A 300 μm Organotypic Bone Slice Culture Model for Temporal Investigation of Endochondral Osteogenesis

Ex vivo organotypic bone slice culture reveals preferential chondrogenesis after sustained growth plate injury

Postnatal bone growth primarily relies on chondrocyte proliferation and osteogenic differentiation within the growth plate (GP) via endochondral ossification. Despite its importance, the GP is vulnerable to injuries, affecting 15-30 % of bone fractures. These injuries may lead to growth discrepancies, influence bone length and shape, and negatively affecting the patient's quality of life. This study aimed to investigate the molecular and cellular physiological and pathophysiological regeneration following sustained growth plate injury (GPI) in an ex vivo rat femur organotypic culture (OTC) model. Specifically, focusing on postnatal endochondral ossification process. 300 μm thick ex vivo bone cultures with a 2 mm long horizontal GPI was utilized. After 15 days of cultivation, gene expression analysis, histological and immunohistochemistry staining's were conducted to analyze key markers of endochondral ossification. In our OTCs we observed a significant increase in Sox9 expression due to GPI at day 15. The Ihh-PTHrP feedback loop was affected, favoring chondrocyte proliferation and maturation. Ihh levels increased significantly on day 7 and day 15, while PTHrP was downregulated on day 7. GPI had no impact on osteoclast number and activity, but gene expression analysis indicated OTCs' efforts to inhibit osteoclast differentiation and activation, thereby reducing bone resorption. In conclusion, our study provides novel insights into the molecular and cellular mechanisms underlying postnatal bone growth and regeneration following growth plate injury (GPI). We demonstrate that chondrocyte proliferation and differentiation play pivotal roles in the regeneration process, with the Ihh-PTHrP feedback loop modulating these processes. Importantly, our ex vivo rat femur organotypic culture model allows for the detailed investigation of these processes, providing a valuable tool for future research in the field of skeletal biology and regenerative medicine.

Proton and Carbon Ion Irradiation Changes the Process of Endochondral Ossification in an Ex Vivo Femur Organotypic Culture Model

Particle therapy (PT) that utilizes protons and carbon ions offers a promising way to reduce the side effects of radiation oncology, especially in pediatric patients. To investigate the influence of PT on growing bone, we exposed an organotypic rat ex vivo femur culture model to PT. After irradiation, histological staining, immunohistochemical staining, and gene expression analysis were conducted following 1 or 14 days of in vitro culture (DIV). Our data indicated a significant loss of proliferating chondrocytes at 1 DIV, which was followed by regeneration attempts through chondrocytic cluster formation at 14 DIV. Accelerated levels of mineralization were observed, which correlated with increased proteoglycan production and secretion into the pericellular matrix. Col2α1 expression, which increased during the cultivation period, was significantly inhibited by PT. Additionally, the decrease in ColX expression over time was more pronounced compared to the non-IR control. The chondrogenic markers BMP2, RUNX2, OPG, and the osteogenic marker ALPL, showed a significant reduction in the increase in expression after 14 DIV due to PT treatment. It was noted that carbon ions had a stronger influence than protons. Our bone model demonstrated the occurrence of pathological and regenerative processes induced by PT, thus building on the current understanding of the biological mechanisms of bone.

A Review of the research methods and progress of biocompatibility evaluation of root canal sealers

The function of root canal sealer was to achieve an appropriate three-dimensional filling effect by filling the root canal and some irregular lumen, thereby inhibiting the residual bacteria. There were many types of sealers, but research to find the most suitable ones was still ongoing. In recent years, researchers had continuously improved the performance of sealers by developing new sealers or adding active ingredients to the sealers. However, most sealers exhibit varying degrees of cytotoxicity and tissue responses, which affect clinical therapy efficacy. This review describes different technical approaches, and recent research progress in the biocompatibility evaluation of root canal sealers and provides brief insights into this field by summarising the performance studies of different root canal sealers.

Disruption of Endochondral Ossification and Extracellular Matrix Maturation in an Ex Vivo Rat Femur Organotypic Slice Model Due to Growth Plate Injury

Postnatal bone fractures of the growth plate (GP) are often associated with regenerative complications such as growth impairment. In order to understand the underlying processes of trauma-associated growth impairment within postnatal bone, an ex vivo rat femur slice model was developed. To achieve this, a 2 mm horizontal cut was made through the GP of rat femur prior to the organotypic culture being cultivated for 15 days in vitro. Histological analysis showed disrupted endochondral ossification, including disordered architecture, increased chondrocyte metabolic activity, and a loss of hypertrophic zone throughout the distal femur. Furthermore, altered expression patterns of Col2α1, Acan, and ColX, and increased chondrocyte metabolic activity in the TZ and MZ at day 7 and day 15 postinjury were observed. STEM revealed the presence of stem cells, fibroblasts, and chondrocytes within the injury site at day 7. In summary, the findings of this study suggest that the ex vivo organotypic GP injury model could be a valuable tool for investigating the underlying mechanisms of GP regeneration post-trauma, as well as other tissue engineering and disease studies.

Culture of vibrating microtome tissue slices as a 3D model in biomedical research

The basic idea behind the use of 3-dimensional (3D) tools in biomedical research is the assumption that the structures under study will perform at the best in vitro if cultivated in an environment that is as similar as possible to their natural in vivo embedding. Tissue slicing fulfills this premise optimally: it is an accessible, unexpensive, imaging-friendly, and technically rather simple procedure which largely preserves the extracellular matrix and includes all or at least most supportive cell types in the correct tissue architecture with little cellular damage. Vibrating microtomes (vibratomes) can further improve the quality of the generated slices because of the lateral, saw-like movement of the blade, which significantly reduces tissue pulling or tearing compared to a straight cut. In spite of its obvious advantages, vibrating microtome slices are rather underrepresented in the current discussion on 3D tools, which is dominated by methods as organoids, organ-on-chip and bioprinting. Here, we review the development of vibrating microtome tissue slices, the major technical features underlying its application, as well as its current use and potential advances, such as a combination with novel microfluidic culture chambers. Once fully integrated into the 3D toolbox, tissue slices may significantly contribute to decrease the use of laboratory animals and is likely to have a strong impact on basic and translational research as well as drug screening.

SIRT1 targeted by miR-211-5p regulated proliferation and apoptosis of Dex-treated growth plate chondrocytes via mediating SOX2

Dexamethasone (Dex) is reported to cause bone growth retardation in children, which is associated with the increased apoptosis and decreased proliferation of growth plate chondrocytes. Sirtuin 1 (SIRT1) plays an important role in chondrocyte function and homeostasis. Thus, we further explored the regulatory mechanism of SIRT1 in Dex-induced growth plate chondrocyte dysfunction. SIRT1 expression was detected in Dex-treated growth plate chondrocytes using RT-qPCR and western blot assay. The modulation of SIRT1 on SOX2 expression was evaluated. Besides, we identified that SIRT1 was targeted by miR-211-5p using TargetScan and RNA pull-down assay. A loss-of-function assay was performed to evaluate the effects of miR-211-5p on Dex-induced growth plate chondrocyte dysfunction in vitro and in vivo. We found that SIRT1 was downregulated in Dex-treated growth plate chondrocytes. The expression of SOX2 was upregulated by overexpression SIRT1. Meanwhile, downregulation of SOX2 weakened the positive function of SIRT1 overexpression on Dex-induced growth plate chondrocytes dysfunction. Subsequently, we confirmed that SIRT1 was targeted by miR-211-5p. MiR-211-5p inhibitor increased the expression levels of SIRT1 and SOX2, and restored the Dex-treated growth plate chondrocyte function. Animal assays further demonstrated that the effects of miR-211-5p on the growth plate chondrogenesis. In conclusion, our data suggest that SIRT1 exerts a protective effect on growth plate chondrocyte under Dex stimulation. MiR-211-5p/SIRT1/SOX2 axis regulates the process of Dex-inhibited growth plate chondrogenesis.

Current knowledge, challenges and innovations in developmental pharmacology: A combined conect4children Expert Group and European Society for Developmental, Perinatal and Paediatric Pharmacology White Paper

Developmental pharmacology describes the impact of maturation on drug disposition (pharmacokinetics, PK) and drug effects (pharmacodynamics, PD) throughout the paediatric age range. This paper, written by a multidisciplinary group of experts, summarizes current knowledge, and provides suggestions to pharmaceutical companies, regulatory agencies and academicians on how to incorporate the latest knowledge regarding developmental pharmacology and innovative techniques into neonatal and paediatric drug development. Biological aspects of drug absorption, distribution, metabolism and excretion throughout development are summarized. Although this area made enormous progress during the last two decades, remaining knowledge gaps were identified. Minimal risk and burden designs allow for optimally informative but minimally invasive PK sampling, while concomitant profiling of drug metabolites may provide additional insight in the unique PK behaviour in children. Furthermore, developmental PD needs to be considered during drug development, which is illustrated by disease- and/or target organ-specific examples. Identifying and testing PD targets and effects in special populations, and application of age- and/or population-specific assessment tools are discussed. Drug development plans also need to incorporate innovative techniques such as preclinical models to study therapeutic strategies, and shift from sequential enrolment of subgroups, to more rational designs. To stimulate appropriate research plans, illustrations of specific PK/PD-related as well as drug safety-related challenges during drug development are provided. The suggestions made in this joint paper of the Innovative Medicines Initiative conect4children Expert group on Developmental Pharmacology and the European Society for Developmental, Perinatal and Paediatric Pharmacology, should facilitate all those involved in drug development.

The Osteogenic Assessment of Mineral Trioxide Aggregate-based Endodontic Sealers in an Organotypic Ex Vivo Bone Development Model

INTRODUCTION

Mineral trioxide aggregate (MTA)-based sealers are endodontic materials with widespread success in distinct clinical applications, potentially embracing direct contact with the bone tissue. Bone response to these materials has been traditionally addressed in vitro. Nonetheless, translational data are limited by the absence of native cell-to-cell and cell-to-matrix interactions that hinder the representativeness of the analysis. Ex vivo organotypic systems, relying on the culture of explanted biological tissues, preserve the cell/tissue composition, reproducing the spatial and organizational in situ complexity. This study was grounded on an innovative research approach, relying on the assessment of an ex vivo organotypic bone tissue culture system to address the osteogenic response to 3 distinct MTA-based sealers.

METHODS

Embryonic chick femurs were isolated and grown ex vivo for 11 days in the presence of MTA Plus (Avalon Biomed Inc, Bradenton, FL), ProRoot MTA (Dentsply Tulsa Dental, Hohnson City, Germany), Biodentine (Septodont, Saint Maurdes Fosses, France), or AH Plus (Dentsply Sirona, Konstanz, Germany); the latter was used as a control material. Femurs were characterized by histologic, histochemical, and histomorphometric analysis. Gene expression assessment of relevant osteogenic markers was conducted by quantitative polymerase chain reaction.

RESULTS

All MTA-based sealers presented an enhanced osteogenic performance compared with AH Plus. Histochemical and histomorphometric analyses support the increased activation of the osteogenic program by MTA-based sealers, with enhanced collagenous matrix deposition and tissue mineralization. Gene expression analysis supported the enhanced activation of the osteogenic program. Comparatively, ProRoot MTA induced the highest osteogenic functionality on the characterized femurs.

CONCLUSIONS

MTA-based sealers enhanced the osteogenic activity within the assayed organotypic bone model, which was found to be a sensitive system for the assessment of osteogenic modulation mediated by endodontic sealers.

Ex vivo Bone Models and Their Potential in Preclinical Evaluation

PURPOSE OF REVIEW

Novel therapies for damaged and diseased bone are being developed in a preclinical testing process consisting of in vitro cell experiments followed by in vivo animal studies. The in vitro results are often not representative of the results observed in vivo. This could be caused by the complexity of the natural bone environment that is missing in vitro. Ex vivo bone explant cultures provide a model in which cells are preserved in their native three-dimensional environment. Herein, it is aimed to review the current status of bone explant culture models in relation to their potential in complementing the preclinical evaluation process with specific attention paid to the incorporation of mechanical loading within ex vivo culture systems.

RECENT FINDINGS

Bone explant cultures are often performed with physiologically less relevant bone, immature bone, and explants derived from rodents, which complicates translatability into clinical practice. Mature bone explants encounter difficulties with maintaining viability, especially in static culture. The integration of mechanical stimuli was able to extend the lifespan of explants and to induce new bone formation. Bone explant cultures provide unique platforms for bone research and mechanical loading was demonstrated to be an important component in achieving osteogenesis ex vivo. However, more research is needed to establish a representative, reliable, and reproducible bone explant culture system that includes both components of bone remodeling, i.e., formation and resorption, in order to bridge the gap between in vitro and in vivo research in preclinical testing.

Fluoride Inhibits Longitudinal Bone Growth by Acting Directly at the Growth Plate in Cultured Neonatal Rat Metatarsal Bones

Excessive intake of fluoride inhibits bone growth in both humans and animals. It is unknown whether fluoride acts directly on the growth plate to inhibit longitudinal bone growth, and its mechanism of action has not been elucidated. In this study, we used an organ culture system and SW1353 cells to evaluate the effects of fluoride on endochondral ossification. Neonatal rat metatarsal bones were dissected and cultured with or without fluoride for 7 days. The total length and width of the metatarsal rudiments and the length of the calcification zone were measured. Chondrocyte proliferation, differentiation, and apoptosis were analyzed by immunohistochemistry and TUNEL assay in sectioned bones. The apoptosis was detected by flow cytometry, and the expression of apoptosis-related proteins Bax, Bcl-2, and Caspase-3 were detected by western blotting in SW1353 cells. Linear measurements demonstrated that fluoride induced a biphasic effect on longitudinal bone growth in organ culture, with a significant growth inhibition at a high concentration (10^-4 M) and a stimulatory action at low concentration (10^-6 M) of fluoride. Histomorphometrical analysis of growth plate from fluoride-exposed metatarsal rudiments showed a significant reduction in the height of the proliferative and hypertrophic chondrocyte zones. Analysis of the Col2α1 and Col10α1 expression by immunohistochemistry revealed fluoride-suppressed metatarsal growth plate chondrocyte proliferation and differentiation. In addition, fluoride increased the number of apoptotic chondrocytes in the metatarsal growth plate. Western blotting showed an up-regulated expression of Caspase-3 and Bax and down-regulated expression of anti-apoptotic protein Bcl-2 after treatment with 5 × 10^-4 M fluoride in SW1353 cells. Our findings indicated that fluoride inhibited longitudinal bone growth by acting directly at the growth plate in cultured neonatal rat metatarsal bones. Such growth inhibition was mediated by suppressing proliferation and differentiation, increasing apoptosis of resting chondrocytes and causing premature cell senescence in the growth plate.

Use of in vitro bone models to screen for altered bone metabolism, osteopathies, and fracture healing: challenges of complex models

Approx. every third hospitalized patient in Europe suffers from musculoskeletal injuries or diseases. Up to 20% of these patients need costly surgical revisions after delayed or impaired fracture healing. Reasons for this are the severity of the trauma, individual factors, e.g, the patients’ age, individual lifestyle, chronic diseases, medication, and, over 70 diseases that negatively affect the bone quality. To investigate the various disease constellations and/or develop new treatment strategies, many in vivo, ex vivo, and in vitro models can be applied. Analyzing these various models more closely, it is obvious that many of them have limits and/or restrictions. Undoubtedly, in vivo models most completely represent the biological situation. Besides possible species-specific differences, ethical concerns may question the use of in vivo models especially for large screening approaches. Challenging whether ex vivo or in vitro bone models can be used as an adequate replacement for such screenings, we here summarize the advantages and challenges of frequently used ex vivo and in vitro bone models to study disturbed bone metabolism and fracture healing. Using own examples, we discuss the common challenge of cell-specific normalization of data obtained from more complex in vitro models as one example of the analytical limits which lower the full potential of these complex model systems.

miR‑10a‑5p inhibits osteogenic differentiation of bone marrow‑derived mesenchymal stem cells

The use of human bone marrow mesenchymal stem cells (hBMSCs) as a tissue engineering application for individuals affected by osteoporosis and other types of bone loss diseases has been well studied in recent years. The osteogenic differentiation of hBMSCs can be regulated by a number of cues. MicroRNAs (miRNAs/miRs) serve as the key regulators of various biological processes; however, to the best of our knowledge, no information exists with regards to the specific modulatory effects of miR-10a-5p on osteogenic differentiation of hBMSCs. The aim of the present study was to investigate the relationship between hBMSCs and miR-10a-5p and, ultimately, to determine how miR-10a-5p affects the osteogenic differentiation process of hBMSCs in vitro and in vivo. The hBMSCs used in the present study were transfected with mirVana™ miRNA inhibitors and mimics, and transfection efficiency was assessed by fluorescence microscopy and reverse transcription-quantitative PCR (RT-qPCR). Viability of hBMSCs following transfection was analyzed using a Cell Counting Kit-8 assay. The mRNA expression levels of specific osteoblast markers, including alkaline phosphatase (ALP) and runt-related transcription factor 2 (RUNX2) were measured using RT-qPCR and western blot analysis. New bone formation was evaluated by Goldner's trichrome staining and micro-CT analysis in vivo. No significant difference in cell viability was observed among the different groups 24 h post-transfection. Overexpression of miR-10a-5p inhibited the expression of osteoblast makers in hBMSCs, whereas inhibition of miR-10a-5p upregulated the expression of ALP and RUNX2 in vitro. Furthermore, miR-10a-5p acted as a suppressor during the process of new bone formation in vivo. In conclusion, the findings of the present study suggested that miR-10a-5p served as a negative regulatory factor during osteoblast differentiation of hBMSCs and may be utilized in a treatment approach for bone repair in osteogenic-related diseases.

Evolving applications of the egg: chorioallantoic membrane assay and ex vivo organotypic culture of materials for bone tissue engineering

The chick chorioallantoic membrane model has been around for over a century, applied in angiogenic, oncology, dental and xenograft research. Despite its often perceived archaic, redolent history, the chorioallantoic membrane assay offers new and exciting opportunities for material and growth factor evaluation in bone tissue engineering. Currently, superior/improved experimental methodology for the chorioallantoic membrane assay are difficult to identify, given an absence of scientific consensus in defining experimental approaches, including timing of inoculation with materials and the analysis of results. In addition, critically, regulatory and welfare issues impact upon experimental designs. Given such disparate points, this review details recent research using the ex vivo chorioallantoic membrane assay and the ex vivo organotypic culture to advance the field of bone tissue engineering, and highlights potential areas of improvement for their application based on recent developments within our group and the tissue engineering field.