105increased differentiation of Sca1-derived cells to the different cell types present in
the heart. The study presents an evidence of continuous replacement of myocardial
cells by Sca1+ CSCs. Using a model of myocardial injury with patent coronary cir-
culation to test the spontaneous regenerative capacity of resident CSCs, in-situ
labeling and genetic tracking the fate of c-kit+ cardiac stem cells and the replace-
ment of the CSCs by transplantation of genetically tagged CSCs, Ellison et al. [ 9 ]
provided an evidence that the CSCs autonomously repair extensive cardiac diffuse
damage, leading to complete cellular, anatomical and functional cardiac recovery.
The study showed that if the eCSCs are ablated, myocardial regeneration and ven-
tricular performance is debilitated causing heart failure unless they are replaced by
exogenous CSCs.
Another group of researchers however believe that CSCs have a non-significant
role in cardiomyocyte renewal even during injury. Senyo et al. [ 24 ] combined two
different pulse-chase approaches—genetic fate-mapping with stable isotope label-
ing and Multi-isotope Imaging Mass Spectrometry. They showed that genesis of
cardiomyocytes occurs at a low rate by division of pre-existing cardiomyocytes
during normal aging, a process that increases by four-fold adjacent to areas of myo-
cardial injury. The study concluded that cardiac progenitors do not play a significant
role in myocardial homeostasis in mammals and suggests that their role after injury
is also limited. Ali et al. [ 25 ] used several transgenic mouse models that enable
clonal analysis of postnatal cardiomyogenesis. They provided a new line of evi-
dence for the differentiated α-myosin heavy chain-expressing cardiomyocyte as the
cell of origin of postnatal cardiomyogenesis using the “mosaic analysis with double
markers” mouse model. The observations of the study also argue against the exis-
tence of robust putative stem cells. The capacity to divide postnatally appears to be
restricted to a small fraction of cardiomyocytes, and this property diminishes over
time and appears insensitive to stimulation by an infarction injury within a 4-week
time period after the infarct [ 25 ]. In another recent study, Berlo et al. [ 26 ] generated
mice in which the Kit locus was used for lineage tracing analysis to examine if and
how frequently c-kit+ cells generate cardiomyocytes in vivo. The study revealed that
c-kit+ cells have the ability to contribute to the cardiomyocyte compartment of the
heart and loss of the Kit gene, which is known to compromise the progenitor and
migration activity of c-kit+ cells, completely prevent cardiomyocyte formation from
c-kit+ cells. However, throughout development, with aging or with cardiac injury,
the percentage of cardiomyocytes emerging from the c-kit+ lineage is very low and
hence highly unlikely to significantly affect cardiac function [ 26 ].
Given the disparity of results regarding the role of CSCs in cardiomyocyte
renewal, it may be argued that the endogenous or transplanted CSCs, even if not
involved in direct cardiovascular differentiation, may contribute towards myocar-
dial repair by secretion of paracrine factors. Moreover, it should be noted that most
of the lineage tracing and radioactive thymidine labeling studies suffer from one or
the other technical limitations such as inappropriate labeling of the cells, measure-
ment of DNA synthesis and not actual cell division, toxicity attributed to thymidine
labeling etc. Most importantly, lineage-tracing protocols cannot be performed in
humans and hence results from animal studies should be cautiously extrapolated to
human CSCs.
7 The Emerging Role of Cardiac Stem Cells in Cardiac Regeneration