Bone is a complex tissue with a variety of functions, such as providing mechanical stability for locomotion, protection of the inner organs, mineral homeostasis and haematopoiesis. To fulfil these diverse roles in the human body, bone consists of a multitude of different cells and an extracellular matrix that is mechanically stable, yet flexible at the same time. Unlike most tissues, bone is under constant renewal facilitated by a coordinated interaction of bone-forming and bone-resorbing cells. It is thus challenging to recreate bone in its complexity in vitro and most current models rather focus on certain aspects of bone biology that are of relevance for the research question addressed. In addition, animal models are still regarded as the gold-standard in the context of bone biology and pathology, especially for the development of novel treatment strategies. However, species-specific differences impede the translation of findings from animal models to humans. The current review summarizes and discusses the latest developments in bone tissue engineering and organoid culture including suitable cell sources, extracellular matrices and microfluidic bioreactor systems. With available technology in mind, a best possible bone model will be hypothesized. Furthermore, the future need and application of such a complex model will be discussed.

Throughout life, continuous remodelling is part of human bone biology and depends on the simultaneous action of physicochemical parameters such as oxygen tension and varying mechanical load. Thus, suitable model systems are needed, which allow concomitant modulation of these factors to recapitulate in vivo bone formation. Here, we report on the development of a first microphysiological system (MPS) that enables perfusion, environment-independent regulation of the oxygen tension as well as precise quantification and control of mechanical load. To demonstrate the use of the MPS for future studies on the (patho-)biology of bone, we built a simplified 3D model for early de novo bone formation. Primary human osteoblasts (OBs), which are the key players during this process, were seeded onto type I collagen scaffolds and cultured in the MPS. We could not only monitor cell viability and metabolism of OBs under varied physicochemical conditions, but also visualise the mineralisation of the extracellular matrix. In summary, we present a MPS that uniquely combines the independent control of physicochemical parameters and allows investigation of their influence on bone biology. We consider our MPS highly valuable to gain deeper insights into (patho-)physiological processes of bone formation in the future.

Osteocytes form an extensive dendritic network within bone tissue that senses mechanical and biochemical stimuli, critical for bone homeostasis. However, the complex interplay between mechanotransduction, oxygen tension, and osteocyte differentiation remains difficult to study due to the lack of physiologically relevant human in vitro models. Previously, we developed a microphysiological system (MPS) that integrates perfusion, mechanical loading, and oxygen control within a culture platform that enables the culture of bone-like three-dimensional (3D) constructs [1]. Here, we demonstrate the application of this MPS as Bonoid-on-a-chip (BoC)-platform to investigate osteoblast-to-early osteocyte transition and mechanosensitive signalling in a human-relevant 3D context, under different physical and chemical conditions. We generated bonoids by bioprinting primary human osteoblasts into a gelatine-hydrogel and cultured them under either static conditions or as dynamic BoC. Different BoC environments consisting in controlled perfusion (0.5 ml min⁻¹), mechanical loading (0% or 10% compression, 1 Hz frequency), and physiological oxygen tension (21% or 12%) over a seven-day period were compared. These different models were additionally treated with dexamethasone (DEXA), to evaluate early osteocyte-targeted effects of drug exposure under these different conditions. We observed high cell viability across all conditions, but clear differences in metabolic activity, morphological changes, and mechanotransduction markers between static and dynamic cultures. Notably, cells within the BoC-platform exhibited extended dendritic processes and significantly increased secretion of prostaglandin E2 and nitrite in response to mechanical stimulation - hallmarks of osteocytic signalling in mechanotransduction. When testing the new BoC as platform for early osteocyte-targeted investigations, we found DEXA exposure disrupting this mechanoresponsive phenotype, highlighting the model’s applicability for pharmacological screening. These findings demonstrate the value of our BoC-platform as a human-relevant model for studying bone mechanobiology and drug responses in vitro. This approach offers a promising alternative to animal models in bone research and opens new avenues for investigating osteocyte function under physiologically dynamic conditions.