Morphological Analysis of the Myelinated Parent Axons that Innervate Rat Upper Molar Pulps in the Trigeminal Ganglion

Previous studies suggested that myelinated axons innervating rat molar pulps undergo morphological changes in their peripheral course. However, little information is available on the morphological feature of the parent axons at the site of origin. We therefore investigated the size of the myelinated parent axons and their morphological features at the proximal sensory root of the trigeminal ganglion by horseradish peroxidase (HRP) injection into rat upper molar pulps and subsequent light and electron microscopy. A total of 248 HRP-labeled myelinated axons investigated were highly variable in the size. Fiber area, fiber diameter, axon area (axoplasm area), axon diameter (axoplasm diameter), and myelin thickness were 11.32 +/- 8.36 microm2 (0.80~53.17 microm2), 3.99 +/- 1.53 microm (1.08~9.26 microm), 8.70 +/- 6.30 microm2 (0.70~41.83 microm2), 3.13 +/- 1.13 microm (0.94~7.20 microm) and 0.43 +/- 0.23 microm (0.07~1.06 microm), respectively. The g-ratio (axon diameter / fiber diameter) of the labeled axons was 0.79 +/- 0.05 (0.61~0.91). Axon diameter was highly correlated with myelin thickness (correlation coefficients,r=0.83) but little correlated with g-ratio (r=-0.33) of individual myelinated parent axons. These results indicate that myelin thickness of the myelinated parent axons innervating rat molar pulps increase with increasing axon diameter, thus maintaining a constant g-ratio.

Ultrastructural analysis of low-threshold mechanoreceptive vibrissa afferent boutons in the cat trigeminal caudal nucleus / 대한해부학회지

Ultrastructural parameters related to synaptic release and their correlation with synaptic connectivity were analyzed in the low-threshold mechanoreceptive vibrissa afferent boutons in laminae III and IV of the trigeminal caudal nucleus (Vc). Rapidly adapting vibrissa afferents were intra-axonally labeled, and quantitative ultrastructural analyses with serial sections were performed on the labeled boutons and their presynaptic endings (p-endings). The volume of the labeled boutons was widely distributed from small to large ones (0.8~12.3 microm3), whereas the p-endings were small and uniform in size. The volume of the labeled boutons was positively correlated with the ultrastructural parameters such as mitochondrial volume (correlation coefficient, r=0.96), active zone area (r=0.82) and apposed surface area (r=0.79). Vesicle density (r=-0.18) showed little correlation to the volume of labeled boutons, suggesting that the total vesicle number of a bouton is proportional to its volume. In addition, the bouton volume was positively correlated with the number of p-endings (r=0.52) and with the number of dendrites postsynaptic to the labeled bouton (r=0.83). These findings suggest that low-threshold mechanoreception conveyed through vibrissa afferents is processed in a bouton size-dependent manner in the Vc, which may contribute to the sensory-motor function of laminae III/IV in Vc.

Morphological analysis of developmental changes in soma area of digastric motoneurons in the rat trigeminal motor nuclei

To analyze the developmental changes in soma diameters of digastric motoneurons, wheat-germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was injected into the digastric muscle and visualized the retrogradely HRP-labeled motoneurons through tungstate/tetramethylbenzidine (TMB) and following diaminobenzidine (DAB) reactions. The results obtained from Sprague-Dawley rats at postnatal days 1 (P1), 10 (P10) and 30 (P30) indicated as follows: firstly, soma diameters of digastric motoneurons showed unimodal distribution in all postnatal days examined; secondly, the period of P1 to P10 (period 1) showed about 2 times faster growth rate than that of P10 to P30 (period 2); thirdly, the smallest soma examined in each postnatal day exhibited slower growth rate with that of the largest one (increase ratio in soma diameters from P1 to P30, smallest vs. largest =1.62 : 1.93); Finally, relative growth rates a day showed again that period 1 had faster growth rate than that of period 2. Consequently, developmental changes in soma diameters of digastric motoneurons resulted in very different growth rates between both periods. This implies that the growth of the soma is almost completing within P10 and thereafter growing slowly. The period 1 and 2 are corresponding to sucking and sucking/masticatory period, respectively. Therefore present study providing morphological changes in soma diameters of digastric motoneurons suggests that both periods and their different growth rates of the motoneurons in each period may closely be related with each other.