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Peripheral nerve regeneration comprises the formation of axonal sprouts, their outgrowth as regenerating axons and the reinnervation of original targets. This review focuses on the morphological features of axonal sprouts at the node of Ranvier and their subsequent outgrowth guided by Schwann cells or by Schwann cell basal laminae. Adhesion molecules such as N-CAM, L1 and N-cadherin are involved in the axon-to-axon and axon-to-Schwann cell attachment, and it is suggested that integrins such as alpha 1 beta 1 and alpha 6 beta 1 mediate the attachment between axons and Schwann cell basal laminae. The presence of synaptic vesicle-associated proteins such as synaptophysin, synaptotagmin and synapsin I in the growth cones of regenerating axons indicates the possibility that exocytotic fusion of vesicles with the surface axolemma supplies the membranous components for the extension of regenerating axons. Almost all the subtypes of protein kinase C have been localized in growth cones both in vivo and in vitro. Protein kinase C and GAP-43 are implicated to be involved in at least some part of the adhesion of growth cones to the substrate and their growth activity. The significance of tyrosine kinase in growth cones is emphasized. Tyrosine kinase plays an important role in intracellular signal transduction of the growth of regenerating axons mediated by both nerve trophic factors and adhesion molecules. Growth factors such as NGF, BDNF, CNTF and bFGF are also discussed mainly in terms of the influence of Schwann cells on regenerating axons.

Ultrastructural localization of protein kinase C (PKC) beta-subspecies in neuromuscular junctions of the rat lumbrical muscle was investigated by the immunoperoxidase and immunofluorescence methods. By light microscopy, PKC beta-like immunoreactivity (PKC beta-LIR) was found in the axon terminal expansions as well as in the preterminal axons. By confocal laser scanning microscopy, the staining for PKC beta-like immunoreactivity was more intense in the presynaptic regions just in contact with the acetylcholine receptor stained by FITC-alpha-bungarotoxin. By electron microscopy, PKC beta-like immunoreactivity was distributed non-uniformly in the terminal expansions. In the terminal expansions, PKC beta-like immunoreactivity was accumulated in the presynaptic regions in contact with the post-synaptic folds. This accumulation was approximately 0.1-0.2 microns in diameter, which comprised a part of the presynaptic plasma membrane and a group of synaptic vesicles adjacent to it. Weak immunoreactivity was also found diffusely in the axoplasmic matrix. The discrete presynaptic accumulation of PKC beta-subspecies may represent the strategical localization specialized for the effective regulation of neurotransmitter release.

Recent studies suggest that the soluble N-ethylmaleimide-sensitive factor attached protein (SNAP) receptor (SNARE)-mediated membrane fusion system is involved in vesicle fusion in the plasma membrane that allows expansion for neurite elongation. There have been several reports analyzing the effects of neurite outgrowth by inhibition of SNAREs. In this study, we took the opposite approach by overexpressing green fluorescent protein (GFP)-fusion SNAREs, including VAMP-2, SNAP-25A, and syntaxin1A, in PC12 cells to investigate the role of SNAREs in the neurite outgrowth of PC12 cells. Neurite outgrowth analysis demonstrated that: (1) GFP-VAMP-2 increased the length of individual neurites, without changing the number of neurites per cell; (2) GFP-SNAP-25A increased the number of neurites per cell, with no change in the length of the individual neurites. In both cases, the total length of neurites per cell was increased; (3) GFP-syntaxin1A resulted in no significant change, either in neurite length, or in the number of neurites per cell. These findings suggest that when overexpressed in PC12 cells, VAMP-2 can promote neurite elongation, while SNAP-25A can stimulate neurite sprouting. On the other hand, overexpression of syntaxin1A neither promotes nor inhibits neurite outgrowth. Thus VAMP-2 and SNAP-25A play different roles in neurite elongation and sprouting.

Excimer laser (KrF excimer laser, 248 nm wavelength) was used to damage cellular components in the dorsal funiculus at the lumbar level (L2) of the rat spinal cord. An open lesion was not found at the irradiation site on the spinal cord. However, the cytological examination revealed that cellular components were damaged to the depth of 200-500 microm from the pial surface. The characteristic feature was that at the border of the lesion, many axons remained naked but intact after their myelin sheaths had been completely disintegrated. Such naked axons were subsequently remyelinated by mature or immature glial cells. Mature oligodendrocytes, while retaining their cytoplasmic processes connected with the myelin sheaths of unaffected axons, extended new cytoplasmic processes on nearby naked axons and made new myelin sheaths around them. In contrast, 7 days after the irradiation, numerous immature glial cells appeared in association with naked axons, and some of them were differentiated into oligodendrocytes forming thin myelin sheaths on naked axons. These findings suggest that demyelinated axons can cause the proliferation and probably dedifferentiation of the oligodendrocyte lineage. The use of lasers provides a unique experimental model of demyelination and remyelination in the central nervous system of adult mammals.

Summary A small piece of muscle tissue, 6–7 mm long, was excised from the biceps femoris of the mouse and treated several times by repetitive freezing and thawing to kill the muscle cells. The tissue so treated was then placed, end to end, in contact with the proximal stump of the transected sciatic nerve of the same animal. The growth of the regenerating nerve fibers through the graft was examined 2, 3, 5, 7, 10, 17 and 25 days after grafting. Two to 5 days after grafting, dead muscle cells had disintegrated and were phagocytized by macrophages, leaving behind the basal laminae in a tube form (basal lamina scaffold) of the original muscle fibers. Seven days after grafting, regenerating nerve fibers were first observed to have grown out through such basal lamina scaffolds in the region near the proximal end of the graft. Ten days after grafting, such regenerating nerve fibers and accompanying Schwann cells became numerous in the basal lamina scaffolds when observed at the middle level of the graft. Few regenerating nerve fibers were found outside the scaffolds, i.e. in the connective tissue compartment. Seventeen days after grafting, regenerating fibers were sorted out and some of the thick fibers were myelinated. Twenty-five days after grafting, the myelination was more advanced, and regenerating fiber bundles in the basal lamina scaffolds were, as a whole, surrounded by cells resembling perineural cells. These findings show that basal lamina scaffolds of muscle cells provide a proper microenvironment for the growth of regenerating nerve fibers. This is discussed from the point of view of the general significance of the basal laminae for nerve regeneration.

The present report reviews the physiological and morphological specializations of Pacinian corpuscles and other mechanoreceptors that are present in the skin and connective tissues of the body as well as the cochlea. The remarkable sensitivity of Pacinian corpuscles is such that the only form of mechanical energy that could be perceived by a Pacinian corpuscle is a sound wave. In fact the human finger as demonstrated by Munger and Ide (1987) can perceive sound waves when water is the coupling agent. The structural specializations are equally remarkable with extensive membrane specializations of both the inner core and inner portion of the outer core. The halves of the inner core are each coupled with gap junctions and the inner portion of the outer core joined with numerous tight junctions. The cleft regions have specializations involving the axolemma that consist of numerous axonal spines containing bundles of filaments projecting into the cleft of the inner core. These structural specializations are thought to represent specializations for mechano-electric transduction analogous in many respects to the hair cells of the cochlea. A hypothesis for mechano-electric transduction is presented that may account in part for the extreme sensitivity of Pacinian corpuscles and other mechanoreceptors.