Fig. 1. The cellular hierarchy of cardiac progenitors and their lineage specification. (A) Multipotent Isl1+ cardiovascular progenitor cells (MICPs) express Isl1, Nkx2.5 and Flk1, and can give rise to all three lineages - cardiac muscle, smooth muscle and endothelium - during embryonic heart development. (B) Loss of Flk1 expression yields Isl1+/Nkx2.5+ cells that are similar to postnatal Isl1+ progenitors and that can generate cardiac or smooth-muscle lineages. (C) Isl1+/Flk1+ cells are more restricted in their ability to differentiate; they can give rise to either endothelial or smooth-muscle cells. Adapted from Moretti et al. (Moretti et al., 2006), with permission.