Large populations (up to 600/cell) of spherical, electron-opaque granules ∼0.3 to 0.4 µ in diameter are characteristically found in muscle fibers of mammalian atria. They are absent in muscle fibers of the ventricles. The granules are concentrated in the sarcoplasmic core and occur in lesser numbers in the sarcoplasmic layers between myofibrils and under the plasma membrane. Their intimate association with a central voluminous Golgi complex and the frequent occurrence of material reminiscent of the granular content within the cisternae of the Golgi complex suggest that the latter is involved in the formation of the atrial granules. Atrial granules are larger and more numerous in smaller species (rat, mouse), and generally smaller and less numerous in larger mammals (dog, cat, human); they are absent from the atrial fibers of very young fetuses (rat) but are present in those of newborn animals. A small population of bodies containing glycogen particles and remnants of the endoplasmic reticulum and mitochondria occurs in the sarcoplasmic cores of atrial as well as ventricular muscle fibers in the rat; they contain acid phosphatase and thus appear to be residual bodies of autolytic foci. Their frequency increases with the age of the animal. Typical lipofuscin pigment granules, which are known to contain acid phosphatase and are found in the sarcoplasmic cores in old animals (cat, dog and human), are presumed to arise by progressive aggregation and fusion of small residual bodies.

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