Percentage of cells expressing Smad-2P in medial and lateral tibial cartilage. Paraffin sections of knee joints of young (5 months old) and old (2 years old) mice were stained immunohistochemically with antibodies against Smad-2P. Subsequently, the number of cells staining positive were scored with a computerized imaging system and corrected for the total number of cells. The Smad-2P expression was reduced with age in both medial and lateral tibial cartilage. Error bars display the standard error. For statistical analysis, a Student's t-test was used. *** = p < 0.0005.