Primary cell culture
Primary cells were obtained by biopsy of the ear skin of calves. The ear skin was chopped into small pieces with a sterile scalpel and then washed several times and incubated at 38.5 °C for 4–18 h in Hank’s Balanced Salt Solution (Gibco, Cat. no. 14175095) supplemented with collagenase (Collagenase type I, Gibco, Cat. no. 17–100–017). The following day the dispersed cells were washed several times in DMEM (Gibco, Cat. no. 21068028) and cultured in DMEM supplemented with 10% fetal calf serum (Gibco, Cat. no. GIB-11150–059), 1% penicillin/streptomycin (Gibco, Cat. no. 15140148), 1% non-essential amino acids (Gibco, Cat. no. 11140050), and 100 mM β-mercaptoethanol (Sigma–Aldrich, Cat. no. M3418).