Percentage of cells expressing transforming growth factor (TGF)-beta receptors (TGF-betaRs). Paraffin sections of knee joints of young (5 months old) and old mice (2 years old) were stained immunohistochemically with antibodies against TGF-betaRI or TGF-betaRII. Subsequently, the number of cells staining positive in the cartilage were scored with a computerized imaging system and corrected for the total number of cells. The expression of both receptors was reduced with age in both (a) medial and (b) lateral tibial cartilage, but the reduced TGF-betaRII was significant only in lateral tibial cartilage. Error bars display the standard error. For statistical analysis, a Student's t-test was used. * = p < 0.05; ** = p < 0.005; *** = p < 0.0005.