Characteristics of the two types of synoviocytes in rat synovial membrane. An ultrastructural study

The characteristics of the two types of synoviocytes (A and S) from the knee joint of rat synovial membrane were analyzed by electron microscopy in order to identify all cellular profiles in single sections of the synovial intima and to interpret changes observed in the synoviocytes under different experimental and diseased conditions. The results demonstrated that type A and type S synoviocytes differed with respect to almost all cell organelles. Type A, which showed features of an absorptive, macrophagic cell, was primarily characterized by a well-developed vacuolar apparatus, a moderate Golgi apparatus, larger pale mitochondria, little rough endoplasmic reticulum showing narrow cisternae, and a heterochromatin-rich nucleus. The type S synoviocyte , which exhibited a cell body and one or two large cell processes reaching the joint cavity, showed ultrastructural features of a secretory cell. It was primarily characterized by an extensively developed rough endoplasmic reticulum showing wide cisternae, a large Golgi apparatus, and small, presumably secretory, granules, but surface caveolae, smaller dense mitochondria, and an euchromatin-rich nucleus were also very characteristic of this cell type. In addition, vesicles and vacuoles were invariably located in the large cell processes. Many S cells showed a solitary cilium. The present demonstration that the A and S cells are ultrastructurally totally different cells, together with the fact that intermediate types of cells do not exist, may support the interpretation that type A and type S synoviocytes represent two distinct cell types with distinct and different functions.