Impact of aging on tendon homeostasis, tendinopathy development, and impaired healing.

Aging is a complex and progressive process where the tissues of the body demonstrate a decreased ability to maintain homeostasis. During aging, there are substantial cellular and molecular changes, with a subsequent increase in susceptibility to pathological degeneration of normal tissue function. In tendon, aging results in well characterized alterations in extracellular matrix (ECM) structure and composition. In addition, the cellular environment of aged tendons is altered, including a marked decrease in cell density and metabolic activity, as well as an increase in cellular senescence. Collectively, these degenerative changes make aging a key risk factor for the development of tendinopathies and can increase the frequency of tendon injuries. However, inconsistencies in the extent of age-related degenerative impairments in tendons have been reported, likely due to differences in how "old" and "young" age-groups have been defined, differences between anatomically distinct tendons, and differences between animal models that have been utilized to study the impact of aging on tendon homeostasis. In this review, we address these issues by summarizing data by well-defined age categories (young adults, middle-aged, and aged) and from anatomically distinct tendon types. We then summarize in detail how aging affects tendon mechanics, structure, composition, and the cellular environment based on current data and underscore what is currently not known. Finally, we discuss gaps in the current understanding of tendon aging and propose key avenues for future research that can shed light on the specific mechanisms of tendon pathogenesis due to aging.