Changes in fine structure of rat skeletal muscle related to cold acclimation.

Mitochondria became larger and some numerous in the internal red fibers of M. quadriceps femoris and neck muscle in cold-acclimated rats than in warm-acclimated ones, while they became less in number in spite of remarkable increase of size in the external white fibers of the former muscle. Mitochondrial cristae and sarcoplasmic reticuli developed better in the muscles of cold-acclimated rats.