Scheme of the agent-based stem cell model and its coupling in the hybrid model. Each cell is characterized by its affiliation to one GE, niche affinity a < amax and cell cycle position c. The two GEs represent functionally different environments: in GE Α affinity a increases with time and is limited by amax. In GE Ω it decreases and cells proliferate. When a drops below the threshold a = amin, the cells lose the ability to switch to Α and leave the stem cell compartment. In the hybrid model they enter the progenitor compartment CG. The effect of chemotherapy CX was modeled as cell loss in GE Ω without affecting GE A.