Light and electron microscopic investigation of equine synovial membrane. A comparison between healthy joints and joints with intraarticular fractures and osteochondrosis dissecans.

Light and electron microscopic examination was made on equine synovial membrane from 23 healthy joints, nine joints with synovitis caused by intraarticular fracture and 10 joints with synovitis caused by osteochondrosis dissecans. Histologically as well as ultrastructurally the equine synovial membrane from healthy joints was of principally the same character as described in other species. Three types of synovial membrane — areolar, fibrous and adipose — and two types of lining cell were distinguished histologically. Ultrastructurally three types of lining cells were distinguished: A and Β type and an intermediate cell type. In healthy joints they were loosely arranged, parallel to the joint surface in an intercellular matrix, which was in direct continuity with the joint space. In joints with intraarticular fracture there was mild inflammation of the synovial membrane. There was elongation and hyperplasia of the lining cells with a relative increase in type A cells. The cell surface of lining cells was increased through filopodia. There was also an increase in cytoplasmic organelles i.e. hyperplasia of rough endoplasmic reticulum and Golgi complexes in Β type cells and an increase in lysosomes, and increased numbers of vesicles of varying types in A cells. In joints with osteochondrosis dissecans the lining cell hyperplasia and the inflammation in the synovial membrane were more prominent. Ultrastructurally the same alterations as in the previous group were seen including a relative increase in the number of A cells but degenerative changes were common in the lining cells. These changes were dilatation and vesiculation of rough endoplasmic reticulum, mitochondrial condensation, dilatation of the nuclear envelope and loss of plasma membranes, leading to disintegration of cells.