There is conflicting evidence as to whether bone resorption and bone formation are coupled in the site‐specific manner that is typical of bone remodeling in the rat. The aim of this study was to elucidate this controversy further by analysis of tibial and vertebral cancellous and endocortical bone in rats of different age groups with a combination of in vivo fluorochrome labeling with cement line staining.

After multiple in vivo fluorochrome labeling, groups of female Fischer‐344 rats were killed at 3, 6, 9, and 12 months of age, and the first lumbar vertebrae and the proximal tibiae were processed undecalcified for bone histomorphometry. By comparing fluorochrome labeling and the contour of cement lines in serial sections, cancellous and endocortical bone formation sites were classified as “remodeling,” “modeling,” or “uncertain.”

In vertebral cancellous bone, remodeling was the main turnover activity in all age groups, increasing from 70.4 ± 2.2% (mean ± SEM) in 3‐month‐old rats to 91.0 ± 2.4% in 12‐month‐old rats. The percentage of modeling sites decreased from 17.1 ± 1.7% at age 3 months to 4.67 ± 1.84% at age 12 months. In the proximal tibial metaphysis of 3‐month‐old rats, 61.6 ± 3.6% of all trabecular bone‐forming sites were classified as modeling and 21.1 ± 3.1% as remodeling sites. In the 12‐month‐old rats, 66.3 ± 3.4% were classified as remodeling and 16.0 ± 3.1% as modeling sites. A similar trend toward augmented portions of remodeling with increasing age was observed in tibial and vertebral endocortical bone‐formation sites.

The present study suggests that, similar to higher mammals, the prevailing activity in vertebral and tibial cancellous bone of aged rats is remodeling. In the rapidly growing proximal tibia of 3‐month‐old rats, however, most of the cancellous bone‐forming sites were minimodeling sites. © 1996 Wiley‐Liss, Inc.