The palisading cells of rheumatoid nodules share certain features with synovial intimal cells. The similarities between the two cell populations have been reassessed using new cytochemical markers.
Cell populations in cryostat sections of non-inflamed, rheumatoid and osteoarthritic synovial tissues, and rheumatoid nodules were assessed for the presence of CD68, prolyl hydroxylase, vascular cell adhesion molecule 1 (VCAM-1), and the alpha 4 and beta 1 integrin chains, and the activity of uridine diphosphoglucose dehydrogenase (UDPGD) and nonspecific esterase.
Synovial intimal cells formed a dual population of macrophages (nonspecific esterase positive, strongly positive for CD68) and fibroblastic cells (prolyl hydroxylase positive). The latter showed prominent VCAM-1 expression and high UDPGD activity as previously reported and also prominent beta 1 integrin chain expression. Palisading cells similarly proved to be a dual population of macrophages and fibroblastic cells. In contrast with synovial intima, however, the fibroblastic cells lacked UDPGD activity and expression of VCAM-1 and showed no preferential expression of the beta 1 integrin chain. The exception to this rule was where nodules contained central clefts, which were lined with cells showing all the features associated with synovial intimal cells.
Palisading cells are a mixture of macrophages and fibroblasts, but the latter show no evidence of synoviocyte differentiation. Cells with features of synoviocytes may occur lining clefts within areas of necrobiosis.