Muscle Fiber Differentiation and Growth Patterns During Hyperplasia and Hypertrophy in the Ricefield Eel Regulated by Myogenic Regulatory Factors

Muscle growth is mainly achieved by two processes, hyperplasia (fiber number increase) and hypertrophy (fiber size increase), and is regulated by the myogenic regulatory factor (MRF) family. The muscle fiber composition and expressional patterns of the four MRFs (Myf5, MRF4, MyoD, and myogenin [MyoG]) were characterized in the white muscle of Ricefield Eels Monopterus albus ranging from 5 to 180g. Results showed that the mean muscle fiber diameters gradually increased as body mass increased from 5 to 114g. The small muscle fibers (diameter < 20m) were found in the highest ratio in 5-45-g eels; most of the fibers ranged from 20 to 60m for 64-104-g fish; and muscle mainly consisted of fibers over 60m in 114-180-g fish. The expression of Myf5 and MyoD increased continuously at the stages mainly consisting of the small and medium muscle fibers. Expression of MRF4 and MyoG was low at the stages with high ratios of small muscle fibers, but their expression levels increased at the stages with high ratios of medium muscle fibers. The expression levels of the four MRFs exhibited a regulatory effect on the process of muscle hyperplasia or hypertrophy. The data indicated that hyperplasia was the major event at the earlier stages of the Ricefield Eel, whereas hypertrophy occurred at later developmental stages. The results of our study provide valuable knowledge regarding muscle development in Ricefield Eels.