Changes in Marrow Composition and Osteogenic Poten-Tial during the Development of Osteoporosis in Ovariec-Tomized Rats

This experiment was designed to observe the sequential bone changes dur-ing the development of osteoporosis in ovariectomized rats and to evaluate the in vitro osteogenic potential of marrow stromal cells after ovariectomy. Forty-eight female SD rats were subjected to either bilateral ovariectmoy or sham surgery. Sequential bone and marrow changes were observed using histomor-phometric method at 2 weeks, 4 weeks, and 3 months after surgery. In vitro bone marrow cell cultures were performed during matched time schedules. Ovariectomized (OVX) rats gained significantly more weight than the control rast did. Osteogenic potentials of marrow stromal cells were essentially the same between matched groups. The planting efficiencies (PE) and proportions of alka-line phosphatase (ALP) positive colony were similar between matched groups in three time schedules. However, the PEs and ratios of colony with ALP positive were significantly correlated with ages of the rats. Histomorphometric study showed rapid bone loss of the proximal tibia (4 weeks) and the iliac bone (2 weeks) after ovariectomy. The adipose tissue volume (ATV) of the proximal tibia did not show significant changes until 3 months after ovariectomy. The ATV of the iliac bone was only correlated with ages of the rats. This study demonstrated that rapid bone loss occurred soon after ovariectomy. The ATV changes fol-lowed, rather than preceded the onset of bone loss. Marrow fat volume change was a result, not a cause,of osteoporosis. Our findings also suggested that the etiology of osteoporosis did not involve a primary failure of proliferation of the mesenchymal cells at the endosteum.