Fig. 1: Development of a hybrid HCA of the naïve and myeloma bone microenvironment.

a Bone-lining osteoblast lineage cells release RANKL inducing fusion and maturation of osteoclasts (1). Osteoclasts resorb the bone, releasing stored bone-derived factors (BDFs) such as TGF-β (2). TGF-β recruits local MSCs and stimulates asymmetric division in to preosteoblasts (2). When TGF-β levels remain high, preosteoblasts rapidly proliferate. Following osteoclast apoptosis, release of TGF-β falls and preosteoblasts differentiate to mature bone producing osteoblasts (3). While TGF-β levels remain low, osteoblasts produce new bone (4). As bone returns to normal, a fraction of the osteoblasts is buried within the matrix becoming terminally differentiated osteoblasts (5), the remaining osteoblasts undergo apoptosis, or become quiescent bone-lining cells (6). Myeloma cells enhance the formation of osteoclasts (7), enhanced bone resorption produces higher levels of BDFs which fuel myeloma growth (8) and inhibit osteoblast differentiation (8) and activity (9). Created with biorender.com. b Interaction diagram between cell types in the HCA and factors such as BDFs and RANKL (created with biorender.com). A more detailed interaction diagram with references can be found in supplementary fig. 1. c Flowcharts describing the sequence of steps followed by preosteoclasts, osteoclasts, MSCs, preosteoblasts, osteoblasts, and myeloma cells.