The nature and origins of synovium: experimental approaches to the study of synoviocyte differentiation.

Synovium is characterised by an intimal layer of cells, now recognised to be a mixture of bone marrow-derived macrophages and specialised fibroblast-like cells. The fibroblast-like cells, or synoviocytes, differ from other fibroblasts in a number of respects, including high activity of uridine diphosphoglucose dehydrogenase (UDPGD) and constitutive expression of VCAM-1. Experiments have been devised to try to establish the factors that control these specialised features. Both high UDPGD activity and VCAM-1 expression can be seen in adventitious or regenerate connective tissue linings tissue under certain circumstances. Mechanical factors may be implicated in the induction of UDPGD activity and VCAM-1 expression, but there is evidence that they are controlled independently. The factors involved in synoviocyte differentiation both in the embryo and under conditions of regeneration or generation ab initio at adventitious sites in the adult require further investigation.