Objective: Fibroblast-like type B synoviocytes in the temporomandibular joint (TMJ) play important roles in metabolism of synovial fluid to make jaw movement smooth possible. Our previous studies have suggested heat shock protein 25 (Hsp25) is a specific cell marker for type B synoviocytes in the rat TMJ. The present study examined the development of type B synoviocytes in the TMJ by immunocytochemistry for Hsp25 both at the light and electron microscopic levels. Methods: Male Wistar rats at embryonic day 17 (E17), E19, E21, postnatal day 1 (P1), P3, P7 and P15 were used. Serial frozen sections including in TMJ were processed for the ABC method using an antibody to Hsp25. Some immunostained sections were embedded in epoxy resin, and examined under a transmission electron microscope. Results: A few Hsp25-positive cells appeared at the upper portion of the developing condyle at E19. At E21 when the formation of upper articular cavity had commenced, the developing articular cavity was lined with flat cells positive for Hsp25. Immunopositive cells with flat or round profiles arranged on the surface of the enlarged upper articular cavity at P1. At P3, the formation of synovial folds had begun in the posterior portion. In addition to the immunopositive cells located on the cavity surface, some immunopositive cells were found in the deeper layer; they extended long cytoplasmic processes towards to the articular cavity. At P7, the Hsp25-positive cells with well-developed cell organelle increased in their number with the enlargement of synovial fold developed rich cell organelle. At P15, immunopositive type B synoviocytes showed a sheet-like arrangement on the synovial surface. Conclusion: These findings indicate the development of type B synoviocytes is closely related to the formation of the articular cavity and synovial fold in TMJ. This works was supported by MEXT Grant (No. 14771001).

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