Glial outgrowth and central-type myelination of regenerating axons in spinal nerve roots following transection and suture: light and electron microscopic study in the pig

Neural reconstruction methods of restoring bladder function

During the past century, diverse studies have focused on the development of surgical strategies to restore function of a decentralized bladder after spinal cord or spinal root injury via repair of the original roots or by transferring new axonal sources. The techniques included end-to-end sacral root repairs, transfer of roots from other spinal segments to sacral roots, transfer of intercostal nerves to sacral roots, transfer of various somatic nerves to the pelvic or pudendal nerve, direct reinnervation of the detrusor muscle, or creation of an artificial reflex pathway between the skin and the bladder via the central nervous system. All of these surgical techniques have demonstrated specific strengths and limitations. The findings made to date already indicate appropriate patient populations for each procedure, but a comprehensive assessment of the effectiveness of each technique to restore urinary function after bladder decentralization is required to guide future research and potential clinical application.

The transitional zone and CNS regeneration

Most nerves are attached to the neuraxis by rootlets. The CNS-PNS transitional zone (TZ) is that length of rootlet containing both central and peripheral nervous tissue. The 2 tissues are separated by a very irregular but clearly defined interface, consisting of the surface of the astrocytic tissue comprising the central component of the TZ. Central to this, myelin sheaths are formed by oligodendrocytes and the supporting tissue is astrocytic. Peripheral to it, sheaths are formed by Schwann cells which are enveloped in endoneurium. The features of transitional nodes are a composite of those of central and peripheral type. The interface is penetrated only by axons. It is absent at first. It is formed by growth of processes into the axon bundle from glial cell bodies around its perimeter. These form a barrier across the bundle which fully segregates prospectively myelinated axons. Rat spinal dorsal root TZs have been used extensively to study CNS axon regeneration. The CNS part of the TZ responds to primary afferent axon degeneration and to regenerating axons in ways which constitute a satisfactory model of the gliotic tissue response which occurs in CNS lesions. It undergoes gliosis and the gliotic TZ tissue expands distally along the root. In mature animals axons can regenerate satisfactorily through the endoneurial tubes of the root but cease growth on reaching the gliotic tissue. The general objective of experimental studies is to achieve axon regeneration from the PNS through this outgrowth and into the dorsal spinal cord. Since immature tissue has a greater capacity for regeneration than that of the adult, one approach includes the transplantation of embryonic or fetal dorsal root ganglia into the locus of an extirpated adult ganglion. Axons grow centrally from the transplanted ganglion cells and some enter the cord. Other approaches include alteration of the TZ environment to facilitate axon regeneration, for example, by the application of tropic, trophic, or other molecular factors, and also by transplantation of cultured olfactory ensheathing cells (OECs) into the TZ region. OECs, by association with growing axons, facilitate their extensive regeneration into the cord. Unusually, ventral motoneuron axons may undergo some degree of unaided CNS regeneration. When interrupted in the spinal cord white matter, some grow out to the ventral rootlet TZ and thence distally in the PNS. The DRTZ is especially useful for quantitative studies on regeneration. Since the tissue is anisometric, individual parameters such as axon numbers, axon size and glial ensheathment can be readily measured and compared in the CNS and PNS environments, thereby yielding indices of regeneration across the interface for different sets of experimental conditions.

The ability of human Schwann cell grafts to promote regeneration in the transected nude rat spinal cord

Advances in the purification and expansion of Schwann cells (SCs) from adult human peripheral nerve, together with biomaterials development, have made the construction of unique grafts with defined properties possible. We have utilized PAN/PVC guidance channels to form solid human SC grafts which can be transplanted either with or without the channel. We studied the ability of grafts placed with and without channels to support regeneration and to influence functional recovery; characteristics of the graft and host/graft interface were also compared. The T9-T10 spinal cord of nude rats was resected and a graft was placed across the gap; methylprednisolone was delivered acutely to decrease secondary injury. Channels minimized the immigration of connective tissue into grafts but contributed to some necrotic tissue loss, especially in the distal spinal cord. Grafts without channels contained more myelinated axons (x = 2129 +/- 785) vs (x = 1442 +/- 514) and were larger in cross-sectional area ( x = 1.53 +/- 0.24 mm2) vs (x = 0.95 +/- 0.86 mm2). The interfaces formed between the host spinal cord and the grafts placed without channels were highly interdigitated and resembled CNS-PNS transition zones; chondroitin sulfate proteoglycans was deposited there. Whereas several neuronal populations including propriospinal, sensory, motoneuronal, and brainstem neurons regenerated into human SC grafts, only propriospinal and sensory neurons were observed to reenter the host spinal cord. Using combinations of anterograde and retrograde tracers, we observed regeneration of propriospinal neurons up to 2.6 mm beyond grafts. We estimate that 1% of the fibers that enter grafts reenter the host spinal cord by 45 days after grafting. Following retrograde tracing from the distal spinal cord, more labeled neurons were unexpectedly found in the region of the dextran amine anterograde tracer injection site where a marked inflammatory reaction had occurred. Animals with bridging grafts obtained modestly higher scores during open field [(x = 8.2 +/- 0.35) vs (x = 6.8 +/- 0.42), P = 0.02] and inclined plane testing (x = 38.6 +/- 0. 542) vs (x = 36.3 +/- 0.53), P = 0.006] than animals with similar grafts in distally capped channels. In summary, this study showed that in the nude rat given methylprednisolone in combination with human SC grafts, some regenerative growth occurred beyond the graft and a modest improvement in function was observed.

Interaction of regrowing PNS axons with transplanted aggregates of cultured CNS glia in vivo

Aggregates of cultured neonatal mouse cerebellar astrocytes were implanted into adult mouse sciatic nerves. Two different experimental models were used: aggregates were either placed between proximal and distal stumps of totally transected nerves, or were placed in gaps in partially transected nerves in direct apposition with the cut surface of the proximal stumps. In the model where aggregates were not placed in contact with the proximal stump, regrowing axons rarely entered the aggregates. Where aggregates were placed in contact with the proximal stumps, axons entered the astrocyte-rich environment. Experimental depression of the supply of Schwann cells available to comigrate with regenerating axons proved to be unnecessary: astrocytes provided an alternative substrate for axons. Some axons became myelinated by oligodendrocytes which differentiated within the aggregates; however, few axons remained, unmyelinated, in long-term association with the transplanted astrocytes.

A comparison of the regeneration potential of dorsal root fibers into gray or white matter of the adult rat spinal cord

To assess the role of white matter inhibition as a barrier to neurite outgrowth in vivo, we unilaterally transected three consecutive lumbar dorsal roots (L4-L6), incised the spinal cord, and transplanted the peripheral stump of L4 either medially onto the white matter of the dorsal columns or laterally, just superficial to the gray matter of the dorsal horn at the level of L5. Three weeks to seven months later, the translocated root was retransected, and its central stump was anterogradely labeled with HRP. The staining pattern demonstrated that regenerating sensory axons had entered the spinal cord from both medially and laterally placed roots. Axonal staining from medially placed dorsal roots (onto the white matter of the dorsal columns) was sparse and limited to the white matter. Staining of laterally placed roots revealed a small subpopulation of regenerating axons which had entered the gray matter and formed terminal arbors. Successful axonal regeneration into the gray matter, albeit minimal, was associated with a localized and limited inflammatory response near the sites of axonal ingrowth.

The response of regenerating peripheral neurites to a grafted optic nerve

Optic nerves, both viable (fresh or pre-degenerate) or non-viable (frozen-thawed) were grafted between the proximal and distal stumps of freshly transected sciatic nerves, using either 10/0 sutures or strips of nitrocellulose paper. The majority of regenerating peripheral neurites, always in association with Schwann cells, avoided the viable optic nerve grafts, growing along the outside of the grafts in well vascularized minifascicles until they gained the distal stumps. A very small number of axons entered the grafts and grew, for distances typically less than 2 mm, between layers of astrocyte processes. The number of axons entering was not increased by using predegenerate grafts or by blocking Schwann cell proliferation in the proximal stumps by pre-treating the latter with mitomycin C. There was no evidence of a continuous cellular-acellular partition between graft and host during the outgrowth phase of the neurites: it was concluded that axons failed to enter the grafts as a result of inhibitory interactions between Schwann cells and astrocytes. When grafts were rendered acellular, all structured debris, including recognizable components of the extracellular matrix, was rapidly removed and the space thus vacated was invaded by manifascicles of Schwann cells and regenerating neurites. Glial fibrillary acidic protein-positive astrocytes and carbonic anhydrase II-positive oligodendrocytes persisted within viable grafts for 17 months; they did not migrate into the surrounding nerve.

Glial bundle formation in spinal roots following experimental neuronopathy

Spinal motor neurons and dorsal root ganglion cells were caused to degenerate by axoplasmic transport of toxic lectins via the sciatic nerve. Within a few months, loss of axons and glial bundle formation were seen in the proximal portions of the L4-6 ventral and dorsal roots. The glial bundles observed were structurally the same as those described in humans, but they were occasionally also invested with Schwann cell cytoplasmic processes, the cell membrane of which was directly apposed to that of astrocytes. Basal lamina surrounding the outer surface of the bundles was shared by these two as a continuous sheet. This unique complex of astroglial and Schwann cell processes, completely covered by basal lamina, may be a newly formed interface between the central and peripheral nervous systems and thus may severe as a guide and potential pathway for regenerating neurites.

Migration of host astrocytes into superior cervical sympathetic ganglia autografted into the septal nuclei or choroid fissure of adult rats

Adult astrocytes and their processes, identified by glial fibrillary acidic protein immunohistochemistry and by electron microscopy, migrate into superior cervical ganglia auto-transplanted into the choroid fissure or septal nuclei of adult rats. Migration routes were along the blood vessels, and along the Schwann cell bundles of the transplant. Ultrastructurally, astrocytic processes could be seen to lie in direct contact with Schwann cell processes within the basal lamina enwrapping the Schwann cell and its associated axons. Around the region of the host/transplant interface, the astrocytes were transformed into flattened cells with many short, irregular, fringe-like processes, but within the depths of the transplant mass they resumed a more stellate configuration. Glial fibrillary acidic protein immunoreactivity was present within the intrinsic satellite and Schwann cells of the grafted ganglia, but at a much lower level than in the host astrocytes. It is concluded that reactive astrocytes from adult host central nervous system migrate into peripheral ganglionic transplants, where they differentiate and establish organized arrangements with the ganglionic elements.

Glial bundles in spinal nerve roots. An immunocytochemical study stressing their nonspecificity in various spinal cord and peripheral nerve diseases

Glial bundles (GBs) in spinal nerve roots in 86 autopsy cases with various spinal lesions were examined using the peroxidase-antiperoxidase technique for glial fibrillary acidic protein (GFAP). In 19 of 22 cases of Werding-Hoffmann disease (WHD), GBs were present in the anterior roots (ARs) but absent in the youngest age group (age less than 1.5 months at death). GBs were numerous in classical cases (age 3-24 months), accompanying severe damage of the anterior horns and roots, but were less prominent in most cases of protracted course (age 2-8.5 years). Thus, development of GBs in the ARs of motor neuron disease at a young age seems to depend on the clinical type (age at onset and disease duration) and degree of damage to motor neurons and ARs. Varying numbers of GBs were found also in the posterior roots (PRs) of 12 cases of WHD. In 13 patients with amyotrophic lateral sclerosis (ALS), few GBs were observed in the ARs of two and PRs of five cases without apparent relation to other clinicopathologic data. GBs in the PRs of both WHD and ALS might indicate spreading of the degenerative process to sensory neurons despite the absence of pathology detectable by routine histological stains. Numerous GBs were found also in adults affected with polymyelitis in childhood. Varying numbers of GBs were present, however, in many different diseases, such as Friedreich ataxia, Guillain-Barré syndrome, various polyneuropathies, cervical spondylosis, ataxia telangiectasia, metachromatic leukodystrophy, and Leigh syndrome.(ABSTRACT TRUNCATED AT 250 WORDS)

Glial bundles in spinal nerve roots: a form of isomorphic gliosis at the junction of the central and peripheral nervous system

A morphologic study of the spinal nerve roots was undertaken in three cases of Werdnig-Hoffmann disease to investigate the phenomenon of glial bundle formation. The glial elements extended along the ventral roots as discrete cylindrical bundles comprising a large number of parallel astrocytic processes and sparsely scattered cell bodies all enclosed by a basal lamina. The bundles tapered off at a variable distance from the root exit zones. The early stage of glial bundle formation was characterized by the protrusion of astrocytes into the neurilemmal tubes containing degenerated myelinated axons. It was concluded that axonal degeneration, evoking a glial reaction, was the initial event in this process. Subsequently, the reactive astrocytes from the vicinity of the root exit zones enter the neurilemmal tubes previously occupied by myelinated axons and migrated into the domain of the peripheral nervous system in an orderly fashion. Thus glial bundle formation might be considered a special form of isomorphic gliosis occurring in Werdnig-Hoffmann disease and also in several other conditions all sharing a common feature, namely, degeneration of axons within the spinal nerve roots.

Scanning electron microscopic observations of normal and regenerating nerve roots in the cat

The surface morphology of normal and regenerated nerve roots was studied using correlated scanning and transmission electron microscopic methods. Nerve roots of the cauda equina were either cut and rejoined or crossed from a segment above to a segment below. Good regeneration was observed in both experimental procedures. The regenerated nerve root sheath had alterations in surface structure created by extensive growth of collagen. Despite this collagen formation, regenerated axons crossed the anastomotic site with relative ease. Surface features of the regenerated axons were similar in appearance to those of the normal axon. Schwann cells were easily recognized, as were the collagen fibers of the endoneurium, although the endoneurium was more prominent and occupied more of the interaxonal space. Macrophages were identified as round structures with a laminated surface or as a honeycomb structure. Internal features of the regenerating axons were more difficult to identify, but mitochondria and a fibrous network were observed. These studies have demonstrated the application of scanning electron microscopic methods to visualize surface structures and cells in regenerated nerve roots.

The central-peripheral transition zone of cervical spinal nerve roots in Jimpy mutant and normal mice. Light- and electron-microscopic study

Comparative morphological and ultrastructural investigations on the cervical dorsal and ventral central-peripheral transition zones (CPTZs) of Jimpys and control mice have been performed at early and advanced myelination stages. After postnatal development a characteristic cone-shaped glial outgrowth extends into the proximal part of the dorsal roots, while the ventral roots exhibit short Schwann cell and peripheral nervous tissue invaginations into the spinal cord at the ventral root-spinal cord junction in both animal groups. In Jimpys, although there is marked central myelin deficiency and absence of oligodendroglial development on the CNS side, the normal general aspect of the CPTZs is maintained. Previously postulated astrocytic and neuroaxonal abnormalities in the mutants do not alter the central-peripheral borderline, and Schwann cell migration from the spinal nerve roots into the cord does not occur.

Glial outgrowth along spinal nerve roots in amyotrophic lateral sclerosis

Formation of glial bundles in the proximal portion of the ventral nerve roots is described in a 51-year-old patient with the sporadic form of amyotrophic lateral sclerosis (ALS). Although the bundles were relatively fewer, they were identical in morphology and distribution to those consistently found in Werdnig-Hoffmann disease (WHD). The occurrence of glial bundles in ALS, albeit rare, indicates that this phenomenon is not a unique feature of WHD. Similar changes have been observed in several other unrelated conditions, always in association with degeneration of neurons or axons. Thus, outgrowth of astrocytes in the form of glial bundles should be considered a special type of astrocytic reaction at the interface of the central and peripheral nervous systems.

Congenital hypomyelination neuropathy: glial bundles in cranial and spinal nerve roots

Autopsy examination of a 3 1/4-year-old child with a severe congenital hypomyelination neuropathy showed the anterior spinal nerve roots and motor cranial nerves to be almost devoid of myelin in their subarachnoid course. The posterior spinal nerve roots and peripheral nerves were less severely affected. Onion bulb formation was minimal and was present only in the sural nerve. There was extensive glial overgrowth in cranial nerves and spinal nerve roots adjacent to the brainstem and spinal cord. The extent and severity of glial overgrowth were similar to that described in Werdnig-Hoffmann disease and morphologically appeared as glial bundles. These glial bundles are most likely secondary to chronic myelin and axonal damage.

Growth of nerve-cell body and myelinogenesis in mouse trigemnal ganglion and root: a combined cytofluorometric and morphometric study

Postnatal growth of mouse trigeminal ganglion cells and myelinogenesis in the central and peripheral portions of the trigeminal root were studied in animals aged 0-120 days. The trigeminal ganglion cells were dispersed into single cell suspensions. The growth of individual nerve cells was quantitated by measuring total protein content with a new cytofluorometric method based on o-phthaldialdehyde binding to cells fixed in a mixture of ethanol and acetic acid. White matter from the C.N.S. protrudes from the brainstem into the trigeminal root, comes into direct contact with the P.N.S. in a transitional region. C.N.S. and P.N.S. and myelinogenesis were studied in the same population of trigeminal sensory nerve fibres. Myelinogenesis was quantitated at the ultrastuctural level by morphometric techniques. A prominent peak in nerve cell body growth occurred between 3 and 6 days. Myelinogenesis in terms of established contacts between axons and their myelinating cells started at the same time in C.N.S. and P.N.S. and the transformation from nonmyelinated to promyelinated and myelinated fibres occurred concurrently in the central and peripheral parts of the trigeminal root. The growth of the myelin sheath, that is, the addition of myelin lamellae, was faster and more intense in P.N.S. than in C.N.S. This could reflect the fact that a Schwann cell myelinates only one internode, whereas an oligodendrocyte provides myelin for several internodes in different axons. These results support the concept of a common 'signal' for myelinogenesis in C.N.S. and P.N.S.

Neuropathology of "spinning syndrome" induced by prenatal intoxication with a PCB in mice

A striking motor dysfunction, "spinning syndrome," developed with a high frequency in weaning mice whose dams received oral 3,4,3',4'-tetrachlorobiphenyl (4-CB) during gestation (day 10 through day 16). The syndrome is permanent and is characterized by swift circling movements sustained in one direction at a minimal rate of 40 turns/min (usually 50 to 150 turns/min), restlessness, and hyperkinesia. Twenty-four spinners and 4-CB nonspinners and age-matched controls were subjected to histopathologic, histofluorescent, histochemical, and electron microscopic studies. The most reliable histopathologic marker for prenatal 4-CB injury to the CNS is the presence of cylindrical CNS peninsulas (CCPs) in the spinal and cranial nerve roots. The CCPs consist of either CNS-type myelinated fibers, unmyelinated fibers, or astroglial bundles in varying proportions, and are enclosed by a basement membrane. The CCPs are also observed in 4-CB nonspinners but in none of 12 controls studied. A selective defect in synaptogenesis induced prenatally by 4-CB is proposed as the primary event pursuant to the development of the CCPs, while interference with synaptogenesis may have occurred selectively in the striatonigral dopaminergic system. This is suggested by electron microscopy on the nucleus accumbens and also by the responses to administration of dopaminergic agonists and antagonist. The 4-CB induced clinico-pathologic anomaly may serve as a singular model for understanding human neurologic disorders, in particular, Werdnig-Hoffmann disease and minimal brain dysfunction syndrome.

Studies on the control of myelinogenesis. 3. Signalling of oligodendrocyte myelination by regenerating peripheral axons

Continuing from earlier work which demonstrated the peripheral axonal regulation of Schwann cell myelination, this study has investigated the possibility that a peripheral axon can stimulate oligodendrocyte myelination. To test this hypothesis, regenerating PNS axons were allowed to interact with uncommitted oligodendrocytes by transecting a rat peroneal nerve and inserting a segment of the autologous optic nerve between the cut ends. Grafts were maintained for 4-28 weeks and then examined by light and electron microscopy. A few regenerating peripheral myelinated nerve fibers penetrated the optic nerve graft. Some axons penetrated the outer margin of the graft, were myelinated by Schwann cells, and surrounded by astrocyte processes bordered by basal lamina. More centrally in the optic nerve graft, regenerating peripheral axons displayed myelin of CNS type. The outer myelin lamella abutted directly on the plasmalemma surface of surrounding astrocytic processes and was expanded focally to form a glial tongue. These observations demonstrate the experimental induction of central myelination by regenerating peripheral axons and suggest the existence of a common neuronal mechanism to stimulate myelin formation by both the Schwann cell and the oligodendrocyte.

Ensheathment and myelination of regenerating PNS fibres by transplanted optic nerve glia

Interactions between PNS axons and CNS glia were studied morphologically by transplanting optic nerves into peripheral nerves in groups of rats and mice. Four to 11 months after grafting, small numbers of axons from the peripheral nerves had penetrated the CNS grafts where they became ensheathed and myelinated by CNS glia. Glial protuberances observed at the CNS-PNS interfaces suggested that there had been an active glial response to innervation by PNS axons. These findings provide experimental evidence that denervated CNS glia can be reinnervated and form myelin.