Studies assessing cardiac myocyte renewal and myocardial repair in the adult mammalian myocardium

The initial aims of this thesis were to investigate whether the myocardium contains resident progenitor cells that contribute to myocardial renewal and whether extra-cardiac bone marrow derived cells contribute to myocardial regeneration. I reveal that the myocardium has the capacity to produce humoral factors that enable extra-cardiac progenitors to survive in vitro though this was insufficient to induce cardiac differentiation. I have shown that the myocardium has the capacity to produce a heterogeneous population of cells in vitro, some of which express cardiac related markers but do not adopt a full cardiac phenotype. When these cells are transplanted into a normal or injured heart they integrate into the myocardium but fail to develop a full mature functional cardiac phenotype. I have set up the frame work for demonstrating and defining the qualitative histological structure of the myocardium using lineage tracing techniques and strengthening the criteria for defining various cell lineages in the heart and therefore demonstrated the deficiencies of seminal studies that claimed that adult stem cells had the capacity to differentiate into cardiomyocytes and secondly that cultured heart explants produce cardiac progenitors. From this work it is clear that more needs to be done to identify the various cell lineages and roles of endogenous cardiac cells. The identification of clusters of perivascular cells expressing cardiac markers using 3 dimensional confocal imaging by two photon molecular excitation provided a different approach for identifying putative cardiac progenitors. This in combination with lineage tracing techniques and cell isolation is now required to identify the role of these interesting perivascular cells in cardiac homeostasis.