Morphological evidence of a polypeptide-like secretory function of the B cells in the mouse synovial membrane

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.

A developmental study of the synovial membrane of the rat temporomandibular joint: changes in the three-dimensional configuration during postnatal development

The development of synovial membranes in the posterior synovial portion of the rat temporomandibular joint was studied and the three-dimensional structure of the posterior synovial portion reconstructed from sagittal semithin sections. Reconstructions showed that the synovial membrane expanded and that synovial folds increased in number and became complicated in shape with the growth of the joint. Using transmission-electron microscopy, it was observed that the synovial lining cells degenerated, that the synovial membrane split to make further synovial folds, and that the folded-end structures consisted of synovial lining cells that extended into the subsynovial connective tissue. It is suggested that in the development of the three-dimensional configuration of the synovial membrane, several processes proceed simultaneously to form the synovial folds: a splitting of the synovial membrane, infolding of the synovial membrane into the subsynovial connective tissue, and outgrowth of the synovial folds towards the synovial cavity.

Ultrastructure of the mouse synovial membrane. Development and organization of the extracellular matrix

The synovial membrane of the mouse knee joint was examined by electron microscopy and electron microscopic histochemistry, with special reference to the development of the extracellular matrix. In the embryonic synovium, the intercellular spaces were filled with hyaluronate and chondroitin sulfate. The formation of the early joint cavity appeared to be initiated by accumulation of hyaluronate and chondroitin sulfate in the synovial primordium. At the postnatal stage, the synovial primordium differentiated into a true synovial intima that could be easily identified by the presence of two distinct lining cells: fibroblast-like cells (B cells) and phagocytic cells (A cells). Simultaneously, the synovial intima provided the specialized extracellular matrix that was characterized by organized structures of microfibrils, collagen fibers, and fibrous long spacing fibers embedded in a large number of glycoproteins.

Secretory granules of B-cells in the synovial membrane. An ultrastructural and cytochemical study

Ultrastructural and cytochemical studies have been made on secretory granules of B-cells (fibroblast-like cells) in the knee-joint synovium. The secretory granules were membrane-bounded spherical or slightly elongated bodies, 150 to 350 nm (average 230 nm) in diameter and had a homogenous matrix with several cores. These granules were found in B-cells of all animal species examined; they were numerous in mice and rats, and few in guinea pigs, rabbits and man. Ultrastructural and cytochemical examinations revealed that the Golgi apparatus was involved in the formation of the secretory granules. Unlike lysosomes, they showed no acid phosphatase activity. The granule matrix was positively stained by Thiéry's periodic acid-thiocarbohydrazide-silver proteinate technique, and the cores were digested by protease. These findings suggest that the granule matrix contains mucopolysaccharide(s) and/or glycoprotein(s) and the core material is largely proteinaceous in nature.

B-cells of the synovial membrane. I. A comparative ultrastructural study in some mammals

Comparative ultrastructural study of the B-cells in the intimal layer of the synovial membrane in mouse, rat, rabbit, guinea-pig and man clearly distinguishes these cells from both the histiocytic A-cells (macrophage-like cells) and the fibroblasts. In addition to the marked development of the rough endoplasmic reticulum and Golgi apparatus, typical dense secretory vesicles apparently of Golgi origin are always found in mouse B-cells and frequently in those of the rat. These secretory characteristics clearly relate these cells to glandular cells engaged in polypeptidic secretion. The variations in the other species studied concern only the figuration of secretory material. Thus, the B-cells appear to constitute a category of secretory cells specific to the synovial membrane, but the function of which has yet to be determined.