Hematopoiesis: Reconciling Historic Controversies about the Niche.

The niche, as first conceptualized, was in conflict with the prevailing wisdom that stem cells have internal logic. The niche hypothesis has been indisputably confirmed. Yet, recent findings indicate little plasticity of epigenetically scripted hematopoietic stem/progenitors. Reconciling this conflict requires re-envisioning the niche as an enabler, not designer, of cell fate.