title: Age-related changes and the possible adaptability of rat jaw muscle spindles: immunohistochemical and fine structural studies.
abstract: Afferent signals from jaw muscle spindles contribute to the feedback mechanism that regulates mastication. The integrity and adaptability of this proprioceptor to age-related changes of the surrounding structures are therefore essential to maintain an appropriate masticatory function throughout life. In this study, we examined muscle spindles obtained from temporal and masseter muscles of 10-week-, 12-, 18-, and 24-month-old Wistar rats, employing immunohistochemistry for protein gene product 9.5 (PGP 9.5) or growth-associated protein (GAP-43) in addition to transmission electron microscopy, in order to investigate their morphological changes in relation to the effect of aging on the adaptive potential of the receptors. Immunohistochemistry for PGP 9.5 showed virtually similar reactions at sensory nerve terminals in all age groups. On the other hand, immunoreactivity for GAP-43 in the sensory nerve ending of the muscle spindles was found 2 and 3 weeks after birth but became almost undetectable by 10 weeks. However GAP-43 immunoreactions occasionally reappeared in those of spindles in 12- and 18-month old animals, and vanished again by 24 months of age. Electron microscopic observations also revealed age-related morphological changes in the intrafusal muscle fibers of the rats in 12-month and older groups. The extent of degenerative and/or atrophic alterations of intrafusal fibers increased with age and involved the nerve elements of spindles by 24 months. These findings indicate that the adaptation potential of rat jaw muscle spindles is well preserved until middle age, but diminishes in elderly animals. Structural changes of muscle spindles in elderly animals probably contribute to the deterioration of the muscular function.
authors.affiliation: Orthodontic Science, Department of Orofacial Development and Function, Tokyo Medical and Dental University Graduate School, Tokyo, Japan.
authors.forename: Ladda
authors.initials: L
authors.lastname: Winarakwong
authors.forename: Takeshi
authors.initials: T
authors.lastname: Muramoto
authors.forename: Kunimichi
authors.initials: K
authors.lastname: Soma
authors.forename: Yoshiro
authors.initials: Y
authors.lastname: Takano
mesh_terms: D000367:Age Factors; D000818:Animals; D019922:GAP-43 Protein / Q000378:metabolism*; D007150:Immunohistochemistry; D007568:Jaw / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D008297:Male; D046529:Microscopy, Electron, Transmission; D009470:Muscle Spindles / Q000378:metabolism / Q000648:ultrastructure*; D009475:Neurons, Afferent / Q000378:metabolism / Q000648:ultrastructure; D011859:Radiography; D051381:Rats; D017208:Rats, Wistar; D043222:Ubiquitin Thiolesterase / Q000378:metabolism*
chemical_list: D019922:GAP-43 Protein; C489034:UCHL1 protein, rat; D043222:Ubiquitin Thiolesterase