Quantitative study of the osteoblasts and osteoclasts in the bones of rats during the simulation of weightlessness

Tibia and vertebrae of rats exposed to hypokinesia or head-down suspension were investigated by quantitative histomorphometry. It was found that 35- and 60-day hypokinesia as well as 35-day suspension caused osteoporosis in the tibial and vertebral spongiosa. The development of osteoporosis was accompanied by a significant reduction in the number of osteoblasts in the primary spongiosa of tibia and vertebrae whereas no noticeable changes in osteoclasts were observed either in hypokinetic or suspended rats. The only exception was lumbar vertebrae in which the amount of osteoclasts decreased as a result of 60-day hypokinesia. It is assumed that the reduction in the number and activity of osteoblasts plays the major part in the development of osteoporosis during hypokinesia and suspension.