts supports the concept that osteoclasts are more active following estrogen withdrawal in agreement with theoretical arguments advanced previously. Understanding the cellular and biochemical mechanisms responsible for increased depth of osteoclast resorption when estrogen is withdrawn may allow the development of new strategies for preventing postmenopausal bone loss.

The pathophysiology and treatment of postmenopausal osteoporosis. An evidence-based approach to estrogen replacement therapy

Osteoporosis is one of the most common and debilitating diseases of postmenopausal women. Recent advances in understanding the bone remodeling unit have clarified the pathophysiologic processes that contribute to bone loss after the onset of estrogen deprivation. Epidemiologic studies have suggested a protective effect from long-term estrogen replacement therapy on fracture risk. This article examines the key role estrogens play in bone remodeling and the current evidence that estrogen treatment in postmenopausal women reduces the likelihood of osteoporotic fractures.

Transgenic mice expressing soluble tumor necrosis factor-receptor are protected against bone loss caused by estrogen deficiency

To evaluate the role of tumor necrosis factor (TNF alpha) in bone loss resulting from estrogen deficiency, the effects of ovariectomy were explored in six-month-old transgenic mice expressing high blood levels of a soluble TNF receptor type I (sTNFR1)-FcIgG3 fusion protein, which neutralizes TNF alpha, and in their nontransgenic littermates used as controls. These transgenic mice were identical to control mice in bone mass (evaluated by bone mineral density and content) and strength. 12 weeks after ovariectomy, the decrease in bone mass and increase in osteocalcin (marker of bone turnover) found in control mice were not observed in transgenic mice, which were not different from sham-operated mice, transgenic or not. This observation suggests a critical role for TNF alpha in the pathogenesis of bone loss induced by estrogen deficiency, a common cause of morbidity in postmenopausal women.

Bone biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. Bone modelling is the process associated with growth and re-shaping of bones in childhood and adolescence. This is distinguished from bone remodelling, which describes the lifelong process whereby skeletal tissue is continually being resorbed and replaced in order to maintain skeletal integrity, shape and mass. Bone remodelling is controlled by systemic hormones and cytokines and is an integral part of the calcium homeostatic system. The maintenance of a normal, healthy skeletal mass depends on interactions between osteoblasts, osteoclasts and constituents of the bone matrix to keep the process of bone resorption and formation in balance. The factors, local and systemic, which regulate these processes are discussed.

Proliferating cells in the primary spongiosa express osteoblastic phenotype in vitro

We have shown that intermittent parathyroid hormone (PTH) treatment targets proliferating cells in the primary spongiosa of trabecular bone of young rats, resulting in an increased number of osteoblasts. To further characterize these proliferating osteoprogenitor cells, bromodeoxyuridine (BrdUrd) incorporated in vivo, was used as a marker to identify and isolate cells for in vitro studies. Proliferating cells were labeled in vivo in young rats with BrdUrd and 24 h later were isolated by trypsinization of sections of the primary spongiosa of the distal femur metaphysis. Within 12 h of isolation, BrdUrd+ cells formed distinct foci containing 20-500 cells with fibroblast morphology. Stimulation of proliferation as determined by [3H]-thymidine incorporation was observed for these cells in response to fetal bovine serum, platelet derived growth factor, and transforming growth factor beta-1. Neither insulin-like growth factor-1 (IGF-1) nor insulin stimulated proliferation PTH (1-34) and dexamethasone inhibited proliferation. The effects of PTH and dexamethasone were additive. Cells expressed the osteoblast phenotype as evidenced by synthesis of type I collagen, expression of high alkaline phosphatase activity, and production of increased intracellular cAMP in response to PTH (1-34). Confluent cell aggregates spontaneously formed mineralized nodules within 4-7 days, in the absence of inducers. These observations suggest that the primary spongiosa cells recapitulates the differentiation process in vitro in an accelerated fashion and may serve as a useful model to study osteoblast differentiation.

Embedding of bone samples in methylmethacrylate: an improved method suitable for bone histomorphometry, histochemistry, and immunohistochemistry

Methylmethacrylate (MMA) embedding of undecalcified bone biopsies is a technique widely used for bone histomorphometry. However, conventional MMA embedding causes almost complete loss of enzyme activity and protein antigenicity in the tissues. Recently, an MMA embedding technique has been reported that preserves enzyme activity and antigenic determinants in bone tissue. We describe here a modification of this embedding method. For our modified MMA embedding process, commercially available methacrylates can be used without purification, and the histologic quality of bone sections is comparable to that of conventionally MMA-embedded bone specimens. The technique reported here can be employed for embedding of larger bone samples and is suitable for histochemical and immunohistological applications as well as for routine bone histomorphometry. By addition of methylbenzoate during infiltration and polymerization of the plastic, the antigenicity of the tissue was improved. As applications of this novel technique, demonstration of alkaline phosphatase and tartrate-resistant acid phosphatase as well as positive labeling of Kupffer cells and osteoclasts with the monoclonal antibody ED1 in sections of liver, tibiae, and vertebrae of 3-month-old rats was demonstrated. The method described here might be useful for the inclusion of histochemical and immunohistological methods into bone histomorphometry.