Microgravity and Radiation Effects on Astronaut Intervertebral Disc Health.

The effects of spaceflight on the intervertebral disc (IVD) have not been thoroughly studied, despite the knowledge that spaceflight increases the risk of herniation of IVDs in astronauts upon return to Earth. However, as long duration missions become more common, fully characterizing the mechanisms behind space-induced IVD degeneration becomes increasingly imperative for mission success. This review therefore surveys current literature to outline the results of human, animal, and cell-level studies investigating the effect of microgravity and radiation exposure on IVD health. Overall, recurring study findings include increases in IVD height in microgravity conditions, upregulation of catabolic proteases leading to a weakening extracellular matrix (ECM), and both nucleus pulposus (NP) swelling and loss of annulus fibrosus (AF) fiber alignment which are hypothesized to contribute to the increased risk of herniation when reloading is experienced. However, the limitations of current studies are also discussed. For example, human studies do not allow for invasive measures of the underpinning biochemical mechanisms, correlating animal model results to the human condition may be difficult, and cellular studies lack incorporation of ECM and other complexities that mimic the native IVD microarchitecture and environment. Moving forward, the use of three-dimensional organoid culture models that incorporate IVD-specific human cells, ECM, and signals as well as the development of cell- and ECM-level computational models may further improve our understanding of the impacts that spaceflight has on astronaut IVD health.