Although diffusion has been shown to be the major contributing mechanism for molecular transport in the extravascular spaces of organs and soft tissues, it is unlikely that diffusion alone can account for molecular transport in the porous, yet relatively impermeable matrix of bone. Rather, it has been proposed that fluid flow induced by the deformations that bone is subjected to during daily activities may promote molecular transport through convective mixing of fluids or enhancement of molecular transport from the capillaries to the outermost osteocytes within a given osteon. As the relative contribution of diffusive and convective transport in the bone matrix has not yet been elucidated, we conducted experiments to study the primary role of diffusion for molecular transport within bone and to establish a baseline for fluid transport whereby mechanical loading effects are negligible. Procion red and microperoxidase were utilized as short-term (i.e., low MW, transported on the order of minutes) and long-term (i.e., comparatively high MW, transported on the order of hours) molecular tracers, respectively, to elucidate in vivo the pathways and extent of transport in the metacarpus and tibia of 60-day-old (i.e., skeletally immature) and 180-day-old (i.e., skeletally mature) animals. The tracers were introduced intravenously and the animals were maintained in an anesthetized state for the duration of the experiment to prevent physiological loading. In short-term studies, procion red tracer distribution was highly dependent on bone structure, demarcating spaces apposing the vascular pathways in the trabecular bone of immature animals and vascular and extravascular pathways (i.e., specifically, the lacunocanalicular system) within compact bone of mature animals. In longer term studies using microperoxidase, reaction product was concentrated in soft tissues as well as along a subperiosteal and subendosteal band of bone. In contrast, little peroxidase reaction product was observed in the metacarpal and tibial cortices of either immature or mature animals. Based on the results of these studies, diffusive transport mechanisms may suffice to insure an adequate supply of small molecules, such as amino acids, to osteocytes in the midcortex within minutes. In contrast, diffusion alone may not be efficient for transport of larger molecules. Thus, another mechanism of transport, such as convective transport by means of load-induced fluid flow, may be necessary to provide a sufficient supply of larger molecules, such as proteins to osteocytes for the maintenance of metabolic activity, as well as for activation or suppression of modeling processes.
Bone; Molecular transport; Tracer; Fluid flow; Osteocyte