The occurrence of a sequential bone remodeling activity, similar to what is observed in human bone, is demonstrated in rat trabecular bone at the level of the secondary spongiosa. A complete dynamic histomorphometric analysis of the remodeling activity, using undecalcified sections and double fluorescent labels, has consequently been performed in young adults (220 g, 8 weeks old) and in more mature animals (320 g, 12 weeks old). The results showed that, despite a similar trabecular bone volume, younger animals had a five times higher bone formation rate and five times more osteoclasts than more mature animals. The higher bone formation rate was due in part to a threefold higher extent of double-labeled trabecular bone surface and in part to a 1.5-fold faster mineralization rate. These results therefore demonstrate a marked slowing down of bone turnover during skeletal maturation in the rat. The values obtained in this study have been compared with measurements made in other parts of the skeleton in the same species (Vignery and Baron 1978, 1980b; Tran Van et al., 1982a) or in humans. This comparison indicated that 12-week-old rats had a turnover rate very similar to values observed in iliac crest trabecular bone in adult humans. The rat is therefore a good experimental animal for the study of trabecular bone remodeling but since large variations occur during skeletal maturation, care should be taken in the selection of an age group relevant to the type of questions being asked.

Although widely used for the assessment of bone remodeling activity in humans, dynamic histomorphometric techniques have seldom been applied to small laboratory animals like the rat. Partial studies have been published but they deal either only with cortical bone (Baylink et al., 1970; Tam and Anderson, 1980) or with limited measurements of trabecular bone activity (Schenk et al., 1973; Miller and Jee, 1975; Kimmel and Jee, 1980; Wronski and Morey, 1982) often made in the growth plate area. In addition, and because of the prolonged growth of this species, the fact that bone remodeling occurs in the rat similar to that in human trabecular bone, is still controversial (Frost, 1976). We have demonstrated in previous studies (Baron, 1973; Vignery and Baron, 1980b) that bone remodeling occurs in rat alveolar bone and follows the same sequence as that described in humans (Frost, 1964; Rasmussen and Bordier, 1974). We have also recently