Tradition considers that mammalian heart consists of about 70% non-myocytes (interstitial cells) and 30% cardiomyocytes (CMs). Anyway, the presence of telocytes (TCs) has been overlooked, since they were described in 2010 (visit www.telocytes.com). Also,
the number of cardiac stem cells (CSCs) has not 
accurately estimated in humans during ageing
. We used electron microscopy to identify and estimate the number of cells in human atrial myocardium (appendages). Three age-related groups were studied: newborns (17 days-1 year), children (6-17 years) and adults (34-60 years). Morphometry was performed on low-magnification electron microscope images using computer-assisted technology. We found that interstitial area gradually increases with age from 31.3 ± 4.9% in newborns to 41 ± 5.2% in adults. Also, the number of blood capillaries (per mm(2)) increased with several hundreds in children and adults versus newborns. CMs are the most numerous cells, representing 76% in newborns, 88% in children and 86% in adults. Images of CMs mitoses were seen in the 17-day newborns. Interestingly, no lipofuscin granules were found in CMs of human newborns and children. The percentage of cells that occupy interstitium were (depending on age): endothelial cells 52-62%; vascular smooth muscle cells and pericytes 22-28%, Schwann cells with nerve endings 6-7%, fibroblasts 3-10%, macrophages 1-8%, TCs about 1% and stem cells less than 1%.
We cannot confirm the popular belief that cardiac 
fibroblasts are the most prevalent cell type in the heart and account for about 
20% of myocardial volume
. Numerically, TCs represent a small fraction of human cardiac interstitial cells, but because of their extensive telopodes, they achieve a 3D network that, for instance, supports CSCs.
The myocardial (very) low 
capability to regenerate may be explained by the number of CSCs, which decreases 
fivefold by age (from 0.5% to 0.1% in newborns versus adults)
.