We consider a heterogeneous cellular network (HetNet) where a macrocell tier with a large antenna array base station (BS) is overlaid with a dense tier of small cells (SCs). We inves- tigate the potential benefits of incorporating a mas- sive MIMO BS in a TDD-based HetNet and we pro- vide analytical expressions for the coverage proba- bility and the area spectral efficiency using stochas- tic geometry. The duplexing mode in which SCs should operate during uplink macrocell transmis- sions is optimized. Furthermore, we consider a re- verse TDD scheme, in which the massive MIMO BS can estimate the SC interference covariance ma- trix. Our results suggest that significant throughput improvement can be achieved by exploiting inter- ference nulling and implicit coordination across the tiers due to flexible and asymmetric TDD operation.