Synovium of treated Tg197 mice. (a) Nontransgenic wild-type group (control) at age 8 weeks: type A (sa) and type B (sb) synoviocytes are arranged loosely in the synovium (syn). Type A cells are characterized by many thin filopodia (arrow), while type B cells contain many instances of rough endoplasmic reticulum (r-ER), small vesicles, and basement membrane structures (arrowhead); (b) P-NT.II-treated Tg197 group at 5–8 weeks of age: synoviocyte A cells (sa) with characteristic cytoplasmic processes intermingled with those of neighboring cells (arrow) and B cells (sb), seen at age 6–8 weeks of age (i.e. 2–4 weeks post-treatment), appear unmodified during the time course (1–4 weeks) of treatment. The ultrastructural features are similar to those seen in the ankle joints of wild-type controls in (a). (c) Scrambled-P-NT.II-treated Tg197 group at age 8 weeks: type A (sa) and B (sb) synovial cells were seen lining up close together, and many cell fragments (arrowheads) resulting from fibrous degeneration of endothelial cells were present in the synovium (syn), along with infiltrating mast cells (arrows). N = 4 joints/group; mean percentage of degenerating synoviocytes = 24% and 75% of total in (b) and (c), respectively. Bars = 2 μm.