Fibroblasts resident in fibrous articular tissues support stable expression of exogenous transgenes. Following intra-articular injection of lentivirus-GFP or Ad.GFP into the knees of nude rats, groups of animals were sacrificed at days 5 and 168. The knee joints and surrounding tissues were harvested intact, decalcified, and processed for histology. For each joint, the approximate positions of fluorescent cells identified in serial, sagittal whole-knee sections were tabulated in green on knee joint diagrams similar to those shown on the left. The diagrams shown are representative of the results observed with both viruses at the respective times. On the right, images characteristic of the appearance of the GFP+ cells in tissue sections at the different times are shown (×20 magnification). Lines indicate the approximate regions represented by the tissue sections. The numbers of GFP+ cells in the synovium and subsynovium were reduced dramatically at day 168. The density and distribution of GFP+ cells in the tendon, ligament, and fibrous synovium were largely unchanged over the duration of the experiment. No fluorescent cells were seen in the articular cartilage with either virus at any time point. B, bone; GFP, green fluorescent protein; M, muscle; P, patella. Reprinted with permission .