Figure 4: Histopathological examination of the heart by hematoxylin and eosin staining in the different studied groups: (a) sections from the control group showed normal cardiac muscle with skewed fusiform cells with an eosinophilic cytoplasm and spindle-shaped nucleus. (b) Sections from the isoprenaline (ISO)-treated group showed an area of infarction with homogenous pink structureless material with karyorrhexis (fragmented nuclei). (c) Sections from rats pretreated with quercetin showed moderate cardiac muscle changes in the form of cardiac muscle swelling and cytoplasmic edema. (d) Sections from the trimetazidine (TMZ)–ISO-treated group showed mild cardiac muscle degeneration with myocyte vacuolization – these were large, clear, intracellular spaces, probably containing water. (e) Sections from the quercetin-treated and TMZ–ISO-treated group showed minimal cardiac muscle degeneration in the form of cloudy cardiac muscle swelling with a pale esinophilic cytoplasm.