Age-dependent morphometric alterations in the distal femora of male and female rats.

Morphologic parameters, bone area, bone-to-bone + marrow ratio, periosteal-to-periosteal + endocortical surface ratio, mean trabecular thickness, and surface-to-volume ratio were studied in the epiphysis and metaphysis of the distal femora of male and female rats (Heiligenberg strain) between birth and the end of the lifespan. With increasing age, bone area, bone-to-bone + marrow ratio, and mean trabecular thickness increases, whereas periosteal-to-periosteal + endocortical surface ratio and surface-to-volume ratio decreases in both parts of bone during the first 150 days. Afterwards, periosteal-to-periosteal + endocortical surface ratio, mean trabecular thickness, and surface-to-volume ratio remain constant, whereas the bone area and the bone-to-bone + marrow ratio decrease. Modeling data were measured by use of the vital labeling technique with calcein. From the stained bone area, the bone formation, the bone resorption, and the periosteal mineral apposition rates have been calculated. The bone formation rate, about 13,000%/year in the metaphysis and 2,000%/year in the epiphysis, respectively, is greatest after birth and decreases continuously with increasing age to 3.5%/year for both bone regions. During the first 150 days the bone resorption rate is lower than the bone formation rate, leading to an increase in bone area, but afterwards it is higher so that the area decreases. Likewise the periosteal mineral apposition rate is greater in the metaphysis (24 microns/day at day 50) than in the epiphysis (14 microns/day at day 50), but after 700 days it is comparable for both bone regions (0.07 microns/day). The absolute values of body weight, femur length, and bone area of epiphysis and metaphysis are greater in male rats; only the mean trabecular thickness and the periosteal mineral apposition rate are comparable in both sexes. The relative values of bone-to-bone + marrow ratio, periosteal-to-periosteal + endocortical surface ratio, bone formation rate, and bone resorption rate are comparable for both sexes.