Table 1.

Approaches to determining the cardiac potential of progenitor cells.

	Advantages	Limitations
Fate mapping using chemical label (e.g. BrdU, fluorescent dye, iron particles) to track progenitor cells	Convenient in many co-culture or transplantation studies Often only alternative when genetic labels are unavailable Iron particles potentially compatible with human studies	Susceptible to misinterpretation due to cell fusion Requires phenotyping by immunocytochemistry or flow cytometry, which are susceptible to artifacts (non-specific antibody binding, autofluorescence, and errors of co-localization) Susceptible to false positives with transfer of the label to co-cultured or host cells
Fate mapping using in situ hybridization for species or gender-specific DNA sequence (e.g. Y-chromosome) to track progenitor cells	Convenient in many co-culture or transplantation studies Provides a stable, non-transferable lineage marker Compatible with some human transplantation studies (i.e. gender-mismatched transplants)	Technically challenging Susceptible to misinterpretation due to cell fusion Requires phenotyping by immunocytochemistry or flow cytometry, which are susceptible to artifacts
Fate mapping using genetic reporter (e.g. GFP) to track progenitor cells	Less readily transferred than chemical labels Provides additional phenotypic or temporal specificity if transgene includes a genetic reporter driven by a conditional promoter (e.g. the cardiac-specific αMHC→GFP transgene)	Susceptible to misinterpretation due to cell fusion Requires phenotyping by immunocytochemistry or flow cytometry, which are susceptible to artifacts Not compatible with all progenitor cell types (e.g. difficult to transduce cell types) or hosts (e.g. human patients) Genetic reporter can potentially interfere with viability or biological function of labeled cells
Fate mapping using independent genetic reporters to track progenitor cells and neighboring host/co-cultured cells	Helps distinguish differentiation/transdifferentiation events from cell fusion (this readout is especially straightforward with the Cre-lox recombination system, e.g. Cre- expressing graft cells into a ‘floxed’ reporter host) Provides additional phenotypic or temporal specificity if transgene involves a genetic reporter driven by a conditional promoter	Requires phenotyping by immunocytochemistry or flow cytometry, which are susceptible to artifacts Not compatible with all progenitor cell types (e.g. difficult to transduce cell types) or hosts (e.g. human patients) Genetic reporter can potentially interfere with biological function of labeled cells
Species-specific RT-PCR for cardiac transcripts	Simple and comparatively high throughput	Limited to xenotransplantation or co-culture experiments involving different species Provides limited mechanistic insights Requires careful primer design and rigorous control studies
Direct assessment of in vivo graft function (e.g. intravital calcium imaging studies of GFP- tagged grafts)	Determines electromechanical integration and functional phenotype of progenitor-derived graft, rather than mere histological outcome(s)	Technically challenging and requires instrumentation that is not widely available Generally requires genetically-tagged cells and so is not compatible with all progenitor cell types or hosts Generally not feasible in large-animal or human studies
Assessment of cardiac function (e.g. echocardiography)	Examines the parameters that determine patient outcomes Available in human studies	Provides limited mechanistic insights, as improved cardiac function ≠ de novo formation of new myocardium