On Mechanosensation in Bone Under Microgravity

Recent research has illuminated the biological response of bone to mechanical loading at the cellular level, but the precise mechanosensory system that signals bone cells to deposit or resorb tissue has not been identified. The purpose of this paper is to describe the current status of this research and to suggest some possible mechanosensory systems by which bone cells might sense environmental loads. The question of whether the mechanosensory system in bone tissue is at the level of the cell or whether it is at the tissue level and involving the cells is considered here. More precisely, the following question is addressed: can an osteocyte or an osteoblast read the gravitational field changes directly (and independent of changes in its environment), or does it detect those changes indirectly from its environment by contact stresses as it must detect other changes in mechanical loading on the surface of the earth? Our strategies for coping with the decay of the musculoskeletal system in long term space flight are somewhat dependent upon the answer to this question.