The lack of mass transfer in bone lacunar-canalicular system may be the decisive factor of osteoporosis under microgravity

During spaceflight, astronauts experience 1–1.5% bone loss per month, especially in the lumbar spine, pelvis and lower limbs. The bone loss leads to osteoporosis and increased the risk of fracture. Current researches focus on anti-osteoporosis under microgravity mainly by inhibiting bone resorption of osteoclasts and / or increasing bone formation of osteoblasts. However, studies on the effects of mass transfer in the bone lacunar-canalicular system (LCS) on osteoporosis are lacking. Osteocytes reside in the lacunae and communicate with other osteocytes, osteoblasts and osteoclasts through the LCS in the bone matrix. Osteocytes are mainly responsible for mechanosensing and signal regulation in bone, and the LCS is the basic structure for signaling, mass transfer and mechanical stimulation. Microgravity causes deficient mass transfer in the LCS, especially in the outer layer of osteon. Osteocytes far away from the Haversian canals are inhibited or accelerated apoptosis to stimulate osteoclasts which result in bone loss. Deficient mass transfer in the LCS may be a determinant of human osteoporosis under microgravity, which will open up a new way to treat osteoporosis in space.