# The Telocytes in the Subepicardial Niche
## 4 Discussion
### The Epicardial and Subepicardial Expression of Podoplanin  
Although in the central nervous system (CNS) and thymus, endothelial cells and pericytes result from ectoderm-derived neural crest cells, they are mesoderm- and mesothelium-derived in coelomic organs [51]. This supports the (re)activated mechanism of mesothelial-to-endothelial/pericitary transformation in epicardial cells, which is actually an epithelial-mesenchymal transformation (EMT), ensuring that either subepicardial vasculogenesis or subepicardial lymphwasculogenesis occurs. This reasonably correlates with our evidence of podoplanin-expressing mesothelial cells and submesothelial lymphatics and isolated cells. We also found PDGFR-\(\alpha\)-expressing epicardial mesothelial cells; this is normal, as signaling through both PDGFRs, \(\alpha\) and \(\beta\), is needed for epicardial EMT and the formation of EPDCs [52]. In these regards, testing the PDGFR expression in mesothelial cells would support a potential of EMT in the samples studied. PDGFR-\(\alpha\) is crucial in the differentiation of cardiac fibroblasts from EPDCs [53], so a PDGFR-\(\alpha\)-positive subepicardial stroma would clearly indicate a subepicardium with reparatory potential, regardless of whether the respective stromal cells do or do not have telopodes. A strong expression of PDGFR-\(\alpha\) in human adult cardiac tissue was demonstrated in the interstitial cells of the subepicardium, myocardium, endocardium, coronary smooth muscle and endothelialcells, as we also found, and cardiomyocytes express this marker [13; 54]. PDGFR-\(\alpha\) was not tested in previous studies on epicardial and subepicardial TCs, although [PERSON] and [PERSON] indicated that 'CD34/PDGFR-[\(\alpha\) double immunohistochemistry can orientate the diagnosis' [55]. This double expression was used for TC identification in other normal and pathological non-cardiac tissues [56; 57; 58; 59; 60]. Cardiac TCs were indicated to have a CD34-positive/PDGFR-\(\alpha\)-positive phenotype [61], but this resulted in cell cultures that did not use any sorting molecules; cells were sorted solely based on morphological criteria. Moreover, the rodent hearts analysed 'were minced into 1 mm\({}^{3}\) pieces and incubated on an orbital shaker' [61], which could not permit the discrimination of epicardium-specific cells from other cell populations in those cardiac samples. Podoplanin (or GP36, E11 antigen, oncofetal antigens M2A and T1A-2, aggrus) is a mucin-type transmembrane glycoprotein first identified as podoplanin in podocytes [62; 63; 64] but as the E11 antigen in osteoblasts and osteocytes [65; 66]. Podoplanin is homologous to the oncofetal antigen M2A, which is recognized by the D2-40 antibody [67]. Lymphatic endothelial cells express high levels of podoplanin [68]. Podoplanin has homologues in humans, mice, rats, dogs and hamsters and is relatively well-conserved between species [68]. Podoplanin is a cell morphological regulator [67] and is required for normal heart development, specifically for EMT in epicardium-derived cells [68], and our evidence reasonably suggests that this role of podoplanin also exists in adult tissues. During development, podoplanin is expressed in the epithelial lining of the coelomic wall of the pericardio-peritoneal canal and in the cells lining the pleural and pericardial cavities [66]. Podoplanin is expressed in epithelial and mesothelial cells, such as those in the intestinal epithelium, alveolar type I cells, podocytes, the mesothelium of the visceral peritoneum [66] and, as we demonstrated, in epicardial mesothelial cells. Several markers could be used to identify lymphatic endothelial cells on slides, such as CD31 (the pan-endothelial marker), podoplanin (D2-40), Vascular Endothelial Growth Factor Receptor-3 (VEGFR-3), Lymphatic Vascular Endothelial Hyaluronan Receptor-1 (LYVE-1) and Prospero related homeobox1 (PROX1) [69]. Therefore, further co-staining experiments could be helpful in better assessing the lymphatic anatomy of subepicardium.