Sources of cells for cardiac repair, and routes of their administration. (A) Cells in current human trials include skeletal muscle myoblasts, unfractionated bone marrow, and circulating (endothelial) progenitor cells. Cells in preclinical studies include bone marrow MSCs, multipotent cells from other sources, and novel progenitor or stem cells discovered in the adult myocardium; see text for details. (B) Existing trials use intracoronary delivery routes (over-the-wire balloon catheters), intramuscular delivery via catheters (e.g., the NOGA system for electromechanical mapping), or direct injection during cardiac surgery. Not represented here are the theoretical potential for systemic delivery, suggested by the homing of some cell types to infarcted myocardium (39), and strategies to mobilize endogenous cells from other tissue sites to the heart.