Morphological and immunocytochemical characterization of cultured fibroblast-like cells derived from adult human synovial membrane

The synovial membrane (SM) is a source of multipotent mesenchymal stem cells (MSCs), which appeared microscopically to be a relatively homogeneous population of fibroblast-like cells (FCs) in culture (De Bari et al., 2001). The aim of this study was to investigate phenotypic characteristics of the SM-derived FCs (SD-FCs) that could elucidate their origin inside the synovial tissue. Morphological characterization of SD-FCs was assessed by electron microscopy and by expression of surfactant protein A (SPA). This study, yielded substantial evidence that SD-FCs show ultrastructural and immunocytochemical features of type B synoviocytes; they contained characteristic lamellar bodies (LBs) that are secreted by exocytosis. LB secretion ability was maintained upon passaging (P3-P10). Immunocytochemistry showed that SD-FCs express surfactant protein A (SP-A). Taken together, these results indicate that multipotent SD-MSCs may originate from the synovial lining, having a phenotype highly similar to that of type B synoviocytes. We believe our data highlight the potent ability of type B synoviocytes to have a multilineage differentiation potential.