| Accumulating evidences show that a series of changes occurred during the transition from sucking to mastication, including central and periphery organs related to mastication. Neuromuscular junction (NMJ) is a typical cholinergic chemical synapse, and is a key structure for communication between motor neuron and muscles. During different developmental periods and under various neuromuscular activities, NMJ shows high structural and functional plasticity, the mechanism of which has been an area received much attention. However, up to date, the research about the change and plasticity of NMJ during development in masticatory muscles remain scared, and the impacts of food characteristics on the development of NMJ are also unknown. To address that, the NMJ of masseter muscle, which is the main jaw-closing muscle, was chosen, in order to observe the effect of food characteristics on the development of NMJ in masseter muscle, the rats after weaning were fed with soft food or solid food, the changes about morphology, composition, and some related factors of NMJ during the different periods of mastication formation were observed.1. The effects of soft-diet feeding on the development of masster muscle fibrein ratObjective: To observe the impacts of soft diet on the development of masseter muscle fibre.Methods: Forty 18-day-old male rats were equally divided into solid or soft group, each of which was fed with standard solid diet or soft diet containing the same components as the standard pellet. At 3,4, 6, 9 postnatal weeks, the rats were weighted and sacrificed, the superficial masseter muscle were dissected. ATPase staining and NADH-TR staining were selected to evaluate the effects of soft diet on fibre composition and fibre size of masseter muscle fibre.Results:1. Compared with 3 postnatal weeks, the body weight of both groups increased 68.4%, 282%, 532% at 4, 6, and 9 postnatal weeks respectively; there was no significant difference in body weight between two groups.2. The superficial masseter muscle is composed of type 2a and 2b fibre, from 3 to 9 postnatal weeks, the mean cross section area of both fibres in two groups increased dramatically.3. At 3, 4, 6 postnatal weeks, the mean cross section area of type 2a fibre in rat of soft group were significantly smaller than solid group, at 6, 9 postnatal weeks, the mean cross section area of type 2b fibre in rat of soft group were significantly smaller than solid group.4. At 9 postnatal weeks, 2b fibre comprised 62.13% of whole area in powder group in contrast to 55.13% in solid group, powder group showed a significantly higher portion of 2b fibre.Conclusion: soft-diet feeding don't influence the body weight, however, it decreases the mean cross section area of masseter muscle fiber, and enhances the composition of 2b fibre.2. The effects of soft-diet feeding on the development of NMJ of massetermuscle in ratObjective: To observe the effects of soft-diet feeding on the endplate morphology, nicotinic acetylcholine receptor (nAchR) subunit expression, and acetylcholinesterase (AchE) content of rat masseter muscle during development.Methods: Cytofluorescence staining was used to visualize morphology of NMJ, RT-PCR was used to detect the expression of nAchR subunit, and AchE content was analyzed by AchE staining.Results: 1. The endplate morphology of rat masseter muscle gradually became maturedfrom 3 to 9 weeks after birth. The size of endplate increased little from 3 to 4postnatal weeks, but it increased dramatically from 6 to 9 postnatal weeks, thestaining area increased 53.8%, 71.9% respectively.2. At 9 postnatal weeks, the staining area and polygon area of endplate in soft group were only 87.3% and 80.3% of that in solid group respectively, significantly smaller than solid group.3. At 4 postnatal weeks, while no nAchR γ subunit mRNA was detected in masseter muscle in solid group, weak expression still existed in soft group.4. At 4 postnatal weeks, the expression of ε subunit mRNA significantly decreased in both groups compared with 3 postnatal weeks, there was no obvious change from 4 to 9 postnatal weeks, no significant differences were observed between two groups.5. From 3 to 9 postnatal weeks, while the expression of δ subunit mRNA in both groups showed gradual decrease, the soft group decreased more, and became significantly lower than solid group at 9 postnatal weeks.6. From 3 to 6 postnatal weeks, the AchE content in masseter muscle kept increasing, and decreased little at 9 postnatal weeks, no significant differences were observed between two groups.Conclusions: During the formation of mastication, soft diet can affect the morphological maturation of endplate of masseter muscle in rat, lower the size of endplate, and decrease the expression of δ subunit; the replacement of γ subunit by ε subunit is also delayed. These results mean characteristics of food could inflence the normal development of NMJ.3. The effects of soft-diet feeding on the expression of neurotrophins and its receptor mRNA in rat masseter muscle during developmentObjective: To observe the effects of soft diet on the expression of BDNF/TrkB and NT3/ TrkC mRNA in rat masseter muscle during development.Methods: RT-PCR method was chosen to observe the developmental changes of expression of BDNF/TrkB and NT3/TrkC mRNA in rat masseter muscle, and the effects of soft diet were also observed.Results:1. From 3 to 9 postnatal weeks, the expression of NT3 mRNA in both groups decreased, the soft group showed more decrease, and significant lower expression at 6 postnatal weeks.2. From 3 to 9 postnatal weeks, the expression of TrkC mRNA in both groups kept decreasing, no significant differences were observed between two groups.3. At 4 postnatal weeks, the expression of BDNF mRNA in solid and soft group showed 701.9%, 334.1% increase respectively compared with that at 3 postnatal weeks, and the expression in solid group was higher than that in powder group. At 6, 9 postnatal weeks, the expression in both groups decreased, no significant differences were observed compared with that at 3 postnatal weeks.4. At 4 postnatal weeks, the expression of TrkB mRNA in solid group increased significantly compared with that at 3 postnatal weeks, and soft group showed little increase, significantly lower than solid group. At 6, 9 postnatal weeks, the expression of TrkB mRNA in both groups decreased, no significant differences were observed between two groups or compared with that at 3 postnatal weeks.Conclusions: During the critical period of mastication formation (from 3 to 4 postnatal weeks), BDNF/TrkB mRNA increased dramatically, and soft diet could lower their expression, BDNF/TrkB may have a closer relationship with NMJ, NT3 may have a role in the early development of NMJ.
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