It is time for agent-based modelling (ABM) to trespass the "me too" stage, so as to deliver up to its promises. To this end, agent-based techniques should be tested against scenarios that have proven to be too complex for traditional approaches. In this paper, we take a complex problem in the biological domain — lineage commitment of Hematopoietic Stem Cells (HSC) —, and introduce an articulated agent-based model accounting for both intra- and inter-cellular aspects. An implementation of the model in MASON is presented, and the first simulation results are discussed.