Subsets of telocytes: Myocardial telocytes

Telocytes (TCs) are morphologically defined as small-sized cells with long, thin, moniliform processes called telopodes (Tps). Numerous papers imply that TCs are a distinctive cell type, and that transmission electron microscopy (TEM) is the gold standard tool for their identification. We aimed to reproduce previous studies on myocardial TCs to check their validity. For this purpose we performed an immunohistochemical study on human cardiac samples from six autopsied donor cadavers, using antibodies against CD10, CD31, CD34, CD146, Ki67, alpha-smooth muscle actin (α-SMA), Platelet-Derived Growth Factor Receptor-alpha (PDGFRα) and laminin. Additionally we performed a TEM study on cardiac samples from three human autopsied donor cadavers and five adult Sprague-Dawley rats. We found endothelial cells (ECs), cords, and filopodia-projecting endothelial tip cells (ETCs) that expressed CD10, CD31, CD34, CD146, and PDGFR-α. Often, endothelial cells closely neighbored the sarcolemmal basal laminae. Endothelial progenitor cells, as well as nascent capillaries, were CD31+/CD34+. Proliferative endothelial cells expressed Ki67. In larger vessels we found pericytes that expressed CD146 and α-SMA; scarce α-SMA-expressing spindle-shaped cells lining cardiomyocytes were suggestive of a pericytic role in angiogenic sprout guidance. The TEM study showed that endothelial tubes are almost exclusively found in the narrow myocardial interstitia. ECs that built them up appeared identical to the cells that previous TEM studies have suggested to be myocardial telocytes. A subset of stromal cells with TC-like phenotype and telopodes-like processes actually seem to configure blood vessels, and therefore belong to the endothelial lineage. This study shows that data presented in previous studies on myocardial telocytes is not enough to allow the reproducibility of the results. At least a subset of cells considered to be TCs might belong to the endothelial lineage.