Abstract 296: Bone Marrow Derived Cells Contribute to Cell Turnover in Ageing Murine Hearts
Background: The paradigm that the heart is a terminally differentiated organ has recently been challenged since some studies reported the ability of bone marrow (BM) derived cells to transdifferentiate into cardiomyocytes after myocardial damage. However, the physiological role of bone marrow derived cells during the lifespan of an undamaged heart is widely unknown. We therefore examined the quantity and phenotype of bone marrow derived cells in aged murine hearts.
Methods: 12-week-old mice (n=20) were sublethally irradiated and BM from enhanced green fluorescent-transgenic (eGFP) littermates was transplanted. After 1 month 5 mice were sacrificed and served as control. The remaining mice were sacrificed after 18.2±1.1 months. Immunohistochemistry was performed in 10 hearts using titin antibodies to identify cardiomyocytes, vimentin for fibroblasts, sMemb for myofibroblasts, α-smooth muscle actin for smooth muscle cells, F4/80 for macrophages, BS-1 and CD31 for endothelial cells. Additionally, anti-eGFP immunostaining was used to exclude autofluorescence. Sections were analyzed using fluorescence and confocal laser microscopy. The remaining 5 hearts were digested with collagenase and cell sorting was performed for a quantification of BM-derived cells in relation to eGFP negative cells.
Results: BM transplantation was successful as FACS analysis showed 92±5% eGFP expressing leukocytes after 1 month and 78±6% after 18 months. In the juvenile hearts only few eGFP-positive cells were detected (<1 cell/mm2). Numerous eGFP-positive cells were found in left ventricular sections in the old hearts. Histological quantification revealed 9.3±3.3 cells/mm2 to be derived from BM cells. Most of these cells were fibroblasts and myofibroblasts. In addition, numerous endothelial cells and smooth muscle cells contributing to neoangiogenesis were detected. Few eGFP-positive cardiomyocytes could be identified. The cell sorting of eGFP-positive cells documented 4.8±1.9% of all cardiac cells to be derived of BM cells.
Conclusion: The present study demonstrates for the first time a substantial recruitment and accumulation of BM derived cells in the ageing myocardium suggesting their contribution in cell turnover of the heart during the lifespan of mice.