Simulations suggest that the formation of runaway electron (RE) beams that follow ITER disruptions is strongly dependent on the characteristics of a pre-disruption RE population [1]. The effect of a RE seed on RE beam formation was investigated in the TCV experiment and a threshold for RE beam formation on the RE critical field (proportional to the pre-disruption electron density) identified [2]. The pre-disruption RE population dynamics is studied in a series of dedicated experiments in TCV. RE generation, acceleration, and transport mechanisms are investigated in the plasma start-up and flat-top phases by varying the plasma density and/or the ohmic drive and monitoring the MHD activity. Strong hysteresis is observed in the RE population upon the plasma density, highlighting the importance of controlling plasma conditions in the early stages of the discharge. A clear density threshold is identified for RE seed generation during the start-up phase. [1] Hesslow et al. Physical Review Letters (2017) [2] Decker et al. Nucl. Fusion (2022)