Ultrastructural changes of skeletal muscle evoked by heat were examined microscopically by means of the azan method. The specimens used had been collected from several different muscles of animals of some species. They were boiled or frozen by various methods at different temperatures.
The results obtained are summarized as follows.
1. As a whole, boiling process had an effect of making the ultrastructure of muscle fiber coarse, while freezing process had an effect of making it dense. The gradient of ultrastructural changes, however, varied to some extent with the kind and type of muscle. It is assumed that the ultrastructure of muscle became coarse by boiling, mainly because the cell elements had been coagulated and partly because they had been disintegrated, and that it became dense by freezing, because the cell elements had been dehydrated.
2. The following relationships were found between boiling temperature and ultrastructural changes of muscle fiber.
a. The ultrastructural density of muscle fiber was not changed by boiling at a temperature below 30°C and became slightly lower at 40°C and the lowest at a temperature from 50 to 60°C. The ultrastructure of muscle fiber boiled at a temperature over 70°C, however, was not any longer so coarse as that of muscle fiber boiled at 60°C. The ultrastructure of muscle fiber boiled at 100°C was coarser than that of unboiled muscle fiber.
b. White muscle fiber inclined to become coarse in ultrastructure by boiling at lower temperature than red muscle fiber.
c. Fish muscle fiber inclined to become coarse in ultrastructure more rapidly than muscle of any warm-blooded animal examined.
3. The following relationships were found between method of freezing and ultrastructural changes of muscle fiber.
a. Rapid freezing made the ultrastructure of muscle fiber denser than slow freezing.
b. Fish muscle inclined to become ultrastructurally denser through freezing process than muscle of any warm-blooded animal examined.
c. No demonstrable difference was found in the ultrastructure after freezing process between red and white muscle.