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

The mechanical or metabolic function of secondary osteonal bone in the monkey Macaca fascicularis

Secondary osteonal bone is believed by many to serve a mechanical function, altering the properties and/or orientation of bone in response to fluctuating mechanical demands or in the prevention and/or repair of fatigue microdamage. Based on this belief, secondary osteons should be concentrated mainly in regions experiencing high peak-strain conditions. Others contend that secondary osteonal bone functions primarily in meeting the body's calcium needs, and should be expected to form principally in low peak-strain regions so as to avoid compromising the mechanical strength of the bone. These two hypotheses were tested by examining the distribution of secondary osteonal bone in both relatively high- and low-strain regions of the macaque face. Previous strain-gauge studies have demonstrated a steep strain gradient in the macaque face, with relatively high peak strains in the anterior portion of the zygomatic arch and in the mandibular corpus. Relatively low peak strains have been found in the posterior portion of the zygomatic arch and supraorbital bar. Results presented here show that in the mature macaques, there is no consistent relation between newly forming secondary osteons (i.e. those labelled with fluorescent dyes) and peak strain levels. From these data it is concluded that, in the non-perturbed adult, either mechanical and metabolic factors contribute equally to the observed pattern or that metabolically driven remodelling is initiated without regard to strain levels. In immature macaques, however, the relation between peak strain levels and secondary osteon density is positive, with a significantly higher density of labelled osteons in the high strain regions. From these data it is concluded that, in immature individuals, mechanical factors are predominantly responsible for the initiation of secondary osteonal remodelling.

Mice lacking tartrate-resistant acid phosphatase (Acp 5) have disrupted endochondral ossification and mild osteopetrosis

Mature osteoclasts specifically express the purple, band 5 isozyme (Acp 5) of tartrate-resistant acid phosphatase, a binuclear metalloenzyme that can generate reactive oxygen species. The function of Acp 5 was investigated by targeted disruption of the gene in mice. Animals homozygous for the null Acp 5 allele had progressive foreshortening and deformity of the long bones and axial skeleton but apparently normal tooth eruption and skull plate development, indicating a role for Acp 5 in endochondral ossification. Histomorphometry and mineralization density analysis of backscattered electron imaging revealed widened and disorganized epiphyseal growth plates with delayed mineralization of cartilage in 6- to 8-week-old mutant mice. The membrane bones of the skull showed increased density at all ages examined, indicating defective osteoclastic bone turnover. Increased mineralization density was observed in the long bones of older animals which showed modelling deformities at their extremities: heterozygotes and homozygous Acp 5 mutant mice had tissue that was more mineralized and occupied a greater proportion of the bone in all regions. Thus the findings reflect a mild osteopetrosis due to an intrinsic defect of osteoclastic modelling activity that was confirmed in the resorption pit assay in vitro. We conclude that this bifunctional metalloprotein of the osteoclast is required for normal mineralization of cartilage in developing bones; it also maintains integrity and turnover of the adult skeleton by a critical contribution to bone matrix resorption.

Trabecular and endocortical bone surfaces in the rat: modeling or remodeling?

BACKGROUND

There is conflicting evidence as to whether bone resorption and bone formation are coupled in the site-specific manner that is typical of bone remodeling in the rat. The aim of this study was to elucidate this controversy further by analysis of tibial and vertebral cancellous and endocortical bone in rats of different age groups with a combination of in vivo fluorochrome labeling with cement line staining.

METHODS

After multiple in vivo fluorochrome labeling, groups of female Fischer-344 rats were killed at 3, 6, 9, and 12 months of age, and the first lumbar vertebrae and the proximal tibiae were processed undecalcified for bone histomorphometry. By comparing fluorochrome labeling and the contour of cement lines in serial sections, cancellous and endocortical bone formation sites were classified as "remodeling," "modeling," or "uncertain."

RESULTS

In vertebral cancellous bone, remodeling was the main turnover activity in all age groups, increasing from 70.4 +/- 2.2% (mean +/- SEM) in 3-month-old rats to 91.0 +/- 2.4% in 12-month-old rats. The percentage of modeling sites decreased from 17.1 +/- 1.7% at age 3 months to 4.67 +/- 1.84% at age 12 months. In the proximal tibial metaphysis of 3-month-old rats, 61.6 +/- 3.6% of all trabecular bone-forming sites were classified as modeling and 21.1 +/- 3.1% as remodeling sites. In the 12-month-old rats, 66.3 +/- 3.4% were classified as remodeling and 16.0 +/- 3.1% as modeling sites. A similar trend toward augmented portions of remodeling with increasing age was observed in tibial and vertebral endocortical bone-formation sites.

CONCLUSIONS

The present study suggests that, similar to higher mammals, the prevailing activity in vertebral and tibial cancellous bone of aged rats is remodeling. In the rapidly growing proximal tibia of 3-month-old rats, however, most of the cancellous bone-forming sites were minimodeling sites.

Tissue adaptation as a dynamical process far from equilibrium

It is well established that tissue growth, maintenance, and degeneration are biochemically regulated processes influenced by mechanical function. Biomechanical models have been developed to predict adaptive processes; for example, computer simulation of bone remodeling around orthopaedic implants can accurately predict the effect of certain implant design variables. However, the same success remains to be achieved with other adaptive processes such as joint morphogenesis or osteoporosis. We propose that, to become capable of stimulating such adaptive processes, biomechanical models should capture the inherently irreversible nature of tissue adaptation and therefore should not rely on the assumption of a "homeostatic" equilibrium. In this article, it is proposed that tissue adaptation is an unstable process of moving between tissue states that are far from the equilibrium state--and that to simulate it, independent sensors and positive feedback stimuli should be employed.

Development and characterization of a conditionally transformed adult human osteoblastic cell line

Many osteoblastic cell lines are currently in use, but these have limitations either in terms of their relevance to adult human biology and disease or in terms of their suitability for biochemical and molecular analyses. Consequently, we undertook the development of conditionally transformed adult human osteoblastic cell lines. Osteoblasts were obtained from a normal explant cancellous bone chip culture. These cells were infected with adenovirus-ori-SV40 tsA 209, which encodes a temperature-sensitive large T-antigen mutant. Cells immortalized with this virus express a transformed phenotype at the permissive temperature of 34 degrees C but revert to a normal phenotype at the nonpermissive temperature of 40 degrees C. Using this approach, we have isolated several cell clones and describe the characterization of one that was designated HOB-02-C1. Immunocytochemistry revealed that > 95% of the cells express the large T-antigen at both temperatures. These cells exponentially proliferate at 34 degrees C with a doubling time of approximately 2 days but irreversibly stop dividing at 40 degrees C. However, cell volume increases > 2-fold when the cells are maintained for 6 days at the higher temperature. This clone expresses alpha 1 type (I) procollagen mRNA and secretes type I procollagen C-peptide at both temperatures, although the levels were slightly elevated at 40 degrees C. The cell line expresses alkaline phosphatase activity at 34 degrees C, and the basal level of this enzyme increases 2- to 6-fold at 40 degrees C. Alkaline phosphatase activity is induced 4- to 8-fold by 1 alpha,25-dihydroxyvitamin D3 (vitamin D3) at both temperatures, but transforming growth factor-beta 1 (TGF-beta 1) suppresses enzyme expression > 90% at 40 degrees C. Vitamin D3 also induces a 10-fold increase in osteocalcin secretion when the clone is maintained at 34 degrees C, and this induction is enhanced > 8-fold at 40 degrees C. Parathyroid hormone and forskolin stimulate a 4- to 6-fold increase in the production of intracellular cyclic AMP (cAMP) by the cells at 34 degrees C, and this stimulation is enhanced 2- to 4-fold at 40 degrees C. In contrast, prostaglandin E2 stimulates a 7- to 8-fold increase in cAMP only when the cells are maintained at 34 degrees C. This cell line secretes TGF-beta 1 and interleukin-6 (IL-6) at 34 degrees C, but only the basal secretion of IL-6 increases 70% at 40 degrees C. Finally, alizarin red-S histochemical staining demonstrates that these cells produce mineralized nodules at both temperatures. In summary, the results of this study indicate that the HOB-02-C1 cells have a mature osteoblastic phenotype. Consequently, this new cell line and others obtained in a similar fashion should be valuable in vitro tools for cellular, biochemical, and molecular studies of adult human osteoblast biology.

Efficacy of pamidronate in reducing skeletal events in patients with advanced multiple myeloma. Myeloma Aredia Study Group

BACKGROUND

Skeletal complications are a major clinical manifestation of multiple myeloma. These complications are caused by soluble factors that stimulate osteoclasts to resorb bone. Bisphosphonates such as pamidronate inhibit osteoclastic activity and reduce bone resorption.

METHODS

Patients with stage III multiple myeloma and at least one lytic lesion received either placebo or pamidronate (90 mg) as a four-hour intravenous infusion given every four weeks for nine cycles in addition to antimyeloma therapy. The patients were stratified according to whether they were receiving first-line (stratum 1) or second-line (stratum 2) antimyeloma chemotherapy at entry into the study. Skeletal events (pathologic fracture, irradiation of or surgery on bone, and spinal cord compression), hypercalcemia (symptoms or a serum calcium concentration > or = 12 mg per deciliter [3.0 mmol per liter]), bone pain, analgesic-drug use, performance status, and quality of life were assessed monthly.

RESULTS

Among 392 treated patients, the efficacy of treatment could be evaluated in 196 who received pamidronate and 181 who received placebo. The proportion of patients who had any skeletal events was significantly lower in the pamidronate group (24 percent) than in the placebo group (41 percent, P < 0.001), and the reduction was evident in both stratum 1 (P = 0.04) and stratum 2 (P = 0.004). The patients who received pamidronate had significant decreases in bone pain and no deterioration in performance status and quality of life. Pamidronate was tolerated well.

CONCLUSIONS

Monthly infusions of pamidronate provide significant protection against skeletal complications and improve the quality of life of patients with stage III multiple myeloma.

Increased expression of TGF-beta 2 in osteoblasts results in an osteoporosis-like phenotype

The development of the skeleton requires the coordinated activities of bone-forming osteoblasts and bone-resorbing osteoclasts. The activities of these two cell types are likely to be regulated by TGF-beta, which is abundant in bone matrix. We have used transgenic mice to evaluate the role of TGF-beta 2 in bone development and turnover. Osteoblast-specific overexpression of TGF-beta 2 from the osteocalcin promoter resulted in progressive bone loss associated with increases in osteoblastic matrix deposition and osteoclastic bone resorption. This phenotype closely resembles the bone abnormalities seen in human hyperparathyroidism and osteoporosis. Furthermore, a high level of TGF-beta 2 overexpression resulted in defective bone mineralization and severe hypoplasia of the clavicles, a hallmark of the developmental disease cleidocranial dysplasia. Our results suggest that TGF-beta 2 functions as a local positive regulator of bone remodeling and that alterations in TGF-beta 2 synthesis by bone cells, or in their responsiveness to TGF-beta 2, may contribute to the pathogenesis of metabolic bone disease.

Relations between histologic indices of bone formation: implications for the pathogenesis of spinal osteoporosis

Wall thickness, a major determinant of trabecular thickness, falls with age and falls further in osteoporosis. To estimate the importance of defective osteoblast recruitment in the pathogenesis of this defect, we compared various histologic indices of bone formation in iliac bone biopsies in three groups of subjects--healthy premenopausal women, healthy postmenopausal women, and patients with postmenopausal osteoporosis and at least one non-traumatic vertebral compression fracture. Indices that reflect the frequency of activation of bone remodeling and consequent birth rate of new teams of osteoblasts (osteoid surface, mineralizing surface, osteoblast surface, and bone formation rate, all expressed per unit of bone surface) were each higher in healthy subjects who were postmenopausal than in those who were premenopausal, but lower in osteoporotic than in normal postmenopausal women. In each group, the primary surface measurements were significantly correlated with each other, but the correlation was less close in those with osteoporosis. Indices that reflect the average collective performance of individual teams of osteoblasts (mineralizing surface and osteoblast surface per unit of osteoid surface, mineral apposition rate, adjusted apposition rate, and wall thickness) were all lower in postmenopausal than in premenopausal normal subjects, and even lower in those with postmenopausal osteoporosis. The parameters of the regression lines relating bone formation rate to osteoblast surface were essentially the same in each group, indicating that bone formation rate per unit of osteoblast surface was unaffected by age or menopause, and was the same in osteoporosis as in healthy subjects of similar age.(ABSTRACT TRUNCATED AT 250 WORDS)

Bone marrow, cytokines, and bone remodeling. Emerging insights into the pathophysiology of osteoporosis

Both osteoblasts and osteoclasts are derived from progenitors that reside in the bone marrow; osteoblasts belong to the mesenchymal lineage of the marrow stroma, and osteoclasts to the hematopoietic lineage. The development of osteoclasts from their progenitors is dependent on stromal-osteoblastic cells, which are a major source of cytokines that are critical in osteoclastogenesis, such as interleukin-6 and interleukin-11. The production of interleukin-6 by stromal osteoblastic cells, as well as the responsiveness of bone marrow cells to cytokines such as interleukin-6 and interleukin-11, is regulated by sex steroids. When gonadal function is lost, the formation of osteoclasts as well as osteoblasts increases in the marrow, both changes apparently mediated by an increase in the production of interleukin-6 and perhaps by an increase in the responsiveness of bone marrow progenitor cells not only to interleukin-6 but also to other cytokines with osteoclastogenic and osteoblastogenic properties. The cellular activity of the bone marrow is also altered by the process of aging. Specifically, senescence may decrease the ability of the marrow to form osteoblast precursors. The association between the dysregulation of osteoclast or osteoblast development in the marrow and the disruption of the balance between bone resorption and bone formation, resulting in the loss of bone, leads to the following notion. Like homeostasis of other regenerating tissues, homeostasis of bone depends on the orderly replenishment of its cellular constituents. Excessive osteoclastogenesis and inadequate osteoblastogenesis are responsible for the mismatch between the formation and resorption of bone in postmenopausal and age-related osteopenia. The recognition that changes in the numbers of bone cells, rather than changes in the activity of individual cells, form the pathogenetic basis of osteoporosis is a major advance in understanding the mechanism of this disease.