Histological differences in healing following experimental transmural infarction in rats

Mechanisms of cardiac regeneration following transmural myocardial infarction were analysed in rat hearts using immunohistochemistry for a-SMA, caspase-3, Ki-67 and nestin markers. Seven weeks after experimental myocardial infarction, two different types of healing processes were revealed in rats with and without aneurysmatic bulging of the left ventricular wall. Besides thinning of the ventricular wall, three zones characterized both types of scars: the scar zone (divided into central and peripheral parts), the peri-infarct zone and the border zone. The main difference between the types of scars was the presence of a central necrotic zone inside the aneurysmatic wall, while connective tissue with myofibroblasts characterized the same zone in non-bulging wall. Apoptotic caspase-3 positive cells were found in the granulation tissue of the border zone in aneurysmatic scar, while in non-bulging scar they characterized all three zones. Proliferating Ki-67 positive cells displayed reverse expression pattern compared to apoptotic cells. Quantification of a-SMA positive cells revealed 60% a-SMA positive cells inside the central part of the aneurysmatic scar zone and 39% in invaginating areas, versus 19% in non-invaginating areas of the peripheral zone, but only 30% in the peripheral part of the non-bulging scar zone. Nestin positive cells were found in both types of scars, but with different distribution. These results suggest that even seven weeks after myocardial infarction, the healing processes in non-bulging scars are in chronic phase, while aneurysmatic scars are still in subacute phase. Histological differences in scar healing might be important for functional properties of the heart wall and for heart recovery prognosis.