EGFR and osteoarthritis: For example, in vitro, articular cartilage superficial zone cells have been shown to proliferate and express progenitor or superficial zone markers in response to TGF-β1 [50] and to transient activation of canonical Wnt signaling [18]; and in vivo, transient activation of β-catenin signaling, which like the EGFR has typically been associated with osteoarthritis [51] also causes articular cartilage thickening in postnatal mice [52].