The sarcosomes of the heart.

Abstract

The electron microscopic picture of the heart muscle differs from that of the skeletal muscle of the same animal, not so much by significant differences in the structure of the myofibrils, but rather by the amount of sarcosomes (1) present in the muscle fiber. In the skeletal muscle there are only a few sarcosomes in each field between the myofibrils, whereas in the heart muscle at the same magnifications there are innumerable masses, filling the space between the individual myofibrils and the space between sarcolemma and myofibrils. This may explain the ability of the heart muscle to work for a lifetime without longer rest, because the sarcosomes contain most important metabolic enzymes. It also explains why, in the electron microscopic picture, the myofibrils of the heart are never seen so tightly packed as in the skeletal muscle of the same animal. As a rule, these sarcosomes are of irregular, spherical, or ellipsoid shape in the heart, whereas in the skeletal muscle they are more frequently oblong, or ribbon-shaped. The inner structure of the sarcosomes of the heart shows the usual lamellae or cristae of osmiophilic material in a less osmiophilic matrix. These cristae are normally tightly packed, parallel, sometimes concentrically arranged. In some of the sarcosomes no definite lamellar structure can be found, which may reflect functional changes in the sarcosomes. A certain difference seems to exist between the sarcosomes of the ventricles and the auricles of the same animal. In the auricles, in addition to the sarcosomes resembling those of the ventricles (diameter up to 1 micron), there are a great number of much smaller sarcosomes, whose electron microscopic investigation is still in progress. In the auricle of the heart we have also found much more frequently than in the ventricle, sarcosomes of the ribbon-like, mitochondrial type. The inner structure of these can be seen in Fig~ 4. In striated muscle (leg muscle) of the rat very small spherical sarcosomes with a diameter of about 1~ micron have been seen, besides the usual normal sized sarcosomes. These small sarcosomes are typically located near the Z-bands, singly or in twos, between the individual myofibrils. It cannot yet be decided