Testosterone-induced neuronal recruitment to the adult songbird vocal control center, HVC, requires the androgenic induction of VEGF, followed by VEGF-triggered MMP2 expression and activation, which further augments VEGF concentration by releasing matrix-bound VEGF and initiates angiogenesis. The expanded microvascular bed acts as a source of BDNF, which supports the immigration of new neurons derived from neuronal progenitors in the overlying ventricular zone. This figure is derived and modified from (Goldman and Chen, 2011). AR, androgen receptor; ER, estrogen receptor; Ncad, N-cadherin; Hu, Hu protein; IGF-1, insulin growth factor 1.