The nucleus (Fig. A1), with its conspicuous pores, is located in the center of the cell body. At both nuclear poles between the diverging and converging myofibrils (Fig. A2), sarcoplasm-rich endoplasm (Fig. A3) develops with organelles, glycogen granules, lipid droplets, and—in the musculature of the atria—specific atrial granules (arrow, Fig.Bl). The latter are highly osmiophilic, membrane-bound granules, 0.3–0.4 μm across, and contain hormone cardiodilatin. Since cardiac muscle cells belong to the group of cells that are incapable of expelling residual bodies or lipofuscin granules, these accumulate in the course of life, for which reason large numbers of lipofuscin granules (Fig. A4) can be found in the endoplasm of older individuals.
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