Mesenchymal stem cells isolated from the Wharton’s jelly of human umbilical cords (WJ-MSC) are of increasing interest for cell therapies, but scalable cell production in stirred tank bioreactors (STR) still requires further investigations in order to be more efficient and with decreased costs. To handle the problem of cell confluence on microcarriers leading to cell aggregation, a new strategy of microcarriers addition was proposed. The ’bead-to-bead transfer’ ability of WJ-MSC was indeed used to maintain constant the number of cells per microcarriers. However, the resulting increase of bead shocks frequency could also negatively impact cell quantity and quality. Until now, no quantitative study describing the impact of bead interactions on WJ-MSC death was reported. In this study the influence of microcarriers addition as well as mixing characteristics on cell viability were determined. Obtained results showed that, when particle mixing is below the just-suspended state condition (Njs), local increase of particle volume fraction promotes a significant cell death in an agitation mode of orbital stirring. However, an increase in agitation rate at Njs is clearly beneficial to cell viability and growth. These effects were magnified during microcarrier addition due to the increase of mean volume fraction of particles. The present study also demonstrates the critical influence of Njs and particle distributions within the bioreactor on WJ-MSC culture performances.