Differences between esophageal body and lower esophageal sphincter in mitochondria of smooth muscle in opossum.

In opossum, contractions of smooth muscle of the circular layer of the esophageal body resist oxygen deprivation, while tone of the lower esophageal sphincter requires oxygen. Thus, morphologic differences in mitochondria could distinguish the two types of muscle. Transverse strips, cut at regular intervals along the smooth-muscle segment of the esophageal body, were physiologically identified as muscle of body or sphincter. Transverse sections of the circular muscle layer were examined by transmission electron microscopy. Muscle cell profiles located between the ends of the nucleus and the tips of the cells were used to evaluate the number of mitochondrial profiles per muscle cell profile, mitochondrial mass (relative total area of mitochondrial profiles in each muscle cell profile), mitochondrial position (distance from the center of each mitochondrial profile to the nearest point on the cell membrane relative to mean radius of that muscle cell profile), and mass of smooth endoplasmic reticulum (relative total area of profiles in each muscle cell profile). In the nonnucleated part of the muscle cells, sphincter muscle had a greater ratio of mitochondrial profile area to cell profile area (p less than 0.05), mitochondrial profiles in sphincter muscle were more central in location than in esophageal body muscle (p less than 0.05), and sphincter muscle contained more endoplasmic reticulum than body muscle (p less than 0.05). Sphincter muscle did not differ from body muscle in mean muscle cell profile area and number of mitochondrial profiles per muscle cell profile in this region of the muscle cell (p greater than 0.05).