[Secretory aspects and differentiation of B cells of the synovial membrane in mice].

The intimal layer of the synovial membrane (SM) includes essentially 2 cell types: A-cells with macrophagic functions and B-cells generally assimilated to fibroblasts. A comparative study of the B-cells in some mammals revealed that these cells possess specific polypeptidergic secretory features which are particularly clear in the mouse. The B-cells, which are more numerous than A-cells and often entirely line the synovial cavity, probably play an essential role in the metabolism of the SM. A study of the development of the SM in the mouse shows that the primitive cleft is formed before any differenciation of the synovial mesenchyme through degradation of the fine mesenchymal layer in direct contact with the chondrogenic layers. The differenciation of the SM coincides with the clarification and dilatation of the synovial cavity. As soon as the SM organizes into intimal and sub-intimal layers (6 days of life), the 2 intimal cell types can be identified by their macrophagic (A-cells) and secretory (B-cells) features. The B-cells, however, only show an appreciable content of typical secretory vesciles at 13 days. Our ultrastructural observations suggest that the B-cells are the source of some specific proteins of the synovial fluid and that they participate in the maintenance of the peculiar structure of the intimal interstitial tissue.