Structural adaptation and intracortical bone turnover in an ovine model of osteoporosis

Biological Deciphering of the "Kidney Governing Bones" Theory in Traditional Chinese Medicine

The description of the "kidney" was entirely different from modern medicine. In traditional Chinese medicine (TCM), the kidney was a functional concept regulating water metabolism, which was closely related to the urinary system, reproductive system, nervous system, endocrine, skeleton, hearing, metabolism, immunity, etc. In particular, the kidney in TCM plays an important regulatory role in the processes of growth, development, prime, aging, and reproduction. Hence, "Kidney Governing Bone" (KGB) was a classical theory in TCM, which hypothesized that the function of the kidney was responsible for bone health. However, the related modern physiological mechanisms of this TCM theory are unclear. This present paper proposed a new understanding and explored the biological basis of the KGB theory. After searching through plenty of reported literature, we discovered that the functions of the kidney in TCM were closely associated with the hypothalamic-pituitary-gonadal (HPG) axis in modern science. The physiological mechanism of the KGB was regulated by sex hormones and their receptors. This review deciphered the connotation of the KGB theory in modern medicine and further verified the scientificity of the basic TCM theory.

Preclinical and Translational Studies in Small Ruminants (Sheep and Goat) as Models for Osteoporosis Research

PURPOSE OF THE REVIEW

This review summarizes research on the use of sheep and goats as large animal models of human osteoporosis for preclinical and translational studies.

RECENT FINDINGS

The most frequent osteoporotic sheep model used is the ovariectomized sheep with 12 months post-operatively or more and the combined treatment of ovariectomized sheep associated to calcium/vitamin D-deficient diet and glucocorticoid applications for 6 months, but other methods are also described, like pinealectomy or hypothalamic-pituitary disconnection in ovariectomized sheep. The goat model for osteoporosis research has been used in a very limited number of studies in osteoporosis research relative to sheep. These osteoporotic small ruminant models are applied for biomaterial research, bone augmentation, efficacy of implant fixation, fragility fracture-healing process improvement, or bone-defect repair studies in the osteopenic or osteoporotic bone. Sheep are a recognized large animal model for preclinical and translational studies in osteoporosis research and the goat to a lesser extent. Recently, the pathophysiological mechanism underlying induction of osteoporosis in glucocorticoid-treated ovariectomized aged sheep was clarified, being similar to what occurs in postmenopausal women with glucocorticoid-induced osteoporosis. It was also concluded that the receptor activator of NF-κB ligand was stimulated in the late progressive phase of the osteoporosis induced by steroids in sheep. The knowledge of the pathophysiological mechanisms at the cellular and molecular levels of the induction of osteoporosis in small ruminants, if identical to humans, will allow in the future, the use of these animal models with greater confidence in the preclinical and translational studies for osteoporosis research.

Bone: A Fertile Soil for Cancer Metastasis

Bone is one of the most common and most dangerous sites for metastatic growth across cancer types, and bone metastasis remains incurable. Unfortunately, the processes by which cancers preferentially metastasize to bone are still not well understood. In this review, we summarize the morphological features, physical properties, and cell signaling events that make bone a unique site for metastasis and bone remodeling. The signaling crosstalk between the tumor cells and bone cells begins a vicious cycle - a self-sustaining feedback loop between the tumor cells and the bone microenvironment composed of osteoclasts, osteoblasts, other bone marrow cells, bone matrix, and vasculature to support both tumor growth and bone destruction. Through this crosstalk, bone provides a fertile microenvironment that can harbor dormant tumor cells, sometimes for long periods, and support their growth by releasing cytokines as the bone matrix is destroyed, similar to providing nutrients for a seed to germinate in soil. However, few models exist to study the late stages of bone colonization by metastatic tumor cells. We describe some of the current methodologies used to study bone metastasis, highlighting the limitations of these methods and alternative future strategies to be used to study bone metastasis. While <i>in vivo</i> animal and patient studies may provide the gold standard for studying metastasis, <i>ex vivo</i> models can be used as an alternative to enable more controlled experiments designed to study the late stages of bone metastasis.

Mechanisms of osteocyte stimulation in osteoporosis

Experimental studies have shown that primary osteoporosis caused by oestrogen-deficiency results in localised alterations in bone tissue properties and mineral composition. Additionally, changes to the lacunar-canalicular architecture surrounding the mechanosensitive osteocyte have been observed in animal models of the disease. Recently, it has also been demonstrated that the mechanical stimulation sensed by osteocytes changes significantly during osteoporosis. Specifically, it was shown that osteoporotic bone cells experience higher maximum strains than healthy bone cells after short durations of oestrogen deficiency. However, in long-term oestrogen deficiency there was no significant difference between bone cells in healthy and normal bone. The mechanisms by which these changes arise are unknown. In this study, we test the hypothesis that complex changes in tissue composition and lacunar-canalicular architecture during osteoporosis alter the mechanical stimulation of the osteocyte. The objective of this research is to employ computational methods to investigate the relationship between changes in bone tissue composition and microstructure and the mechanical stimulation of osteocytes during osteoporosis. By simulating physiological loading, it was observed that an initial decrease in tissue stiffness (of 0.425GPa) and mineral content (of 0.66wt% Ca) relative to controls could explain the mechanical stimulation observed at the early stages of oestrogen deficiency (5 weeks post-OVX) during in situ bone cell loading in an oestrogen-deficient rat model of post-menopausal osteoporosis (Verbruggen et al., 2015). Moreover, it was found that a later increase in stiffness (of 1.175GPa) and mineral content (of 1.64wt% Ca) during long-term osteoporosis (34 weeks post-OVX), could explain the mechanical stimuli previously observed at a later time point due to the progression of osteoporosis. Furthermore, changes in canalicular tortuosity arising during osteoporosis were shown to result in increased osteogenic strain stimulation, though to a lesser extent than has been observed experimentally. The findings of this study indicate that changes in the extracellular environment during osteoporosis, arising from altered mineralisation and lacunar-canalicular architecture, lead to altered mechanical stimulation of osteocytes, and provide an enhanced understanding of changes in bone mechanobiology during osteoporosis.

Can we induce osteoporosis in animals comparable to the human situation?

Osteoporosis is a chronic systemic bone disease of growing relevance due to the on-going demographic change. Since the underlying regulatory mechanisms of this critical illness are still not fully understood and treatment options are not satisfactorily resolved, there is still a great need for osteoporosis research in general and animal models in particular. Ovariectomized rodents are standard animal models for postmenopausal osteoporosis and highly attractive due to the possibility to specifically modify their genetic background. However, some aspects can only be addressed in large animal models; such as metaphyseal fracture healing and advancement of orthopedic implants. Among other large animal models sheep in particular have been proven invaluable for osteoporosis research in this context. In conclusion, today we are able to influence the bone metabolism in animals causing a more or less pronounced systemic bone loss and structural deterioration comparable to the situation found in patients suffering from osteoporosis. However, there is no perfect model for osteoporosis, but a variety of models appropriate for answering specific questions. Though, the appropriateness of an animal model is not only defined in regard to the similarity to human physiology and the disease itself, but also in regard to acquisition, housing requirements, handling, costs, and particularly ethical concerns and animal welfare.

Altered mechanical environment of bone cells in an animal model of short- and long-term osteoporosis

Alterations in bone tissue composition during osteoporosis likely disrupt the mechanical environment of bone cells and may thereby initiate a mechanobiological response. It has proved challenging to characterize the mechanical environment of bone cells in vivo, and the mechanical environment of osteoporotic bone cells is not known. The objective of this research is to characterize the local mechanical environment of osteocytes and osteoblasts from healthy and osteoporotic bone in a rat model of osteoporosis. Using a custom-designed micromechanical loading device, we apply strains representative of a range of physical activity (up to 3000 με) to fluorescently stained femur samples from normal and ovariectomized rats. Confocal imaging was simultaneously performed, and digital image correlation techniques were applied to characterize cellular strains. In healthy bone tissue, osteocytes experience higher maximum strains (31,028 ± 4213 με) than osteoblasts (24,921 ± 3,832 με), whereas a larger proportion of the osteoblast experiences strains >10,000 με. Most interestingly, we show that osteoporotic bone cells experience similar or higher maximum strains than healthy bone cells after short durations of estrogen deficiency (5 weeks), and exceeded the osteogenic strain threshold (10,000 με) in a similar or significantly larger proportion of the cell (osteoblast, 12.68% vs. 13.68%; osteocyte, 15.74% vs. 5.37%). However, in long-term estrogen deficiency (34 weeks), there was no significant difference between bone cells in healthy and osteoporotic bone. These results suggest that the mechanical environment of bone cells is altered during early-stage osteoporosis, and that mechanobiological responses act to restore the mechanical environment of the bone tissue after it has been perturbed by ovariectomy.

Temporal changes in bone composition, architecture, and strength following estrogen deficiency in osteoporosis

Using an ovariectomized (OVX) ovine model, we provide an analysis of the timing of changes in bone following estrogen deficiency. The expression of genes known to regulate osteoclastogenesis, matrix production, and mineralization, as measured by real-time RT-PCR, was significantly increased by 12 months; and increased expression was maintained through to 31 months post-OVX compared to controls. FTIR spectroscopy confirmed that mineralized crystals were less mature than in controls 12 months post-OVX and were even less so by 31 months. The mineral-to-matrix ratio was significantly reduced by 31 months, while the ratio of mature to immature collagen cross-linking was initially increased at 12 months and subsequently reduced at 31 months post-OVX. In contrast, trabecular number, thickness, and separation were unchanged at 12 months. Significant reductions in trabecular number and thickness and a significant increase in trabecular separation were observed 31 months after OVX. Most notably perhaps these combined changes led to a significant reduction in the compressive strength of trabecular bone after 31 months. The results indicate that there is an initial increase in bone turnover, which is accompanied by a change in bone composition. This is followed by a continued increase in bone resorption and relative reduction in bone formation, leading to deterioration in bone microarchitecture. Ultimately, these cumulative changes led to a significant reduction in the compressive strength of bones following 31 months of estrogen deficiency. These findings provide important insight into the time sequence of changes during osteoporosis.

Changes in proximal femur bone properties following ovariectomy and their association with resistance to fracture

Bone strength depends on several material and structural properties, but findings concerning the best predictors of bone mechanical performance are conflicting. The aim of this study was to investigate how a broad set of bone properties in the proximal femur are influenced by age and hormonal status, and how these properties together determine bone strength. Twenty-five Wistar rats were ovariectomized (OVX, n = 13) or sham operated (SHAM, n = 12) at 5 months of age, and killed after 9 months. Another group of rats was killed at 5 months as baseline control (BSL, n = 7). At sacrifice, serum 17β-estradiol and bone turnover marker concentrations were determined in the serum. Both femurs were collected for assessment of trabecular microarchitecture, femoral neck geometry, radiographic absorptiometry, calcium and phosphate content, and biomechanical properties. While stiffness was mostly associated with proximal femur trabecular microarchitecture and mineralization degree, bone strength was mostly linked to bone size and femoral neck geometry, which predicted almost 50% of its variance. Despite the decrease in cortical and trabecular bone as well as in mineralization degree following estrogen loss, bone strength was not reduced in OVX animals compared to BSL or sham-operated rats. This was due to a change in femoral neck geometry as well as to an increase in femur size in OVX, which apparently compensated their lower bone volume and mineral content, thereby preserving bone strength. Estrogen loss leads to a deterioration of bone tissue quality, but bone strength was preserved at the expense of geometric adaptations.

Effects of estrogen deficiency and bisphosphonate therapy on osteocyte viability and microdamage accumulation in an ovine model of osteoporosis

It has been proposed that osteocyte viability plays an important role in bone integrity, and that bone loss in osteoporosis may be partially due to osteocyte cell death following estrogen depletion. Osteoporosis treatments such as bisphosphonates can inhibit osteocyte apoptosis which in turn may also reduce remodeling. Consequently, microcracks in bone which are normally repaired by bone remodeling may accumulate. This study used an ovine model of osteoporosis to examine the effects of estrogen depletion and bisphosphonates on osteocyte apoptosis and microdamage accumulation. Skeletally mature ewes were randomly assigned into two equal groups; ovariectomy (OVX) and a non-treatment group (control). Half of these animals were sacrificed 12 months post-OVX. Twenty months post-OVX, a number of OVX animals were randomly selected and each received a supra-pharmacological dose of the bisphosphonate, zoledronic acid (Zol). This group and all the remaining animals were sacrificed 31 months post-OVX. A compact bone specimen was removed from the left metacarpal of each animal; half was used for osteocyte apoptosis detection and the remainder for microdamage analysis. Estrogen deficiency resulted in significant increases in the levels of osteocyte apoptosis while zoledronic acid significantly reduced the level of apoptosis in osteocytes. Zoledronic acid treatment resulted in the formation of more microcracks. However, these cracks were shorter than in control or OVX groups which may provide one explanation as to why increased damage levels following bisphosphonate treatment have not lead to increased fractures. This study also provides additional evidence of the importance of estrogen in preserving the osteocyte network.

The effects of estrogen deficiency and bisphosphonate treatment on tissue mineralisation and stiffness in an ovine model of osteoporosis

While much research has been dedicated to understanding osteoporosis, the nature of mineral distribution and the mechanical property variation in diseased bone is poorly understood. The current study aimed to determine the effect of estrogen deficiency and bisphosphonate therapy on bone tissue properties using an ovine model of osteoporosis. Skeletally mature animals (4+ years) were divided into an ovariectomy group (ovx, n=20) and a non treatment control group (control, n=20). A zoledronic acid treated group was also included in which animals were estrogen deficient for 20 months prior to receiving treatment (Zol, n=4). Half of the control and ovx groups were euthanized 12 or 31 months post-operatively and all Zol animals were euthanised at 31 months. Individual trabeculae were removed from the proximal femur and were analysed at specific locations across the width of the trabeculae. The mineral content was measured using quantitative backscatter electron imaging and the modulus was measured using nanoindentation. The spatial distribution of tissue modulus and mineral content in bone from ovariectomised animals was similar to control. However, ovariectomy significantly reduced the overall mineral content and tissue modulus relative to the control group after 12 months. Interestingly, significant differences were not maintained 31 months post-OVX. Treatment with zoledronic acid increased the mineral content and tissue modulus relative to both the ovariectomised and control groups. Zoledronic acid was also found to alter the mineral and modulus gradients normally associated with healthy bone tissue. The current study provides evidence that both estrogen deficiency and zoledronic acid therapy significantly alter mineral content and the mechanical properties of trabecular tissue.

Anatomists and geometers: 16th Samuel Haughton Lecture of the Royal Academy of Medicine in Ireland

This paper is concerned with the interactions between medics and biologists on the one hand, the 'anatomists' of the title, and 'geometers', or engineers and physicists, on the other. It was delivered as the 16th annual Samuel Haughton Lecture on 23rd January 2010 at the Bioengineering in Ireland conference in Malahide. The paper begins with Samuel Haughton, the father of Irish biomechanics, and then discusses how anatomists and geometers have cooperated to solve problems in the areas of bone adaptation, fatigue microdamage, osteoporosis, third-level education and even art.