A model for the opposite effects of VEGFR-1 and VEGFR-2 on lymphoid cell development. (A) VEGF suppresses the progression from common lymphoid progenitors (CLP) to pro-B cells and from pro-B to pre-B cells via VEGFR-2; whereas VEGF promotes the progression from pro-B to pre-B cells, and the exit from bone marrow to spleen via VEGFR-1. (B) VEGF suppresses the progression from triple-negative to double-positive, especially from intermediate single-positive to double-positive thymocytes via VEGFR-2, and VEGF promotes the mobilization of related cells via VEGFR-1 (such as the exit of the mature single positive T cells from thymus to spleen).