Percentage of cells expressing Smad in medial and lateral tibial cartilage. Frozen sections of knee joints of young (5 months old) and old (2 years old) mice were stained immunohistochemically with antibodies against Smad2, Smad3, Smad4, Smad6 or Smad7. Subsequently, the number of cells staining positive were scored with a computerized imaging system and corrected for the total number of cells. (a) In medial tibial cartilage, expression of Smad3, Smad4 and Smad7 increased with age. (b) In lateral tibial cartilage Smad3 and Smad6 expression increased with age. Error bars display the standard error. For statistical analysis, a Student's t-test was used. * = p < 0.05; *** = p < 0.0005.