The purpose of a working group on Bone-on-Chip is to promote interaction, share knowledge, and provide opportunities for collaboration. Bundling of facilities and disciplines will help to develop human bone organoids for both trabecular and cortical bone including osteoblasts, osteocytes and osteoclasts but also preferably vessel formation, mimicking healthy as well as diseased (genetic or (chronically) inflamed) bone and bone marrow compartments. Such bone on chip models can be used as a human measurement model for the investigation of pathophysiology of metabolic inflammatory/cancerous bone diseases and bone defects, for screening of potential pharmacological compounds and for improving bone quality, even on a personalized basis. The bone-on-chip constructs might also be the start of new tissue-engineered graft material.
To reproduce complete cortical and trabecular lamellar bone with all its facets, to mimic human disease in vitro, and to integrate bone-on-chip with other tissues on chip which are in close connection to the bone such as muscle, tendon, bone marrow and vessels. Several issues need to be resolved:
• Combination of cell types from different lineages
• Post-mitotic status of osteocytes
• Natural hypoxic environment
• Shear-stress/microfluidics effects on bone cell behavior
• Large 3D cultures because of the size of defects that need TE bone
• Matrix components (collagenous, non-collagenous, mineralization)
• Trabecular vs cortical structure/ osteonal bone/ lamellar structure
• Mechanical aspects (competence as well as mechanoresponsiveness)
• Enchondral bone formation
- 16 June 2023 (UTwente)
- 14 December 2023 (Radboudumc)
- 7 December 2022 (Kick-off, Amsterdam)
Globally, bone is the second most transplanted tissue, i.e. more than 2 million bone graft procedures are performed annually. Nevertheless, one of the most important processes during this surgical procedure, bone regeneration, is largely not understood. This lack of knowledge can be attributed to the fact that in vitro models are scarce, largely not understood, mostly not meeting the microenvironmental and cell type complexity to resemble physiological and clinical reality and are thus not reliable for predicting clinical outcome.
Bone is an organ that is extremely dynamic and adaptive to its environment. Three types of cells orchestrate bone metabolism: osteoblasts (which build bone tissue), osteoclasts (which take bone away) and osteocytes (which regulate the building and breaking down of bone). Osteocytes in vivo are embedded in heavily mineralized extracellular matrix, which affects both mechanosensation and mechanotransduction. Moreover, the calcified matrix as micro-environment causes impeded oxygen diffusion, which differs from regular cell culture conditions. The composition of the matrix as well as the degree of mineralization are different between conditions such as metabolic bone diseases or aging and consequently influence the function of the bone cells. Bone formation is therefore a complex process that requires not only differentiation of cell types but also the formation of a calcified collagenous matrix and an intricate network of intercellular communication. On top of that a 3 dimensional osteonal organization of lamellae in a cortical or trabecular structure is needed to obtain sufficient bone strength.
The most physiological situation would be to culture the osteocytes in their native matrix, but such cultures can survive no longer then 10 to 15 days, which is not long enough to study bone adaptation (1). Thus far in vitro cultures of bone started as separate osteogenesis and osteoclastogenesis monolayer cultures or co-cultures. Though osteoblastic monolayer cultures can result in calcified nodules, these cultures lack the specific properties of osteocytes and bone matrix. Scaffolds, mostly using biphasic calcium phosphate (BCP) microbeads or granules, have been developed to create a 3D environment for osteoblasts and osteocytes. In these models a fully developed mineralized collagen matrix was also not observed. A step forward was made when mouse mesenchymal stromal cells were differentiated towards functional osteocytes (2). These cells however were not able to produce sclerostin, a typical marker for osteocytes. In addition, these cells do not demonstrate a 3D bone-like mineralized matrix showing intrafibrillar mineralization under biological control. In 2021 the first organoid of woven bone was shown using human mesenchymal stromal cells (3). Further developments in this perspective should lead to complete cortical or trabecular lamellar bone with all its facets, including bone marrow components, such as blood cells and marrow adipocytes as well as vasculature.