Number of cells in medial and lateral tibial cartilage of 5 month and 2 year old C57Bl/6 mice. Paraffin sections of knee joints of young (5 months old) and old (2 years old) mice were stained with hematoxylin and eosin after which a blinded observer used a computerized imaging system to count the number of chondrocytes in the tibial cartilage. Old mice have a lower number of cells in their cartilage than young mice. The reduction in cell number is more pronounced on the medial side of the joint. 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.