Change in the mRNA expression of osteocalcin (top), type I collagen α1 (middle), and type II collagen (bottom) with time after mid-diaphyseal femoral fracture in rats at 6 weeks (younger) and 15 months (older) of age. The mRNA was extracted, reverse transcribed, and amplified by the polymerase chain reaction. The amplimers were separated by electrophoresis, blotted to nylon membranes and hybridized with 32P-labeled internal oligonucleotides. The level of radioactivity of each sample was measured in units of photo stimulated luminescence (PSL). The data are shown as mean ± SEM for three to four samples per point except for 2 (n = 2) and 6 (n = 1) week older rats. SEM smaller than the radius of the point are not shown.