Morphometric and histological changes in cardiac nodes after acute spontaneous myocardial infarction in humans and pigs

Background and Aim: The sinoatrial node is responsible for the intrinsic electrical activation that in mammals leads to coordinated rhythmic contractions of the heart, from where it is distributed through the atrial tissue to the atrioventricular node. This study aimed to conduct a histological and morphometric study of the components and cells in cardiac nodes altered by myocardial infarction (MI) and compare them with normal tissues in humans and pigs. Materials and Methods: We analyzed 10 human hearts and 10 pig hearts that died from MI and compared them with 10 healthy control hearts from each species. Histological sections of 5 μm thickness were obtained using a microtome and stained with hematoxylin-eosin and Masson's trichrome. The identification and assessment of the percentage of connective tissue and cellularity in the cardiac nodes were performed. Results: We observed a decreased size of cardiac nodes in humans and pigs, as well as an increased percentage of fibrosis inside the nodes, and changes in the size of the nodal cells and surrounding cardiomyocytes (decrease or hypertrophy) were observed. Cartilaginous metaplasia was also found in the cardiac skeleton of all pig samples. Conclusion: In the present study, a significant increase in collagen fibers and a decrease in cellularity were found in cardiac nodes in samples from humans and pigs with MI. These findings would explain the presence of arrhythmias, which often lead to death.