# The Telocytes in the Subepicardial Niche
## 4 Discussion
### C-Kit-Positive Cardiac Cells  
The c-kit protein (CD117) is a type III tyrosine-protein kinase that recognises the stem cell factor (SCF) ligand [34]. During development, the c-kit receptor is expressed both in cardiac progenitors capable of cardiomyocytesogenesis and in others with divergent evolution patterns [35]. This receptor-ligand interaction activates signaling pathways that are responsible for cellular proliferation, development and differentiation. We found c-kit expressed equally in epicardial mesothelial and subepicardial stromal cells, which could indicate the presence of a stem/progenitor niche and transdifferentiation processes.  
The cardiac stem cells expressing c-kit have been studied extensively (see [35]). Approximately 1.1-1.8% of the myocardial cell population in humans are c-kit-positive [36; 37]. Approximately half of these cells are multipotent cells, and the other half are early committed cells [38]. Interestingly, most of the detected c-kit-positive cells were found to also express two other proteins: CD45 and CD34, the pan-leukocyte and endothelial/hematopoietic progenitor markers, respectively [39; 40]. With rare exceptions, freshly isolated c-kit-positive cells do not express myocytes (\(\alpha\)-sarcomeric actin, cardiac myosin, desmin, \(\alpha\)-cardiac actin and connexin 43), endothelial cells (von Willebrand factor, CD31 and vimentin), smooth muscle cells (\(\alpha\)-smooth muscle actin and desmin) or fibroblast cytoplasmic proteins (fibronectin, procollagen I and vimentin) [41]. In vitro, isolated, c-kit-positive cardiac stem cells are spindle-shaped, like cardiac fibroblasts, and co-express GATA4 and NKX2.5 [36; 42].  
The pattern of markers displayed by c-kit-positive cells raised doubts with regards to their origin. [PERSON] et al. found that endothelial progenitor cells (EPCs) express both c-kit and CD34,  
Figure 5: Human adult cardiac wall. Immediate submesothelial expression of CD34 includes nascent endothelial tubes (arrows).  
while c-kit-positive stromal cells are negative for the endothelial/hematopoietic progenitor marker CD34 [41]. They are also different from bone-marrow-derived cells, which express CD45 [41]. Interestingly, several studies demonstrated that most c-kit-positive stromal cardiac cells are mast cells that co-express CD45 [43; 44], while c-kit-positive/CD45 stromal cells express mRNA of the endothelial lineage [45]. Accordingly, either c-kit-expressing TCs will not express CD34 or CD34-positive TCs are not c-kit-expressing cells. The expressions of c-kit and CD34 in cardiac interstitial are mutually exclusive and not concomitant. If they are found to be concomitant, the immunohistochemical method and its results should be checked or repeated to avoid producing inaccurate data (Figure 6).  
Figure 6: Reprinted with permission from John Wiley and Sons (License Number: 4562340930830) from [27]. The original legend in the *.pdf version of the article indicates _“Figure 3(A) Immunostaining for CD34 (brown). CD34 positivity might be expressed by telocytes and endothelial cells. (B) Localization of double positive cells for c-Kit (red) and CD34 (brown). (C) Human epicardium, deep layer. Vimentin positivity is stronger than c-Kit positivity by sandwich method. (D) Cells expressing T protein (arrows). (A–D) Original magnification, 40X”_. One could observe from the figure and the legend that the thin arrow added by the authors in (**A**) was not assigned to endothelial tubes, nor was it regarded as a telocyte, but it rather belongs to an endothelial tube (we added an additional white arrow). The cells indicated as “telocytes” in (**B**,**C**) do not display the peculiar telopodes. The expression of Tau-protein demonstrated in (**D**) is rather pan-tisular due to a consistent background staining. The double-labelling result in (**B**) indicating a c-kit+/CD34+ phenotype of telocytes (TCs) is highly speculative due to an invalid immunohistochemical method that determined the pan-labelling of the slide.