Abstract
Directed differentiation of embryonic stem cells indicates that mesodermal lineages in the mammalian heart (cardiac, endothelial, and smooth muscle cells) develop from a common, multipotent cardiovascular precursor. To isolate and characterize the lineage potential of a resident pool of cardiovascular progenitor cells (CPcs), we developed BAC transgenic mice in which enhanced green fluorescent protein (EGFP) is placed under control of the c-kit locus (c-kit(BAC)-EGFP mice). Discrete c-kit-EGFP(+) cells were observed at different stages of differentiation in embryonic hearts, increasing in number to a maximum at about postnatal day (PN) 2; thereafter, EGFP(+) cells declined and were rarely observed in the adult heart. EGFP(+) cells purified from PN 0-5 hearts were nestin(+) and expanded in culture; 67% of cells were fluorescent after 9 days. Purified cells differentiated into endothelial, cardiac, and smooth muscle cells, and differentiation could be directed by specific growth factors. CPc-derived cardiac myocytes displayed rhythmic beating and action potentials characteristic of multiple cardiac cell types, similar to ES cell-derived cardiomyocytes. Single-cell dilution studies confirmed the potential of individual CPcs to form all 3 cardiovascular lineages. In adult hearts, cryoablation resulted in c-kit-EGFP(+) expression, peaking 7 days postcryolesion. Expression occurred in endothelial and smooth muscle cells in the revascularizing infarct, and in terminally differentiated cardiomyocytes in the border zone surrounding the infarct. Thus, c-kit expression marks CPc in the neonatal heart that are capable of directed differentiation in vitro; however, c-kit expression in cardiomyocytes in the adult heart after injury does not identify cardiac myogenesis.