Both pluripotent stem cell types were cultured, maintained, and differentiated as previously described [14 (link)]. Human iPSCs and ESCs were cultured in E8 media (Life Technologies) with daily media replenishment. Cells were replated using 0.5 mM EDTA at 1:10 or 1:12 ratios every four days, at which time they reached ~85% confluence. During differentiation, basal medium RPMI 1640 was supplemented with B27 without insulin (Life Technologies). The medium was changed every other day (48 hr). For day 0 to day 2, the basal medium was supplemented with 6 μM CHIR99021 (LC Laboratories). On day 2, the supplement was changed to 2 μM Wnt-C59 (Selleck Chemicals). On day 4 onward, the basal medium was changed every other day without any supplement. Contracting cells were usually observed after 7 days.