The interwoven fates of blood and bone

The intertwined fates of bone and bone marrow are tightly coupled with haematopoiesis –defined as the production of all blood cells in the body. Multipotent haematopoietic stem cells (HSCs) are central to this process as they are responsible for replenishing the entire haematopoietic system. Interestingly, HSCs have not been definitively identified and challenges remain in preserving their functionality ex vivo. The HSCs referred to in current literature are in actuality, heterogeneous populations of cells, as they are prospectively isolated based on expression of collective panels of different cell surface markers. HSCs reside in and are regulated at specialised microenvironments or niches, and one such example is the endosteal niche. The endosteal niche localises to the interface between the bone and bone marrow. The intimate, functional and reciprocal relationship shared between the bone and bone marrow extends beyond their anatomical juxtaposition. Indeed, there are emerging evidence to support the role of bone cells in regulating various aspects of haematopoiesis such as HSC proliferation and differentiation. However, one of the most riveting and relatively unexplored questions is, when does bone begin to play an instrumental role in haematopoietic development? To address this question, one would consider the interface between the bone and bone marrow. In an established or adult setting, this microenvironment may be described as an intricate tapestry composed of a myriad of cells, growth factors, signalling molecules and the extracellular matrix. The complexity of this system poses a significant challenge in investigating when bone begins to modulate haematopoiesis. This thesis exploits the tissue regenerative capacity of an acellular biological scaffold, known as demineralised bone matrix (DBM), in two models to study adult haematopoiesis. The first model investigates the spatial and temporal events leading up to the genesis of ectopic bone and bone marrow. The second model (vascularised chamber model) develops and characterises an entire bone and bone marrow unit that is henceforth referred to as the organoid. Three key findings can be inferred from the ectopic model. Firstly, neovascularisation is an important event for the subsequent cellular repopulation of the matrix. Secondly, cartilage mineralisation precedes the detection of haematopoietic progenitors within the nodule. Thirdly, as the matrix remodels, new bone forms and it eventually encompasses a centralised bone marrow cavity within the mature nodule. Collectively, these results suggest that both cartilage and bone may play instrumental roles in shaping the haematopoietic microenvironment at different developmental stages. Four principal findings can also be extrapolated from the vascularised chamber model. Firstly, the model highlights the importance of neovascularisation and demonstrates that the repopulating cells are of blood-borne origin. Secondly, the incorporation of an osteogenic growth factor to the acellular DBM results in a concomitant expansion of both bone and bone marrow compartments within the organoid, accompanied by augmented production of haematopoietic progenitors. Thirdly, the developed bone and bone marrow organoid demonstrates responsiveness to systemic pathophysiological changes by expanding the pool of haematopoietic progenitors within its own marrow. Finally, the transplanted organoid demonstrated functionality as evidenced by peripheral blood chimerism and modest haematopoietic recovery of irradiated recipients. Taken together, the findings reported in this thesis provide a deeper insight into the interwoven fates of blood and bone