Rationale In adult cardiomyocytes there are known to be 3 different sub-populations of mitochondria: Subsarcolemmal (SS), Interfibrillar (IF), and Perinuclear (PN), which appear to have distinctive properties and shapes. Here, we investigate the mitochondrial morphology of these 3 different mitochondrial subgroups and examine the effect of ischaemia on these parameters.
Methodology We investigated mitochondrial morphology in adult rodent hearts at baseline and in response to ischemia using electron microscopy (x3000 magnification). In six randomly selected electron micrographs of longitudinally-arranged cardiomyocytes, the following characteristics were determined (approx. 500 mitochondria in total): (1) Surface area (SA); (2) Fret’s diameter (the longest distance between any points in a given mitochondrion); (3) Perimeter; (4) Aspect ratio: the length-to-width ratio; (5) Roundness: 4. SA/π.major axis2; (6) Circularity: 4π. SA/perimeter2 (1.0 indicates a perfect circle and as the value approaches zero it indicates an elongated shape).
Results In adult rat cardiomyocytes (n = 3 animals), IF mitochondria have a significantly greater SA [μm2] (IF 0.37 ± 1.08 vs. SS 0.30 ± 1.07, PN 0.27 ± 1.03; P < 0.0001), Fret’s diameter (IF 0.93 ± 0.02 vs. SS 0.83 ± 0.02, PN 0.79 ± 0.18, P < 0.0001), and Perimeter (IF 2.48 ± 1.08 vs. SS 2.21 ± 1.07 PN 2.11 ± 1.03, P < 0.0001), when compared to SSM and PN mitochondria. However, there were no major differences in Aspect ratio, Roundness or Circularity. IF mitochondria in the ischaemic adult murine heart had a smaller Area, Perimeter, Fret’s diameter, Aspect ratio, and a greater degree of Roundness and Circularity.
Conclusions Interfibrillar mitochondria are generally larger and longer than SSM and PN mitochondria. In response to ischaemia, the IF mitochondria undergo fragmentation, generating smaller more rounded mitochondria.
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