Ultrastructure of rabbit synovial membrane

Ultrastructural localization of hyaluronic acid in the synovium of the goat knee joint

In the Japanese miniature (Shiba) goat, the synovial membrane contains synoviocytes referred to as type A (macrophage-like cells) and type B cells (fibroblast-like cells) in the intimal layer. Small capillaries and blood vessels of varying sizes were located in the extracellular matrix in the synovial subintima. The type A cells in the synovium possessed numerous vesicles, vacuoles and lysosomes as well as pinocytotic vesicles. These ultrastructural features indicating phagocytosis showed distinct positive reactions following hyaluronan staining. On the other hand, in the type B cells, hyaluronic acids were present in the surface coat of the plasma membrane and its periphery. Additionally, perivascular connective tissue of the small capillaries and blood vessels and interfibrous matrix contained hyaluronan. The results suggest that hyaluronic acid, in the synovial tissue, is synthesized on the plasma membrane of type B cells, and taken up by type A cells. Moreover, hyaluronan is involved in cellular functions in the synovial connective tissue.

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.

Morphology and functional roles of synoviocytes in the joint

The joint capsule exhibits a unique cellular lining in the luminal surface of the synovial membrane. The synovial intimal cells, termed synoviocytes, are believed to be responsible for the production of synovial fluid components, for absorption from the joint cavity, and for blood/synovial fluid exchanges, but their detailed structure and function as well as pathological changes remain unclear. Two types of synoviocytes, macrophagic cells (type A cells) and fibroblast-like cells (type B cells) have been identified. Type A synoviocytes are non-fixed cells that can phagocytose actively cell debris and wastes in the joint cavity, and possess an antigen-presenting ability. These type A cells, derived from blood-borne mononuclear cells, can be considered resident macrophages (tissue macrophages) like hepatic Kupffer cells. Type B synoviocytes are characterized by the rich existence of rough endoplasmic reticulum, and dendritic processes which form a regular network in the luminal surface of the synovial membrane. Their complex three-dimensional architecture was first revealed by our recent scanning electron microscopy of macerated samples. The type B cells, which are proper synoviocytes, are involved in production of specialized matrix constituents including hyaluronan, collagens and fibronectin for the intimal interstitium and synovial fluid. The proliferative potentials of type B cells in loco are much higher than type A cells, although the transformation of subintimal fibroblasts into type B cells can not be excluded. In some mammals, type B cells show features suggesting endocrine and sensory functions, but these are not recognized in other species. The synoviocytes, which form a discontinuous cell layer, develop both fragmented basement membranes around the cells and junctional apparatus such as desmosomes and gap junctions. For an exact understanding of the mechanism of arthritis, we need to establish the morphological background of synoviocytes as well as their functions under normal conditions.

Reversible depletion of synovial lining cells after intra-articular treatment with liposome-encapsulated dichloromethylene diphosphonate

We studied the depletion and repopulation of synovial lining cells in mice. A single intra-articular injection of liposomes encapsulating the drug dichloromethylene diphosphonate (CL2MDP) in the mouse knee joint caused selective elimination of synovial lining cells. Depletion of cells occurred within a few days as evidenced by light microscopic, electronmicroscopic and immunohistochemical studies. Maximal depletion was seen on day 7. Repopulation was observed in the following weeks, starting at the bone side of the joint. Until day 30, full recovery (60% recovery) was not observed in the lining lying adjacent to the dermis. Side effects on cartilage metabolism, such as inhibition of proteoglycan synthesis or degradation of proteoglycans from the matrix was minor but significant, 1 and 2 days after liposome treatment but thereafter full recovery was observed. Selective elimination of lining cells from the joint enabled us to study the in vivo role of these cells in the onset and subsequent pathology of experimental arthritis. An immune-complex-mediated experimental arthritis elicited in lining cell depleted joints that had received CL2MDP-liposomes 7 days earlier prevented inflammation as compared to controls.

Aging of the human synovium: an in vivo and ex vivo morphological study

Age-associated changes of the human synovium have been investigated by microarthroscopy, optical and electron microscopy, immunohistochemistry, and cytochemistry. The knee joints of nineteen 15- to 56-year-old subjects, classified as normal by inspection, were carefully examined by microarthroscopy; small synovial tissue biopsy specimens from both the suprapatellar pouch and the medial tibiofemoral gutter were taken. Microarthroscopy showed that the villi were more numerous and the vascular network and cell distribution and profiles less regular in aged individuals. These data were confirmed by scanning electron microscopy, which also showed large areas of the synovial surface devoid of cells and collagen bundles in contact with the joint cavity in aged subjects. Light and transmission electron microscopy confirmed these data and allowed evaluation of the number, distribution, shape, and internal organization of cells as well as the distribution of vessels and the organization of the extracellular matrix in the full thickness of the synovium (down to 2 mm). Particular attention was paid to synovial lining cells, among which three main phenotypes could be recognized: synthetic type (present at all ages and hypertrophied in aged subjects), macrophagelike (increasing with age), and fibroblastlike. Collagen increased with age. Further studies are needed for comprehensive understanding of age-associated changes in the human synovium.

Analysis of cell populations in rheumatoid arthritis synovial tissues

Knee synovium, taken from patients with rheumatoid arthritis at the time of arthroplasty, was studied immunohistologically. Focal perivascular lymphoid infiltrates of different sizes were examined in detail to evaluate changes in cell populations as the infiltrate size increased. T cells formed the largest component of mononuclear cells of all aggregates. The large grade 3 aggregates contained substantial numbers of B cells arranged around a central venule and cells bearing the CD45RA+ phenotype. In contrast, the small grade 1 aggregates contained few B cells and the T-cell population contained relatively greater numbers of CD8+ cells. Cells bearing the CD45RO+ phenotype exceeded CD45RA+ cells in grade 1 aggregates. Detailed analysis of mononuclear cell aggregates of different sizes in the rheumatoid synovium suggests that the composition of each aggregate depends on the total number of mononuclear cells it contains.

Morphologic and morphometric changes in synovial membrane associated with mechanically induced osteoarthrosis

We sought to determine whether synovial leukocytic inflammation is a primary event in mechanically induced osteoarthrosis. Repetitive impulse loading (50 ms duration at 60 Hz for 40 minutes each day) was applied to the right hindlimbs of 24 New Zealand white rabbits for 3, 6, or 9 weeks. The synovial membrane from the medial suprapatellar area was examined qualitatively using transmission electron microscopy and quantitatively using light microscopic morphometry. The results indicate that synovial inflammation is not a primary event in this mechanically induced osteoarthrosis, but synovial hyperplasia occurs prior to histologically evident cartilage destruction at 6-9 weeks.

Immunocytochemical analysis of human synovial lining cells: phenotypic relation to other marrow derived cells

The antigenic phenotype of human synovial lining cells in normal and hyperplastic synovium intima was determined with a panel of monoclonal antibodies directed against a large number of well defined myeloid (macrophage/granulocyte associated) antigens. Synovial lining cells express numerous macrophage associated antigens, including CD11b (CR3), CD13, CD14, CD16 (FcRIII), CD18, CD32 (FcRII), CD45 (leucocyte common antigen), CD54 (ICAM-1), CD64 (FcRI), CD68, and CD71 (transferrin receptor). Few synovial lining cells expressed CD11a (LFA-1) and CD11c (p150,95). Subintimal macrophages expressed all the macrophage associated antigens which were present on synovial lining cells and, in addition, expressed CD15a, CD25 (interleukin-2 receptor), CD34, and CD35 (C3b receptor), none of which was present on synovial lining cells. Synovial lining cell expression of a wide range of macrophage antigens argues in favour of their marrow origin and membership of the mononuclear phagocyte system.

Ultrastructure of transport pathways in stressed synovium of the knee in anaesthetized rabbits

1. The hydraulic conductance of the synovial lining of a rabbit knee increases greatly when intra-articular pressure (IAP) is raised above approximately 9 cmH2O (yield point). To investigate the cause, synovium was fixed in situ by perfusion at controlled IAP and prepared for transmission electron microscopy. Micrographs of synovium fixed below yield pressure (atmospheric pressure and 5 cmH2O IAP, ten joints) and above it (25 cmH2O IAP, five joints) were analysed by morphometry. 2. The discontinuous cellular lining consisted of fibroblast-like cells (67%) and macrophage-like cells (33%) separated by interstitium-filled gaps. Interstitium formed 26-36% of the surface below yield pressure. Depending on sample site the surface gaps averaged 1.9 +/- 0.2 to 2.4 +/- 0.2 microns wide below yield pressure (mean +/- S.E.M. throughout). Above yield pressure the mean gap width increased by 42-64% (P less than 0.05, analysis of variance). 3. The qualitative and quantitative composition of the lining varied with distance below the surface. In a plane 5 microns deep, the intercellular distances and interstitial area fraction were almost double those at the surface. Classic periodic collagen fibrils (diameter 50 +/- 3 nm) abounded at 5 microns depth whereas the surface interstitium was richer in Ruthenium Red-staining microfibrils (diameter 9.3 +/- 0.7 nm) associated with 93 nm period fibrous long-spacing bundles. 4. Averaging over all the tissue between the surface and the 5 microns deep plane, the mean interstitial volume fraction was 0.61 +/- 0.05 at 5 cmH2O and 0.67 +/- 0.02 at 25 cmH2O (n.s.). 5. Capillary fenestrae (8.5 +/- 1.1 per fenestrated profile) and intercellular junctions were unaltered at high IAP. The tortuosity of the capillary-to-joint cavity path was 1.50 +/- 0.01 below yield pressure and 1.86 +/- 0.24 at 25 cmH2O (n.s.). 6. Intra-articular tracers (ferrocyanide, ferritin and glycogen) permeated synovial interstitium without evidence of preferential pathways. Ferrocyanide delineated the capillary intercellular junction as a permeable channel. Ferritin and glycogen were phagocytosed by the macrophages. 7. In suprapatellar areolar synovium, the most extensive and most altered tissue, the ratio of interstitial area to path length increased maximally 4.1 times between 5 and 25 cmH2O IAP. This represents a substantial contribution to the physiologically estimated rise in interstitial conductance (14 x) but does not wholly explain it.

Synovial lining cells

The cells making up the lining of the synovium have long been known as type A and B synoviocytes, with an intermediate form sometimes also described. Accumulating evidence shows that the type A cells are macrophages and the type B cells are fibroblasts. Recently, a definite orientation of these cells within the synovial lining has been observed. The number of synovial lining cells increases in joint disease, and this now seems more likely to be due to cellular recruitment rather than local proliferation.

Leukocyte migration in synovial tissue. Leukocyte distribution, orientation, and migratory pattern after immune complex deposition in rabbit knee joints

Complement-activating bovine serum albumin (BSA)-anti-BSA immune complexes (ICs) were injected into rabbit knee joint cavities; the contralateral control joint was injected with BSA together with normal rabbit serum. The migration of leukocytes from the synovial venules into the joint cavity was analyzed with light microscopy (LM), scanning (SEM) and transmission (TEM) electron microscopy. EM autoradiography was used to study the endocytosis of ICs by leukocytes. The shape, orientation, and distribution of migrating polymorphonuclear granulocytes (PMNGs) were analyzed by LM morphometry. PMNGs accumulated in the joints injected with ICs. The peak of the number of PMNGs in the synovial tissue was reached after 4 hours, in the joint cavity after 6 hours. PMNGs in the synovial tissue were concentrated in the intimal layer. Migrating PMNGs were polarized, as judged by the ratio between the long (D max) and short (D min) axes of the cells. There was a close association between the migrating PMNGs and the collagen fibers. The morphometric data showed that the nonflattened, cylindrically-shaped PMNGs were oriented along the collagen bundles, running parallel to the synovial surface, and did not migrate in the straight direction of a theoretic leukotactic gradient originating in the joint cavity after IC deposition. SEM and TEM showed that the PMNGs were aligned along the collagen fibers and interacted activity with the collagen by pseudopods and cytoplasmic projections. EM autoradiography showed that the PMNGs in the joint cavity had ingested 125I-labeled ICs and were degranulated. In contrast, the PMNGs within the synovial membrane did not show any signs of IC endocytosis or any apparent degranulation. Synovial type A cells were found to contain ICs. This study indicates that the response of PMNGs in IC-induced synovitis consists of two distinct phases: an initial, mainly migratory phase in the synovial membrane where the PMNGs appear to use the collagen fibres as a climbing framework, and a second phase, in the joint cavity, characterized by PMNG metabolic activation, endocytosis of ICs, and degranulation. The apparent inability of PMNGs in the synovial membrane to ingest ICs and become degranulated might be due to not only concentration differences of ICs and leukotactic factors between the joint cavity and the synovial tissue but also might be related to the apparently active interaction with collagen.

The immunohistology of synovial lining cells in normal and inflamed synovium

The immunohistology of synovial lining cells (SLCs) in normal and inflamed hyperplastic synovium was investigated using monoclonal antibodies directed against leucocyte common antigen (LCA) HLA-DR and other macrophage components. We found that some SLCs in normal synovium express LCA, HLA-DR, and monocyte/macrophage-associated antigens. The number of SLCs expressing these antigens is increased in hyperplastic osteoarthritic (OA) and rheumatoid (RA) synovium. Some SLCs which did not react for LCA or other macrophage markers but were positive for HLA-DR were also noted in normal synovium and some segments of hyperplastic OA synovium. SLCs which are positive for LCA, HLA-DR, and macrophage markers contribute to the intimal hyperplasia in RA where they account for the majority of SLCs in the synovial intima. In OA synovium, the distribution of SLCs showing this pattern of reactivity was less uniform with numerous SLCs which were positive for HLA-DR but negative for LCA and other macrophage markers also present in the synovial intima. These findings indicate that there are some SLCs of bone marrow origin in normal and hyperplastic synovium. They also suggest that recruitment of SLCs of marrow origin is important in the production of intimal hyperplasia in both RA and OA and that there is also a significant local proliferation of non-marrow derived SLCs in OA.

Synovisol as an irrigant for electrosurgery of joints

The appropriate irrigation fluid for use during arthroscopic procedures should be selected on the basis of fluid-tissue and fluid-instrumentation compatibility. This study describes a new irrigation fluid, Synovisol, that is isomolar, nonhemolytic, nonantigenic, has a low viscosity, and is nonconductive. The rabbit knee was used as a model to demonstrate the immediate and long-term (3 weeks) effects of irrigation with Synovisol compared with water, normal saline, mannitol, sorbitol, glycine, and with nonirrigated controls. Results were assessed by light- and electron-microscopic evaluation of synovium and cartilage dissected from rabbit knees. While the greatest amount of damage was seen in water-treated samples, significant effects were noted with other irrigants compared with Synovisol and controls. Long-term animals showed recovery of tissues in all cases. Systemic effects evaluated by measuring plasma glycerol levels showed a transient increase that peaked at 20 min. No hemolysis was detected and kidney morphology was normal. The physiological, electrocompatible, simplicity of formula, low cost, and long-shelf-life properties of this solution makes it a fluid uniquely suited to arthroscopy.

An ultrastructural study of age-associated changes in the rabbit synovial membrane

The synovial membranes of 5 young rabbits about 6 weeks old and 5 older rabbits about 3-4 years old were studied with the light and electron microscopes, with a view to discovering any age-associated changes that might occur. Several age-associated changes were noted. These included: (1) a reduction in the overall population of synovial intimal cells; (2) a statistically significant reduction in type B cells and a relative increase in type A cells; (3) the emergence of atrophic cells poorly endowed with organelles; (4) decreased vascularity; and (5) fibrosis.

An ultrastructural study of the marginal transitional zone in the rabbit knee joint

The ultrastructure of the marginal transitional zone of femoral articular cartilage has been studied in the rabbit knee. There is an abrupt boundary between the convex margin of the cartilage and the synovial membrane. This is due to the arrangement and amount of collagen and of cells, because cell ultrastructure changes gradually from synovium to cartilage. The densely fibrous marginal synovium contains scattered fibrocytic cells with sparse cytoplasm and long filopodia. Near the synovium/cartilage interface, oval boundary cells containing more abundant cytoplasm abut on the cartilage matrix. In the periphery of the cartilage, an edge-belt of collagen fibrils runs obliquely from articular surface to subchondral bone. Chondrocytes near the edge-belt, whatever their depth from the articular surface, ultrastructurally resemble middle zone (zone II) cells of articular cartilage generally. The synovial surface of the marginal zone is smooth and resembles articular cartilage surfaces. Most intimal cells contain plentiful granular endoplasmic reticulum and Golgi membranes and hence are intermediate between A and B synoviocytes commonly found elsewhere. Non-fenestrated (type I) capillaries lie in a superficial stratum beneath the synovial surface and in a deep stratum near the synovium/cartilage boundary, and are surrounded by pericytes. No mast cells, macrophages, lymph vessels or nerves could be identified in the marginal zone. Contrary to earlier accounts of collagen orientation in this zone, most of the fibrils in the marginal synovium appear to run around the perimeter of the cartilage and only a few bundles run radially from the synovium towards the cartilage. It is suggested that the circumferential collagen both contains the marginal cartilage and prevents displacement of synovial tissue on to the articular surface. The radial strata of collagen serve to anchor the circumferential collagen to the cartilage edge-belt. In agreement with earlier investigators, it is considered that the edge-belt withstands tensile stresses arising from deformation of the articular surface. The role of the marginal synovium is also discussed in relation to synovial fluid formation and cartilage nutrition.

[Hemarthrosis and the cruciate ligaments - morphological studies. 2]

Some morphological investigations were conducted with cruciate ligaments of rabbits which had been exposed to post-traumatic hemarthrosis for different periods of time and submitted to an additional lesion of the synovial sheath. Already one week later, histologic examination under the microscope showed significant modifications of the fibrous structure of these ligaments. These modifications persisted for several months and consisted in bloatings of the individual fibres, disaggregations of the fibrous structure, partly even cystic necrosis. The electron microscope allowed to give a more precise definition of the modifications and to demonstrate morphologically the structure alteration of collagenic fibres caused by hemarthrosis. The visible alterations of the fibrous structure of such cruciate ligaments will partly persist still for a long time after the restitution of their full mechanical loading capacity.

Ultrastructure of human tendon sheath and synovium: implications for tumor histogenesis

Normal human tendon sheath and synovium were studied by scanning an transmission electron microscopy. The lining cells of the two tissues appear to be identical ultrastructurally. The most superficial cells (B-cells) possess long cytoplasmic extensions that clothe the membrane surface. Intermingled with deeper B-cells are the so-called A-cells, which have similar cytoplasmic features but lack long processes and instead have many filopodia. the frequent occurrence of intermediate forms indicates that the two cells form part of a morphologic spectrum. Comparison with cells of tumors that have been ascribed to synovium or tendon sheath (synovial sarcoma, epithelioid sarcoma, clear cell sarcoma) do not reveal any close similarities that might support a histogenetic relationship.

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.

The origin of phagocytic cells in the joint and bone

Studies dealing with the concept of the mononuclear phagocyte system are described, and the origin of synovial type A cells and osteoclasts is discussed in some detail. All of the available evidence indicates that both derive from precursor cells in the bone marrow. It seems justifiable to conclude that monocytes transform into type A synovial cells and fuse to form osteoclasts.

Contributions of the lymphatic and microvascular systems to fluid absorption from the synovial cavity of the rabbit knee

1. The trans-synovial flow (Qs) of Ringer solution from the cavity of immobile knee (stifle) joints was determined in anaesthetized rabbits when intra-articular hydrostatic pressure (PJ) was elevated in steps from 2 to 25 cm H2O. 2. It has been demonstrated previously (Levick, 1978) that slope DQs/dPJ shows an abrupt sixfold increase at a 'breaking point' (PB) around 9 . 5 cm H2O, rising from a mean of 0 . 49 microliter.min-1 cm H2O-1 (PJ less than PB) to 2 . 81 microliter.min-1 cm H2O-1 (PJ greater than PB). 3. Perforation of the synovial intima by an intra-articular cannula increased dQs/dPJ below breaking pressure and thus largely abolished the breaking point phenomenon, indicating that the phenomenon might be simulated by a break-down in synovial resistance to flow. 4. Ligation of the femoral lymph trunks draining the joint did not significantly alter the relationship between Qs and PJ. The slope dQs/dPJ was 0 . 60 +/- 0 . 17 microliter.min-1 cm H2O-1 (mean +/- S.E.) below a breaking pressure of 8 . 8--10.5 cm H2O, and 2 . 90 +/- 0 . 64 microliter.min-1 cm H2O-1 above breaking pressure. Thus changes in synovial lymph flow did not explain the breaking point phenomenon. 5. Interruption of synovial blood flow by vascular clamps or by killing the animal reduced, but did not abolish fluid absorption; nor was the breaking point phenomenon abolished. Slope dQs/dPJ increased from 0 . 37 +/- 0 . 06 microliter.min-1 cm H2O-1 below breaking point (10 . 5 +/- 1 . 0 cm H2O) to between 1 . 82 and 0 . 96 +/- 0 . 15 microliter.min-1 cm H2O-1 above breaking pressure. Fluid accumulated in extra-synovial interstitial spaces. 6. When the synovial intima was divested of its surrounding tissues, lymphatic and vascular supplies by extensive dissection, the denuded synovium still showed a marked increase in hydraulic conductivity at normal breaking pressures. The breaking point phenomenon was therefore not caused by changes in extra-synovial interstitial pressure or compliance. 7. It is concluded that fluid absorption from the joint cavity occurs by two parallel pathways viz. the synovial capillary bed and the extra-synovial interstitial spaces. A simple analysis of the system indicates that the breaking point phenomenon cannot be explained by an abrupt increase in synovial conductivity (cf. Edlund, 1949) but is explicable if synovial conductivity (and possibly interstitial conductivity) becomes a continuous function of PJ above breaking pressure. This hypothesis reconciles the non-linear pressure-flow relationship with Starling's hypothesis for fluid absorption from connective tissue spaces (1896).

Overview article: the articular territory of the reticuloendothelial system

It has long been recognized that synovial intimal cells are phagocytic and that they are capable of picking up colloidal or particulate material injected into the circulation. Hence they have been described as the "articular territory of the reticuloendothelial system." Ultrastructural studies have added a wealth new knowledge and details about the remarkable endocytotic powers of synovial cells. It has been shown that they can endocytose not only small particulate substances like colloidal iron, colloidal gold, and thorotrast but also relatively large objects like masses of fibrin and entire erythrocytes. Controversy has arisen as to whether it is the Type A or Type B cell that is the main scavenger of the joint. Evidence will be presented to show that this is a somewhat fictitious controversy and that these are not distinct and different races of cells with different functions but merely cells whose differences in morphology reflects the function they are performing at a given moment.

Fate of antigen after intravenous and intraarticular injection into rabbits

To determine the manner in which small protein antigens enter the synovium and are localized, horseradish peroxidase (molecular weight 40,000) was injected by intravenous and intraarticular routes. Passage of antigen from capillary lumen to joint space and vice versa occurred primarily across fenestrated vessels. Antigen localized to collagen fibers and lining cells. At low concentrations of antigen, antigen was taken up predominantly by marcophages, whereas at higher concentrations both macrophages and fibroblast-like lining cells accumulated antigen.

Long-term effects of myochrysine on the synovial membrane and aurosomes

Mysochrysine injected into the rabbit knee joint produced regressive and destructive changes in the synovial membrane, ultimately leading to fibrosis. Aurosomes, containing characteristic electron-dense deposits indicating the presence of gold, formed in the synovial intimal cells and subsynovial macrophages. The number of aurosomes decreased with the passage of time but some were found even 2 yr after the injection of Myochrysine. Electron-probe X-ray analysis showed that the aurosomes contain gold, sulphur and phosphorus. A comparison was made between the atomic ratios of these elements in 3-day and 18-mth-old aurosomes but no significant difference was detected.

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.

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

In the synovial membrane of the mouse, morphological features associated with active secretion are unusually well developed in cells immediately subjacent to the lining layer (in the position of B cells), comparable to those of cells known to elaborate polypeptides.

Experimental restoration of the digital synovial sheath

The digital synovial sheath constitutes an important component of the delicate mechanism of flexor tendon nutrition and gliding function, In the present study the true nature of the inner cell layers of secondary healed defects in the tendon sheath as well as of free tendon sheath autografts were studied. Leghorn chickens were used as experimental animals and the gradual development of the pseudosheath as well as the healing of sheath autografts were studied both macroscopically and histologically including transmission electron miscroscopy. Synovial regeneration by extension from intact parts of the sheath was never observed and the pseudosheath formed around silastic rods consisted of granulation tissue with fibroblasts and macrophages. The free tendon sheath autografts demonstrated a normal process of healing at the edges of the defect. Synovial regeneration appeared to be that of metaplasia and proliferation of fibroblasts and macrophages. This phenomenon was demonstrable both in the secondary healed defects and more convincingly in the sheath autografts. Further, the silastic rod was found to induce foreign body reaction in the healing synovium. It is concluded that grafting of autologous tendon sheath tissue seems to be a promising method for restoration of defects in the digital tendon sheath.

Fine structural analysis of rabbit synovial cells. II. Fine structure and rosette-forming cells of explant and monolayer cultures

The fine structure and plasma membrane receptors of explant and monolayer cells of normal rabbit synovium were studied. In explants about 10% of the cells were round and formed rosettes with IgC and C3 markers, whereas the remaining cells were stellate, resembled young fibroblasts, and had no receptors for IgG and complement. Monolayer cells looked like fibroblasts, produced fine extracellular fibrils and hyaluronate, and formed no rosettes. Thus early cultures contain both macrophages and fibroblasts but only the latter persist in monolayer.

Cellular contacts in the synovial membrane of the cat and the rabbit: an ultrastructural study

Regularly, four different types of cellular contacts are found in synovial folds and villi of the cobital joint of the cat (interdigitations, desmosomes, intermediate junctions, gap junctions). The same types of contact--with the exception of intermediate junctions--occur sporadically also in synovial fat folds of the knee joint of the rabbit. In both species, hemidesmosomes and discontinuous basement membranes are seen in the synovial lining layer. Cellular contacts predominate between A-cells and cells of the intermediate type, hemidesmosomes and incomplete basement membranes predominate in intermediate cells and B-cells. The latter are rare in A-cells. The importance of such contacts for mechanical, metabolic and electrical interactions of cellular elements in the synovial membrane is discussed. No unanimous concept as to their function can be advanced at present.

Fine structure of the synovial membrane in Mycoplasma hyorhinis arthritis of swine

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Ultrastructure of synovial membrane in osteo-arthritis

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Ultrastructure of synovial membrane in rheumatoid arthritis

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Ultrastructure of normal rat synovial membrane

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Pathology of experimental haemarthrosis

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