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 samples obtained from the human lumbar facet joint. Ultrastructural changes in synoviocytes were studied after loading synovial samples with 5-hydroxydopamine (5-OHDA) in an oxygenated Krebs' solution, prior to fixation. Synoviocytes were set loosely in the intimal matrix and classified into type A (phagocytic) and type B (secretory) cells. In general, type A cells populated the surface of the synovial lining, whereas type B cells were located deeper in the tissue, extending a process into the synovial fluid. Type B cells in control samples contained sparse secretory granules. Free nerve endings were not found in the synovial intima. In response to incubation in 5-OHDA, a precursor of biogenic monoamines, synoviocytes clustered and established contact. The ultrastructure of type B cells in the loaded group clearly differed from controls. They possessed typical membrane-bound vesicles, containing an electron dense interior surrounded by a lucent space. The size of these dense core vesicles ranged from 100 to 260 nm (on average 180 nm). They were 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 altered. The present observations provide morphological evidence for the amine-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.