Masayoshi Kumegawa

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.