*2.1. c-KIT-Expressing CPCs*

The first-ever detected CPCs were isolated from female rats and were characterized by the expression of the stem cell surface marker c-KIT [28]. These CPCs are present throughout the ventricular and atrial myocardium, particularly in the atria and the ventricular apex [28]. These progenitor cells also express the cardiac transcription factors NKX2.5, GATA4, and MEF2C, and are negative for hematopoietic lineage markers, such as CD45, CD34, CD3, CD14, CD16, CD19, CD20 and CD56 [50–52]. They are self-renewing, clonogenic and are able to differentiate into the three cardiac cell types in vitro and in vivo [28,53]. The c-KIT receptor binds to the Stem Cell Factor (SCF) which leads to the activation of the signaling pathways Phosphoinositide 3-kinase/Protein Kinase B (PI3K/AKT) and p38 Mitogen-Activated Protein Kinase (MAPK) [54,55]. These pathways regulate a variety of CPC functions like self-renewal, proliferation, survival, and migration [54–57]. Even though c-KIT CPCs contribute to the generation of cardiomyocytes at earlier stages of embryonic development and right after birth, this ability is mostly lost in the adult heart and very low percentages of new cardiomyocytes seem to originate from these CPCs [58–60]. Therefore, the improvement of cardiac function by c-KIT CPCs might be a result of paracrine factors rather than the production of de novo cardiomyocytes [58,61]. Furthermore, c-KIT expression is considered necessary but not sufficient to define CPCs [62].

#### *2.2. SCA1-Expressing CPCs*

Another CPC population present in adult hearts expresses the Stem Cell Antigen 1 (SCA1). The cells were first identified in adult mouse hearts [11] and are predominantly located in the atrium, the intra-atrial septum, the atrium-ventricular boundary and scattered within the epicardial layer [37]. SCA1 is a cell surface protein of the Ly6 gene family and it was initially used to isolate hematopoietic stem cells [63]. Additionally, SCA1 is widely expressed by stem and progenitor cells from a variety of tissues, including the heart, and it has roles in cell survival, proliferation and differentiation [63]. Several studies have shown that SCA1 CPCs are negative for hematopoietic lineage markers and are able to differentiate into the three cardiac lineages [11,64]. These CPCs also have the ability of homing in response to injury and contribute to neovascularization in vivo [11,65,66]. Although this CPC population seems promising for cardiac regeneration, their translational relevance is not without caveats. First, all the SCA1 CPC populations identified to date display different gene expression profiles and distinct differentiation potential [37,66–71]. In addition, several studies have shown that the benefits resulted from the transplantation of these CPCs might be predominantly due to paracrine mechanisms as these cells differentiate into cardiomyocytes with very low efficiency [66,68,70,71]. Finally, SCA1 is only present in murine cells and a human ortholog of SCA1 has yet to be identified [63]. Therefore, the nature of the epitope target of SCA1 in humans and the nature of regeneration of the associated CPC population have yet to be determined.

#### *2.3. MESP1*/*2-Expressing CPCs*

During the development of mesoderm, embryonic cells express the transcription factor Mesoderm Posterior Protein 1/2 (MESP1/2), which is essential for proper cell migration [15,72–74]. MESP1/2 expression marks the first step in the commitment of the nascent mesoderm into the myocardial lineages, and it describes the first population of multipotent cardiac progenitor cells that produce the various cardiac cell types of the heart [72,75]. Although MESP1/2 CPCs show increased cardiac potential, in comparison to other CPC types, they are not irreversibly committed to the cardiac fate [76]. Consequently, there is a possibility that these cells will differentiate into derivates of the paraxial

mesoderm and skeletal muscle [77,78]. Furthermore, MESP1/2 is only transiently expressed during embryonic development, which increases the difficulty of tracking the expansion and differentiation of the CPCs [79,80].
