PLT bioreactor models major components of BM. Primary mouse MKs are shown. (A) The upper and lower channels can be selectively coated with ECM proteins to reproduce osteoblastic and vascular niche composition. (B) MKs trap at gaps and extend proPLTs into the lower channel (white arrow). MKs can be selectively embedded in alginate or Matrigel gels, modeling three-dimensional ECM organization and physiological BM stiffness (250 Pa). Vascular flow is retained in the lower channel as demonstrated by 0.02 µm fluorescent bead streaking and fluorescein isothiocyanate-dextran fluorescence. (C) HUVECs can be selectively cultured in ECM-coated channels to reproduce blood vessel physiology. (D) MK trapping, BM stiffness, ECM composition, micro-channel size, hemodynamic vascular shear stress, and endothelial cell contacts can be combined to reproduce human BM in vitro. (E) Shear rate distribution along the length of the channel. Arrows indicate the magnitude and direction of the velocity field. (F) Fluid shear rates are well-characterized and can be tightly regulated across the bioreactor as a function of flow rate. (G) Regardless of the number of occupied slits, trapped MKs experience physiological shear stresses at gap junctions. Media viscosity is 1.20 mPa · s. Scale bars represent 50 µm (A-D).