Birthdating studies reshape models for pituitary gland cell specification

► The majority of the anterior lobe cell types in the mouse pituitary gland begin differentiation between e11.5 and e13.5. ► The majority of intermediate lobe melanotropes begin differentiation after the anterior lobe cell types between e12.5 and e15.5. ► Regional enrichment of specialized, anterior lobe cell types at birth is not correlated with the timing of cell cycle exit, and potentially involves formation of homotypic networks. ► Intrinsic signaling factors within Rathke's pouch are likely to drive anterior lobe cell specification instead of proposed extrinsic gradients of BMP and FGF signaling.