Treadmill running exercise results in the presence of numerous myofibroblasts in mouse patellar tendons

Mechanical loading is known to alter tendon structure, but its cellular mechanisms are unclear. This study aimed to determine the effect of mechanical loading on tendon cells in vivo. C57BL/6J female mice were used in a treadmill running study. The treadmill running protocol consisted of treadmill training for 1 week, followed by sustained moderate running at 13 m/min for 50 min/day, 5 days/week, for 3 weeks. Immunohistochemical staining of tendon sections of mice after treadmill running revealed that numerous cells in the tendon section expressed α-SMA, whereas in the tendon sections of control mice, only a few cells exhibited weak α-SMA signals. Furthermore, mouse patellar tendon cells (MPTCs) derived from treadmill running mice were generally larger in culture, proliferated faster, expressed a higher level of α-SMA, and formed more abundant stress fibers compared to MPTCs from control mice. In addition, MPTCs from treadmill running mice generated larger traction forces (169 ± 66.1 Pa) than those from control mice (102 ± 34.2 Pa). Finally, cells from treadmill running mice produced higher levels of total collagen (516.4 ± 92.7 µg/10,000 cells) than their counterparts (303.9 ± 34.8 µg/10,000 cells). Thus, mechanical loading via treadmill running increased the presence of myofibroblasts in mouse patellar tendons. As myofibroblasts are activated fibroblasts, their presence in the tendon following treadmill running indicates that they actively repair and remodel tendon tissue under strenuous mechanical loading, leading to known changes in tendon structure. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1373–1378, 2009