The determination of bone viability: a histochemical method for identification of lactate dehydrogenase activity in osteocytes in fresh calcified and decalcified sections of human bone

Characterization of an advanced viable bone allograft with preserved native bone-forming cells

Bone grafts are widely used to successfully restore structure and function to patients with a broad range of musculoskeletal ailments and bone defects. Autogenous bone grafts are historically preferred because they theoretically contain the three essential components of bone healing (ie, osteoconductivity, osteoinductivity, and osteogenicity), but they have inherent limitations. Allograft bone derived from deceased human donors is one alternative that is also capable of providing both an osteoconductive scaffold and osteoinductive potential but, until recently, lacked the osteogenic component of bone healing. Relatively new, cellular bone allografts (CBAs) were designed to address this need by preserving viable cells. Although most commercially-available CBAs feature mesenchymal stem cells (MSCs), osteogenic differentiation is time-consuming and complex. A more advanced graft, a viable bone allograft (VBA), was thus developed to preserve lineage-committed bone-forming cells, which may be more suitable than MSCs to promote bone fusion. The purpose of this paper was to present the results of preclinical research characterizing VBA. Through a comprehensive series of in vitro and in vivo assays, the present results demonstrate that VBA in its final form is capable of providing all three essential bone remodeling properties and contains viable lineage-committed bone-forming cells, which do not elicit an immune response. The results are discussed in the context of clinical evidence published to date that further supports VBA as a potential alternative to autograft without the associated drawbacks.

Cell viability assays: introduction

The measurement of cell viability plays a fundamental role in all forms of cell culture. Sometimes it is the main purpose of the experiment, such as in toxicity assays. Alternatively, cell viability can be used to -correlate cell behaviour to cell number, providing a more accurate picture of, for example, anabolic -activity. There are wide arrays of cell viability methods which range from the most routine trypan blue dye exclusion assay to highly complex analysis of individual cells, such as using RAMAN microscopy. The cost, speed, and complexity of equipment required will all play a role in determining the assay used. This chapter aims to provide an overview of many of the assays available today.

Viability assessment of osteocytes using histological lactate dehydrogenase activity staining on human cancellous bone sections

The assessment of viable osteocytes within bone tissue is of crucial importance. Osteocytes are the most abundant cells in bone. Due to their interconnectivity in the bone matrix they are hypothesised to play an important role in the maintenance of the extracellular matrix of bone. The death of osteocytes and the resulting disturbance of the osteocyte-canalicular network are responsible for the "loss of function" seen in several bone diseases. The lactate dehydrogenase (LDH) assay is a popular method to detect cell viability in bone sections. The major advantage of the LDH assay is the stability of the LDH enzyme for up to 36 h after cell death, eliminating any false negative viability results due to processing of the tissue. Here, we present a quick, reliable, and easy modification of the LDH assay using non-decalcified, thick, unfixed cancellous bone sections for the quantification of osteocyte viability.

Biomimetic bone mechanotransduction modeling in neonatal rat femur organ cultures: structural verification of proof of concept

The goal of this work was to develop and validate a whole bone organ culture model to be utilized in biomimetic mechanotransduction research. Femurs harvested from 2-day-old neonatal rat pups were maintained in culture for 1 week post-harvest and assessed for growth and viability. For stimulation studies, femurs were physiologically stimulated for 350 cycles 24 h post-harvest then maintained in culture for 1 week at which time structural tests were conducted. Comparing 1 and 8 days in culture, bones grew significantly in size over the 7-day culture period. In addition, histology supported adequate diffusion and organ viability at 2 weeks in culture. For stimulation studies, 350 cycles of physiologic loading 24 h post-harvest resulted in increased bone strength over the 7-day culture period. In this work, structural proof of concept was established for the use of whole bone organ cultures as mechanotransduction models. Specifically, this work established that these cultures grow and remain viable in culture, are adequately nourished via diffusion and are capable of responding to a brief bout of mechanical stimulation with an increase in strength.

Hypoxia and HIF-1alpha expression in the epiphyseal cartilage following ischemic injury to the immature femoral head

HIF-1alpha has been shown to be a central mediator of cellular response to hypoxia. The role it plays after ischemic injury to the immature femoral head is unknown. The purpose of this study was to determine the region of the femoral head affected by hypoxia following ischemic injury to the immature femoral head and to determine the site of HIF-1alpha activation and revascularization. We hypothesize that the epiphyseal cartilage, rather than the bony epiphysis, is the site of HIF-1alpha activation following ischemic osteonecrosis and that the epiphyseal cartilage plays an important role in the revascularization process.

MATERIALS AND METHODS

Femoral head osteonecrosis was surgically induced in 56 immature pigs. Hypoxyprobe staining, cell viability assay, HIF-1alpha western blot, RT-qPCR of HIF-1alpha, VEGF, VEGFR2, and PECAM, and micro-CT assessments of microfil-infused femoral heads were performed.

RESULTS

Severe hypoxia was present in the bony epiphysis and the lower part of the epiphyseal cartilage following ischemia. In the bony epiphysis, extensive cell death and tissue necrosis was observed with degradation of proteins and RNAs which precluded further analysis. In the epiphyseal cartilage, the loss of cell viability was limited to its deep layer with the remainder of the cartilage remaining viable. Furthermore, the cartilage from the ischemic side showed a significant increase in HIF-1alpha protein level and HIF-1alpha expression. VEGF expression in the cartilage was dramatically and significantly increased at 24 h, 2 and 4 weeks (p<0.05 for all) with 5 to 10 fold increase being observed on the ischemic side compared to the normal side. PECAM and VEGFR2 expressions in the cartilage were both significantly decreased at 24 h but returned to the normal levels by 2 and 4 weeks, respectively. Micro-CT showed revascularization of the cartilage on the ischemic side with the vessel volume/total volume equaling the normal side by 4 weeks.

CONCLUSIONS

Acute ischemic injury to the immature femoral head induced severe hypoxia and cell death in the bony epiphysis and the deep layer of the epiphyseal cartilage. Viable chondrocytes in the superficial layer of the epiphyseal cartilage showed HIF-1alpha activation and VEGF upregulation with subsequent revascularization occurring in the cartilage.

Effects of disruption of epiphyseal vasculature on the proximal femoral growth plate

BACKGROUND

Proximal femoral growth disturbance is a major complication associated with ischemic osteonecrotic conditions, such as Legg-Calvè-Perthes disease. The extent of ischemic damage and the mechanisms by which ischemic injury to the growing femoral head produces growth disturbance of the proximal femoral growth plate remain unclear. The purpose of this study was to investigate the effects of disruption of the epiphyseal vasculature on the morphology and function of the proximal femoral growth plate in a porcine model.

METHODS

Ischemic osteonecrosis of the femoral head was surgically induced in sixty-five piglets by placing a ligature tightly around the femoral neck. Radiographic, histological, micro-computed tomographic, cellular viability, hypoxia marker, and cellular proliferation studies were performed.

RESULTS

Disruption of the epiphyseal vasculature did not lead to diffuse growth plate damage in the majority of the ischemic femoral heads. One of the twelve femoral heads analyzed at four weeks and six of the twenty-six femoral heads analyzed at eight weeks had severe disruption of the growth plate that precluded histological assessment of the growth plate zones. In the remaining animals, the proximal part of the femur continued to elongate following induction of ischemia, albeit at a slower rate than on the normal side. Histologically, normal developmental thinning of the growth plate was seen to be absent on the ischemic side. Severe hypoxia and cellular death were limited to the area of the growth plate bordering on the infarcted osseous epiphysis. Normal chondrocytic organization and continued proliferation were observed in the proliferative zone of the growth plate.

CONCLUSIONS

In our porcine model, the proximal femoral growth plate was not diffusely damaged following disruption of the epiphyseal vasculature in the majority of the ischemic femoral heads. The majority of the growth plates remained viable and were able to function despite total disruption of the epiphyseal vasculature. These findings suggest that the source of nutrition for the proximal femoral growth plate is not solely the epiphyseal vasculature as has been traditionally believed.

Modifications of the lactate dehydrogenase assay, a histochemical determinant of osteocyte viability--a qualitative study

We have refined a technique for assessment of osteocyte viability in a canine and murine model using a modification of the lactate dehydrogenase assay (LDH). With this method, viable osteocytes react to form non-reversible tetrazolium-formazan granules, while non-viable osteocytes are distinguished by a methyl green stain. LDH assay in canine and murine models have not been reported and our initial efforts were not successful. We examined the effect of (a) concentration of coenzyme and tetrazole (b) bone specimen thickness (c) ability to use frozen sections and (d) incubation time/dilution. We concluded that a 1000-fold increase in the concentration of coenzyme and tetrazole were required. Fresh bone produced optimal results and near-complete viability. Special considerations must be taken with smaller, more fragile specimens (e.g., mouse bone), such as increasing specimen thickness, dilution of incubation medium and/or the reduction of incubation time. Sections from thawed frozen bone resulted in a diffuse reaction. Osteocyte viability can be assessed via LDH assay in both dog and mouse bones; however, this approach requires modifications from the previous published method.

Novel explant model to study mechanotransduction and cell-cell communication

To understand in situ behavior of osteocytes, we characterized a model of osteocytes in their native bone matrix and demonstrated real-time biologic activity of osteocytes while bending the bone matrix. Using 43 male Sprague-Dawley rats, dumbbell-shaped explants were harvested from stainless steel femoral implants after 6-12 weeks and incubated in culture medium or fixed. Sixteen specimens were used to determine bone volume density (BV/TV), volumetric bone mineral density (BMD) and histology for different implantation periods. Osteocyte viability was evaluated by L-lactate dehydrogenase (LDH) activity in 12 cultured explants. Confocal microscopy was used to assess tracer diffusion in three explants and changes in osteocyte pH of a mechanically loaded explant. From 6 to 12 weeks, explant BV/TV and volumetric BMD trended up 92.5% and 101%, respectively. They were significantly and highly correlated. Tissues were uniformly intramembranous and all bone cell types were present. Explants maintained LDH activity through culture day 8. Diffusion at 200 microM was limited to 1,209 Da. Explants appeared capable of reproducing complex bone biology. This model may be useful in understanding osteocyte mechanotransduction in the context of a physiologically relevant bone matrix.

Osteocyte-based image analysis for quantitation of histologically apparent femoral head osteonecrosis: application to an emu model

Femoral head osteonecrosis is often characterized histologically by the presence of empty lacunae in the affected bony regions. The shape, size and location of a necrotic lesion influences prognosis, and can, in principle, be quantified by mapping the distribution of empty lacunae within a femoral head. An algorithm is here described that automatically identifies the locations of osteocyte-filled vs. empty lacunae. The algorithm is applied to necrotic lesions surgically induced in the emu, a large bipedal animal model in which osteonecrosis progresses to collapse, as occurs in humans. The animals' femoral heads were harvested at sacrifice, and hematoxylin and eosin-stained histological preparations of the coronal midsections were digitized and image-analyzed. The algorithm's performance in detecting empty lacunae was validated by comparing its results to corresponding assessments by six trained histologists. The percentage of osteocyte-filled lacunae identified by the algorithm vs. by the human readers was statistically indistinguishable.

Osteocyte viability with glucocorticoid treatment: relation to histomorphometry

BACKGROUND

Glucocorticoid induced osteoporosis is a common clinical problem.

OBJECTIVE

To determine the pathophysiology of glucocorticoid induced osteoarthritis at the organ level.

METHODS

Iliac crest biopsy specimens were obtained from nine patients receiving prednisone treatment for rheumatoid arthritis. Osteocyte viability and histomorphometric indices were assessed.

RESULTS

Compared with controls, glucocorticoid treated subjects had reduced trabecular thickness and increased erosion. The number of viable osteocytes was significantly decreased in glucocorticoid treated patients compared with controls.

CONCLUSION

The impaired bone formation, increased erosion and, importantly, loss of viable osteocytes are all likely to contribute to the increased risk of fracture in these patients.

Evaluation of ligament fibroblast viability in ruptured cranial cruciate ligament of dogs

OBJECTIVE

To determine fibroblast viability, assess development of apoptosis, and evaluate tissue hypoxia via histochemical, in-situ hybridization, or immunohistochemical staining in ruptured and intact cranial cruciate ligaments (CCLs) of dogs.

ANIMALS

32 dogs with ruptured CCLs, and 8 aged and 19 young dogs with intact CCLs.

PROCEDURE

Markers of cell viability (lactate dehydrogenase [LDH]), apoptosis (terminal deoxynucleatidyl transferase-mediated deoxyuridine triphosphate-nick end labeling [TUNEL] method), and hypoxia (hypoxia-inducible factor-1alpha [HIF-1alpha] monoclonal antibody) were applied to CCL specimens; positive cells were assessed objectively (LDH) and subjectively (TUNEL and HIF-1alpha) in the main axial tissue component (core) and synovial intima and subintima (epiligamentous tissue).

RESULTS

Viable fibroblasts were seen in all intact and ruptured CCLs. More nonviable cells were found in the core regions of ruptured CCLs and intact CCLs of young dogs than in the epiligamentous regions. Number of nonviable cells in the core region of ruptured CCLs was greater than that in intact CCLs of young and aged dogs, whereas the number in the epiligamentous region was similar in all specimens. The TUNEL and HIF-1alpha staining was only found in the epiligamentous region of ruptured CCLs.

CONCLUSIONS AND CLINICAL RELEVANCE

Ruptured CCLs contained a high number of nonviable cells but not a great number of apoptotic cells. Repair processes in the epiligamentous region of the CCL include a metabolic response to hypoxia, suggesting that necrosis of ligament fibroblasts and transformation of surviving cells to a spheroid phenotype may be a response to hypoxia cause by microinjury or inadequate blood flow.

Effect of microwave oven induced mild hyperthermia on bone viability and strength

Extracorporeal hyperthermia treatment of bone followed by reimplantation may be an option for treating bone tumors. However, intensive heat treatment, such as autoclaving, causes a decline of mechanical and biologic functions of bone tissue. In the current study, a microwave oven was used for minimal hyperthermic treatment, and it was found that complete eradication of all viable cells in rat bone could be achieved with minimal reduction in mechanical function. When the cells were evaluated histologically by special lactate dehydrogenase activity staining, complete bone cell death occurred after 60 seconds of heating in an empty Petri dish and after 30 seconds when heated in a Petri dish containing normal saline. Mechanical stiffness and strength of the bones, tested in three-point bending, showed no decrease after this heating. Microwave oven induced hyperthermia eradication of viable cells without significant damage to the mechanical properties may have clinical relevance in limb salvage tumor surgery.

Intracortical remodeling in adult rat long bones after fatigue loading

Intracortical remodeling in the adult skeleton removes and replaces areas of compact bone that have sustained microdamage. Although studies have been performed in animal species in which there is an existing baseline of remodeling activity, laboratory rodents have been considered to have limited suitability as models for cortical bone turnover processes because of a lack of haversian remodeling activity. Supraphysiological cyclic axial loading of the ulna in vivo was used to induce bending with consequent fatigue and microdamage. Right ulnae of adult Sprague-Dawley rats were fatigue-loaded to a prefailure stopping point of 30% decrease in ulnae whole bone stiffness. Ten days after the first loading, left ulnae were fatigued in the same way. Ulnae were harvested immediately to allow comparison of the immediate response of the left ulna to the fatigue loads, and the biological response of the right leg to the fatigue challenge. Histomorphometry and confocal microscopy of basic fuchsin-stained bone sections were used to assess intracortical remodeling activity, microdamage, and osteocyte integrity. Bone microdamage (linear microcracks, as well as patches of diffuse basic fuchsin staining within the cortex) occurred in fatigue-loaded ulnar diaphyses. Ten days after fatigue loading, intracortical resorption was activated in ulnar cortices. Intracortical resorption occurred in preferential association with linear-type microcracks, with microcrack number density reduced almost 40% by 10 days after fatigue. Resorption spaces were also consistently observed within areas of the cortex in which no bone matrix damage could be detected. Confocal microscopy studies showed alterations of osteocyte and canalicular integrity around these resorption spaces. These studies reveal that: (1) rat bone undergoes intracortical remodeling in response to high levels of cyclic strain, which induce microdamage in the cortex; and (2) intracortical resorption is associated both with bone microdamage and with regions of altered osteocyte integrity. From these studies, we conclude that rats can initiate haversian remodeling in long bones in response to fatigue, and that osteocyte death or damage may provide one of the stimuli for this process.

Effect of moderate bone hyperthermia on cell viability and mechanical function

Extracorporeal hyperthermia treatment of bone followed by its reimplantation may be an optional treatment of bone tumors. In this study, the authors examined the minimal hyperthermic condition in which complete eradication of all viable cells in rat bone can be achieved and the mechanical effect of this treatment on the tested bone. When the results were evaluated histologically by special lactate dehydrogenase activity staining, it was found that complete bone cell death occurred after 30 minutes of heating at 60 degrees C. Cartilage cells, including those of the epiphysis, were more resistant to thermal damage. When the ability of the specimens to proliferate in cell cultures was tested, no growth was observed after heating at temperatures of 50 degrees C or greater. The mechanical stiffness tested in the Instron machine showed decreased bone stiffness at 70 degrees C but no change in the breaking load of the bones. Controlled hyperthermia's ability to eradicate viable cells without significant damage to the mechanical properties may have clinical relevance in limb salvage tumor surgery.

Osteocyte death and hip fracture

The viability of osteocytes can be demonstrated in sawn decalcified sections of bone by their lactate dehydrogenase activity. In the cancellous bone of the femoral head, the proportion of lacunae containing viable osteocytes decreased from 88 +/- 7% (mean +/- SD) at 10-29 years to 58 +/- 12% (P < 0.001) by 70-89 years. Viability in the second lumbar vertebra was 88 +/- 3% in subjects aged 25-90 years and did not decrease with age. Mean osteocyte viability in the femoral head of 21 hip fracture patients aged 72-94 years was 58 +/- 21%, similar to controls of a similar age, though there was greater variation and, in five patients, osteocyte viability was less than 25%. In hip fracture patients, microfracture callus incidence correlated positively with osteocyte viability, with little or no fracture callus observed if the bone viability was low. Ultimate compressive strength did not correlate with osteocyte viability. In the femoral head there is gradual, age-related reduction in osteocyte viability that can be more pronounced in hip fracture. Osteocyte death may affect bone quality by impairing repair of fatigue damage.

Bone death in hip fracture in the elderly

We examined femoral head bone from 50 cadavers and from 21 patients who had suffered pathologic fracture of the femoral neck. We used a histochemical technique for lactate dehydrogenase (LDH) activity to demonstrate osteocyte viability. The femoral heads were removed within 36 hours of death or fracture, as LDH activity persists in the cytoplasm of viable cells for this time at 37 degrees after interruption of the blood supply. In the controls, there was an age-related reduction in mean osteocyte viability, from 88 +/- 7% (mean +/- SD) at age 10-29 years to 58 +/- 12% at age 70-89 years. In the hip fracture patients, mean osteocyte viability was 58 +/- 21% but there was much variability in both osteocyte viability and bone mass. In 5 fracture patients, there was extensive osteocyte death, suggesting that most of the femoral head bone was nonviable; these patients had little microfracture callus. Others had predominantly viable bone which was usually osteoporotic, and their bone frequently showed microfracture callus. Osteomalacia was not seen in any patient. It is suggested that bone death, in addition to osteoporosis, may sometimes contribute to hip fracture in the elderly.