The synovial lining layer in the temporomandibular joint (TMJ) consists of macrophage-like type A cells and fibroblast-like type B cells. Their identification, however, has been difficult because of the lack of a specific cell marker. This review summarizes the characteristic configuration and development of the type B cell in the murine TMJ. Immunocytochemistry for 25kDa-heat shock protein (Hsp25) revealed two profiles of the fibroblast-like type B cells with cytoplasmic processes in the adult rat TMJ: one projected horizontally slender processes which covered the synovial membrane, and the other extended a thick, long process towards the articular cavity. The former appeared earlier than the latter during the development of synovial membrane, in close relation with the formation of the articular cavity and the commencement of active jaw movement. Since immature type B cells with Hsp25-immunoreactivity were found in the mesenchymal tissue which corresponded to the future articular cavity, type B cells may differentiate directly from mesenchymal cells. © 2004, Japanese Association for Oral Biology. All rights reserved.