However our findings confirmed that despite the high levels
However, our findings confirmed that despite the high levels of cross-engraftment of hematopoietic cells, the analysis of the heart tissue from parabiotic animals, regardless of physical training, revealed very little evidence of extracardiac oxymetazoline hydrochloride that are targeted to the heart. The same result was observed in another study that followed parabiotic mice for a period of 6–7months, aiming to investigate the post-grafting rate, which was shown to be negligible (Wagers et al., 2002). Thus, similarly to Wagers et al. (2002), we believe that trans-differentiation of HSCs into nonhematopoietic cells is not a common outcome of the HSC developmental program, and the steady-state tissue regeneration appears to predominantly result from tissue-resident cells rather than from circulating cells (Wagers et al., 2002). Taken together, these data and the insignificant participation of cells that are attracted to the heart, which migrated through the blood circulation, clearly show that the increase in the amount of c-Kit+Lin− cells that followed physiological cardiac hypertrophy in the trained animals was, in fact, due to endogenous CSCs. Furthermore our results cannot demonstrate whether the resident stem cells presented in higher numbers in the trained hearts had an increased rate of cell division or conversely, whether the cell death was decreased due to a protected microenvironment generated by the growth factors released during the cardiac remodeling and growth. The investigation of mechanisms that are possibly responsible for the CSC proliferation and/or cell death protection, as well as the establishment of the cardiac profile that was modulated by the factors released during physical activity, which affects endogenous CSCs, will yield results that are relevant to this study, and remain under investigation. Colony forming efficiency is generally considered to be an estimate of the number of stem/progenitor cells within a tissue, specifically for cardiac muscle-derived stromal stem/progenitor cells (Javazon et al., 2004). The colony forming efficiency results from cardiac tissue lead us to believe that the number of CMSC population remains unchanged in the heart after swimming exercise training. The colony forming efficiency from extracardiac sources was also investigated. Spleen-CFU-F was studied because this organ represents the largest extramedullary hematopoiesis site in pathological conditions (Morita et al., 2011). In turn, to represent the major reservoir of adult stem cells, bone-marrow-CFU-F was also investigated. Circulating CFU-F population was examined using peripheral blood. The absence of blood-CFU-F confirmed previous findings that in general, stromal-like cells appear to be absent in the circulation under physiological conditions (da Silva Meirelles et al., 2006). In contrast, studies of pathological processes indicate that colonies formed by stromal-like cells in peripheral blood can be detected (Chen et al., 2010; Rochefort et al., 2006). It remains uncertain whether stromal-like cells can be mobilized in a way similar to hematopoietic stem cells under stressful conditions and recruited to the injured sites. However, it was previously demonstrated that there was no interchange between bone marrow total cells and cardiac-CFU-Fs even after injury induced by myocardial infarction (Chong et al., 2011). Probably the diverse origins for CFU-Fs define distinct differentiation fates that could also influence their capacity for participating in tissue repair (Chong et al., 2011). Since no difference between the Sedentary and Trained groups was found regarding the CFU-F efficiency assay, it seems that physical activity does not modulate the number of CFU-Fs in any of the different sources tested. An important limitation of our study regarding CMSCs was the lack of immunophenotyping of these cells in order to better characterize them in both sedentary and trained conditions. Taking into account recent papers by Chong at al. (2011) and Pelekanos et al. (2012), we can presume that these cells are not similar to c-Kit+Lin− CSCs, since they are completely negative for c-kit expression, are adherent on uncoated plastic surfaces and are able to grow in unriched medium like DMEM (Chong at al., 2011; Pelekanos et al., 2012). On the other hand, regarding sca-1 marker, there is the possibility that CMSCs can share some common properties with Sca-1+Lin− CSCs, since both cells are positive for sca-1 marker and negative for hematopietic and endothelial lineage markers, can grow in unriched medium and can adhere to plastic surfaces (Oh et al., 2003; Chong at al., 2011; Pelekanos et al., 2012), arguing that these cells can be the same cell population. Curiously, our results during exercise training in which both cells are not modified by training protocol favor this assertion. However, more experiments using for example lineage tracing protocols should be designed and performed in a near future to resolve this issue.