Control of direction of growth during the elongation of neurites

Norman K. Wessells: a life in science

"In its triple role as locomotory organelle, as a site of deposition of new surface material for the elongating axon, and a source of microspikes (sensory probes), the growth cone becomes the key to axon elongation" Yamada et al. (1971).

Immunocytochemical demonstration of early appearing astroglial structures that form boundaries and pathways along axon tracts in the fetal brain

During normal development of the mammalian forebrain, the paired cerebral hemispheres are initially separated midsagittally by the connective tissue-filled longitudinal fissure. During subsequent stages, the hemispheres fuse as basal lamina is remodeled and fibroblasts are eliminated from the fissure to create new central nervous system (CNS) territory in the midline. Two axon pathways, the corpus callosum and dorsal callosal stria, eventually use this region as part of their pathway. In order to assess the possible role of glial cells in the fusion process and in the guidance of axons in this and several other areas of the forebrain, we have analyzed the developing brain in timed cat and mouse embryos with immunohistochemical and morphological techniques. With the use of astroglial-specific antibodies and electron microscopy, we have visualized two distinct, primitive astroglial structures associated with the cerebral midline, and seven more associated with other specific brain regions. The way in which one of these structures moves as a column along the hemispheric midline in synchrony with seam formation suggests the possibility that during morphogenesis of the telencephalon, astrocytes may aid in the fusion process. In addition, the compact assemblage, early appearance and location of this and the other glial structures in relation to well defined neuroanatomical landmarks or axon pathways suggest that they may transiently compartmentalize relatively large regions of the CNS and organize certain developing fiber systems by acting as guides or barriers at critical stages of ontogeny.

New plastic plates which enhance neurite extension in culture: roles of bisphenol-A and tricyclodecanyl units for growth and orientation of neurites on plastic plates with microstructures

In order to study molecular mechanisms of contact guidance of neurites, dissociated culture of adult mouse dorsal root ganglion (DRG) cells was performed for 6 species of plastic plates different in monomer compositions. Microstructures (grooves of 5-10 microns wide and 1 micron deep) were grafted on the plates so as to guide neurite growth in their axial direction. Neurites were longer and more oriented on plates with larger water contact angle. The distribution of neurites was uneven between grooves and steps of the microstructures, exhibiting a bell-shaped curve in relation to the water contact angle of the plastic. These indicate that hydrophilic properties of the plastic plate surface, due to the presence of bisphenol-A and tricyclodecanyl units, are crucially involved in the elongation and orientation of neurites.

Embryonic mesencephalic and striatal co-grafts: development of grafted dopamine neurons and functional recovery

Previous neural grafting studies have shown that embryonic dopamine neurons survive transplantation into the parenchyma of the brain; however, fiber outgrowth from those cells is often limited to the immediate vicinity of the graft. More extensive outgrowth is desirable for promoting and maintaining functional recovery of damaged neural systems in animal models as well as human neurodegenerative disorders. The present study examined the possibility of stimulating fiber outgrowth of grafted neurons by simultaneously grafting dopamine neurons with their embryonic target cells. Subsequent functional recovery was evaluated in concert with morphological characteristics of these grafts. Co-grafts of embryonic mesencephalic and striatal cells were implanted into the DA-denervated striatum of rats previously given unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway. Two types of co-grafts were implanted into the DA-denervated striatum: mixed or separate cell suspensions. Tyrosine hydroxylase immunocytochemical analysis of brain sections containing co-grafts revealed extensive arborization of TH-positive neurons in both types of co-grafts. When mesencephalic and striatal nerve cells were implanted into separate sites, TH-positive neurons extended projections that appeared to preferentially reach regions occupied by embryonic striatal neurons. Moreover, the average size of TH-positive cell bodies found in mixed or separate co-grafts was significantly larger than the size of those found in single mesencephalic grafts. Amphetamine-induced rotational behavior was used to assess the degree of functional recovery. In the majority of co-grafted animals, rotational behavior was attenuated by 3 weeks and reversed (amphetamine-induced contralateral rotation) by 5 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of the rat corticospinal tract through an altered glial environment

The major corticospinal tract (CST) in the rat is located at the base of the dorsal funiculus. It is a late-developing tract, and the growth of its axons into the lumbosacral region of the spinal cord does not occur until postnatal days 5 and 6. This delay is taken advantage of in this study in order to evaluate the effects of a markedly reduced glial population on ingrowth of the CST axons into the lumbosacral spinal cord. A reduction of the glial population is achieved by exposure of this region of spinal cord to X-radiation at 3 days of age. Growth of CST axons into and through the lumbosacral spinal cord in rats in which this region has undergone a radiation-induced depletion of glial cells is compared with that in their non-irradiated littermate controls by axonal tracing techniques using horseradish peroxidase (HRP). The HRP was applied directly to the motor cortices of normal and irradiated rats, and at all ages studied, there was anterograde filling of CST axons and their growth cones. At 3 days postnatally, the age when the lumbosacral spinal cord was irradiated in the experimental animals, CST axons were present in the more rostral thoracic levels. CST axons were observed in the lumbar region of non-irradiated rats on day 5, and by day 7 they were present at sacral levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of laminin B1 mRNA in retinal ganglion cells by in situ hybridization

In the nervous system, neuronal migration and axonal growth are dependent on specific interactions with extracellular matrix proteins. During development of the vertebrate retina, ganglion cell axons extend along the internal limiting (basement) membrane and form the optic nerve. Laminin, a major component of basement membranes, is known to be present in the internal limiting membrane, and might be involved in the growth of ganglion cell axons. The identity of the cells that produce retinal laminin, however, has not been established. In the present study, we have used in situ hybridization to localize the sites of laminin B1 mRNA synthesis in the developing mouse retina. Our results show that there are at least two principal sites of laminin B1 mRNA synthesis: (a) the hyaloid vessels and the lens during the period of major axonal outgrowth, and (b) the retinal ganglion cells at later development stages. Müller (glial) cells, the major class of nonneuronal cells in the retina, do not appear to express laminin B1 mRNA either during development or in the adult retina. In Northern blots, we found a single transcript of approximately 6-kb size that encodes the laminin B1 chain in the retina. Moreover, laminin B1 mRNA level was four- to fivefold higher in the postnatal retina compared to that in the adult. Our results show that in addition to nonneuronal cells, retinal ganglion cells also synthesize laminin. The function of laminin in postnatal retinas, however, remains to be elucidated. Nevertheless, our findings raise the possibility that neurons in other parts of the nervous system might also synthesize extracellular matrix proteins.

Molecular and cellular characterization of the glial roof plate of the spinal cord and optic tectum: a possible role for a proteoglycan in the development of an axon barrier

Certain types of glial structures, located at strategic positions along axon pathways, may provide the mechanical and/or chemical elements for the construction of barriers which can grossly direct the elongation of axons during development. The roof plate, a putative axon barrier, is located along the dorsal midline of the developing spinal cord and may be important for the guidance of the commissural and dorsal column axons. We examined the roof plate to determine the developmental morphology of the region and to determine which molecules were correlated with the barrier function when axons were growing nearby. Light and electron microscopic observations of the roof plate revealed that this glial domain undergoes a dramatic change in shape from a "wedge" with large extracellular spaces between the cell apices at E12.5 to a thin, dense septum with reduced extracellular space at E15.5. Immunocytochemical techniques demonstrated that highly sialylated neural cell adhesion molecule (N-CAM), the carbohydrate recognized by L2 monoclonal antibody, cholinesterase, stage-specific embryonic antigen 1, and a ligand that binds tetragonolobus purpureas agglutinin are expressed by the roof plate. These molecules, however, were also found in other regions of the spinal cord which are permissive or attractive to axon growth. A molecule which is unique to the roof plate when axons grow close to, but do not cross, the dorsal midline is a glycosaminoglycan (GAG), keratan sulfate. Keratan sulfate is also present in the tectal midline and in other noninnervated regions such as the outer epidermis and developing cartilage. Our data suggest that keratan sulfate, alone or in combination with other molecules expressed by the roof plate, may be responsible, in part, for the inhibition of axon elongation through the roof plate in the embryonic spinal cord.

Effect of different surgical repair modalities on regeneration of the rabbit mandibular nerve

A study was designed to evaluate the ability of the rabbit mandibular nerve to regenerate when exposed to crush and resection injuries, as well as to determine how differently sized resection injuries healed when repaired with either autogenous grafts or laminin-lined collagen tubulization. The nerve demonstrated a regenerative capacity over a 1-cm defect, with morphology and function that approximated normals, but could not span a 2-cm gap defect unaided. Crush injuries produced findings that were inferior to both those in normal nerves and in those with resections. In 1-cm defects, both grafting and tubular repairs produced similar results, with substantial recovery of neural function after 16 weeks. In 2-cm defects, autogenous grafting was superior to tubulization by both morphologic and functional assessment. Replacement of the lateral cortex of the mandible after nerve repair was shown to be unnecessary. The implications of these findings as they relate to nerve injury and repair in humans is discussed.

Is astrocyte laminin involved in axon guidance in the mammalian CNS?

This paper provides evidence for the expression of laminin on glia in correlation with axon elongation and nerve pathway formation during embryonic development of the mouse optic nerve and other parts of the central nervous system (CNS). We show that punctate deposits of laminin on immature glial cells precede the entrance of the first optic axons into the nerve, and remain in close association with growing axons. Furthermore, we show that in one particular region of the optic pathway that the retinal ganglion cell axons avoid in normal animals (i.e., the pigmented area of the distal nerve) the punctate laminin matrix is missing. As the optic nerve matures punctate laminin deposits disappear, and laminin is reduced in the astroglial cytoplasm. The close correlation of the punctate form of laminin with early axonal growth is true not only in the optic nerve but also in some other parts of the CNS. We demonstrate such punctate laminin deposits in a model of astrocyte-induced regeneration of the corpus callosum in acallosal mice (G. Smith, R. Miller, and J. Silver, 1986, J. Comp. Neurol. 251, 23-43), and in glia associated with several normal developing axon trajectories, such as the corpus callosum, fornix, and pathways in the embryonic hindbrain. In all of these regions punctate laminin deposits are found on astroglia that are associated with early growing axons. Our results indicate that the punctate form of laminin, produced by astrocytes, may be an important factor involved in axon elongation and nerve pathway formation in the mammalian CNS.

Collagen tube repair of the mandibular nerve: a preliminary investigation in the rat

A technique for repair of the inferior alveolar nerve of the rat involving a collagen tube conduit with laminin gel lining was compared with unguided regeneration in both large and small nerve gaps. No significant morphologic differences were noted between the groups with small defects. With large defects, however, only the tubular repairs were effective in restoring neural continuity. The implications of the results of this study are discussed.

Growth cone localization of neural cell adhesion molecule on central nervous system neurons in vitro

Ultrastructural analysis of colloidal gold immunocytochemical staining and immunofluorescence microscopy has been used to study the presence of neural cell adhesion molecule (NCAM) on the surface of neuronal growth cones. The studies were carried out with cultures of rat hypothalamic and ventral mesencephalic cells, using morphology and expression of tyrosine hydroxylase, neurofilaments, and glial fibrillary acidic protein as differential markers for neurons and glia. NCAM was found on all plasmalemmal surfaces of neurons including perikarya and neurites. The density of NCAM varied for different neurons growing in the same culture dish, and neurons had at least 25 times more colloidal gold particles on their plasmalemmal membranes than astroglia. Of particular interest in the present study was a strong labeling for NCAM on all parts of neuritic growth cones, including the lamellar and filopodial processes that extend from the tip of the axon. The density of NCAM was similar on different filopodia of the same growth cone. Therefore, in situations where homophilic (NCAM-NCAM) binding might contribute to axon pathfinding, a choice in direction is more likely to reflect differences in the NCAM content of the environment, rather than the distribution of NCAM within a growth cone. On the other hand, the variation in NCAM levels between single neurons in culture was significant and could provide a basis for selective responses of growing neurites.

Growth patterns of primary cultures dissociated from 3-day-old chick embryos: morphological and biochemical comparisons

Cultures were prepared by dissociating 3-day-old whole chick embryos and plating the dispersed cells on poly-L-lysine-coated dishes in Dulbecco's Modified Eagle's Medium with 10% fetal calf serum. By 48 hr in culture, aggregates and neuritic sprouting were observed. Long neuritic bundles connecting cell aggregates were evident by 4 days in culture. Consistent patterns throughout the lifespan of the cultures were contacts between neurites, and flat isolated cells, presumptively glial, emerged. Throughout the lifespan of the cultures, the cholinergic cell population was characterized histochemically by the method of Karnovsky and Roots and biochemically by assaying choline acetyltransferase. By 4 days in culture, all aggregates showed light cholinesterase-positive staining; however, with days in culture, several aggregates had no staining, and some positive-stained aggregates were interconnected with other aggregates showing only spotted positive staining. Choline acetyltransferase activity showed a developmental profile in agreement with the histological findings. The early presence of choline acetyltransferase activity is taken as indication of the early commitment of cholinergic neurons.

Nerve regeneration through biodegradable polyester tubes

One approach to repair of transected nerves is to attempt extrinsic guidance of axons across the gaps. We inserted the proximal and distal stumps of severed mouse sciatic nerves into opposite ends of biodegradable polyester tubes. The nerves and ensheathing tubes were examined after postoperative survival times of as long as 2 years. Myelinated fiber number in each successfully regenerated nerve was measured and correlated with the tube's residual lumen size. In selected regenerated nerves axonal sizes and myelin sheath widths were sampled and compared with control values. Swelling and deformation of tube walls occurred in nearly all tubes. Successful regeneration was obtained through more than half of the implants, and was more probable in tubes with larger initial lumens. Myelinated fiber number in regenerated nerves ranged from 231 to 3561 (normally 3900 to 4200); larger values again were found in tubes with larger initial lumens. Mean axonal areas in regenerated nerves were roughly half of normal, though myelin sheaths became appropriately thick. We concluded that the more biodegradable a tube, the more likely it was to incur distortion and luminal narrowing. Tube composition per se seemed of importance mainly as it related to maintenance of adequate luminal size over the length of the degrading tubes; luminal adequacy, not tube composition, seemed paramount in determining the extent of nerve regeneration.

An in vivo model to quantify motor and sensory peripheral nerve regeneration using bioresorbable nerve guide tubes

An in vivo preparation is presented to study the rate and time course of motor and sensory axonal regeneration. The cut ends of a transected sciatic nerve were inserted into each end of a 5-6 mm non-toxic and bioresorbable nerve guide tube to create a 4 mm nerve gap in adult mice. Subsequently, cell bodies in the ventral spinal cord and L3-L5 dorsal root ganglia that had regenerated axons across the gap were retrogradely labeled with horseradish peroxidase (HRP). The HRP was applied 3 mm distal to the nerve guide and was accessible only to axons that had regenerated through the nerve guide. Labeled cells were counted in 40 micron serial sections at 2, 4 and 6 weeks after initial nerve transection. The results indicate a significant increase in the number of labeled motor and sensory cell bodies over time. By 6 weeks after transection, approximately two thirds as many ventral horn motor cells and one third as many dorsal root ganglion sensory cells were labeled as in control non-transected animals. These data serve as a baseline to compare differential effects of additives to the nerve guide lumen in terms of sensory and motor neuron response.

Peripheral nerve grafts to the frog optic tectum: a morphological study of foreign axon regeneration in the central nervous system

The proximal stump of a transected mandibular nerve was grafted onto the rostrodorsal surface of the optic tectum in adult Rana pipiens to investigate the morphologic characteristics of nonspecific axonal regeneration in a highly organized region of central nervous system (CNS). Within the first 3 weeks postgraft surgery (WPS), the nerve-tectum interface became firmly established. Concomitant with this was an invasion of the host tectum by a small number of fine "pioneerlike" axons from the nerve. By 6 WPS there developed a concerted instreaming of a large number of peripheral fibers. Once within the CNS, the foreign axons distributed themselves throughout the rostrocaudal extent of the tectum, but primarily its dorsal aspect within superficial layers 8 and 9. Presence of intact optic fibers at the time of mandibular fiber invasion served somewhat to restrict the regenerating aberrant axons in their course through layer 9. This restriction could be avoided by removal of the optic input either before or during peripheral ingrowth. However, once peripheral fibers had entered and established themselves in the host environment, no subsequent manipulation of the retinotectal projection had any effect. The aberrant growth pattern, which appeared remarkably stable after 6 WPS, consisted of a plexus of medium- and fine-caliber peripheral axons. Many of these fibers had numerous branches and "en passant" varicosities, the latter encompassing a variety of shapes and sizes. Terminal swellings and arborizations were also found. When comparing the regeneration of optic and mandibular nerve fibers in the tectum, two distinctions were made. Whereas optic axons revealed a fascicular and layered organization, mandibular axons showed a highly segregated and disordered growth pattern. These characteristic differences were maintained even when the two fiber systems were allowed to coregenerate into the same target tectum. Thus, each of the two groups of axons interacts with the tectal substrate in a distinct manner, apparently independent of the other.

Promoting functional plasticity in the damaged nervous system

Damage to the central and peripheral nervous system often produces lasting functional deficits. A major focus of neuroscience research has been to enhance functional restitution of the damaged nervous system and thereby produce recovery of behavioral or physiological processes. Promising procedures include surgical, physical, and chemical manipulations to reduce scar formation and minimize the disruption of support elements, administration of growth-stimulating substances, tissue grafts to bridge gaps in fiber pathways, and embryonic brain tissue grafts to provide new cells with the potential to generate fiber systems. Two elements are required for functional nervous system repair: (i) neurons with the capacity to extend processes must be present, and (ii) the regenerating neurites must find a continuous, unbroken pathway to appropriate targets through a supportive milieu.

Cultured peripheral nervous system cells support peripheral nerve regeneration through tubes in the absence of distal nerve stump

Axons of a cut peripheral nerve will grow across a gap (less than or equal to 10 mm in adult rodents) formed when the proximal and distal stumps are placed at opposite ends of an impermeable, inert tube, but will not grow to the end of a blind-ended tube in the absence of the distal stump [Williams et al, 1984]. Work reported here demonstrates that cultured peripheral nervous system (PNS) cells suspended in a collagen matrix will provide an effective milieu that directs and supports axonal regeneration from a severed nerve into a blind-ended tube in the absence of a distal stump. Adult mouse sciatic nerves were cut and the proximal stumps were inserted into close-ended tubes that contained either a collagen matrix containing dissociated cells from embryonic mouse dorsal root ganglia (DRG), a collagen matrix saturated with medium conditioned by cultured DRG cells, or a collagen matrix saturated with fresh medium. In all three cases cellular cables formed that ran the full length of the tubes, but myelinated and unmyelinated axons regenerated the length of the tubes only when cultured cells had been added. The critical factor in influencing axonal regeneration through the length of the tubes was the presence of cultured cells, since collagen alone or collagen saturated with conditioned medium did not support axonal regrowth even though cells had migrated into the chambers from the proximal stumps in all cases. Ordered structure was not a requisite for axonal growth, since the cultures consisted of random arrays of dissociated cells.

Influences of the neural cell adhesion molecule on axon growth and guidance

The neural cell adhesion molecule (NCAM) has been shown to be a ligand in the formation of cell-cell bonds. This molecule is present on essentially the entire surface of differentiated nerve cells, including the cell body, neurite shaft, and growth cone. In mediating membrane-membrane adhesion, NCAM appears to ligate with itself, and one of its most obvious functions is in the self-association of nerve fibers to form fascicles. In most cases fasciculation occurs by the successive elongation of axons along other axons and, therefore, is likely to represent a growth cone-neurite shaft interaction as well as a shaft-to-shaft adhesion. Competition between neurite shafts and the surrounding substrate for growth cone adhesion probably represents a major factor in the branching of nerve bundles. In addition to neurons, NCAM appears on some glial and muscle cells. Recent experiments suggest that this molecule is involved in growth cone guidance along adhesive pathways on glial precursors in the vertebrate central nervous system, and in the initial interaction of axons with muscle prior to synaptogenesis.

Guidance of optic axons in vivo by a preformed adhesive pathway on neuroepithelial endfeet

Antibodies against the neural cell adhesion molecule (NCAM) were used in vivo both to localize NCAM antigenic determinants in developing tissues of the chicken visual system and to perturb cell-cell adhesion during growth of optic fibers to the tectum. The immunohistochemical studies revealed a staining pattern on neuroepithelial cells which coincided with certain regions of the presumptive route for optic axons, not only with respect to the overall pathway from the eye to the tectum, but also in the preferential distribution of the antigen on the marginal endfeet which are contacted by optic axon growth cones. The antibody-perturbation studies, which involved intraocular injection of anti-NCAM Fab at embryonic Day 3.5, demonstrated that inhibition of NCAM-mediated adhesion results in a dramatic distortion of growth cone-neuroepithelial cell relationships and consequently of the optic pathway. Together, these studies suggest that guidance of optic axons along the margin of the brain is at least in part influenced by a preformed adhesive pathway on neuroepithelial cells associated with NCAM antigens.

The possible role of "sibling neurite bias" in the coordination of neurite extension, branching, and survival

In this review we consider a novel mechanism, "sibling neurite bias," which may explain aspects of the coordination of elongation, branching, and resorption among different neurites growing from the same neuronal cell body. In this model, growing neurites which incorporate structural precursors at higher rates would deplete the cellular pool of precursors available to their "sibling" neurites; neurites would compete for survival, but in addition they would bias each other's behavior during active growth. Evidence is reviewed that "sibling neurite bias" may contribute to the establishment and stabilization of specific neural connections. Specific examples examined include the loss of polyinnervation at the developing neuromuscular junction, contextual mapping in the retino-tectal system, and selective neurite growth patterns and synaptic connections in nerve tissue culture model systems.

Heparan sulfate proteoglycans of human neuroblastoma cells: affinity fractionation on columns of platelet factor-4+

Human neuroblastoma cells (Platt) were detached from tissue culture substrata with a Ca2+ chelating agent, and then the suspended cells were extracted with a sodium dodecyl sulfate (SDS)-containing buffer to maximally solubilize their sulfate-radiolabeled proteoglycans. The majority of the high-molecular-weight material in these dissociative extracts was heparan sulfate proteoglycan, which resolves into two heterodisperse size classes upon gel filtration on columns of Sepharose CL4B. After removal of SDS from these extracts by hydrophobic chromatography on Sep-Pak C18 cartridges, extracts were further fractionated on various affinity matrices. All of the sulfate-radiolabeled material eluted as one peak from DEAE-Sephadex ion-exchange columns. In contrast, affinity fractionation on Sepharose columns derivatized with the heparan sulfate-binding protein, platelet factor-4, resolved three major and one minor subsets of these components. The nonbinding fraction contained some heparan sulfate proteoglycan and some chondroitin sulfate. The weak-binding fraction contained principally heparan sulfate proteoglycan, as well as a small amount of chondroitin sulfate proteoglycan; the gel-filtration properties of these proteoglycans before or after alkaline borohydride treatment indicated that they were small in size, containing perhaps 2 to 4 glycosaminoglycan chains. The high-affinity fraction eluted from platelet factor 4-Sepharose was composed entirely of "single-chain" heparan sulfate. A portion of the heparan sulfate proteoglycan of the original extract bound to the hydrophobic affinity matrix, octyl-Sepharose, and this hydrophobic proteoglycan partitioned into the nonbinding and weak-binding fractions of the platelet factor 4-Sepharose affinity columns. These studies reveal that the majority of the proteoglycan made by these neuronal cells in culture is of the heparan sulfate class, is small in size when compared to other characterized proteoglycans, and can be resolved into several overlapping subsets when fractionated on affinity matrices.

Monoclonal antibody to Thy-1 enhances regeneration of processes by rat retinal ganglion cells in culture

Ganglion cells were dissociated from postnatal rat retinas, identified by specific fluorescent labels, and maintained in culture on a variety of substrates. Regeneration of processes by retinal ganglion cells was enhanced when the cells were plated on glass coated with a monoclonal antibody against the Thy-1 determinant. Plain glass and glass coated with polylysine, collagen, fibronectin, or other monoclonal antibodies supported the growth of neural processes, but were less effective than antibody to Thy-1.

Astrocytic membrane morphology: differences between mammalian and amphibian astrocytes after axotomy

Previous studies have shown that astrocytes in some nonmammalian species provide a favorable environment for axonal elongation, whereas mammalian astrocytes are thought to inhibit fiber outgrowth. The present study was performed to determine whether any plasma membrane differences exist between these glial elements which could account for their contrasting effects upon axonal outgrowth. Astrocytic scars were formed in optic nerves of rats, newts, and frogs by enucleation. Subsequently, the astrocytic membranes were examined with the freeze-fracture technique. Orthogonal arrays of small intramembranous particles (IMPs) are a prominent component of the plasma membranes of normal mammalian astrocytes; these arrays are most numerous in astrocytic membranes that form an interface between the CNS and nonneural tissue. Astrocytic membranes within the normal CNS parenchyma, however, possess much lower densities of arrays. Following axotomy and Wallerian degeneration, the density of arrays increased threefold within the parenchyma of the optic nerve, while remaining constant at the glia limitans. In striking contrast, only a few aggregates of IMPs that resembled orthogonal arrays could be found in normal and reactive astrocytes of amphibians, although the cytology of these glial cells and density of the scars are otherwise similar to those of their mammalian counterparts. These findings suggest (1) that a proliferation of orthogonal arrays in astrocytic plasma membranes is a prominent feature of gliosis in the mammalian CNS and (2) that differences in the composition of reactive mammalian and amphibian astrocytic membranes may account for variations in axonal-glial interactions within the injured CNS.

Conditioned medium from a clonal Rous sarcoma virus transformed cerebellar cell line induces process extension in glial lines

A Rous Sarcoma virus transformed cerebellar cell line, BC6, secretes a factor which causes clonal glial cell lines to rapidly (1 h) extend processes. The factor shows a degree of specificity since only 3 out of 10 lines which exhibit either glial or combined glial and neuronal properties respond. The active factor appears to be a soluble protein since it remains in the supernatant after centrifugation at 100,000 g for 2 h and is trypsin-sensitive. When conditioned medium is fractionated on a Sephadex G-100 column, activity elutes in and just behind the void volume. By several criteria the factor is distinct from glia maturation factor.

Studies on cell migration and axon guidance in the developing distal auditory system of the mouse

The events that take place along the potential route of distal auditory axons (future vestibular component) prior to and during their outgrowth were examined morphologically using timed mouse embryos. During embryonic (E) day 9.5 a discrete zone of cell death appears in the rostrolateral wall of the otic cup. Necrosis is accompanied by outward migration of epitheloid cells from the same region of the otic wall. Temporally and spatially correlated with these two events is the widening of extracellular spaces between otic neuroepithelial cells and the breakdown of basement membrane. During E 10.5 migrating epitheloid cells condense to form a funnel-shaped configuration. This cellular "funnel" begins narrowly at the dorsorostrolateral wall of the otocyst and broadens as it reaches the auditory ganglion. During E 11.5 through E 12.5, "pioneer" distal auditory axons take a circuitous route and ascend from the auditory ganglion to enter the otocyst. Axons extend toward the otocyst moving along cells of the "funnel," maintaining an orientation similar to that of the cells that compose it. Axon growth cones enter the otocyst at sites devoid of basement membrane and invade the wall of the otocyst moving tangentially along radially arranged cells that bridge the otocyst and the "funnel." These observations demonstrate that a preformed, funnel-shaped tissue exists along the future route of the auditory fibers. We suggest that the "funnel" may influence the growth and directionality of distal auditory axons as they extend from the auditory ganglion to the wall of the otocyst. At the otic wall, the transition provided by "bridge" epitheloid cells, together with the absence of basement membrane at specific sites of the otic wall, provide the auditory axons with a route into the otocyst.

Axonal interactions with connective tissue and glial substrata during optic nerve regeneration in Xenopus larvae and adults

Axonal elongation through connective tissue and glial environments was compared following resection of the optic nerves in Xenopus tadpoles and frogs. During initial stages of fiber outgrowth, axons encountered connective-tissue matrices of varying degrees of complexity in the ablation gaps. Many of the neuritic sprouts were randomly directed after leaving the retinal stump, and a neuroma-like swelling ultimately formed at the cut edge. Although a large number of axons managed to traverse the lesion and associate with the cranial stump, many other fibers were less appropriately directed, especially in the frog where a greater infiltration of dense collagen occurred between the separated segments of the optic nerve. Axons often deviated from their cranially oriented pattern of outgrowth after entering the lesion and invaded surrounding extraocular muscles; others advanced along neighboring blood vessels and cranial nerve branches. In more extreme circumstances, fibers were completely misdirected at the cut end of the retinal stump and ultimately extended adjacent to the retinal segment back toward the eye. A more organized pattern of axonal elongation was observed in the presence of the glial substratum of the central stump, and growth cones appeared to associate preferentially with astrocyte endfeet in both tadpoles and frogs. These observations show that axons in the regenerating optic nerve of the amphibian can interact with a variety of cells and tissues and that the general direction of their outgrowth, at least in more peripheral regions of the visual pathway, appears to be dependent upon the orientation and, possibly, molecular properties of the terrain which they contact. In general, the basic environmental factors which either foster or impede axonal elongation in this regenerating system appear analogous to those influencing fiber outgrowth during regeneration in the peripheral nervous system of various species.

Axonal guidance during development of the great cerebral commissures: descriptive and experimental studies, in vivo, on the role of preformed glial pathways

Do structures exist within the embryonic central nervous system that guide axons across the midline during development of the great cerebral commissures (corpus callosum, anterior commissure)? With the use of serial section and reconstructive computer graphic techniques we have found that during normal ontogeny of the mouse forebrain and before the arrival of the pioneer fibers of the corpus callosum at the midline, a population of primitive glial cells migrates medially (through the fused walls of the dorsal septum) from the ependymal zones of each hemisphere. At the midline, and well rostral to the lamina terminalis, these cells unite to form a bridgelike structure or "sling" suspended below the longitudinal cerebral fissure. The first callosal axons grow along the surface of this cellular bridge as they travel toward the contralateral side of the brain. The "sling" disappears neonatally. The fibers of the anterior commissure grow within the lamina terminalis along a different type of preformed glial structure. Movement of these axons occurs through an aligned system of glial processes separated by wide extracellular spaces. Do these transient glial tissues actually provide guidance cues to the commissural axons? Analyses of three situations in which the glial "sling" is genetically or surgically impaired or nonexistent indicate that this structure does, indeed, play an essential role in the development of the corpus callosum. We have analyzed (1) the embryonic stages of a congenitally acallosal mouse mutant (strain BALB/cCF), (2) several pouch stages of a primitive acallosal marsupial, Didelphys virginiana (opossum), and (3) animals in which the "sling" had been lesioned surgically through the uterine wall in the normal embryo (strain C57BL/6J). In the acallosal mouse mutant fusion of the septal midline is delayed by about 72 hours and the "sling" does not form. Although the would-be callosal axons approach the midline on schedule, they do not cross. Instead, the callosal fibers whirl into a pair of large neuromas adjacent to the longitudinal fissure. Similarly, in the opossum, fusion of the medial septal walls and formation of the glial "sling" are also lacking. However, in this species, instead of traveling dorsally, the "callosal" axons turn ventrally and pass contralaterally by way of the anterior commissure pathway. Surgical disunion of the glial "sling" also resulted in acallosal individuals. The callosal pathology in these affected animals mimicked exactly that of the genetically lesioned mutant. Our observations suggest that many different types of oriented glial tissues exist within the embryonic neural anlage. We propose that such tissues have the ability to influence the directionality of axonal movements and, thereby, play a crucial role in establishing orderly fiber projections within the developing central nervous system.

Intrinsic and extrinsic determinants of neuronal development: relation to infantile autism

This paper attempts to view the autistic syndrome in the context of a disorder of brain development. The authors review some of the known or suspected causes of the autistic syndrome: maternal rubella, metabolic diseases, and heredity. Some basic principles of cellular neuroanatomy and chemical neurotransmission are sketched. The stages of human brain development from neurulation through histogenesis, cell migration, and elaboration of dendritic trees and axonal projections are described. The authors conclude that there are a limited number of developmental loci that could be disrupted and lead to the autistic syndrome, and that these most probably occur in the end stages of neuronal development, after the migrating neurons have reached their final place in the brain and have begun to elaborate communicative processes. Finally, the authors speculate on how neurochemical disturbances might alter end stage neuronal differentiation leading to the pathology of infantile autism.

Early development of the circumferential axonal pathway in mouse and chick spinal cord

The early development of the circumferential axonal pathway in the brachial and lumbar spinal cord of mouse and chick embryos was studied by scanning and transmission electron microscopy. The cellular processes which comprise this pathway grow in the transverse plane and along the lateral margin of the marginal zone (i.e., circumferentially oriented), as typified by the early embryonic commissural axons. The first formative event observed was in the ventrolateral margin of the primitive spinal cord ventricular zone. Cellular processes were found near the external limiting membrane that appeared to grow a variable distance either dorsally or ventrally. Later in development, presumptive motor column neurons migrated into the ventrolateral region, distal to these early circumferentially oriented processes. Concurrently, other circumferentially oriented perikarya and processes appeared along the dorsolateral margin. Due to their aligned sites of origin and parallel growth, the circumferential processes formed a more or less continuous line or pathway, which in about 10% of the scanned specimens could be followed along the entire lateral margin of the embryonic spinal cord. Several specimens later in development had two sets of aligned circumferential processes in the ventral region. Large numbers of circumferential axons were then found to follow the preformed pathway by fasciculation, after the primitive motor column had become established. Since the earliest circumferential processes appeared to differentiate into axons and were found nearly 24 hours prior to growth of most circumferential axons, their role in guidance as pioneering axons was suggested.

Nerve regeneration--evidence for early sprout formation

Following microneurorrhaphy, sharply transected rat sciatic nerves were harvested 2, 5, 12, 16, 21, and 30 days after repair. Transverse sections of these specimens were made 3, 5, 7, 9 and 11 mm distal to the repair and examined with the scanning electron microscope. Myelinated sprouts were found 3 mm distal to the repair on the fifth day. Sprout migration occurred even though old myelin sheaths were present. Sprouts did not appear in old myelin sheath lumens. Many 5 U myelinated sprouts were found 11 mm distally 21 and 30 days after repair. This study demonstrates that nerve regeneration begins early, before degeneration is complete.

The aberrant retino-retinal projection during optic nerve regeneration in the frog. II. Anterograde labeling with horseradish peroxidase

Previous experiments have shown that a substantial number of regenerating optic axons in adult frogs (Rana pipiens) are misrouted into the opposite optic nerve and retina during early stages of regeneration. This projection is maximal at 5 and 6 weeks after optic nerve crush. To further characterize this anomalous projection, small quantities of horseradish peroxidase (HRP) were injected into the right eye or right optic nerve 5 or 6 weeks after right optic nerve crush. Twenty-four hours later the animals were killed and regenerating axons anterogradely filled with HRP were reacted with the tetramethyl-benzidine method or a diaminobenzidine-CoCl2 method. Serial reconstruction tracing the course of individual axons through the optic chiasm showed that few of the axons projecting into the opposite optic nerve were collaterals of axons projecting centrally. Instead, the majority of labeled axons misdirected into the opposite nerve or contributing to an expanded projection into the ipsilateral optic tract turned out of the chiasm without branching. Many of the labeled regenerating axons had unusual trajectories within the chiasm, making abrupt turns or changing their direction of growth. Most of the axons misrouted into the opposite nerve came from portions of the chiasm nearest to the nerve of other eye. In three of eight frogs with an intact optic nerve, a small number of HRP-labeled axons were found in the left nerve after right nerve injection, but there was no indication that these axons reached the left eye. The results from this investigation suggest that the most parsimonious explanation for the chiasmal misrouting of regenerating frog optic axons is that axons are mechanically deflected into inappropriate pathways.

Polyornithine-attached neurite-promoting factors (PNPFs). Culture sources and responsive neurons

We have recently reported the existence within chick embryo heart cell conditioned medium (HCM) of two distinct and independently assayable factors. One agent, ciliary neuronotrophic factor (CNTF), supports the in vitro survival of 8-day chick embryo ciliary ganglionic (CG) neurons. The other factor, polyornithine-attachable neurite promoting factor (PNPF) is required for extensive neuritic growth from these same CNTF-supported CG neurons. In the present study we have examined the occurrence of PNPF activity within nearly 100 different conditioned media using our previously described chick CG bioassay system. From this screening we conclude that: (1) PNPF production is a rather widespread property of cultured neural as well as non-neural cells; and (2) the chick bioassay is sensitive to PNPF activity from all the species examined, including mouse, rat, human and chick cells. We next examined the effects of 3 representative PNPF-containing conditioned media (from chick heart, mouse Schwann and rat Schwannoma) on neurite production from 3 other peripheral ganglionic neuronal cultures (8-day chick dorsal root, 11-day chick sympathetic, and neonatal mouse dorsal root ganglia) as well as 4 central neuronal cultures (8-day chick embryo telencephalon, optic lobe and spinal cord and neonatal mouse cerebellum). The results of these studies indicate: (1) that the peripheral neurons exhibit a dramatic increase in neurite production in response to PNPF which can be easily recognized both qualitatively and quantitatively; whereas (2) the CNS neurons showed essentially no PNPF-induced increase in neurite production. The sole exception to the latter was the appearance within the chick spinal cord cultures of a neuronal population which extended very long neurites in response to PNPF.

Neurites grow faster towards the cathode than the anode in a steady field

We explanted fragments of embryonic chick dorsal root ganglia on to polylysine coated glass and cultured them in a medium containing one unit of nerve growth factor plus enough methylcellulose to give viscosities from 0.01-3,000 poise. We allowed them to grow out in the absence of a field, and then selected explants with halos of neurites which were relatively dense, relatively symmetrical, and practically free of glial cells. These selected explants were then exposed to electrical fields of up to 140 mV/mm for some hours. In media with viscosities of one poise or less, the field some times dragged the central cell mass of an explant towards the anode. However, in cases where the central cell mass did not move, fields of 70-140 mV/mm induced that sector of each neurite halo which faced the cathode to grow out several times faster than the one facing the anode.

The culture of chick embryo dorsal root ganglionic cells on polylysine-coated plastic

Polylysine-coated culture surfaces are strongly adhesive for neural cells, restrict locomotion on nonneuronal elements, but do not inhibit neurite elongation. In the present study, culture dishes were pre-treated with poly-D-lysine (PDL) at various concentrations, seed with dissociates from 8-day chick embryo dorsal root ganglia, and incubated under conditions that normally support both neuronal survival and nonneuronal proliferation. Pretreatment with low (0.1 mg/ml) PDL concentrations had no effect on neuronal survival and neuritic growth, but entirely prevented an increase in ganglionic nonneurons, yielding a numericallly stable culture greatly enriched in neurons. Higher PDL concentrations caused increasing losses in both cell classes. The 50% levels of cell loss were achieved at about the same PDL dose, but earlier for neurons that nonneurons and still with no impairment of neuritic growth from the surviving neurons. A procedure was developed to compare acid-soluble and acid-precipitable accumulation of radioactivity under 1-hr pulses of [3H]uridine, which was applicable even to poorly attached cells. The cytotoxic effects of higher PDL pretreatments was revealed as early as 6 hr after seeding by 2- to 4-fold lower radioaccumulation. The data are discussed in terms of possible regulations of cell permeability and metabolism by adhesive interactions between cells and their substratum, or other cells.

The environment of axonal migration in the developing chick retina: a scanning electron microscopic (SEM) study

We have made a SEM study of the basal intercellular spaces of the retina in chick embryos of different developmental stages. Since this is the environment where optic axons grow, the structural characteristics of this region might play some role in the orientation of axonal migration towards the choroid fissure. The basal region of undifferentiated retinas is formed by the vitreal expansions of neuroepithelial cells. In pre-axonal stages, the intercellular spaces between these expansions do not show any preferential orientation towards the fissure. The growth cones of ganglion cell axons appear in an apicobasal direction and turn towards the fissure immediately beneath the vitreal surface. Fasciculation is an early event during development and, in the more advanced stages, the vitreal expansions from retinal cells are placed in rows following the same orientation as the axon bundles. These observations are discussed in relationship to current hypotheses on axonal migration and orientation.

The role of ependyma in regeneration of the spinal cord in the urodele amphibian tail

The new spinal cord formed during tail regeneration in the newt first develops as a caudal extension of the ependymal tube. Neuroblasts and neuroglia subsequently differentiate from cells of the ependymal tube in a proximal-caudal sweep. Descending axons from the cord rostral to the lesion and from newly differentiating neurons travel in channels which are present prior to the ingrowth of axons. The present study confirms previous observations from our laboratory and presents details of the ultrastructural relations of axons and ependymal processes within the cord. The ependymal cell surface facing channels typically forms numerous digitor sheet-like protuberances which extend into the channel lumen. As axons enter the channels in increasing numbers these protuberances partially subdivide the axons into smaller groupings, even occasionally segregating individual axons. At levels where fibers have not yet entered or have most recently entered the ependymal channesl two specializations appear on the ependymal plasmalemma facing the channels and their axons: coated membranes and hemidesmosome-like structures. At more mature levels, where many fibers have already entered the channels, axons in contact with ependymal processes sometimes show synapse-lide vesicle accumulations with associated membrane densities. Coated membranes and hemidesmosome-like structures are lacking at these levels. Our observations suggest that ependymal processes, in addition to providing substrate and direction for regenerating spinal cord axons, may also furnish or exchange more specific information at the morphologically identifiable specializations described above.

Histogenesis of mouse cerebellum in microwell cultures. Cell reaggregation and migration, fiber and synapse formation

A microwell culture system was developed for analysis of cell movements and interactions during nervous system histogenesis. Cells from trypsinized 7-day-old mouse cerebellum reaggregated within hours into clusters which later developed interconnections consisting of either sheets of migrating cells and cell processes or cables of fiber bundles with cells migrating along their surfaces. Granule cells in several stages of differentiation, basket and/or stellate neurons, some larger neurons, and two types of neuroglial cells were identified in reproducible, nonrandom patterns by scanning and transmission electron microscopy. Axonal and dendritic processes, both with growth cones, and numerous synapses were generated in vitro.

Clockwise growth of neurites from retinal explants

When retinal explants from goldfish are grown on a polycation substratum, a marked tendency for directionality of neurite outgrowth is observed. While the direct relevance to nerve growth in vivo is not known, the phenomenon is interpreted as reflecting an inherent helicity of the neurites.