Recruitment and fate of circulated stem cells into heart valves

Valve interstitial cells (fibroblasts, myofibroblasts, smooth muscle cells) are considered to be derived from the epicardium and cells of endocardial orgin that undergo an EMT during embryogenesis. Our studies of hematopoietic stem cell (HSC) potential suggest that adult valve interstitial cells also have an HSC origin. To demonstrate this, we combined limited clonal expansion of single EGFP+, Lin‐, c‐kit+, Sca‐1+, CD34‐ bone marrow cells with transplantation of clonal populations into congenic non‐EGFP mice. This strategy permits the potential of a single HSC to be evaluated in vivo. Analyses of cardiac valves from mice with high levels of multilineage hematopoietic reconstitution revealed numerous EGFP+ cells within all valve leaflets. To evaluate fibroblastic phenotype in EGFP+ cells, in situ hybridization studies were conducted to determine if the cells were engaged in ECM production. Our finding that a subpopulation of the EGFP+ cells expressed mRNA for procollagen 1α1 suggests that engrafted EGFP+ cells represent a cell type not traditionally associated with the hematopoietic lineage. To exclude fusion as a mechanism for HSC contribution to valve cell populations, Y‐chromosome FISH analysis was conducted using female‐to‐male transplanted mice. That nuclei of EGFP+ cells do not contain a Y‐chromosome clearly established that these cells were the result of HSC differentiation. These findings and our previous works demonstrating HSC contribution to other cell populations with fibroblastic/myofibroblastic properties (kidney mesangial cells, brain microglial cells) suggest that HSCs contribute to the adult valve fibroblast/myofibroblast population.