Each year over 800,000 people in the United States have a heart attack or myocardial infarction (MI) which involves occlusion of coronary artery blood flow, myocardial ischemia, and death of the downstream cardiac muscle [1]. After coronary artery occlusion, the affected myocardium undergoes a robust inflammatory response that ultimately leads to significant changes in tissue content and structure. This highly dynamic response is frequently separated into three healing phases: the inflammatory or necrotic phase, the proliferative or fibrotic phase, and the maturation or long-term remodeling phase [[2], [3], [4]]. First, as cardiomyocytes become necrotic, they release damage-associated molecular patterns and pro-inflammatory cytokines that attract neutrophils and macrophages to the infarction site. These infiltrating immune cells then secrete matrix metalloproteinases (MMPs), which degrade the surrounding extracellular matrix (ECM), enabling them to degrade and remove necrotic cardiomyocytes [5]. The robust native ECM is initially replaced with a weak provisional matrix [3,6]. A marked rise in fibroblast infiltration and collagen deposition signifies the transition to the fibrotic phase of healing.

While the post-MI inflammatory response is critical to healing, it renders the infarcted myocardium vulnerable to risk of cardiac rupture [[7], [8], [9], [10]].Reperfusion therapy, the restoration of blood flow, is one of the most effective treatment strategies for MI. Widespread use of reperfusion therapy has been corelated with a dramatic decrease in cardiac rupture [[11], [12], [13], [14], [15], [16], [17]] even when administered late, hours following MI and after cardiomyocyte death [15,[18], [19], [20], [21], [22], [23], [24]]. Late reperfusion therapy does not reduce infarct size or transmurality [[25], [26], [27], [28], [29]]. Increased MMPs secretion and lower collagen content has widely been proposed as a potential rupture predictor [[30], [31], [32], [33]], but detailed studies on rodents do not support it [[34], [35], [36], [37]]. Thus, the underlying mechanisms of late reperfusion’s success in reducing rupture incidence are not well understood [15,[38], [39], [40]]. Unfortunately, while the mortality and complication rate of MI have dropped remarkably in the reperfusion era, hospital fatality rates following rupture have remained stagnant [41]. We hypothesize that late reperfusion therapy leads to smoother regional changes in the tissue content and structure following an MI [[42], [43], [44]] involving a less stark contrast between infarct core and border and increasing the loading threshold for a tear to form and propagate through the tissue. Furthermore, spatiotemporal heterogeneity in infarct healing is an active area of investigation, particularly for therapies aimed at reducing long term heart failure risk [4,45,46]. Following an MI significant regional heterogeneity is present, however, there has been substantial debate as to what constitutes the infarct core, border, and peripheral regions [47,48]. The existence of and size of the border zone has had conflicting results since the 1960s, with definitions ranging from 1 cell in width to 5 mm in width in rats [[48], [49], [50]]. Traditionally the border has been defined by the presence of hypo-contractile myocytes, in contrast to non-contractile myocytes in the infarct core and healthy contractile myocytes in the periphery. Later in healing, the core is defined by the absence of myocytes, consisting instead of a collagenous-rich scar that transitions to contractile or hypertrophied peripheral myocytes. Studies have also defined the spatial bounds of infarct border according to myocardial deformation and strain [50,51], alterations in metabolism [49], coronary arterial anatomy [52], electrophysiological activity [[53], [54], [55]], and transcriptomics [47].”

In this study, we quantified the temporal changes in regional heterogeneity following permanent coronary occlusion and late reperfusion therapy (temporary coronary occlusion) in rats. We developed a semi-automated image processing approach to quantify differences in both cellular and collagen density in the longitudinal-circumferential plane of the ventricular wall during the first 5 days following an MI. Additionally, we investigated the impact of conservative and liberal variations in the definition of the infarct core, border, and peripheral regions on this spatial distribution of these measurements. Therefore, we were able to identify both temporal and regional differences throughout the healing process between the permanent occlusion and late reperfusion groups.