Fundamental study on humoral factors regulating postnatal development, aging and maintenance of masticatory system

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The various factors are involved in the development, aging and maintenance of the masticatory system. In the present study, we investigated effects of various humoral factors, and those of the lack of the sensory input from the sensory endings in the periodontal ligaments, resulted from tooth-loss on the regulatory system of the jaw movement. We obtained following results and conclusions : early partial loss of dentition induce the inevitable decrease in the sensory input to the Mo5 motoneurons, via Me5 and Su5 from periodontal sensory endings, then might result in the precocious reduction in masticatory motoneurons in the Mo5 that innervate the masseter muscle, and significant effects on the decrease in the number may be easily caused only by the partial tooth-loss during an early limited period in the postnatal development of animals. Moreover, it is reasonably suggested by the present results that a functional decrease in the masseter muscle, due to the atrophy of the extrafusal muscle fibers, secondarily cause the reduction in the number of Me5 neurons innervating muscle spindles in the masseter muscle. The present study also suggests that the degeneration of periodontal afferents may start at a comparatively early period during the postnatal life even in healthy mice. In these mice, we found that various humoral factors including sex hormones and FGFs are involved in aging and the maintenance of the masticatory system.