This aim of this study was to determine the characteristic differences in tendinocytes derived from three sites of the equine superficial digital flexor tendon (SDFT)-proximally the myotendinous junction (MTJ), mid-metacarpal (mM) and osteotendinous junction (OTJ)-in morphology, proliferation, and ability for synthesis of collagen and matrix metalloproteinases (MMPs). Little difference was observed in cell proliferation. Addition of tumor necrosis factor (TNF) α to the culture medium resulted in increased collagen synthesis by tendinocytes from all three sites. The amount of collagen synthesized by tendinocytes derived from the mM and OTJ was much larger than that synthesized by untreated tendinocytes. A collagen zymogram revealed that proMMP-13 synthesis was increased towards the distal site. However, TNFα treatment resulted in a significant decrease in the amount of proMMP-13 synthesized by tendinocytes from all three sites. On the other hand, a gelatin zymogram showed that the synthesis level of proMMP-9 tended to decrease towards the distal site, but there was little difference between synthesis levels of proMMP-9 before and after TNFα treatment. These results indicated that tendinocytes in the same tendon have different characteristics and that these characterisities would reflect the function of tendinocytes in vivo. Also, the isolated tendinocytes provided much information on the characteristics and properties of tendons for the ECM turnover system and on the responsiveness of tendinocytes to complex inflammatory responses in a tendinopathy condition.Tendon injury, especially in the superficial digital flexor tendon (SDFT), has proved to be a major problem for racehorses. Recent studies have shown that about 10-30% of racehorses suffer from tendonitis (8, 30). Interestingly, tendonitis occurs more frequently in the forelimb, especially in the SDFT rather than the deep digital flexor tendon (DDFT), and rarely occurs in the common digital extensor tendon (CDET) (1, 30). The vulnerability of the SDFT is related to its functions in weight supporting and in accumulation and transmission of elastic force for effective movement. Damage to the SDFT usually occurs during high-speed work or racing competition in which the forelimbs are pulled up to the highest position to bear maximum load or straining power (14,35). Moreover, tendonitis in the SDFT occurs in the mid-metacarpal (mM) site in most cases (1). Therefore, structural properties of the tendons might reflect the occurrence ratio of tendonitis (10). The etiology of tendonitis has been discussed in many reports; however, there is little information regarding the mechanism of tendon degradation. Extracellular matrix (ECM) turnover in a tendon might occur repeatedly and routinely as in other connective tissues (13), and matrix metalloprotein-

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