Failure of differentiation into mature myotubes by muscle precursor cells with the side-population phenotype after injection into irreversibly damaged striated urethral sphincter

We previously showed that the injection of a heterogeneous population of muscle precursor cells (MPCs) into striated urethral sphincter irreversibly damaged by electrocoagulation results in the formation of functional myotubes. To gain further insights into the role played by the different types of MPCs, we elected to sort the MPCs by Hoechst 33342 staining/fluorescence-activated cell-sorter analysis before injection. We found that the side population (SP) cells (muscle stem cells) injected in isolation survived, whereas the main population (MP) cells did not. However, the SP cells failed to differentiate into mature myotubes, as observed previously with unfractionated MPCs containing both SP and MP cells. This result suggests that interactions between SP and MP cells are required for the formation of myotubes in a nonregenerating muscle. In the setting of sphincter insufficiency, injection of MPCs at different stages of maturation may be a better option than purified muscle stem cells.