At the cellular level, the main components of the postnatal heart are cardiomyocytes, cardiac fibroblasts, endothelial cells and vascular smooth muscle cells (Banerjee I, fusel JW, 2007). In a normal adult heart, cardiomyocytes occupy most of the tissue volume, but they are only 30% of the total number of cells. The remaining 70% consists of non-cardiomyocyte cell types, among which cardiac fibroblasts represent the vast majority (Jugdutt, 2003).
The heart is organized into a complex array of cellular and non-cellular components. Cardiomyocytes are diplayed in laminae (AA Young, IJ Legrice, 1998), which in turn are organized in layers of cardiomyocytes from two to five cells thick and surrounded by a network of endomysial collagen. In the other hand, the fibroblasts form a network of interconnected cells distributed among the endomysial collagen surrounding groups of cardiomyocytes (Borg TK, Ranson WF, 1981 - Camelliti P, Borg TK, 2005 - Goldsmith EC, Hoffman A, 2004 - Young AA, Legrice IJ, 1998). The distribution of endothelial cells and vascular smooth muscle is restricted to the vasculature. Based on this organization, it has been estimated that all cardiomyocytes would be connected directly to one to five cardiac fibroblasts (Kohl P, Camelliti P, 2005). Direct intercellular contacts of fibroblasts seem to be supported by at least two types of cell to cell connection molecules: connexins and cadherins. In rats, cardiac fibroblasts are connected by both cadherins and connexin-40, while connexin-45 serves as a connection between fibroblasts and cardiomyocytes. However, the distribution of different connexins may vary significantly between species (Banerjee I, K Yekkala, 2006 - Camelliti P, Green CR, 2004). Additionally, integrins mediate the connection between cardiac fibroblasts and the extracellular matrix.