Comparison of long bones and vertebrae in growing male rats: rate of growth, mineralization, and uptake of 3H-tetracycline at the organ level.

Two vertebral and two long bones of the growing male rat were separated as an intact anatomical unit (at the organ level) and analyzed for mass of dry weight, calcium, and collagen. The rate of growth (mass) and mineralization (uptake of 3H-tetracycline) was determined in rats from 3 to 36 weeks of age. Groups of rats age 3, 6, 8, 10, 14, 20, and 36 weeks were given a single dose of 3H-tetracycline 4 days before sacrifice. Whole bones were assayed for calcium, collagen, and 3H-tetracycline radioactivity. Growth of vertebrae and long bones is similar from 3 to 6 weeks of age. From age 6 to 20 weeks vertebrae grow at a faster rate than long bones. While long bones reach a peak dry mass at 20 weeks of age, the dry mass of vertebrae continues to increase. At ages 3 and 4 weeks, mineralization is significantly higher in vertebrae than in long bones. Peak calcification occurs at 8 weeks for long bones and at 10 weeks for vertebrae. These differences appear to be associated with the difference in age at which closing of growth plates occur as well as differences in formation rates between vertebrae and long bones.