Ontogeny of cardiomyocytes: ultrastructure optimization to meet the demand for tight communication in excitation–contraction coupling and energy transfer

Figures

• 

  Figure 1.. Schematic of (a) fish cardiomyocyte, (b) neonatal mammalian cardiomyocyte, and (c) adult mammalian cardiomyocyte drawn on the basis of the references in table 1. The sarcolemma is shown in green, the myofilaments in grey, the SR in blue, and mitochondria in red. Neonatal mammalian and fish cardiomyocytes have a smooth sarcolemma, which is shown on the top and bottom, because their diameter is relatively small. The myofilaments are situated peripherally, as a ring surrounding the central core of mitochondria. The SR is more irregular in fish cardiomyocytes, where it has no specific relation to the myofibrillar bands, whereas in neonatal cardiomyocytes, the SR is peripheral with the periodicity corresponding to the z-lines and m-band. Adult mammalian cardiomyocytes are thicker and with more internal membrane structures. The sarcolemma invaginates to form transverse tubules (t-tubules). It is only shown on the top side of the figure, because the cell is four to five times wider than fish and neonatal cardiomyocytes. There are multiple, parallel, interchanging rows of myofilaments and mitochondria. The SR wraps around t-tubules, myofilaments and mitochondria.

• 

  Figure 2.. Organization of mitochondria and t-tubules in adult rat cardiomyocytes. (a) Overall confocal image of a live cardiomyocyte. Mitochondria (red) were labelled for 10 min with 250 nM MitoTracker® Deep Red FM (ThermoFisher). The sarcolemma (green), including t-tubules, was labelled with 500 nM CellMask™ Orange (ThermoFisher). (b–d) Zoom of the white rectangle in panel (a), showing the t-tubules (b), the mitochondria (c) and the merged image (d). Note the regular pattern of mitochondria, the highly organized network of t-tubules, and how densely these structures are packed.

• 

  Figure 3.. Schematic illustration of a section of an adult mammalian cardiomyocyte. On the left, the cell is shown in full width, but not full length, with sarcolemma (green) on the top and bottom invaginating to form t-tubules. The many parallel rows of mitochondria (red) and myofilaments (grey) in contact with t-tubules as well as the SR (blue) form modules within the cell. Whereas the diffusion coefficient in solution is 200 µm2 s−1, the overall diffusion coefficient across several modules is 80–90% lower (24 and 35 µm2 s−1 in the transversal and longitudinal direction, respectively). On the right is shown an enlargement of the black square illustrating a single module, i.e. a sarcomere surrounded by mitochondria, t-tubules and SR. Within each module, the diffusion coefficient is estimated to be 80% of the coefficient in solution, i.e. 160 µm2 s−1. Diffusion coefficients are from Illaste et al. [112].