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Objective: Masticatory muscle tendon‐aponeurosis hyperplasia, which is associated with limited mouth opening, progresses very slowly from adolescence. The prevalence rates of this disease are higher among women than among men, suggesting oestrogen involvement. As parafunctional habits are frequently observed, mechanical stress is likely involved in the pathogenesis and advancement of this disease. To elucidate the pathological condition, we examined the effect of oestrogen on tenocyte function and the relationship between mechanical stress and crystallin beta A4 (Cryba4), using murine TT‐D6 tenocytes. Materials and Methods: Cell proliferation assays, RT‐PCR, real‐time RT‐PCR, Western blot analysis and mechanical loading experiments were performed. Results: The physiological dose of oestrogen increased the levels of scleraxis and tenomodulin in TT‐D6 tenocytes. In contrast, forced expression of Cryba4 inhibited scleraxis expression in these cells. Surprisingly, oestrogen significantly promoted cell differentiation in the Cryba4‐overexpressing TT‐D6 tenocytes. Moreover, tensile force induced Cryba4 expression in these tendon cells. Conclusion: Oestrogen and Cryba4 may be associated with the progression of masticatory muscle tendon‐aponeurosis hyperplasia. [ABSTRACT FROM AUTHOR]