Objective: To observe the effect of simulated microgravity on cytoskeleton, proliferation, cell cycle, and apoptosis of osteoblasts. Methods: Osteoblasts were isolated and cultured from newborn rat cranial bones, and were cultured in the rotary clinostat for 24h, 48h, 72h, 96h, with control group in 1G condition. The distribution and direction of microtubules were observed by indirect immunofluorescence. The survival rate of osteoblasts was detected by MTT assay. And cell cycle and the percentages of apoptosis were measured through flow cytometry. Results: The microtubules distribution was changed significantly, which became obscured and showed no clear directions under simulated microgravity within 24h, but the growth of osteoblasts increased tinily. While cell proliferation were inhibited and the apoptosis rate increased, the proportion of G(1)/G(0) phase cells was higher, and S, G(2)+M phase cells were lower correspondingly when cells were under simulated microgravity for more than 48h. With the prolonged rotation time, the differences became more obvious. Conclusion: Simulated microgravity has many effects on rat osteoblasts structure and growth. Those effects may interaction to influence the functions of osteoblasts, leading to bone loss under microgravity condition.

Space flight and simulated microgravity lead to suppression of mammalian spermatogenesis and decreased plasma testosterone level. In order to explain the mechanism behind the depression, we used rat tail-suspended model to simulate weightless conditions. To prevent cryptorchidism caused by tail-suspension, some experimental animals received inguinal canal ligation. The results showed that mass of testis decreased significantly and seminiferous tubules became atrophied in rats after tail-suspension. The levels of plasma testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) in tail-suspended rats with or without inguinal canal ligation decreased significantly compared with controls, and an increased level of plasma estradiol (E2) was revealed in tail-suspended rats. The results indicate that besides the direct influence of fluid shift upon testis under short-term simulated microgravity, the pituitary function is also disturbed as a result of either immobilization stress or weight loss during tail-suspension treatment, which is responsible to some extent for the decreased testosterone secretion level and the atrophia of testis. The conversion of testosterone into E2 under simulated microgravity is another possible cause for the decline of plasma testosterone.