Subsets of telocytes: the progenitor cells in the human endocardial niche

Telocytes (TCs) are cells defined by their long and moniliform processes termed telopodes. They were previously identified in the endocardium and express neural markers, such as nestin and neuron-specific enolase (NSE). Previous studies found a positive expression of neuro-filaments in endocardial endothelial cells, and a positive expression of nestin in cardiac side population (CSP) progenitor cells, which allowed us to hypothesize that TCs in the endocardial stem niche could be in fact progenitors that express nestin.We used cardiac samples from 10 human adult donor cadavers. Endocardial endothelial cells expressed CD146, alpha-smooth muscle actin (α-SMA), smooth muscle myosin (SMM), nestin and, scarcely, neurofilaments. Within atrial and ventricular samples, we found an endocardial discontinuous smooth muscle layer expressing, similar to pericytes and vascular smooth muscle cells, α-SMA, nestin, SMM, and CD146. We assessed a similar phenotype in the subendocardial muscle layer, which also expressed neuron-specific enolase. The expression of nestin and CD146 strongly indicates a progenitor phenotype, which, in turn, supports the hypothesis that, in humans, an endocardial stem niche supplied by an endothelial-mesenchymal transition should be considered, although it mimics a primitive supply from the cardiac neural crest with dormant cardiac side population progenitor cells. Nevertheless, the endocardial niche could indeed harbor precursor cells that are morphologically similar to TCs.