A COMPARISON OF MICROANATOMIC BASIS FOR COUPLING BETWEEN BONE-FORMATION AND BONE-RESORPTION IN MAN AND THE RAT

The trabecular bone of the secondary spongiosa of mature rats shows a coupling of bone formation to resorption. In adult man the coupling of formation and resorption involves a site-specific sequence of events, in which bone resorption is normally followed, at the same site, by bone formation. Whether the coupled processes of bone resorption and formation also occur at the same site in the rat is unknown. To elucidate the spatial relationship between bone formation and resorption in the rat, we compared the percentage of crenated and non-crenated cement lines with the percentage of crenated and non-crenated bone surfaces in the proximal tibia of adult rats aged 16 weeks - two years. A similar comparison was also made using bone from adult human iliac crest. We found that the trabecular bones of 16-week and seven-month-old rats exhibited a low percentage of crenated cement lines. In contrast, the surfaces of rat bone trabeculae showed a similar low proportion of crenated surface to human bone. However, in older (two years) rats, in which bones have ceased to grow in length, the percentage of cement lines that were crenated increased to towards human levels. These results imply that most of the bone formed in the secondary spongiosa of growing rats occurs on non-resorbed surfaces. Thus, although there is substantial evidence that bone formation is coupled to resorption in the rat, such that increased resorption is associated with increased formation, and suppression of resorption suppresses bone formation, bone formation does not necessarily occur on a previously resorbed site. This suggests that in the rat, the processes are not coupled by local release of cytokines or local cell interactions but by some other signal, such as mechanical stimulation. Since site-specificity appears not to be crucial to the coupling of formation to resorption, the greater site-specificity of coupling in man, and in older rats, may be attributable not to a different mechanism to that in growing rats, but to a more static skeleton, which engenders a closer spatial correlation between bone formation and the resorption that induced it.