Long-term effect of HB-EGF on MSC proliferation. Dose-dependent enhancement of MSC proliferation induced by HB-EGF persists for several weeks, starting from both 5 × 10³ (A) and 10⁴ (B) cells. No HB indicates MSC proliferation without HB-EGF; HB 5, HB 25, and HB 50: MSC proliferation with 5, 25, and 50 ng/mL HB-EGF, respectively. The figure shows a representative case from those analyzed in Figure 2.