Assessment of skeletal aging using the 3H-proline topographic labeling method.

The present report represents an extension of initial 3H-proline autoradiographic studies designed to provide, at both the cytologic and histologic levels, an indelible topographic record of skeletal events in aging mice. 3H-proline was administered in multiple time-spaced doses and the animals were serially killed between 6 and 14 days later. Whole femora sectioned in longitudinal and three cross-sectional planes were prepared for autoradiography and subsequent grain count and micrometric analysis of the silver topographic bands which form over mineralizing surfaces. These bands, coincidental with the cellular uptake and turnover of radiotracer, allowed for the assessment of cellular rates of labeled matrical precursor production and deposition from which the processes of bone growth and remodeling and the effects of aging on the skeletal system can be evaluated. In addition, composite tracings of topographic data derived from multiple sections prepared at different geometric planes allowed for three-dimensional mapping of skeletal events. The results showed that the matrical rates of bone production in micrometers per day varied considerably from one surface to the next and decreased significantly with increasing age. The rates at posterior periosteal surfaces were twice those observed at the anterior periosteal surfaces of 5-week-old mice. By 26 weeks the rates decreased to 1/8 times, respectively. Between 52 and 104 weeks of age, no significant matrix production occurred at the periosteal surfaces. Endosteal activity was greater than twice the posterior periosteal rate at 5 weeks, 1/4 times at 26 weeks, and 1/8 times at 104 weeks of age. It was concluded that endosteal matrix production continued throughout life via active remodeling processes, but periosteal activity was turned-off by 52 weeks. Furthermore, the study unexpectedly showed that in old animals, subendosteal bone formation exhibiting hematopoietic activity is activated at 52 weeks in focal areas...