evans , using a mouse mutant for the lim homeodomain transcription factor ( islet1 , isl1 ) , demonstrated that an isl1 positive ( isl1 + ) population is localised in the secondary heart field ( shf ) , contributing to the outflow tract , right ventricle , atria and ( to a small extent ) the left ventricle .
isl1 is considered a marker of shf cells that contribute to the venous and arterial poles of the cardiac tube , while the other cells from the shf contribute only to the arterial pole .
the shf or anterior heat field ( ahf ) was identified as a splanchnic mesoderm underlying the caudal pharynx , which provides myocardium to the outflow tract during tube looping . in 2005 ,
the same research group also identified isl1+/c - kit negative ( c - kit-)/stem cell antigen-1 negative ( sca-1- ) progenitors in the postnatal heart , demonstrating that these cells persist after birth and that these organ - specific progenitor cells may be isolated , purified , expanded and differentiated into mature cardiac myocytes for future cardiac applications .
our research group identified isl1 + cells during gestation until the postnatal age , and demonstrated the contemporary expression of isl1 and c - kit in the interstitial cells in the ventricular myocardium in human foetal and postnatal hearts .
this paper demonstrated , by confocal analysis , that all isl1 + cells were also c - kit+ , while c - kit+ cells were not necessarily isl1 + , as if isl1 + cells were a subpopulation of c - kit+ cells .
it has been supposed that there may be a primordial cardiovascular precursor in the shf that expresses the three markers isl1 , nkx2.5 and fetal liver kinase 1 ( flk1 ) and can generate three lineages : cardiac muscle , smooth muscle and endothelium .
the shf cooperates with the first heart field ( fhf ) and the cardiac neural crest cells during the development of the vertebrate heart .
the fhf lineage forms the early cardiac tube and generates the left ventricle , while the shf contributes additional cells to the maturing heart and , along with the cardiac neural crest cells , generates the outflow tract vessels and valves .
the presence of cardiovascular progenitor cells positive for isl1 and/or nkx2.5 in the shf has been well confirmed by a recent work published in science .
the authors suggested that there are two distinct populations , one for the shf and the other for the fhf ; the first population is clearly positive for isl1 and expresses mirna 199a / b , while the second has no distinct markers and expresses mirna 200a / b .
progenitor cells of the shf depend on the cardiac specific transcription factors isl1 and nkx2.5 , which play a key role at an early stage of cardiac progenitor formation .
their expression is regulated by wnt / beta - catenin signalling , which in turn also regulates bmp4 signalling .
this last signalling pathway activates gata4 and srf . despite the key role of isl1 ,
this transcription factor is not considered an indubitable marker of this subpopulation of shf progenitor cells .
it has been demonstrated very recently that isl1 labels both shf precursors and some cardiac neural crest cells and that mef2c is a more efficient marker of the shf progenitor cells in labelling experiments .
this paper indirectly suggests both a review of past literature in view of these new data and using a different marker in studies of the shf progenitor cells .
moreover , considering heart development in different species , such as chicken , mouse and xenopus , it appears that isl1 is expressed in a common cardiac cell population that splits into fhf and shf lineages during heart development and that this common progenitor also contributes cells to the cardiovascular system .
anversa s research group described rat cardiac stem cells for the first time , many research groups have claimed to have discovered new and important cardiac progenitor / stem cells in the adult heart .. since 2005 , the scientific community talked about adult c - kit+ and/or sca-1 + and/or mdr-1 + cells , and embryonic isl1 + cells .
these two populations have always been considered different entities and are described separately in many research papers and reviews , although expression of the isl1 transcription factor by cardiac precursor cells has also been reported in adult hearts .
di nardo , described for the first time the concomitant expression of the three markers , i.e. , c - kit , sca-1 and isl1 , in the same cardiac precursor cell . prof .
di nardo s research group described the concomitant expression of the isl1 , c - kit and sca-1 markers in adult mouse cardiac progenitor cells in 2008 , while in 2009 our research group confirmed the expression of the three markers in adult rat cpcs . in a systematic work published in 2011 about the identification of isl1 + cells in the mouse heart from postnatal day 1 to young adulthood in different strains , the authors found clusters of positive cells in the cardiac ganglia of the studied strains , while found very a few clusters of isl1 + cardiac precursors only in 129svj or balb / c strains and in animals not older than 4 months .
recently genead et al . demonstrated the contemporary expression of c - kit and isl1 markers in rat adult hearts in normal , pregnant and infarcted individuals .
they reported the expression of both markers in the entire heart and in the right ventricle , the left ventricle , the outflow tract and the peri - infarct and peri - ischemia regions .
unfortunately , the study was based on real - time pcr analysis , and whenever immunocytochemistry was shown , no double staining for both markers had been performed . other evidence supports the hypothesis that isl1 + cells are not a different population from sca-1 + cpcs .
the contemporary expression of sca-1 and isl1 has been described both in a subpopulation of sca-1+/c - kit- cells identified and isolated from adult mouse hearts and in sca-1 + cardiosphere - derived cells obtained from cardiac explants from normal , sham - operated or post - myocardial infarct hearts . in this study , only sca-1+/cd45- cells were also positive for isl1 and increased in number only after an acute myocardial infarct . considering that other authors identified isl1 + cells in adult murine and rat hearts , weinberger et al . used heterozygous isl1-lacz mice , a more sensitive genetic approach , to investigate the presence and localisation of these precursor cells in 30 animals at different time points after birth ( 10 weeks to 18 months ) .
they found four different populations of isl1 + cells : i ) clusters of isl1 + neurons were found in the cardiac parasympathetic ganglia of the posterior side of the heart and in the nervous plexus surrounding the pulmonary veins ; ii ) clusters of isl1 + smooth muscle cells were found in the muscular layer above the aortic and pulmonary valve in the proximal part of the aorta and the trunk of the pulmonary artery ( only a few positive cells were present in the aortic valve leaflets ) ; iii ) clusters of isl1 + cardiomyocytes were found in the left ventricular outflow tract region ; iv ) clusters of isl1 + sinoatrial node ( san ) cells were found in the muscular wall between right atrium and vena cava superior .
these results support the hypothesis that in the adult heart many cell populations may derive from isl1 + embryonic precursors , as smooth muscle cells , parasympathetic neurons , san cells , but also that , even if reduced in number , isl1 + cardiomyogenic progenitors , may be present in the adult myocardium .
many papers have been published on cpcs , and many research groups claimed to have discovered different populations of progenitor cells .
it is strange to think that the cardiac tissue possesses a larger number of stem / progenitor cells compared to the other tissues found in our body , and it is hopeful that cpcs may vary in number with external stimuli or with age as it happens for other adult stem cells .
the hypothesis that the myocardium is a non - regenerative tissue is not valid anymore , and much evidence supports the new idea that there is at least one cardiac progenitor cell . isl1 + and c - kit+/sca-1 + cells were considered to be two different populations until 2012 , when prof .
sussmann , citing our recent paper published in the european journal of histochemistry in an editorial in circulation research , suggested considering the possibility of studying a population of cells positive for both isl1 and c - kit instead of choosing to study only one side of the coin . in the review and in the research paper published in 2009 , we reported the concomitant expression of isl1 , c - kit and sca-1 in a population of cpcs isolated without any sorting or selection and suggested for the first time that there could be a unique population of cells with several subpopulations and that the several markers identified could be only the effect of different cultivation conditions in the different laboratories .
other research groups later described the co - expression of isl1 , sca-1 and/or c - kit and we further demonstrated that , in samples from foetal and postnatal human hearts , there was also an isl1 + subpopulation of c - kit+ cells .
anversa among the authors , on a study of the presence of c - kit+ cardiac stem cells in embryonic , foetal and neonatal mouse hearts .
they demonstrated that these cells can form cardiomyocytes and coronary vessels and , as we have demonstrated in our paper , they are more abundant than isl1 + cells . moreover , they stated that isl1 + cells are present only in the outflow tract , atria and part of the right ventricle and that they are a population of committed cells derived from an immature primitive cell .
we can say that there is strong evidence suggesting that c - kit , sca-1 and isl1 may be markers of subpopulations of cpcs that are present in the embryonic , foetal and neonatal heart and persist from after birth until adulthood .
the identification of the real immature cardiac precursor expressing the four cell markers ( c - kit , sca-1 , mdr-1 and isl1 ) and above all the identification of a unique isolation protocol is very important in the clinical practice .
this immature common cardiac precursor may represent the perfect candidate for cardiac tissue engineering and , hopefully , may be driven pharmacologically to one phenotype or the other . to identify a standardised protocol
is extremely important to go further in the studies on cardiac tissue regeneration , and common guidelines are needed in cardiac stem cell therapy .
the isolation of stem cells by cell sorting or immunobead selection using surface markers such as c - kit , sca-1 or mdr-1 is important to identify a standardised protocol of isolation , however isl1 may be useful in basic research to follow the fate of cpcs in heart development and myocardium differentiation .
moreover , it is extremely important to understand which is the predominant precursor population in the different stages of life and which population should be used in a paediatric patient instead of an adult one .
if fate of immature cardiac precursors may be determined pharmacologically , the possibility to regenerate san cells is very important in curing arrhythmia s both in patients with ischemia and malformations .
the identification of the different subpopulations in the developing heart may be a fundamental step in identifying the optimal cell line for cell - based therapies .
 cardiac progenitor cells are multipotent stem cells isolated from both embryonic and adult hearts in several species and are able to differentiate at least into smooth muscle cells , endothelial cells and cardiomyocytes . 
 the embryonic origin of these cells has not yet been demonstrated , but it has been suggested that these cells may derive from the first and secondary heart fields and from the neural crest . in the last decade , 
 two different populations of cardiac progenitor or stem cells have been identified and isolated , i.e. , the islet1 positive ( isl1 + ) and c - kit positive ( c - kit+)/stem cell antigen-1 positive ( sca-1 + ) cells . until 2012 , 
 these two populations have been considered two separate entities with different roles and a different origin , but new evidence now suggests a connection between the two populations and that the two populations may represent two subpopulations of a unique pool of cardiac stem cells , derived from a common immature primitive cell . to find 
 a common consensus on this concept is very important in furthering the application of stem cells to cardiac tissue engineering . 
Islet1 positive cells during heart development
Isl1 positive cells in the adult heart
Concluding remarks