Ontogenetic development of synovial A cells in fetal and neonatal rat knee joints

Macrophages in renal development, injury, and repair

Macrophages have long been regarded as classic mediators of innate immunity because of their production of proinflammatory cytokines and their ability to induce apoptotic cell death. As a result of such activities and the detrimental long-term effect of kidney inflammation, macrophages principally have been regarded as mediators of glomerular damage, tubular cell death, and the downstream fibrotic events leading to chronic kidney disease. Although this has been the accepted consequence of macrophage infiltration in kidney disease, macrophages also play a critical role in normal organ development, cell turnover, and recovery from injury in many organs, including the kidney. There is also a growing awareness that there is considerable heterogeneity of phenotype and function within the macrophage population and that a greater understanding of these different states of activation may result in the development of therapies specifically designed to capitalize on this variation in phenotype and cellular responses. In this review, we discuss the current understanding of induction and consequences of classic versus alternative macrophage activation and highlight what additional therapeutic options this may provide for the management of both acute and chronic kidney disease as well as renal cancer.

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.

Three-dimensional ultrastructure of synoviocytes in the horse joint as revealed by the scanning electron microscope

The synovial membrane displays a superficial cellular lining composed of two types of synoviocytes: "absorptive" macrophages (type A cells) and "secretory" fibroblast-like cells (type B cells). The types are intermingled and extend a variety of processes, rendering the cellular architecture of the synovial membrane difficult to visualize. Previous electron microscopic and histochemical studies failed to demonstrate the entire shape of synoviocytes, except our immunohistochemical study for protein gene product 9.5 in the horse joint. The present SEM study is the first to demonstrate the three-dimensional ultrastructure of synoviocytes as well as their distribution in the synovial membrane, using macerated samples from the horse carpal joints. The equine synovial membrane was largely covered by conspicuously developed synovial villi. Type A synoviocytes were closely similar to macrophages in regard to surface structure, and showed uneven distribution with the densest occurrence around the tips of the synovial villi. In the basal half of villi, type B synoviocytes, which were situated in close proximity to the synovial cavity, projected thick processes horizontally and intertwined to form a regular network of processes on the synovial surface. Those in the upper half of the villi were located in the abluminal layers and protruded an antenna-like process into the joint cavity with tips covered with long microvilli, in addition to forming the superficial plexus of processes. Type B cells were also provided with fine, membranous extensions that tended to cover the surface of synovial intima. The meshwork of horizontal processes, the antenna-like processes, and the membranous processes imply advantages in not only secretion but also sensation and regulation of the barrier function in the synovial membrane.

Unique localization of protein gene product 9.5 in type B synoviocytes in the joints of the horse

Fibroblast-like (Type B) synoviocytes are cells in the synovial membrane that are responsible for production of both synovial fluid and the extracellular matrix in the synovial intima. Immunostaining of the horse synovial membrane for protein gene product (PGP) 9.5, which is a neuron-specific ubiquitin C-terminal hydrolase, demonstrated selective localization of the immunoreactivity in a synoviocyte population different from acid phosphatase-positive Type A synoviocytes. The immunoreactive cells were lined up in the synovial intima and extended dendritic processes towards the joint cavity to form a dense plexus on the surface. Electron microscopic examination clearly identified the PGP 9.5-immunoreactive cells as Type B synoviocytes characterized by developed rough endoplasmic reticulum and free ribosomes. Immunoreactivity for PGP 9.5 was diffusely distributed throughout the cytoplasm, including the tips of fine processes. Western and Northern blot analyses could not distinguish the corresponding protein and mRNA obtained from the brain and synovial membrane. The existence of the neuron-specific PGP 9.5 in Type B synoviocytes suggests a common mechanism regulating the protein metabolism between neurons and synoviocytes, and also provides a new cytochemical marker for identification of the cells.

Development, differentiation, and maturation of Kupffer cells

Primitive macrophages first develop in the murine and human yolk sac and then differentiate into fetal macrophages. Primitive or fetal macrophages enter the blood stream and migrate into the fetal liver. Fetal macrophages possess a high proliferative capacity and express antigens and peroxidase activity of resident macrophages with the progress of gestation; they become mature and then transform into Kupffer cells. In contrast, myelopoiesis and monocytopoiesis are not active in yolk sac hematopoiesis and in the early stages of hepatic hematopoiesis. Precursor cells of primitive or fetal macrophages exist and granulocyte/macrophage colony-forming cells develop in the yolk sac and in the early stages of fetal liver development, whereas macrophage colony-forming cells emerge and increase later in fetal liver development. In vitro, similar colonies were formed from each fetal hematopoietic cell in the presence of different macrophage growth factors. During culturing of the yolk sac cells and hepatic hematopoietic cells on a monolayer of mouse stromal cell line, ST2, primitive or fetal macrophage colonies developed before the formation of monocyte colonies, suggesting the existence of a direct pathway of differentiation from primitive macrophages into fetal macrophages during ontogeny. In severely monocytopenic mice induced by the administration of strontium-89, Kupffer cells have a proliferative capacity and are maintained by self-renewal. In macrophage colony-stimulating factor (M-CSF)-deficient (op/op) mice, the number of Kupffer cells is reduced, and they are characterized by immature morphology and a proliferative potential similar to that of primitive or fetal macrophages during ontogeny. Immediately after the administration of M-CSF to op/op mice, Kupffer cells start proliferating and become mature. This finding indicates that M-CSF plays an important role in the differentiation and proliferation of Kupffer cells.

Development and heterogeneity of macrophages and their related cells through their differentiation pathways

Macrophages are a heterogeneous population differing in their site of location, morphology and function. They develop from hematopoietic stem cells originating in both fetal and bone marrow hematopoiesis. In yolk sac and early hepatic hematopoiesis, primitive/fetal macrophages develop from hematopoietic stem cells, bypassing the stage of monocytic cells (monoblasts, promonocytes and monocytes), possess proliferative capacity and differentiate into resident macrophages in tissues in late ontogeny. Monocytic cells develop in hepatic hematopoiesis after the development of primitive/fetal macrophages, then move into the bone marrow in late ontogeny, forming a monocyte-derived macrophage population in tissues. Like monocytes, the monocyte-derived macrophages have no proliferative potential and are short-lived, whereas the resident macrophages are long-lived in tissue, possess proliferative capacity and can be sustained by self-renewal. In adult life, the bone marrow releases macrophage precursors (immature myeloid cells) and monocytes into peripheral blood, but normally not monoblasts or promonocyts. The myeloid precursor cells migrate into tissues and differentiate into resident macrophages or related cells in situ due to macrophage differentiation or growth factors, such as M-CSF and GM-CSF, produced in situ and/or supplied humorally. Monocytes, however, migrate into tissues in response to inflammatory stimuli and differentiate into exudate macrophages. The distinct differentiation pathways of monocyte/macrophages, resident macrophages, other macrophage subpopulations, and macrophage-related cells are reviewed together with the heterogeneity of macrophage precursor cells.

Development, differentiation, and proliferation of macrophages in the rat yolk sac

Immunohistochemical and immunoelectron microscopic investigation using anti-rat macrophage monoclonal antibody (mAb) RM-1 demonstrated the first emergence of immature macrophages within blood islands of fetal rat yolk sacs at fetal day 9. At fetal day 10, they matured into fetal macrophages, showed intense immunoreactivity to RM-1, developed lysosomal granules, endocytic vesicles or vacuoles, and extended fine cytoplasmic processes. By the rosetting assay with IgG-coated sheep erythrocyte antibody (IgG-EA), both the immature and mature fetal macrophages showed rosette formation and phagocytosis of IgG-EA, but they were negative for peroxidase (PO) reaction. At fetal day 11, fetal macrophages were observed in the mesenchymal layer of the yolk sacs. In the yolk sacs, no promonocytes or monocytes were observed, although there was a very minor population (less than 1%) of immature myeloid cells containing a few small PO-positive cytoplasmic granules from fetal day 11 on. After combination of the vitelline vessels to the embryonic cardiovascular system, fetal macrophages appeared in embryonic rat tissues. By [3H]thymidine autoradiography, yolk sac macrophages were demonstrated to possess a marked proliferative potential. These results suggest that fetal macrophages in the yolk sac differentiate from haematopoietic stem cells without passing through the promonocyte or monocyte stage, proliferate actively, and colonize the embryonic rat tissues.