Population doubling time (PDT) in days of adherent cells cultured in different expansion media between passage 1 (P1) and passage 2 (P2). The media are M1 (10% FBS + FGF2), M2 (10% FBS + 5% HPL), M3 (10% HPL), and M4 (5% HPL). Data shown represent mean ± standard error of the mean from the indicated number of experiments. *Significant difference; P < 0.0007. #No significant difference; P > 0.05.