Factors influencing the growth of regenerating nerve fibers in culture

Biomimetic materials replicating Schwann cell topography enhance neuronal adhesion and neurite alignment in vitro

It is well established that Schwann cells (SCs) promote and enhance axon guidance and nerve regeneration by providing multiple cues, including extracellular matrix, cell surface molecules, neurotrophic factors and cellular topography. Which of the elements of the complex environment associated with SCs provides the essential information for directed nerve growth is unclear, because, until now, it has been impossible to investigate their contributions individually. Our development of biomimetic materials that replicate the micro- and nanoscale topography of SCs has allowed us to investigate for the first time the role of cellular topography in directing nerve growth. Dorsal root ganglion (DRG) neurons were cultured on flat poly(dimethyl siloxane) (PDMS) and on PDMS replicas with protruding SC topography. Image analysis showed that more neurons adhered to the replicas than to the flat substrates, and that neurite growth on the replicas followed the underlying SC pattern. Neuronal alignment was dependent on cell density. Live SCs derived from the DRG also grew along the replica SC pattern. These results suggest that the combination of micro- and nanoscale topographical cues provided by SCs can influence nerve growth and point toward design parameters for future nerve guidance channels.

Altered cutaneous nerve regeneration in a simian immunodeficiency virus / macaque intracutaneous axotomy model

To characterize the regenerative pattern of cutaneous nerves in simian immunodeficiency virus (SIV)-infected and uninfected macaques, excisional axotomies were performed in nonglabrous skin at 14-day intervals. Samples were examined after immunostaining for the pan-axonal marker PGP 9.5 and the Schwann cell marker p75 nerve growth factor receptor. Collateral sprouting of axons from adjacent uninjured superficial dermal nerve bundles was the initial response to axotomy. Both horizontal collateral sprouts and dense vertical regeneration of axons from the deeper dermis led to complete, rapid reinnervation of the epidermis at the axotomy site. In contrast to the slower, incomplete reinnervation previously noted in humans after this technique, in both SIV-infected and uninfected macaques epidermal reinnervation was rapid and completed by 56 days postaxotomy. p75 was densely expressed on the Schwann cells of uninjured nerve bundles along the excision line and on epidermal Schwann cell processes. In both SIV-infected and uninfected macaques, Schwann cell process density was highest at the earliest timepoints postaxotomy and then declined at a similar rate. However, SIV-infection delayed epidermal nerve fiber regeneration and remodeling of new sprouts at every timepoint postaxotomy, and SIV-infected animals consistently had lower mean epidermal Schwann cell densities, suggesting that Schwann cell guidance and support of epidermal nerve fiber regeneration may account for altered nerve regeneration. The relatively rapid regeneration time and the completeness of epidermal reinnervation in this macaque model provides a useful platform for assessing the efficacy of neurotrophic or regenerative drugs for sensory neuropathies including those caused by HIV, diabetes mellitus, medications, and toxins.

Regrowth of axons in lesioned adult rat spinal cord: promotion by implants of cultured Schwann cells

Highly purified populations of Schwann cells were grafted into lesioned adult rat spinal cord to determine if they promote axonal regeneration. Dorsal spinal cord lesions were created by a photochemical lesioning technique. Schwann cells derived from E16 rat dorsal root ganglia, either elongated and associated with their extracellular matrix or dissociated and without matrix, were rolled in polymerized collagen to form an implant 4-6 mm long which was grafted at 5 or 28 days after lesioning. No immunosuppression was used. Acellular collagen rolls served as controls. At 14, 28 and 90 days and 4 and 6 months after grafting, animals were analysed histologically with silver and Toluidine Blue stains and EM. The grafts often filled the lesion and the host borders they apposed exhibited only limited astrogliosis. By 14 days, bundles of unmyelinated and occasional thinly myelinated axons populated the periphery of Schwann cell implants. By 28 days and thereafter, numerous unmyelinated and myelinated axons were present in most grafts. Silver staining revealed sprouted axons at the implant border at 28 days and long bundles of axons within the implant at 90 days. Photographs of entire 1 micron plastic cross-sections of nine grafted areas were assembled into montages to count the number of myelinated axons at the graft midpoint; the number of myelinated axons ranged from 517-3214. Electron microscopy of implants showed typical Schwann cell ensheathment and myelination, increased myelin thickness by 90 days, and a preponderance of unmyelinated over myelinated axons. Random EM sampling of five Schwann cell grafts showed that the ratio of unmyelinated to myelinated axons was highest (20:1) at 28 days. These ratios implied that axons numbered in the thousands at the graft midpoint. Dissociated Schwann cells without matrix promoted axonal ingrowth and longitudinal orientation as effectively as did elongated Schwann cells accompanied by matrix. There was a suggestion that axonal ingrowth was at least as successful, if not more so, when the delay between lesioning and grafting was 28 rather than 5 days. Acellular collagen grafts did not contain axons at 28 days, the only interval assessed. In sum, grafts of Schwann cells in a rolled collagen layer filled the lesion and were well tolerated by the host. The Schwann cells stimulated rapid and abundant growth of axons into grafts and they ensheathed and myelinated these axons in the normal manner.

Release of neurite outgrowth promoting factors by Helisoma central ganglia depends on neural activity

Identified buccal neurons B5 and B19 from the mollusc, Helisoma trivolvis, were plated into cell culture in order to assay for neurite outgrowth promoting factors released from central ring ganglia. The release and attachment of neurite promoting factors to the substratum of poly-lysine coated dishes could be inhibited by blocking spontaneous bioelectric activity in central ring ganglia used to condition the medium and dishes. Bioelectric activity within neurons in central ring ganglia was assayed by intracellular recording and found to be inhibited by exposure to the sodium channel blocker, tetrodotoxin (TTX; 2 x 10(-5) M), or CoCl2 (10 mM). Neither of these agents appeared to be toxic over a three day period since activity within neurons in central ring ganglia was restored following superfusion with saline. To examine the effect of blocking neural activity on the ability of central ring ganglia to release neurite outgrowth promoting factors, we compared the percentage of neurons that extended processes under 5 different conditions: (1) dishes containing conditioned medium and substrate attached growth factors (Super SAM); (2) dishes with substrate attached growth factors only and defined medium (SAM); (3) dishes containing substrate attached growth factors prepared in the presence of TTX; or (4) CoCl2; and (5) dishes containing unconditioned defined medium. The percentage of neurons extending processes under the 5 conditions were: (1) 71% (n = 32); (2) 51% (n = 33); (3) 14% (n = 37); (4) 15% (n = 47); (5) 0% (n = 40), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenetic changes in the regenerative ability of chick retinal ganglion cells as revealed by organ explants

Whereas mature neurons in the central nervous system (CNS) of birds lack the capability of regenerating axons after injury, embryonic nerve cells are able to do so. The time course of this decline of regenerative ability was investigated in ganglion cells from embryonic chick retinae. Retinal strips from 7- to 19-day-old embryos (E7-E19) were explanted and cultured in vitro. The numbers of retinal ganglion cell (RGC) neurites that had extended during the first 22-23 h incubation, their elongation rates, and morphometric parameters of the growth cones were measured to characterize the regenerative behavior. We observed two periods of decline in neuritic growth: the first from E7 to E9, and another from E14 to E19. The first decrease may reflect a gradually disappearing portion of neurons which produced their axons de novo. The second decline coincides with the major period of synaptogenesis by ganglion cell axons in ovo. The time required for initiation of axonal outgrowth increased, accordingly, from less than 3 h in explants from younger retinae (E7-E16) to 10-12 h for E17 and E19 explants. Axonal elongation rates ranged between 36 micron/h and 56 micron/h (mean values) for E7-E13 explants, but were significantly lower for cells from E14-E19 retinae (13-21 microns/h). Morphologically, neurites and growth cones for RGC explanted before E17 were characterized by their high variability. They possessed more filopodia than mature neurons (E17, E19), fasciculated to a higher degree and branched more frequently. In addition, older neurites appeared "stiffer" and were morphologically simpler.(ABSTRACT TRUNCATED AT 250 WORDS)

NGF enhances neurite regeneration from nerve-transected terminals of young adult and aged mouse dorsal root ganglia in vitro

Dorsal root ganglia with nerve fibers, dissected from 3-month-old young adult mice and 26-month-old aged mice, were embedded in collagen gel. In the young adult mice regenerating neurites appeared from nerve-transected terminals after 12-24 h in culture, regardless of the length of fibers, but in the aged mice, regenerating neurites were first seen after 12-48 h. Administration of nerve growth factor to the explants markedly enhanced the growth of regenerating neurites from both the young adult and aged mice.

Implantation of cultured sensory neurons and Schwann cells into lesioned neonatal rat spinal cord. I. Methods for preparing implants from dissociated cells

Our goal was to devise methods of implanting defined populations of the cellular constituents of peripheral nerve into regions of spinal cord injury. This objective derived from the knowledge that the cellular environment of peripheral nerve is known to be supportive of axon regeneration from both central and peripheral neurons. Two of the constituents of the peripheral nerve environment known to influence axonal growth are the Schwann cell and extracellular matrix (particularly basal lamina), both of which can be obtained in culture. We describe here large-scale methods of establishing purified populations of rat sensory neurons to which purified populations of Schwann cells were added. These essentially monolayer preparations were then scrolled and cut into lengths of proper shape and size to provide implants for sites of spinal cord injury in newborn rats. We also describe methods enabling the addition of leptomeningeal components to the implants; this addition contributes a proliferating population of vascular endothelial cells (identified by immunostaining) to the otherwise vasculature-free neuron/Schwann cell implant. Light and electron microscopic observations were made to characterize the implants. When the implant was ready for use, it contained Schwann cells that were differentiated, i.e., had begun to ensheathe axons and form basal lamina. The use of a medium containing human plasma to foster endothelial cell growth led to increased neurite fasciculation and Schwann cell migratory activity in the outgrowth, particularly when the neurons and Schwann cells were cultured on leptomeninges. The second paper in this series reports the deportment of these implants and their influence on corticospinal tract growth after placement into regions of dorsal column injury in neonatal rats (Kuhlengel et al., J. Comp. Neurol 293:74-91, 1990).

Implantation of cultured sensory neurons and Schwann cells into lesioned neonatal rat spinal cord. II. Implant characteristics and examination of corticospinal tract growth

The purpose of this study was to test the effectiveness of implants derived from peripheral neural tissue to serve as bridges following interruption of the developing corticospinal tract (CST). Implants prepared from purified populations of cultured dorsal root ganglion neurons (DRGNs) and Schwann cells (SCs) (Kuhlengel et al., J. Comp. Neurol. 293:63-73, 1990) were placed into thoracolumbar regions of neonatal rat spinal cord from which a 2-mm length of dorsal columns had been removed by suction. These cords were examined by a number of techniques 10 days to 6 months later. The implants, recognizable by their DRGN content, filled the vacated dorsal columns and survived the longest periods examined. The most effective method to maintain implant position was dorsal placement of collagen-coated Nitex filter. Implants were inserted either at the time of lesioning or 5 days later. The implant survival rate was better (72% vs. 50%) and meningeal scarring was less with immediate implantation, but delayed implantation resulted in better implant-cord fusion and the implant better filled the lesion cavity. DRGN/SC implants became well vascularized without leptomeningeal cells; this may explain why implant survival was not improved with leptomeningeal cell addition. Particularly well-differentiated implants (full extracellular matrix production and myelination) did not fuse as well with cord as did those less well differentiated. The addition of nerve growth factor to the Nitex filter collagen coating led to improved survival of DRGNs in implants. Electron microscopy showed that astrocytes populated the implant-cord junction region and migrated into implants. Typical SCs related to nonmyelinated and myelinated axons were present in implants. Close proximity of astrocytes and central myelin to SCs and peripheral myelin demonstrated good implant integration with cord. Clusters of SCs, astrocytes, and axons, all enclosed within a common basal lamina, were observed in implants. Immunostaining for GFAP and laminin confirmed our microscopy findings that SCs did not migrate from implant into host but that astrocytes left host tissue to enter implants. Neuroanatomical tracing of CST neurons with HRP-WGA showed that labeled fibers were not present in the implant but were fasciculated just beneath in gray matter. These fibers remained clustered in gray matter underneath the ventral dorsal columns caudal to the lesion. In lesioned but not implanted rats, labeled fibers were only diffusely distributed in gray matter. Delayed implantation led to more variation in fasciculation compared with immediate implantation.(ABSTRACT TRUNCATED AT 400 WORDS)

Regeneration in the rat optic nerve after cold injury

In order to examine nerve regeneration under conditions in which the basal laminae of the glial limiting membranes (GLM) and blood vessels were preserved intact, the intraorbital segment of adult rat optic nerve was frozen locally. During the next 3 months, degenerative and regenerative changes in axons and glial cells were observed by light and electron microscopy. On the day after treatment, all the myelinated and unmyelinated axons in the central zone of the lesion were damaged. The astrocyte endfeet of the GLM as well as the blood vessels were extensively disrupted, while their basal laminae were preserved apparently intact as a continuous sheet. Three days after treatment, regenerating axons appeared in the central zone of the lesion. They contained various numbers of clear and dense-cored vesicles as well as some smooth endoplasmic reticulum. The regenerating axons gradually increased in number, especially beneath the pial and perivascular surfaces of the lesion, where an abundance of regenerating axons was found 3 months after treatment. A few of these axons were abnormally remyelinated by oligodendrocytes. In addition to this axonal regeneration through the intraoptic nerve compartment, fine regenerating axons were seen growing out through GLM into the pial connective tissue 3 weeks after treatment. Astrocyte endfeet of the GLM became irregular in contour, protruding in a fern-leaf fashion into the pial connective tissue. Fine naked axons grew out through these protrusions and subsequently increased in number, vigorously growing in large bundles both proximally and distally along blood vessels in the pial connective tissue. Bundles of regenerating axons extended as much as 1.5 mm from the site of the lesion 3 months after surgery. These bundles were covered by thin processes of pial or arachnoidal non-neuronal cells, and the regenerating axons remained unmyelinated. The above findings indicate that under well-nourished conditions, adult mammalian optic nerve exhibits considerable regenerative ability.

Regeneration of axons from the adult rat optic nerve: influence of fetal brain grafts, laminin, and artificial basement membrane

After transection of the optic nerve of adult rats, most of the axons in the proximal stump die and the surviving ones are unable to regenerate into the distal optic nerve. Since the fetal brain has an inherent capacity to regenerate axons, we investigated whether fetal (E16) target regions of optic axons (thalamus and tectum) transplanted to the completely transected optic nerve of adult rats would promote axon regeneration. In control operated rats, axon growth beyond the site of transection was restricted to a few fibers that grew irregularly within the connective tissue scar. By contrast, in grafted animals directed outgrowth of optic axons toward the transplant started at 6 days postoperation (p.o.) and reached its maximum 15 days p.o. and later, when numerous single optic fibers and small axon fascicles had grown toward and into the graft, where they formed arborizations and terminal varicosities. Regenerating optic axons were further advanced than GFAP-positive strands of astroglia that emanated from the proximal optic nerve stump. Laminin immunoreactivity appeared at 6 days p.o. in the zone of reactive astroglia in the terminal part of the optic nerve stump. Later it showed a distribution complementary to the pattern of GFAP immunoreactivity, which it seemd to circumscribe. There was no unequivocal codistribution of laminin immunoreactivity with regenerating axons. In further experiments, target regions from different ontogenetic stages (E14 to neonate and adult) and nontarget regions (E16, cerebral cortex or spinal cord) were grafted to the optic nerve stump. With the exception of the adult grafts, all transplants had effects on axon regeneration comparable to those of E16 target regions. In order to test the effects of extracellular matrix molecules on axon regeneration, a basement membrane gel reconstituted from individual components of the Engelbreth-Holm-Sarcoma (EHS) sarcoma was implanted between proximal and distal optic nerve stumps. No axons were induced to regenerate by this matrix. Likewise, laminin adsorbed to nitrocellulose paper and implanted at the lesion site did not stimulate axon growth from the proximal optic nerve stump. These results indicate that fetal brain is able to induce and direct regrowth of axons from the optic nerve toward the graft across a substrate that is not composed of astroglia or basement membrane components like laminin. The directed growth of axons in the absence of a preformed substrate implies a chemotactic growth response along a concentration gradient mediated by neurotropic molecules released from the graft.

Age-dependent changes in the capacity of rat sympathetic neurons to form dendrites in tissue culture

We compared the ability of prenatal and postnatal rat sympathetic neurons to form dendrites in tissue culture. Dendrites were distinguished from axons by light microscopic criteria after intracellular dye injection and by differential immunostaining with antibodies to microtubule-associated protein-2 and to both non-phosphorylated and phosphorylated forms of the M and H neurofilament subunits. When maintained in the absence of serum and non-neuronal cells, most (72%) prenatal neurons were unipolar and had only an axon. In contrast, most (89%) neurons derived from postnatal ganglia were multipolar and extended both axons and dendrites. The dendritic morphology of postnatal neurons was usually simple with cells commonly having 2-5 short (50-200 microns), relatively unbranched dendrites. Thus, as the development of the dendritic arbor progresses in situ, sympathetic neurons acquire an enhanced ability to extend dendrites in tissue culture. To determine whether changes in the capacity to develop dendrites might occur with aging in vitro, ganglia were removed from prenatal rats and grown as explants for 3 weeks in the presence of non-neuronal cells; under these conditions, prenatal neurons within the explant became multipolar. When neurons derived from aged explants were subsequently maintained in dissociated cell culture, most formed dendrites. In cultures treated with an antimitotic agent, neurons typically had 1-4 unbranched dendrites; greater amounts of dendritic growth occurred in cultures in which ganglionic non-neuronal cells were allowed to proliferate. We conclude that: (1) the acquisition of the capacity to form dendrites in dissociated cell culture does not require either normal afferent input or physical contact with the target tissue; and (2) even after aging in vitro, sympathetic neurons remain responsive to the dendrite-promoting activity of ganglionic non-neuronal cells.

Comparison of the Schwann cell surface and Schwann cell extracellular matrix as promoters of neurite growth

The ability of Schwann cells to influence the direction and rate of neurite growth was investigated in a tissue culture model of the bands of Büngner of injured peripheral nerve. The arrangement of this culture system allowed testing of the growth-promoting properties of the Schwann cell surface and extracellular matrix (ECM) assembled by Schwann cells rather than soluble substances secreted into conditioned medium. Various components of peripheral nerve were examined separately as substrata for regenerating neurites: (i) Schwann cells and their ECM; (ii) Schwann cells alone; (iii) Schwann cell ECM alone; (iv) Schwann cells, fibroblasts, and their assembled ECM; (v) Schwann cells, their ECM and neurites; and (vi) purified laminin. Regenerating peripheral neurites were from explants of foetal rat dorsal root ganglia, which had been cultured for several weeks to rid them of accompanying non-neuronal cells, or from explants of foetal rat superior cervical ganglia, which contained non-neuronal cells. CNS neurites from the somatosensory cortex of embryonic rats were also studied; these neurites may be either first growing or regenerating. Neurites from all types of explants studied grew longer and were guided on a substratum of Schwann cells or Schwann cell ECM compared with a collagen substratum. The presence of fibroblasts during ECM assembly did not enhance the neurite growth-promoting activity. The design of the experiments suggested that the factors by which the Schwann cells or their ECM promoted and guided neurite outgrowth were surface-bound rather than medium-borne. Electron microscopic examination showed that neurites grew on either Schwann cell surfaces or basal lamina material. Attempts to define the chemical nature of the neurite growth-promoting effect of ECM by partial enzymatic digestion did not identify any single component as essential. Purified laminin was a more effective promoter of outgrowth of peripheral neurites than were Schwann cells or Schwann cell ECM. Cortical explants also grew on laminin, but neurites were accompanied on this substratum by a massive migration of non-neuronal cells; the neurites appeared to extend primarily on the non-neuronal cells rather than by direct attachment to the laminin substratum. This characteristic outgrowth of cortical non-neuronal cells on laminin was not consistently seen on Schwann cell ECM. In conclusion, either the Schwann cell surface or the ECM produced and assembled by Schwann cells promotes neurite outgrowth and guides that outgrowth from the several types of peripheral and CNS neurons studied in this report.

Geometry of isolated sensory neurons in culture. Effects of embryonic age and culture substratum

Sensory neurons were dissociated from lumbar dorsal root ganglia of embryonic chick and put into culture, either directly or after removing non-neuronal cells by density gradient centrifugation. The cells were grown on culture substrata of various kinds in medium containing nerve growth factor (NGF). After 24 h the cultures were fixed, mounted and analysed. Lengths of neurites were measured, and the numbers of primary processes formed at the cell body and of growth cones were counted. From these values, the rates of growth cone advance and frequency of growth cone branching were calculated. Neuronal outgrowths increased strikingly in length and complexity with embryonic age; there was a 3.5-fold increase in total neurite length and a 3-fold increase in the number of growth cones when neurons from 15-day embryos (E15) were compared with those from 8-day embryos (E8) grown on the same substratum (glass). Growth was markedly greater on surfaces prepared with laminin or conditioned medium compared with plain glass or air-dried collagen. When E15 neurons grown on glass were compared with those grown on laminin, for example, a 2.5-fold increase in total neurite length and a 3-fold increase in the number of growth cones was observed. Calculations showed that a major factor in these changes was an increase in the frequency of growth cone branching. The number of initial processes emanating from the cell body changed with age, but not with the different substrata tested. Non-neuronal cells when present in low numbers and in contact with neurons did not appear to influence neuronal geometry in a systematic way. Our results document the fact that both external factors (in this case, the nature of the culture substratum) and intrinsic factors (stage of development of the neuron) can influence the geometry of neurite outgrowth.