Determination of spheroid growth kinetics
During the spheroid culture process, the growth of MTSs was assessed and recorded using electron microscopy (Life Technology, Carlsbad, CA, USA) every 24 h, and the diameter and roundness of MTSs were calculated by Image Pro Plus 6.0. To determine the active cell number in the spheroid, 3 cell spheroids were taken every 24 h, the supernatant discarded, and then digested with Accutase™ cell digestive solution (40506ES60, Yesen, Shanghai, CN) at 37 ℃ for 30 min to form single cell suspension. The suspension was stained by trypan blue and then counted using Countstar.