Ultrastructural Cytochemical Studies of Horseradish Peroxidase Uptake by Synovial Lining Cells of the Rat Temporomandibular Joint

A study was made of the uptake of intravenously injected horseradish peroxidase (HPO) by rat synovial lining cells in the temporomandibular joint. Synovial membrane was excised from rats sacrificed at various times after the injeciton of HPO, fixed in glutaraldehyde fixative, incubated in diaminobenzidine medium, postfixed in OSO4, and processed for electron microscopy. Within 5 min of HPO injection, reaction product was detected cytochemically in the synovial linin g cells (type A cells). The highest accumulation of reaction product in the type A cells was found at 60 min after the injection of HPO. Furthermore, lysosomes in the type A cells after injection of HPO were visualized by acid phosphatase activity. It is concluded, that exogenous HPO is incorporated into the type A cell rather than the type B cell by endocytosis throuhg the cell membrane and that these endocytotic structures are fused with lysosomes.