The harp seal, Pagophilus groenlandicus (Erxleben, 1777). XXV. Ultrastructure and metabolic adaptation of skeletal muscle.

Abstract

An electron microscopic study of the structure of three skeletal muscles, pectoralis major, psoas, and longissimus dorsi, of the harp seal (Pagophilus groenlandicus) was carried out. Two types of fiber, the dark and the light comparable to the red (type 1) and the white (type 2) of other vertebrate skeletal muscles, were recognized while a third, the intermediate type, could be clearly distinguished only in the pectoralis major. All the fiber types contained considerable amounts of glycogen, while large aggregations of mitochondria and lipid droplets were seen only in the dark fibers.The intermyofibrillar matrix in the fibers of the pup (< 3 months) muscle was found to be more extensive than that of the adult muscle. The dark fibers in M, longissimus dorsi of both the pup and the adult appeared to have higher mitochondrial density than those of the other two muscles studied, thereby indicating that this muscle had a greater capacity for aerobic metabolism than M. psoas, which in turn had a larger population of mitochondria than M. pectoralis major. Though the presence of large mitochondrial aggregations and lipid droplets suggests the possibility of fat being used as fuel for muscular energy, it is postulated that the seal muscle is basically geared for anaerobic use of carbohydrate as an adaptation for the animal's diving habit. However, aerobic metabolism of carbohydrate and of a limited amount of fat may be used during surface swimming. It is also suggested that the mitochondrial aggregations and lipid droplets in the dark fibers of both adult and pup muscles represent an adaptation for thermogenesis comparable to that of the brown adipose tissue.