CHANGE IN NUMBER OF SKELETAL MUSCLE FIBERS DURING POSTNATAL GROWTH OF THE RAT

• Published in: Japanese Journal of Physical Fitness and Sports Medicine Vol. 30; no. 2; pp. 103 - 113
• Main Author: NARUSAWA, MITSUO
• Format: Journal Article
• Language: English
• Published: The Japanese Society of Physical Fitness and Sports Medicine 1981
• ISSN: 0039-906X; 1881-4751
• DOI: 10.7600/jspfsm1949.30.103

It is said that number of muscle fibers does not change in the postnatal period. However, it has been reported that shortly after birth, the uni-motor neuron innervates may muscle fibers (Changeux) . Further it has been shown by Changeux that, during the growth of the immature muscle fiber, various neuromuscular junctions are developed. Also, as Rayne has pointed out, 50% of the total muscle fibers are developed within 6 weeks after birth. In addition it has been shown that during the aging process, the number of muscle fibers decreases. This report, based on the aging phenomena of rats, discusses the change in the number of muscle fibers and the period of differentiation of muscle fiber type. Rats ware used from the one day of birth until the 540th day. Using an optical microgram, the total number of muscle fibers of both EDL and SOL muscle was counted. At the same time using the histochemical method, the differentiation of muscle fiber type was studied. The results are as follows : (1) Differentiation of muscle fiber type in both EDL and SOL muscle occurred within 3 weeks of birth. (2) 70% of the total muscle fibers in both EDL and SQL muscle developed from the first day to the 21st day after birth. (3) From the 21st day to the 105th day after birth, the number of muscle fibers decreased by 10%. There are two possible reasons for the apparent increase in muscle fiber number. (1) Those immature muscle fibers not having sufficient sarcoplasma could not be detected and consequently couldn't be counted. (2) Satellite cells, which are thought to have a relation to myotube fusion, exist in large numbers even after birth and new muscle fibers may be frormed by myoblast fusion. The decrease in the number of muscle fibers may occur as a function of the change from poly-innervation to uni-innervation. It is possible to think that as natural extinction takes place in the case of the former, some of the muscle fibers which were innervated in this manner also become extinct.