Time Course Of Innervation Research Articles

Expression of sonic hedgehog, patched, and Gli1 in developing taste papillae of the mouse.

Lingual taste buds form within taste papillae, which are specialized structures that develop in a characteristic spatial and temporal pattern. To investigate the signaling events responsible for patterning and morphogenesis of taste papillae, the authors examined the time course and distribution of expression of several related developmental signaling genes as well as the time course of innervation of taste papillae in mouse embryos from embryonic day 12 (E12) to E18. Lingual expression of the signaling molecule Sonic hedgehog (Shh), its receptor Patched (Ptc), and the Shh-activated transcription factor Gli1 were assayed by using in situ hybridization. Shh is expressed broadly in the lingual epithelium at E12 but becomes progressively restricted to developing circumvallate and fungiform papillary epithelia. Shh is expressed specifically within the central cells of the papillary epithelium starting at E13.5 and persisting through E18. Ptc and Gli1 expression follow a pattern similar to that of Shh. Compared with Shh, Ptc is expressed in larger regions surrounding the central papillary cells and also in the mesenchyme underlying Shh-expressing epithelium. Innervation of taste papillae was examined by using the panneuronal antibody to ubiquitin carboxyl terminal hydrolase (protein gene product 9.5). Nerves reach the basal lamina of developing taste papillae at E14 to densely innervate the papillary epithelium by E16. Thus, the pattern of Shh expression within developing taste papillae is established prior to innervation, ruling out neuronal induction of papillae. The results suggest that the Shh signaling pathway may be involved in: 1) establishing papillary boundaries in taste papilla morphogenesis, 2) papillary epithelial-mesenchymal interactions, and/or 3) specifying the location or development of taste buds within taste papillae.

Chick wing innervation. I. Time course of innervation and early differentiation of the peripheral nerve pattern.

Anterograde transport of horseradish peroxidase was used to map the initial projection patterns of motor and sensory axons innervating the wing of the chick embryo. Injections which resulted in labeling large numbers of motor and sensory axons, separately or in combination, were used to define the time course of innervation and to visualize the progressive morphogenesis of the peripheral nerve pattern. Motor axons emerged from the spinal cord and accumulated near the ventromedial border of the myotome where they remained for up to 16 hours before growing into the plexus region and limb bud. Despite the known later time of sensory neuron production, the first sensory axons projected to the wing at the same time as motor axons. When axons first entered the wing bud, they were distributed in two loosely organized sheets of axon fascicles, one projecting to dorsal muscle mass, the other to ventral muscle mass. The width of the sheets was between one-third to one-half the width of the wing bud, and this distance was more than twice the diameter of the proximal nerve trunks measured at stage 28. In the proximal limb the basic pattern of peripheral nerves emerged gradually from stages 26 to 28. During these stages, the loosely organized sheets of axonal fascicles seen at younger stages were progressively transformed into several coherent nerve trunks and muscle nerves extended from common nerve trunks. The implication of these observations is that many outgrowing axons appear not to follow preformed pathways corresponding to the mature peripheral nerve branching pattern. This pattern may instead result from axonal recognition of cues within a largely undifferentiated limb bud, and from the subsequent bundling together of loosely organized axon fascicles. These events occur concurrently with limb growth and differentiation.

Regeneration of dystrophic muscle following multiple injections of bupivacaine.

Regenerated myofibers formed subsequent to orthotopic transplantation of young, dystrophic mouse muscle fail to display the extensive histopathological changes characteristics of murine dystrophy. In order to determine whether this modification of the phenotypic expression of murine dystrophy is unique to the transplantation system or whether it can be found when other extreme trauma induces dystrophic muscle to regenerate, the extensor digitorum longus muscles of 4-6-week-old normal (129 ReJ +/+) and dystrophic (129 ReJ dy/dy) mice were given two series of injections of the myotoxin bupivacaine, spaced 12 hours apart. These injections resulted in necrosis of approximately 90% of the original myofibers. At 100 days after injection, the regenerated normal muscle appeared "healthy," whereas the regenerated dystrophic muscle displayed histopathological changes. It is suggested that the differences in the time course of innervation of the myotubes in the transplantation system as compared with that in the bupivacaine system may be a factor in determining whether regenerated dystrophic myofibers express a dystrophic morphology.

Developmental time course of the sympathetic postganglionic innervation of the rat eye

The time course of the innervation of the rat iris by sympathetic postganglionic axons from the superior cervical ganglion (SCG) has been studied in postnatal rats. A retrogradely transported fluorescent dye. Fast blue (FB) was injected bilaterally into the anterior eye chamber of rats within 3 h of birth or at 7 or 14 days postnatal. Littermates were killed at 24 h intervals, both SCG removed, fixed and sectioned. A substantial number of sympathetic neurones were labelled retrogradely from the eye at birth. The number of labelled cells tripled over the first postnatal week to reach proportions equivalent to those labelled in 3-week-old rats. The degree of sympathetic fibre ingrowth to the iris was examined at 3 postnatal ages, using fluorescence histochemistry. This method showed a small proportion of catecholamine-containing fibres in the iris at birth with a more extensive fibre density over the iris by 7 days after birth. A further increase in fibre density had occurred by the end of the 3rd postnatal week even though the number of cells projecting to the iris at these two ages was similar. This difference in fibre density may arise from ramification of fibres which have reached the iris by 7 days postbirth.

Competitive interaction between foreign nerves innervating frog skeletal muscle.

1. Competition between two foreign nerves innervating frog skeletal muscle has been studied by using pairs of somatic motor nerves (s.m.n.s) or one s.m.n. and the preganglionic splanchnic nerve (s.p.n.) implanted into a denervated sartorius muscle that has been transplanted to the lymph sac of the back. 2. A single s.m.n. implanted into the muscle succeeded in innervating essentially every fibre within 2--3 months; tetanic stimulation of the nerve elicited 9--100% of the maximal direct tetanus tension. Most of the e.p.p.s were suprathreshold, since a single indirect stimulus evoked a twitch 60--100% as large as that to a direct stimulus. 3. If two s.m.n.s were implanted simultaneously, tetanic stimulation of either elicited 80--100% of the maximal tension to direct stimulation. If one nerve was implanted 2--3 months before the other, the second, although usually less effective than the first, normally innervated 50--100% of the fibres, with approximately the same time course of innervation as a single s.m.n. 4. Mutual synaptic repression was seen on examination of twitch tensions. With either simultaneous or staggered innervation, stimulation of each s.m.n. resulted in a twitch of 30--50% of the total direct twitch tension, with little overlap between the fields driven by the two nerves. Intracellular recordings showed that the distribution of subthreshold and spike-producing e.p.p.s reflected the existence of separate twitch fields. Even if one s.m.n. was implanted several months before the other and had time to establish suprathreshold junctions on most muscle fibres, an s.m.n. implanted later was able to reduce sharply the effectiveness of many junctions from the earlier nerve while itself innervating most muscle fibres. 5. The subthreshold e.p.p.s had low quantal content, typically ten or fewer quanta/e.p.p. The min e.p.p. frequency was very low, while min e.p.p. amplitude appeared to be normal. 6. In the vast majority of muscle fibres, junctions from the two nerves were not within recording distance of each other. Hence, we infer that the competitive interaction was mediated somehow via the muscle fibre. 7. The preganglionic splanchnic nerve, which also successfully reinnervated frog skeletal muscle, competed with a foreign s.m.n. in ways which differ qualitatively from the competition by a second s.m.n. In the presence of a s.m.n., synapses of the s.p.n. were almost universally subthreshold. However, if the s.p.n. was implanted 2--3 months before the s.m.n., the s.m.n. was prevented for several months from innervating fibres driven by the s.p.n. This delay in s.m.n. reinnervation was greater than if the first nerve implanted was also an s.m.n. 8. After 6--8 months of dual innervation by s.m.n. and s.p.n., the s.m.n. became almost totally dominant. However, if the s.m.n. was then sectioned, the s.p.n. became as effective, within approximately 1 week, as it would have been in the absence of the s.m.n.

Innervation by nerve implants of "fast" and "slow" skeletal muscles of the rat.

AbstractThe time course of innervation by nerve implants of the fast contracting flexor digitorum longus and the slow contracting soleus muscles were studied by the recorcling of isometric twitch tensions in response to stimulation of the implanted nerve and by histochemical visualization of newly‐formed end‐plates. When a foreign motor nerve was implanted into a muscle 30 days prior to sectioning of the original nerve supply, synaptic contacts were established 4–8 days after the section of the nerve, while, with simultaneous implantation and denervation, 16 days were required for the implanted nerve to innervate the muscle. Implantation of the nerve which normally innervates the flexor digitorum longus muscle into the soleus altered the contraction and relaxation speeds of the soleus muscle towards those of a fast contracting muscle, the change being apparent as soon as 4–8 days following sectioning of the original nerve. In muscles with nerve implants and intact normal innervation hypertrophy occurred, amounting to 20–30 per cent within one month.