Space flight: a challenge for normal bone homeostasis.

Space flight results in loss of bone mass, especially in weight-bearing bones, a condition that is suggested to be similar to disuse osteoporosis. As models to elucidate the underlying mechanism, bed rest studies were performed and bone metabolism in the rat both during space flight and during hindlimb unloading was investigated. The general picture is that bone formation is decreased partly as a result of reduced osteoblast function, whereas bone resorption is unaltered or increased. This deficit in bone mass can be replaced, but the time span for restoration exceeds the period of unloading. Changes in blood flow, systemic hormones, and locally produced factors are contributing in a yet undefined way to the response of osteoblastic cells to loading. The pathway by which loading and/or gravity are transduced into biochemical signals is still unknown. In vitro studies with osteoblastic cells show that their differentiation and cell morphology are altered during space flight. Elucidation of the involved signaling pathways has only recently been started. It is hoped that as the mechanisms by which bone responds to mechanical (un)loading are further understood, this insight will influence the treatment of other etiologies of osteoporosis.