Deletion of p57Kip2 increases the proliferation of neural crest cell–derived SA progenitors in the E11 mouse AGM region. The expanded SA progenitors in turn produce more SA cells, which promote the repopulation capacity of hematopoietic stem cells by the catecholamine–β2-adrenergic receptor signaling axis. The neural crest cells also give rise to mesenchymal cells enriched in the ventral part of the aorta in the AGM region. The role of these mesenchymal cells in embryonic hematopoiesis awaits investigation. Professional illustration by Patrick Lane, ScEYEnce Studios.