Development and characterisation of a novel bone-bone marrow organoid

Bone and bone marrow have often been thought of as distinct compartments. However, the link between them is both anatomical and functional, in that a perturbation in one affects the other. Our previous studies using acellular demineralised bone matrix (DBM) demonstrated the development of both an ectopic bone and bone marrow nodule with ultimate characteristics reflecting mature adult long bone. As an extension of this model, we ask the question; can it function as a single organ? Develop and characterise a novel vascularised bone-bone marrow (B-BM) organoid by assessing its response to a) growth factor stimulation; b) stress (blood loss) and c) transplantation potential. B-BM organoid: Male Sprague-Dawley (SD) rats were implanted with a chamber containing a vascular pedicle and filled with DBM in a) the presence or absence of BMP-2; b) SD rats containing the fully developed B-BM organoid were bled (approx. 15% total blood vol.); c) fully formed organoid from male SD rats were transplanted to irradiated female recipients. Blood components were monitored for 4-weeks post transplantation. Histological and morphometric analyses (micro CT), and haematopoietic CFU assays were performed. We have successfully created a vascularised B-BM organoid using acellular DBM. BMP-2 significantly augmented the total B-BM volume and the total number of haematopoietic CFUs compared to control. The B-BM organoid's sensitivity to stress was evidenced by the significantly elevated total CFUs four and seven days post-bleeding, compared to non-bled controls. Successful organoid transplantation was demonstrated by patency of blood vessels for at least 28 days and evidence of earlier haematopoietic recovery compared with irradiated controls. The organoid as a single operational unit detects and responds to growth factors and stress by enhancing production of haematopoietic progenitors. Furthermore, its viability, functionality, and contribution to bone marrow recovery following irradiation highlight a potential for it to be exploited in a clinical setting.