Effects of weightlessness on osseous tissue of the rat after a space flight of 5 days (Cosmos 1514)

Five pregnant female growing rats have been orbited for five days aboard the Cosmos 1514 soviet biological satellite. They were compared to five female rats kept in vivarium and five female conditioned rats in synchronised way. Histomorphometric studies were performed in order to investigate: 1. The early effects of weightlessness on the bone mass in loading (tibiae and femur) and unloading bones (thoracic and lumbar vertebrae). 2. The changes of osteoblastic and osteoclastic activities. A short exposure does not induce changes in the bone mass and the inner structure of loading and unloading bones. These results fit in well with human data available in the literature: they show that weightlessness doesn't change bone mass in the early phase of a spaceflight. However extrapolation of animal results to men is discussed. In unloading bones (vertebrae) osteoblastic activity was not measurable. Osteoclasts detected by histoenzymologic method don't change as far as their number per mm3 of trabecular bone is concerned. However the number per mm2 of trabecular area increases. It seems likely that an increase of the osteoclastic population occurs in trabecular bone. In loading bones, formation activity (appreciated by the measurement of osteoid seam thickness) and total osteoclastic resorption surfaces were not modified. These results are different from those of longer flights.