In vitro loading of human synovial membrane with 5-hydroxydopamine: Evidence for dense core secretory granules in type B cells

Ultrastructural studies of the synovial membrane were performed on tissue s amples obtained from the human lumbar facet: joint, Ultrastructural changes in synoviocytes were studied after loading synovial samples with 5-hydroxy dopamine (5-OHDA) in an oxygenated Krebs' solution, prior to fixation. Synoviocytes were set loosely in the intimal matrix and classified into typ e A (phagocytic) and type B (secretory) cells. In general, type A cells pop ulated the surface of the synovial lining, whereas type B cells were locate d deeper in the tissue, extending a process into the synovial fluid. Type B cells in control samples contained sparse secretory granules, Free nerve e ndings were not found in the synovial intima. In response to incubation in 5-OHDA, a precursor of biogenic monoamines, sy noviocytes clustered and established contact. The ultrastructure of type B cells in the loaded group clearly differed from controls. They possessed ty pical membrane-bound vesicles, containing an electron dense interior surrou nded by a lucent space. The size of these dense core vesicles ranged from 1 00 to 260 nm ton average 180 nm), They mere in relation to microtubules and located preferentially in the marginal area of the cytoplasm, close to the Golgi complex, The ultrastructure of type A cells was not significantly al tered, The present observations provide morphological evidence for the amin e-handling properties of type B cells, indicating that they might be added to the list of 'APUD' cells of the diffuse neuroendocrine system, A recepto -secretory function for type B cells is discussed.