Unique localization of protein gene product 9.5 in type B synoviocytes in the joints of the horse

Fibroblast-like (Type B) synoviocytes are cells in the synovial membrane th at are responsible for production of both synovial fluid and the extracellu lar matrix in the synovial intima. Immunostaining of the horse synovial mem brane for protein gene product (PGP) 9.5, which is a neuron-specific ubiqui tin C-terminal hydrolase, demonstrated selective localization of the immuno reactivity in a synoviocyte population different from acid phosphatase-posi tive Type A synoviocytes. The immunoreactive cells were lined up in the syn ovial intima and extended dendritic processes towards the joint cavity to f orm a dense plexus on the surface. Electron microscopic examination clearly identified the PGP 9.5-immunoreactive cells as Type B synoviocytes charact erized by developed rough endoplasmic reticulum and free ribosomes. Immunor eactivity for PGP 9.5 was diffusely distributed throughout the cytoplasm, i ncluding the tips of fine processes. Western and Northern blot analyses cou ld not distinguish the corresponding protein and mRNA obtained from the bra in and synovial membrane. The existence of the neuron-specific PCP 9.5 in T ype B synoviocytes suggests a common mechanism regulating the protein metab olism between neurons and synoviocytes, and also provides a new cytochemica l marker for identification of the cells.