Oophorectomy-induced osteopenia in rats in relation to age and time postoophorectomy

Oophorectomized (OVX) rats served for many years as a popular model for 'postmenopausal' osteoporosis in spite of the fact that the rat continues to grow during these experiments. We performed OVX in rats at 1, 3, 6 and 10 months of age and compared the histomorphometric (bone size, bone trabecular and cartilage volume in different areas) and chemical (ash, Ca, P and Mg content) parameters at 2, 6, 8 and 20 weeks post-OVX (2-20 weeks) to those of sham-operated rats. Significant differences were observed only in the animals that were OVX at a young or young-mature age, i.e. mainly at 1 and 3 months and some of the rats at 6 months of age. There were no changes in bone ash and mineral contents in the OVX animals in comparison to sham-operated rats, except 2 weeks post-OVX in the 1-month-old rats where these variables were reduced. The most significant finding was a reduction in the metaphyseal bone volume. This was observed in the rats OVX at 1, 3 and 6 months of age, but not in those OVX at 10 months. In the young OVX rats there was also an increase in epiphyseal cartilage volume. The epiphyseal and diaphyseal bone volumes were not different between the groups at any time postsurgery, explaining the lack of differences in bone ash and mineral contents. There were no significant changes in the results of the histomorphometric studies between OVX and sham-operated rats when surgery was performed at 10 months of age. Since OVX exerts significant changes only in young rapidly growing rats, this approach is an inappropriate model for postmenopausal osteoporosis, which occurs long after bone growth has ended.