Tamuro Masuda

Effects of Long-Term Intake of a Fine-Grained Diet on the Mouse Masseter Muscle

Muscle fibers of the masseter muscle of mice which had been fed a fine-grained diet for various periods were studied histochemically and morphometrically. The diameters of both extrafusal and intrafusal muscle fibers decreased with time in mice fed a fine-grained diet, compared with those of control mice. In animals maintained on the special diet for 160 days after weaning at the 20th postnatal day, the effects of the diet on the diameter of muscle spindles were severe, and the diameter of each type of red and white fibers was significantly smaller than those of control animals. But a significant difference was not recognized in the diameter of intermediate fibers between control and treated mice. Unexpectedly, white fibers having a smaller diameter than red fibers were observed in diet-fed mice after the 180th postnatal day, although white fibers having such small diameter were not detectable in control animals. Succinic dehydrogenase activities were decreased in both extrafusal and intrafusal fibers of experimental animals. Moreover, muscle spindles with no annulospiral endings were increased in number in mice fed the diet for 130 and 160 days after weaning, although those spindles also increased in control animals. The diameters of outer capsules and primary endings were also significantly decreased in the animals kept on the diet for a long time. These effects of the fine-grained diet on the mouse masseter muscle became severer with time.

Effects of easily chewable diet and unilateral extraction of upper molars on the masseter muscle in developing mice

The effects of easily chewable diets and unilateral extraction of upper molars on the masseter muscle were studied in developing mice. A liquid diet requiring no mastication suppressed the development of the masseter muscles more than a fine-grained diet, and extraction of unilateral upper molars also caused inhibition of muscle development. Moreover, both unilateral extraction of upper molars and a liquid diet had an additive effect on the suppression of the postnatal development of the masseter muscle, and bilateral suppression of the development of the masseter muscle was induced following unilateral extraction of upper molars. These findings suggest that the sensory input from the sensory endings in the periodontal ligament may also play an important role in the postnatal development of the masseter muscle and that there may be some crossing pathways to convey the sensory input coming from the side of the extracted upper molars to the contralateral motor neurons via the interneuronal circuits.

Postnatal Development of the Anulospiral Endings of Ia Fibers in Muscle Spindles of Mice

The formation of the anulospiral ending of Ia fibers in muscle spindles was investigated in the masseter muscle of developing mice. Before 15 days after birth, the complete anulospiral ending was not observed in almost all of the muscle spindles examined. With the growth of mice, the Ia fiber began to construct the spiral ending, and by the 40th postnatal day after weaning, almost all of the Ia fibers of the muscle spindles had complete coiled endings, though the formation still continued in some spindles. The continuous formation of anulospiral endings for a long period after weaning indicates that muscle spindle morphogenesis may be affected by muscle tension in the masseter muscle due to the movement activated after weaning.

Prolonged Degeneration of Muscle Spindles in the Masseter Muscle after Treatment of Developing Mice with the Local Anesthetic Lidocaine Hydrochloride

The effect of lidocaine-HCl on muscle spindles in the masseter muscle of developing mice was investigated. Repeated injections of mice with anesthetic in the short term decreased the diameters of primary endings, intrafusal muscle fibers and outer capsules in the equatorial regions of muscle spindles, and caused a drop in the succinic dehydrogenase activity in intrafusal muscle fibers of the muscle spindles. In addition, the diameters did not recover to the control value even after about 10 weeks following cessation of anesthetic treatment. Thus, the present results suggest that repeated use of lidocaine-HCl in developing animals may cause dysfunction of the skeletal muscles.

Postnatal Development and Aging of Muscle Spindles in the Mouse Masseter Muscle and Effects of a Fine-Grained Diet on Them

Neuromuscular spindles of skeletal muscles of vertebrates are encapsulated stretch receptors with structurally specialized intrafusal muscle fibers, innervated with sensory and fusimotor nerve endings (Barker and Cope, 1962; Boyd, 1962; Matthews, 1964; Barker et al., 1972). There are many detailed studies on the development of muscle spindles in several animal species and man (Sutton, 1915; Tello, 1917, 1922; Cuajunco 1927, 1940; Hewer, 1935; Zelena, 1957, 1959, 1976; Mavrinskaia, 1960; Bowden, 1963; Schiaffino and Pierobon Bormioli, 1976). Those earlier works showed that the most important event during the development of muscle spindles was the arrival of the sensory nerve fibers into the muscle, and the event occurred in the early stage of gestation. Muscle spindle formation is initiated by the primary afferent nerve contact with a bundle of developing myotubes. In some species, muscle spindle formation is completed during the intrauterine period, but in rats the onset of muscle spindle differentiation is observed to occur at a late stage of gestation (Zelena, 1957, 1959; Marchand and Eldred, 1969; Landon, 1972; Milburn, 1973); and the formation of rat muscle spindles is completed early postnatally.

Postnatal Development of Muscle Spindles and Extrafusal Muscle Fibers in the Mouse Temporal Muscle and Dietary Effect

In 1985, Maeda et al. found that the formation of annulospiral endings in the masseter muscle of developing mice continued after weaning and was completed by the 50th postnatal day. Moreover, Maeda et al. (1987) reported that a long-term intake of a fine-grained diet resulted in degeneration of annulospiral endings of muscle spindles in the masseter muscle of mice.