Microgravity Stress: Bone and Connective Tissue

Abstract

The major alterations in bone and the dense connective tissues in humans and animals exposed to microgravity illustrate the dependency of these tissues' function on normal gravitational loading. Whether these alterations depend solely on the reduced mechanical loading of zero g or are compounded by fluid shifts, altered tissue blood flow, radiation exposure, and altered nutritional status is not yet well defined. Changes in the dense connective tissues and intervertebral disks are generally smaller in magnitude but occur more rapidly than those in mineralized bone with transitions to 0 g and during recovery once back to the loading provided by 1 g conditions. However, joint injuries are projected to occur much more often than the more catastrophic bone fracture during exploration class missions, so protecting the integrity of both tissues is important. This review focuses on the research performed over the last 20 years in humans and animals exposed to actual spaceflight, as well as on knowledge gained from pertinent ground-based models such as bed rest in humans and hindlimb unloading in rodents. Significant progress has been made in our understanding of the mechanisms for alterations in bone and connective tissues with exposure to microgravity, but intriguing questions remain to be solved, particularly with reference to biomedical risks associated with prolonged exploration missions.

Effects of microgravity on bone

“…Bone loss is the most recognized phenomenon related to microgravity. 7 Bone is an active organ that maintains balance of bone formation and bone resorption; in normal conditions. 8 It is considered one anisotropic material, plastic and resistant.…”

Understanding vestibular-related physiological functions could provide clues on adapting to a new gravitational environment

“…Microgravity is well known to induce muscle atrophy and bone loss in astronauts [30][31][32]. A previous study suggested that microgravity-induced muscle wasting and osteopenia are partly due to enhanced bone resorption and reduced calcium absorption in the intestines [4]; however, the effects of gravity change on osteoblastic bone formation and osteoclastic bone resorption remain unclear [33].…”

A Matter of Gravity-Mechanotransduction: How Mechanical Forces Influence Biological Materials

“…One pathway associated with collagen based mechanotransduction involves integrins, a family of cell surface embedded proteins that provide a link between the extracellular environment and intracellular elements [8,10]. Cells are believed to sense intrinsic mechanical properties of ECM by applying traction forces to the collagen fibers [8].…”