Figure 1. Age-related gene expression in mouse Achilles tendons.

(a) Shown are assigned gene ontology groups for a total of 168 differentially expressed genes identified by SSH in healthy-aged C57BL/6 mouse Achilles tendons. (b) Genes coding for extracellular matrix proteins (e.g. Col1a1, Col3a1, Sparc, Lysyl oxidase, Decorin, Biglycan, Fibromodulin, and Thrombospondin 1) are significantly repressed in healthy-aged Achilles tendons. A moderate repression was also observed for the tendon progenitor cell marker Scleraxis. Bars represent mean ± SEM (for 5 individual animals); a fold change of ≥2.0 was assumed as a significant change. (c) Age-related repression of these transcripts is largely maintained in isolated tendon progenitor/stem cells of healthy-aged mouse Achilles tendons (passage = 3) in vitro. Bars represent mean ± SEM (n = 3). (d) Western Blot analysis of Sparc expression in young and healthy aged Achilles tendons (top) and TDSPCs isolated thereof (bottom). (e,f) Representative immunohistochemical images of young and healthy-aged mouse Achilles tendons stained for Sparc demonstrating a significant reduction in Sparc-postive cells in healthy-aged tendons (n ≥2500 cells per group); Scale bars: 100 μm; **P < 0.01, t-test (Mann-Whitney test). (g) Sparc co-distributes with tenomodulin-positive cells within the tendon proper of young Achilles tendons. Scale bar: 50 μm.