Regulation of the elongating nerve fiber

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).

Oxidized galectin-1 stimulates macrophages to promote axonal regeneration in peripheral nerves after axotomy

Various neurotrophic factors that promote axonal regeneration have been investigated in vivo, but the signals that prompt neurons to send out processes in peripheral nerves after axotomy are not well understood. Previously, we have shown oxidized galectin-1 (GAL-1/Ox) promotes initial axonal growth after axotomy in peripheral nerves. However, the mechanism by which GAL-1/Ox promotes axonal regeneration remains unclear and is the subject of the present study. To identify possible target cells of GAL-1/Ox, a fluorescently labeled recombinant human GAL-1/Ox (rhGAL-1/Ox) was incubated with DRG neurons, Schwann cells, and intraperitoneal macrophages from adult rats. Only the cell surfaces of intraperitoneal macrophages bound the rhGAL-1/Ox, suggesting that these cells possess a receptor for GAL-1/Ox. Experiments examining tyrosine phosphorylation revealed that rhGAL-1/Ox stimulated changes in signal transduction pathways in these macrophages. These changes caused macrophages to secrete an axonal growth-promoting factor. This was demonstrated when conditioned media of macrophages stimulated with rhGAL-1/Ox in 48 hr culture strongly enhanced axonal regeneration from transected-nerve sites of DRG explants. Furthermore, activated macrophage-conditioned media also improved Schwann cell migration from the transected-nerve sites. From these results, we propose that axonal regeneration occurs in axotomized peripheral nerves as a result of cytosolic reduced galectin-1 being released from Schwann cells and injured axons, which then becomes oxidized in the extracellular space. Oxidized galectin-1 then stimulates macrophages to secrete a factor that promotes axonal growth and Schwann cell migration, thus enhancing peripheral nerve regeneration.

Identification of oxidized galectin-1 as an initial repair regulatory factor after axotomy in peripheral nerves

Various neurotrophic factors that promote axonal regeneration have been investigated in vivo, but the signals that prompt the axons to send out processes in peripheral nerves after axotomy are not well understood. We have shown using two specific strategies that galectin-1 can play an important role in this initial stage. One used an in vitro nerve regeneration model that allowed us to monitor the initial axon and support cell outgrowth from the proximal nerve stump comparable to the initial stages of nerve repair. The other strategy was to clarify the axonal regeneration-promoting factor from kidney-derived cells. Using these strategies, we discovered that oxidized galectin-1 from the cell (COS1 cell) conditioned media acts as an axonal regeneration-promoting factor without the lectin activity. Oxidized recombinant human galectin-1 (rhGAL-1/Ox) showed the same activity at low concentrations (pg/ml range). A similarly low concentration also effectively promoted axonal regeneration in both transection and crush experiments in vivo. Moreover, the application of functional anti-galectin-1 antibody strongly inhibited the regeneration in vivo. Since galectin-1was shown to be secreted and localized in the regenerating sciatic nerve, this suggests that secreted galectin-1 may be oxidized and change its molecular structure to regulate initial repair after axotomy as a kind of cytokine.

Galectin-1 regulates initial axonal growth in peripheral nerves after axotomy

The signals that prompt the axons to send out processes in peripheral nerves after axotomy are not well understood. Here, we report that galectin-1 can play an important role in this initial stage. We developed an in vitro nerve regeneration model that allows us to monitor the initial axon and support cell outgrowth from the proximal nerve stump, which is comparable to the initial stages of nerve repair. We isolated a factor secreted from COS1 cells that enhanced axonal regeneration, and we identified the factor as galectin-1. Recombinant human galectin-1 (rhGAL-1) showed the same activity at low concentrations (50 pg/ml) that are two orders of magnitude lower than those of lectin activity. A similarly low concentration was also effective in in vivo experiments of axonal regeneration with migrating reactive Schwann cells to a grafted silicone tube after transection of adult rat peripheral nerve. Moreover, the application of functional anti-rhGAL-1 antibody strongly inhibited the regeneration in vivo as well as in vitro. The same effect of rhGAL-1 was confirmed in crush/freeze experiments of the adult mouse sciatic nerve. Because galectin-1 is expressed in the regenerating sciatic nerves as well as in both sensory neurons and motor neurons, we suggest that galectin-1 may regulate initial repair after axotomy. This high activity of the factor applied under nonreducing conditions suggests that galectin-1 may work as a cytokine, not as a lectin.

Acceleration of membrane recycling by axotomy of cultured aplysia neurons

The rapid transition of a stationary axon into a motile growth cone requires the recruitment of membrane and its strategic insertion into the neurolemma. The source of membrane to support the initial rapid growth postaxotomy is not known. Using membrane capacitance measurements, we examined quantitative aspects of membrane dynamics following axotomy of cultured Aplysia neurons. Axotomy activates two processes in parallel: membrane retrieval and exocytosis. Unexpectedly, membrane retrieval is the dominant process in the majority of the experiments. Thus, while a growth cone is vigorously extending, the total neuronal surface area decreases. We suggest that the initial rapid extension phase of the newly formed growth cone postaxotomy is supported by a pool of intracellular membrane that is rapidly retrieved from the neurolemma.

Biochemical properties of Na+/K(+)-ATPase in axonal growth cone particles isolated from fetal rat brain

Axonal growth cones (AGC) isolated from fetal rat brain have an important specific activity of N+/K(+)-ATPase. Kinetic assays of the enzyme in AGC showed that Km values for ATP or K+ are similar to those reported for the adult brain enzyme. For Na+ the affinity (Km) was lower. Vmax for the three substrates was several times lower in AGC as compared to the adult value. We also observed two apparent inhibition constants of Na+/K(+)-ATPase by ouabain, one of low affinity, possibly corresponding to the alpha 1 isoform and another of high affinity which is different to that described for the alpha 2 isoform of the enzyme. These results support an important role for the sodium pump in the maintainance of volume and cationic balance in neuronal differentiating structures. The functional differences observed also suggest that the enzymatic complex of Na+/K(+)-ATPase in AGC is in a transitional state towards the adult configuration.

A review of the morphology of spinal cord malformations and their relation to neuro-embryology

Morphological characteristics of spinal dysrhaphic disorders are briefly described. In addition, attention is focused on an anomaly of the spinal tracts (ascending and descending pathways). Neuro-embryological considerations facilitate greatly the understanding of the pathogenesis of these conditions.

Direct observation in vitro of how neuroblasts migrate: medulla and cochleovestibular ganglion of the chick embryo

The hypothesis that neuroblasts migrate in the nervous system by a locomotory process was tested experimentally. An in vitro preparation permitted direct observation of postmitotic cells migrating from the rhombic lip of the medulla and the anlage of the cochleovestibular ganglion. Cell locomotion was not seen. Instead migration was produced by elongation of a leading process, followed by translocation of the nucleus (perikaryal translocation). On the basis of comparisons with previous observations in situ, we propose that this represents a common mode of migration in the developing nervous system. Cell clusters were explanted from the rhombic lip at the developmental stage when they migrate from the ventricular zone to the acoustico-vestibular anlage in the medulla. Cells from the cochleovestibular ganglion were explanted after migration from the otocyst, but before ganglionic differentiation. Each neuroblast's migration route was formed by an elongating leading process ending in a growth cone. The growth cone attached to other cells and processes or ended freely on an acellular substrate. Nonneuronal cells usually migrated as has been described for fibroblasts, yet with some of the features of perikaryal translocation, but some nonneuronal precursor cells may migrate the way neuroblasts do. Neuroblasts did not migrate preferentially on the processes of nonneuronal cells, although the reverse could be observed. In fact a variety of interactions between migratory cells, neuronal and nonneuronal, were observed. The advantage of the experimental system described here is that one can observe cells migrating spontaneously at the times in development when they normally do so, while preserving the cellular populations present in situ.

Calcium regulation of neuronal differentiation: the role of calcium in GM1-mediated neuritogenesis

Cultures of mouse Neuro-2a neuroblastoma cells treated with 3-6 mM extracellular Ca2+ exhibited enhanced neurite extension characterized by increased neurite numbers and lengths. The ganglioside GM1 potentiated the effect of extracellular Ca2+ by increasing further the number and length of the neurites formed in response to exogenous Ca2+. Maximal neuritic numbers were achieved with 4 mM Ca2+ while the longest neurites were observed in medium containing 4-6 mM Ca2+. Stimulation of the Ca2+ influx with the ionophore A23187 or the amino acid taurine also enhanced neurite formation and GM1 potentiated these actions. Transmission electron microscopy revealed numerous microtubules and neurofilaments in neurites and microfilaments with the spine-like processes along fine neuritic branches and in the filopodia of growth cones. Neuritic varicosities and growth cones contained a variety of vesicles. All of these structures were increased in the presence of GM1 and were increased further by extracellular Ca2+ or A23187. The ability of GM1 to enhance neuritogenesis was diminished by EGTA or Ruthenium red. Similarly, the effect of GM1 was diminished or abolished by Ca2+ channel blockers such as CdCl2 or LaCl3. X-ray microprobe analysis revealed that GM1 alone enhanced intracellular levels of total ionic and membrane bound Ca2+, perhaps accounting for the increased neuritogenesis observed under conditions in which Ca2+ was manipulated. The present study suggest that the neuritogenic action of GM1 is Ca2+ dependent.

Myotome and early neurogenesis in chick embryos

The present study was undertaken in order to verify the identification of profiles of presumptive growth cones in vivo. The developing spinal nerves of chick embryos were studied by light and electron microscopy. We traced the onset of efferent and afferent innervation of the myotome in 2- to 4-day-old chick embryos in order to be sure that we were examining the growing tips of axons. In the process of studying these growing axons, we were able to observe some unique relationships of neural tube, myotome, and differentiating spinal nerves. The neural tube tightly abuts the myotome in Hamburger and Hamilton's (HH) stage 14 chick embryos and cytoplasmic projections from the myotome directly abut the neural tube. The first ventral roots could be identified in HH stage 15 embryos and dorsal roots in HH stage 16 embryos, both under 2 1/2 days of age. The advancing spinal nerve courses toward the anterior or cranial half of the myotome, and growth cones directly contact the medial wall of the myotome. The spinal nerves continue to abut tightly the myotome during the succeeding day of embryonic life, and growth cones enter the substance of the myotome by 3 days, or HH stage 19 embryos. These dorsolaterally directed axons will form the dorsal ramus of the spinal nerves and the ventral ramus continues to be contiguous with the myotome. Invasion of the myotome by axons (putative innervation), and thus innervation of myotomal cells in the 3-day chick embryos, was a totally unexpected finding. The myotome and its potential derivatives thus have extensive neural contact by 3 days of embryonic life in the chick. These findings document a parallel differentiation of afferent and efferent elements of the nervous system and confirm previous accounts identifying growth cones in an intact organism. These findings suggest that afferent as well as efferent nerves may have critical roles in the differentiation of the mesodermal as well as ectodermal derivatives.

Effect of ethanol on the morphohistogenesis and differentiation of cerebellar granule cells in the chick embryo

In this present study we analyse, with the help of the Golgi method, the effect of ethanol on the morphological differentiation of the cerebellar granule cells in the chick embryo. Ethanol seems to affect the process of cell migration from the early stages of differentiation. Some granule cells appear to differentiate in an inverted position. These observations also confirm, on the basis of their axon morphology, the existence of three types of granule cells in the chick cerebellum.

Mosaic Drosophila wings reveal regional heterogeneity in the guidance of ectopic axons

In most studies of axon guidance in the peripheral tissues of insects, the ability of experimentally perturbed axons to pathfind was examined only along their normal pathways. This means that regions normally devoid of axons have not been sampled for their ability to influence axonal trajectories. To examine this question, we have induced the formation of single sensory neurons in a variety of abnormal locations in the developing wing of Drosophila and have examined the course taken by their axons. The axons of such ectopic neurons have a regionally varying tendency to grow in the normal, proximal direction. This proximal bias approaches 100% for neurons located in the distal part of vein L2 and 70% in distal vein L4 but falls to chance (50%) along vein L5. Thus, neurons forming in ectopic regions of the wing, especially those found near the normal axon pathways (veins L1 and L3), have a high probability of growing axons in the correct direction. We conclude that information relevant to axon outgrowth is not restricted to the normal pathways. Whether this information is intrinsic or extrinsic to the neurons, and why its strength shows such conspicuous regional variation, awaits further study.

Role of the growth cone in neuronal differentiation

Nerve growth cones are motile, exploring organelles at the tip of a growing neurite. The growth cone is a highly specialized structure, equipped with a complex machinery for reversible membrane expansion and rapid cytoskeletal reorganization, a machinery required for growth cone motility and neurite elongation. It also contains perception systems that enable the growth cone to respond to external signals, thereby steering the trailing neurite to the correct target. Soluble and substrate bound guidance molecules in the environment modulate growth cone behavior either through direct interaction or classical receptor activation coupled to second messengers. A prominent phosphoprotein of the growth cone is B-50. We propose a role for this growth-associated protein kinase C substrate in signal transduction processes in the growth cone.

Rapid growth cone translocation on laminin is supported by lamellipodial not filopodial structures

To determine the relationship between growth cone structure and motility, we compared the neurite extension rate, the form of individual growth cones, and the organization of f-actin in embryonic (E21) and postnatal (P30) sympathetic neurons in culture. Neurites extended faster on laminin than on collagen, but the P30 nerites were less than half as long as E21 neurites on both substrata. Growth cone shape was classified into one of five categories, ranging from fully lamellipodial to blunt endings. The leading margins of lamellipodia advanced smoothly across the substratum ahead of any filopodial activity and contained meshworks of actin filaments with no linear f-actin bundles, indicating that filopodia need not underlie lamellipodia. Rapid translocation (averaging 0.9-1.4 microns/min) was correlated with the presence of lamellipodia; translocation associated with filopodia averaged only 0.3-0.5 microns/min. This relationship extended to growth cones on a branched neurite where the translocation of each growth cone was dependent on its shape. Growth cones with both filopodial and lamellipodial components moved at intermediate rates. The prevalence of lamellipodial growth cones depended on age of the neurites; early in culture, 70% of E21 growth cones were primarily lamellipodial compared to 38% of P30 growth cones. A high percentage of E21 lamellipodial growth cones were associated with rapid neurite elongation (1.2 mm/day), whereas a week later, only 16% were lamellipodial, and neurites extended at 0.5 mm/day. Age-related differences in neurite extension thus reflected the proportion of lamellipodial growth cones present rather than disparities in basic structure or in the rates at which growth cones of a given type moved at different ages. Filopodia and lamellipodia are each sufficient to advance the neurite margin; however, rapid extension of superior cervical ganglion neurites was supported by lamellipodia independent of filopodial activity.

Growth cone advance mediated by fibronectin-associated filopodia is inhibited by a phorbol ester tumor promoter

In serum-supplemented medium, exposure to the tumor promoter 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) increases the proportion of SH-SY5Y neuroblastoma cells with neurites and increases the average neurite length. In the present study, under serum-free conditions, PMA treatment had the opposite effects, i.e., retarded neurite sprouting and partially inhibited neurite elongation. This inhibition in neurite outgrowth was partially antagonized by the addition of serum fibronectin (FN) to the medium or substratum. In the absence of PMA, SH-SY5Y cells grown under serum-free conditions showed extensive neurite outgrowth as well as the capacity to secrete FN into their microenvironment and form FN-containing substratum-attachment sites. Immunogold labeling and whole mount transmission electron microscopy (WMTEM) demonstrated FN-containing contact pads at sites where filopodia attached to the substratum and focal plaques on the underside of growth cone margins. The appearance and abundance of FN-containing contact pads and focal plaques were increased by the addition of exogenous FN to defined medium. Focal plaques appeared in close association with microfilament bundles, and nearly always with bundles that projected into filopodia attached to the substratum by contact pads. A method for immunolabeling FN in the filopodial contact pads of living cultures provided more direct evidence that filopodia and contact pads have a major role in FN-mediated attachment and are central in determining growth cone shape and the rate and direction of advance. In support of this view, we show that PMA treatment retards neurite sprouting, alters growth cone morphology and motility, and eliminates the appearance of microfilament bundles, filopodia, and FN-containing substratum-attachment plaques.

A thermodynamic model for force integration and microtubule assembly during axonal elongation

We present here a thermodynamic model for tension and compression forces within axons (neurites) of the specific neural-cell line, PC 12, which seems generally applicable to neuronal growth. We suggest that these forces play a crucial role in microtubule assembly during axonal elongation. The Gibbs free energy change for the axonal elongation phase of neuronal growth is modeled as the sum of the extensional work for pulling on a random actin network, work of assembly for compressed microtubules and surface energy terms. This model explains the results of previously published experiments concerning axonal stability and microtubule polymerization and has been used to predict other phenomena.

Effects of p-bromophenacyl bromide on neurite growth at different levels of nerve growth factor

We have previously been shown that the phospholipase inhibitor p-bromophenacyl bromide (BPB) modifies neurite morphology and growth rate in rat dorsal root ganglion explants. Neurite extension is stimulated by 2.5 X 10(-7) M BPB but it is prevented by concentrations higher than 10(-5) M. Under our experimental conditions, no neurites appeared in the absence of added nerve growth factor (NGF). Therefore, we have tested the effect of a low BPB concentration at different levels of exogenous NGF. Thoracic and lumbar ganglia were obtained from 19.5-day-old fetal rats and were cultured for 24 h on a polyornithine substrate. Neurite growth and development of a dense halo were NGF-dependent in both ganglionic populations; however, each of them showed a characteristic dose-response curve. Incubation with BPB induced neurites in the absence of added NGF and stimulated their growth when low levels of this factor were added. In lumbar ganglia incubated with higher levels of NGF, BPB had no detectable effects. On the contrary, similarly treated thoracic ganglia showed an inhibition of neurite extension. Our findings suggest a relationship between the effects of BPB and the initial, membrane-activating actions of NGF.

Effect of laminin and cytoskeletal agents on neurite formation by NG108-15 cells

Laminin promotes attachment and process formation in the neuroblastoma X glioma hybrid cell line NG108-15. As cells attached to laminin, they flattened and remained dispersed rather than associated in clumps. Process formation was observed within 1 hr after exposure to laminin and was dose dependent. Cycloheximide, an inhibitor of protein synthesis, did not block laminin-mediated attachment and neurite formation. Addition of drugs that depolymerize the cytoskeleton led to different behaviors for cells grown on plastic compared with those in the presence of laminin. Cells on plastic treated with either vinblastine or cytochalasin neither flattened nor grew processes. Cells plus laminin and vinblastine retracted processes, but remained flat, suggesting that laminin-induced processes can be destabilized by disrupting microtubules. Cells sequentially treated with laminin and cytochalasin produced processes that were thin and highly branched. Cells in high concentrations of cytochalasin on a laminin substrate formed aberrant processes even when their soma did not flatten. Since laminin counteracted the effect of cytochalasin on process outgrowth but did not alter the effect of cytochalasin on flattening of the cell body, different mechanisms mediated by microfilaments may be involved in cell flattening and in process formation.

Abnormal course of spinal posterior roots and tracts associated with brain malformations. Sensory pathway malformation

An unusual mode of the spinal posterior nerve root entrance and of the course of ascending tracts in the hyperplastic gray matter of the posterior horn was observed in three newborn babies. This "sensory pathway malformation" was combined with severe but unspecific brain anomalies. The common associated brain anomaly was agenesis of the corpus callosum.

Immunocytochemical study of the postnatal development of 5-HT-containing neurons and fibers in the cerebroid ganglia of Cryptomphalus aspersa

In this work we have studied the organization of the serotonergic system in new-hatched snails and the developmental changes that occur postnatally. The serotonergic system of the new-hatched snail is not completely developed. Tracts previously described in the adult were already present at hatching; however immunoreactivity of these tracts was somewhat less intense than that in adults, but, their location was practically the same. At hatch the pro- and postcerebral serotonergic plexi were poorly developed, while the mesocerebral one was nearly absent. The procerebral plexus occupied only the caudal half of the procerebral neuropile. Growth cone profiles were found in all plexi. The paired giant dorsal serotonergic neurons were the only stained neurons constantly found at hatching; they were symmetrically located. The paired medium-sized neurons were not found in all brains examined. Sometimes only one of them was present at hatching; it was located either in the right or in the left ganglion. The small neurons were absent in most cases. They gradually appeared during the first three postnatal months. Following hatching the procerebral plexus had undergone an intense growth. By the third postnatal month it innervated completely the procerebral neuropile. At hatching the area innervated by the procerebral plexus was 4500 micron 2, by the third month this area was about 58,000 micron 2 and in the adult was 80,000 micron 2. The postnatal development of the procerebral serotonergic plexus proceeds through two different modalities of neuritic growth: expansive growth is mediated by centrifugal elongation of terminal branches while the increase in fiber density within the plexus results from lateral sprouting.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate effects on the dynamics of neurite growth in vitro: a quantitative multi-parametric analysis

Embryonic chick dorsal root ganglia were cultured in serum-free medium on natural (collagen, fibronectin and hyaluronic acid) and artificial (polylysine and polyornithine) substrata. The movement of individual growth cones was quantified by measuring five parameters using time-lapse cinematography combined with a digitizing-computer system, and the neurite behaviour was compared between the different substrata with multivariate statistical methods. For each substratum, the morphometry of the growth cone was quantified by measuring six morphological parameters. The most discriminative parameters proved to be mean velocity and straightness index for neurite extension, and projected area and cumulated length of filopodia for growth cone morphometry. A good correlation was obtained between behavioural and morphological parameters and the larger the cone area and the filopodia length, the faster and the straighter the neuritic growth. Both quantitative analyses showed highest values for polyornithine and the lowest for hyaluronic acid, and divided the substrata into two opposite groups, artificial and natural. It is concluded that growth cone behaviour and conformation is modulated by substratum properties.

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.

In situ appearance of the cold-stable microtubules in the growing axons of the tectal plate of mouse investigated immunocytochemically after polyethyleneglycol (PEG) embedding

Tubulin immunostaining of semi-thin sections after polyethylene glycol embedding was used in the tectal plate of the embryonic mouse at 10 days postmating to analyze the effects of cold treatment on the microtubules of the different cell types seen at this stage. Three sets of microtubules are observed. In the radially oriented bipolar columnar cells, dense bundles of microtubules are present in the ventricular processes between the cell nucleus and the ventricular surface. In the mitotic cells, located just at the surface of the ventricle, microtubules are among condensed chromosomal figures. In the apical region, the intermediate zone, tangentially oriented axonal profiles contain dense bundles of microtubules among tangentially oriented young neurons. Cold treatment does not modify the organization of the cells. However, it depolymerizes whole cytoplasmic and mitotic microtubules of the bipolar cells and a large number of microtubules in the growing axons. In the axonal profiles, the cold-stable fraction of microtubules displays the appearance of short fragments. Some of these are regularly organized, suggesting that they could be the remnants of the same individual microtubule. These fragments are approximately 1 micron long and seem to represent nearly 10% of the total microtubules in the axons. These cold-stable fragments might fulfill a function in the axon analogous to the microtubule organizing centers in the perikaryon and their presence can explain some properties of the growing axons suggested by previous studies on the guidance of neurites and growth cones as well as on the growth of isolated axons.

Molecules that make axons grow

The study of neurite growth in tissue culture has been a productive way to identify substances that may control the behavior of axons in vivo. Molecules that promote the outgrowth of neurites include nerve growth factor, laminin, fibronectin, and a protease inhibitor derived from glia. Evidence that these molecules may influence axon growth and guidance in vivo is discussed. The effects these molecules have at the cellular level are compared, in an attempt to identify common mechanisms of action. Several less well-characterized molecules that influence the behavior of neurites are also discussed.

Lipoprotein uptake by neuronal growth cones in vitro

Macrophages that rapidly enter injured peripheral nerve synthesize and secrete large quantities of apolipoprotein E. This protein may be involved in the redistribution of lipid, including cholesterol released during degeneration, to the regenerating axons. To test this postulate, apolipoprotein E-associated lipid particles released from segments of injured rat sciatic nerve and apolipoprotein E-containing lipoproteins from plasma were used to determine whether sprouting neurites, specifically their growth cones, possessed lipoprotein receptors. Pheochromocytoma (PC12) cells, which can be stimulated to produce neurites in vitro, were used as a model system. Apolipoprotein E-containing lipid particles and lipoproteins, which had been labeled with fluorescent dye, were internalized by the neurites and their growth cones; the unmetabolized dye appeared to be localized to the lysosomes. The rapid rate of accumulation in the growth cones precludes the possibility of orthograde transport of the fluorescent particles from the PC12 cell bodies. Thus, receptor-mediated lipoprotein uptake is performed by the apolipoprotein B,E(LDL) (low density lipoprotein) receptors, and in the regenerating peripheral nerve apolipoprotein E may deliver lipids to the neurites and their growth cones for membrane biosynthesis.

Golgi studies on the development of granule cells of the rat olfactory bulb with reference to migration in the subependymal layer

The morphology and the development of the cells in the subependymal layer and of granule cells of the olfactory bulb were examined by Nissl and Golgi staining in postnatal rats. The subependymal layer around the anterior lateral ventricle extends into the center of the olfactory bulb. The mitotic indexes in the subependymal layer are high at the level of the anterior horn of the lateral ventricle and very low inside the olfactory bulb during the first 3 weeks after birth. Golgi-stained subependymal cells are classified into two main groups. One group consists of smoothly contoured bipolar cells with leading processes tipped by large growth cones and with trailing processes. They make up a majority of Golgi-stained subependymal cells during the first 3 weeks of age, and smaller numbers of them continue to exist at 37 and 60 days. They migrate with their growth cones oriented toward the olfactory bulb from the level of the anterior lateral ventricle into the granular layer of the olfactory bulb, where they differentiate into the definitive granule cells: their somata enlarge; the leading processes elongate, branch, sprout many gemmules, and become the peripheral processes; and the trailing processes become the basal dendrites. The other group contains relatively large cells with many cytoplasmic processes that are considered to belong to the glial cell line.

Early changes of surface morphology in monolayer cultures of neural retina cells. The effect of p-bromophenacyl bromide

Scanning electron microscopy was used to study the surface morphology of neural retina cells from fetal rats. Dissociated cells were plated on polyornithine and examined after 1.5-3.0 hr in vitro. A quantitative analysis of the proportion of cells with and without processes was made and the former were classified according to the length and number of their processes. Treatment with p-bromophenacyl bromide (BPB), a selective inhibitor of phospholipase A2 (PLA2), induced significant changes on the early surface activity of retinal cells. An inhibitory effect on cell process formation was observed in monolayers grown for 2 hr in the presence of BPB: process formation was also inhibited when high concentrations (10(-6) M or more) were applied as a 30 min pulse, whereas a similar pulse of a lower concentration (10(-7) M) stimulated the appearance of cells with short processes. These observations suggest that PLA2 or some other BPB-reactive substance is involved in the extension of neural cell processes.

Development of reticulospinal neurons of the zebrafish. II. Early axonal outgrowth and cell body position

The sequence of axonal outgrowth and the early cell body positions of identifiable types of reticulospinal (RS) neurons of the zebrafish (Brachydanio rerio) were determined in order to learn if differences in neuronal characteristics among specific cell types correlate with differences in the times at which the cells develop. The time of axonal outgrowth and the location of the cells in the developing embryo were determined by filling RS neurons retrogradely with horseradish peroxidase from their growing axons over a series of developmental stages. Hindbrain cells developed in two distinct waves separated by about 10 hours. In the first wave, axons initiating growth at nearly the same time and from nearly the some location in the hindbrain diverged in their choice of an ipsilateral or contralateral pathway into the spinal cord. Individual types of RS neurons grew axons into the rostral spinal cord in a similar temporal sequence to that of their birthdays although the lag time between birthday and time of axonal outgrowth was variable (RS neuronal birthdays were reported in the previous paper, Mendelson: J. Comp. Neurol., 251:160-171 1986). All RS somata were initially observed along the ventral surface of the brain and were subsequently displaced dorsally. The displacement could be passive, due to growth of the ventral hindbrain, and could explain the previously observed dorsoventral differences in RS neuronal time of origin.

Microtubule reassembly from nucleating fragments during the regrowth of amputated neurites

We have proposed that stable microtubule (MT) fragments that resist depolymerization may serve as nucleating elements for the local control of MT dynamics in the axon (Heidemann, S. R., M. A. Hamborg, S. J. Thomas, B. Song, S. Lindley, and D. Chu, 1984, J. Cell Biol., 99:1289-1295). Here we report evidence that supports this proposal in studies on the role of MTs in the regrowth of neurites from the distal segments of amputated chick sensory neurites. Amputated neurites collapse to "beads" of axoplasm that rapidly regrow (Shaw, G., and D. Bray, 1977, Exp. Cell Res., 104:55-62). We examined both unarrested regrowth and regrowth after MT disassembly by either cold (-5 degrees C for 2 h) or nocodazole (0.1 microgram/ml for 15-20 min). In all these cases regrowth occurred at 3.5-4.5 micron/min with no delay times other than the times to reach 37 degrees C or rinse out the nocodazole. Electron micrographs of untreated beads show many MTs of varying lengths, while those of cold- and nocodazole-treated beads show markedly shorter MTs. The robust regrowth of neurites from beads containing only very short MTs argues against unfurling of intact MTs from the bead into the growing neurite. Electron micrographs of cold-treated beads lysed under conditions that cause substantial MT depolymerization in untreated intact neurites show persistent MT fragments similar to those in unlysed cold-treated beads. We interpret this as evidence that the MT fragments in cold-treated beads are somehow distinct from the majority of the MT mass that had depolymerized. Collapsed neurites treated with a higher dose of nocodazole (1.0 microgram/ml for 15-20 min) were completely devoid of MTs and regrew only after a 15-20 min delay in two cases but never regrew in 11 other cases. We found that MTs did not return in beads treated with 1.0 microgram/ml nocodazole even 30 min after removal of the drug. It was unlikely that the inability of these beads to reassemble MTs was due to incomplete removal of nocodazole in that a much higher dose (20 micrograms/ml nocodazole) could be quickly rinsed from intact neurites. Beads treated with 1.0 microgram/ml nocodazole could, however, be stimulated to reassemble MTs and regrow neurites by treatment with taxol. We conclude that the immediate, robust regrowth of neurites from collapsed beads of axoplasm requires MT nucleation sites to support MT reassembly.(ABSTRACT TRUNCATED AT 400 WORDS)

The early embryogenesis of papillary (sweat duct) ridges in primate glabrous skin: the dermatotopic map of cutaneous mechanoreceptors and dermatoglyphics

The present study documents the early innervation of the epidermis prior to the onset of differentiation of the papillary (sweat duct) ridge in glabrous digital skin of rhesus monkey embryos measuring 45, 50 and 55 mm (crown-rump) length. We observed small papillary ridges, spaced at a distance of approximately 40 microns, projecting into the dermis in the center of the distal glabrous digital pad of digits 2-5 in the 55-mm embryo. The other digital pads lacked any sign of ridge formation. A two-dimensional, approximately hexagonal grid of afferent nerves was present in the superficial dermis of all digital and palmar pads. At regular intervals of approximately 40 microns, afferent nerves ascended from the superficial dermal nerve plexus and innervated the overlying epidermis. By electron microscopy, axonal growth cones were identified contacting Merkel cells that projected several microns down into the superficial dermis in the digital pad of digit 3. Thus, the earliest wave of differentiated dorsal root ganglion neuroblasts innervates Merkel cells. Schwann cells partially encircled these growing axon tips and could be identified by the presence of rough endoplasmic reticulum and free ribosomes. The youngest embryo studied had no sign of ridge formation; however, axons ascended from the superficial dermal nerve net at 30-40-microns intervals to innervate the epidermis. We conclude that afferent nerve fibers provide a two-dimensional grid that could modulate the spacing and arrangement of the papillary or sweat duct ridges of successive digits. Such an interaction is possible between digits based on the overlapping dermatotopic maps of each rete ridge. An abnormal fingerprint could thus reflect abnormal dorsal root ganglion neuroblasts expressed through mesenchyme and epidermis.

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.

Responses of cultured neural retinal cells to substratum-bound laminin and other extracellular matrix molecules

The responses of cultured chick embryo retinal neurons to several extracellular matrix molecules are described. Retinal cell suspensions in serum-free medium containing the "N1" supplement (J. E. Bottenstein, S. D. Skaper, S. Varon, and J. Sato, 1980, Exp. Cell Res. 125, 183-190) were seeded on tissue culture plastic surfaces pretreated with polyornithine (PORN) and with one of the factors to be tested. Substantial cell survival could be observed after 72 hr in vitro on PORN pretreated with serum or laminin, whereas most cells appeared to be degenerating on untreated PORN, PORN-fibronectin, and PORN-chondronectin. Cell attachment, although quantitatively similar for all these substrata, was temperature-dependent on serum and laminin but not on fibronectin or untreated PORN. In a short-term bioassay, neurite development was abundant on laminin, scarce on serum and fibronectin, and absent on PORN. No positive correlation between cell spreading and neurite production could be seen: cell spreading was more extensive on PORN and fibronectin than on laminin or serum, while on laminin-treated dishes, spreading was similar for neurite-bearing and non-neurite-bearing cells. Laminin effects on retinal neurons were clearly substratum dependent. When bound to tissue culture plastic, laminin showed a dose-dependent inhibitory effect on cell attachment and did not stimulate neurite development. PORN-bound laminin, on the other hand, did not affect cell attachment but caused marked stimulation of neurite development, suggesting that laminin conformation and/or the spatial distribution of active sites play an important role in the neurite-promoting function of this extracellular matrix molecule. Investigation of the embryonic retina with ELISA and immunocytochemical methods showed that laminin is present in this organ during development. Therefore, in vivo and in vitro observations are consistent with the possibility that laminin might influence neuronal development in the retina.

Purification of a factor that promotes neurite outgrowth: isolation of laminin and associated molecules

When culture medium, conditioned by any of several cell types, is applied to a polycationic substratum, a substance is adsorbed that causes neurons cultured on that substratum to extend processes (neurites) rapidly and profusely. We have purified the factor responsible for this effect from medium conditioned by bovine corneal endothelial cells, and have shown that it is composed of the glycoprotein laminin and two associated laminin-binding molecules: a sulfated protein known as entactin, and a large heparan sulfate proteoglycan. Of these molecules, only laminin was found to be present throughout the purification in all fractions possessing neurite outgrowth-promoting activity and absent from all fractions lacking activity. Laminin, purified from other sources, has been shown previously to promote extensive outgrowth by cultured neurons. These and other data presented here support the conclusion that laminin is responsible for the neurite outgrowth-promoting activity of the conditioned medium factor. Evidence is also presented that the association of a proteoglycan with laminin promotes efficient attachment of laminin to polycationic substrata, particularly in the presence of competing molecules.

Sensory neuron growth cones comigrate with posterior lateral line primordial cells in zebrafish

The development of neuromasts and sensory neurons of the posterior lateral line was studied in zebrafish (Brachydanio rerio) in order to determine the relationship between growing axons of sensory neurons and the migratory cellular primordium of midbody line neuromasts. Scanning electron microscopy revealed that a primary system of six neuromasts develops during the second day after fertilization and evidence is presented that these arise from cells of a migratory primordium. The primordium is first detected in the postauditory region immediately adjacent to the developing sensory ganglion. Growth cones of posterior lateral line sensory neurons are found within the premigratory primordium when it is adjacent to the ganglion. At later times growth cones of these sensory neurons are found within the primordium as it migrates caudally along the midbody line. These results demonstrate that although the growth cones of the sensory neurons grow over a considerable distance to their final destination, they are never very far from their target cells (or target cell precursors), which migrate with them and may even lead them.

Growth cones isolated from developing rat forebrain: uptake and release of GABA and noradrenaline

A growth cone-enriched fraction isolated from neonatal rat forebrain was shown to accumulate gamma-amino [3H]butyric acid ([3H]-GABA) and [3H]noradrenaline ([3H]NA). Uptake of both neurotransmitters was sodium- and temperature-dependent and exhibited saturation kinetics with Km values of 17.7 microM and 4.5 microM respectively and Vmax values of 114 pmol/min/mg protein and 59 pmol/min/mg protein respectively. Electron microscopic autoradiography showed that about 50% of isolated growth cones can accumulate [3H]GABA. Inhibitor studies showed that beta-alanine was a relatively weak inhibitor of [3H]GABA uptake compared to nipecotic acid and diamino-butyric acid. Growth cone fractions preloaded with [3H]GABA and [3H]NA demonstrated a K+ (25 mM) -induced release of both neurotransmitters. Of the K+-stimulated release of [3H]GABA 50% was Ca2+-dependent, whereas the release of [3H]NA was entirely Ca2+-independent.

Axon guidance in cultured epithelial fragments of Drosophila wing

Growing axons can be guided by a number of different cues: adhesive substrates, diffusible factors, electrical fields and even factors intrinsic to the neurone itself have all been shown to affect axon orientation and outgrowth in vitro. However, in most intact systems it has proved difficult to test directly the role played by these putative guidance cues. Here, we describe a system, the developing wing of the fruitfly, in which we have tested simultaneously two putative guidance mechanisms, physical constraints to axon growth (channels) and the position of neuronal somata (guideposts), using surgical techniques. We show that pioneer sensory axons can navigate correctly and form their normal stereotyped pattern of axon bundles in wing fragments that apparently lack both physical and neural cues. This technique allows access to the surface along which neuronal pathfinding takes place, making possible a wide range of experimental manipulations on the developing system.

Substrate influences human epidermal melanocyte attachment and spreading in vitro

Previous culture systems for melanocytes have employed serum-supplemented medium and uncoated plastic dishes, prohibiting examination of possible substrate influences on cellular morphology and function. We now report, using a sensitive serum-free system and a quantitative procedure for evaluating cellular morphology, that modification of the plating surface affects human epidermal melanocyte attachment rate and subsequent morphology in vitro. Melanocytes attach and spread more rapidly on surfaces coated with fibronectin or Type I/III collagen or on surfaces previously conditioned by human keratinocytes, dermal fibroblasts, melanocytes, or melanoma cells than do melanocytes on untreated control surfaces. Type IV collagen and laminin, although minimally beneficial for cell attachment, do support a characteristic melanocyte morphology that differs from that seen either on the other coated surfaces or on uncoated plastic controls. Addition of fetal bovine serum at the time of inoculation has no appreciable effect on attachment but markedly improves cell spreading on untreated surfaces, while addition of nerve growth factor with or without serum to this system fails to affect cell attachment or spreading. Our data establish that human epidermal melanocytes are indeed capable of responding morphologically to substrate signals. The ability of several biochemically unrelated surfaces to enhance melanocyte attachment rate and spreading suggests that melanocytes have surface receptors with a variety of specificities. This work is relevant to the development of improved culture systems for melanocytes in vitro and to understanding melanocyte behavior in vivo.

A quantitative study of growth cone filopodial extension

The extension of filopodia from growth cones of regenerating neurites from rat superior cervical ganglion neurons in tissue culture was studied. Cultures were grown on a thin layer of fibrous collagen and maintained in a medium containing serum and nerve growth factor. Time-lapse cinematography and computer-assisted morphometry were used to observe and measure the kinetics of extension of individual filopodia. Filopodia extended from the growth cone margin, trailing neurite, or from each other. Frequently, extension was preceded by the appearance at the cone margin of a nodule of cytoplasm which appeared dense in phase-contrast optics. Branch points between adjacent extending filopodia remained fixed with respect to the growth cone while the filopodia lengthened. The rate of extension was maximum just after initiation (0.12 +/- 0.4 micron/sec; mean +/- SD; n = 36) and declined thereafter until the filopodium collapsed. This initial rate of extension was directly correlated with the eventual length of the filopodium (r = 0.67). Filopodia of growth cones arising from embryonic neurons exhibited higher initial extension rates (range: 0.07 to 0.20 micron/sec; mean = 0.13 micron/sec) than those of postnatal neurons (range: 0.01 to 0.13 micron/sec; mean = 0.09 micron/sec). These data are discussed in relation to a model proposed by Tilney and Inoue [1982] for the extension, by distal addition of G-actin to growing filaments, of another type of elongating process filled with microfilaments, the acrosomal process of Thyone sperm.

Morphological properties and membrane channels of the growth cones induced in PC12 cells by nerve growth factor

Large growth cones were produced in vitro by nerve growth factor (NGF) treatment of multinucleate cells produced by chemical fusion of cells of the neuron-like clone PC12. These endings were studied both at the light microscopic and ultrastructural levels. The activity of ionic channels at growth cones was recorded with intracellular microelectrodes, patch recording of single channels, and whole cone recording from mechanically isolated growth cones. Morphologically, these large growth cones were characterized by the presence of microspikes and filopodia, by the presence of actin demonstrated immunohistochemically, and by the presence of catecholamine fluorescence. At the ultrastructural level they contained a broad spectrum of organelles with a distribution characteristic of neuronal growth cones, including dense core vesicles, abundant smooth membrane cisternae, microtubules, and a filamentous network. The presence of channels capable of generating action potentials was revealed by intracellular microelectrode recording from the growth cone in the presence of locally applied tetraethylammonium (TEA). TEA appeared to block outward current channels that could effectively shunt inward current activated by depolarization. Action potentials elicited by depolarizing current in the presence of TEA could be blocked reversibly by Cd2+, a specific blocker of Ca channels. These action potentials were often followed by a long after-hyperpolarization lasting hundreds of milliseconds. This after-hyperpolarization was similar to that recorded in the cell body of PC12 cells where it appears to be mediated by Ca-activated K current. Single channel recording from outside-out excised patches of membrane from the growth cones perfused with KF revealed the presence of voltage sensitive Na channels, Ca-activated K channels, and K channels resembling delayed rectifier K channels. Macroscopic currents recorded from mechanically isolated growth cones in the "whole cone" configuration showed rapid inward currents at potentials greater than or equal to -40 mV, followed by delayed outward currents at more positive potentials, a finding providing additional evidence for the presence of Na and K channels in growth cones.

Parameters of neuritic growth from ciliary ganglion neurons in vitro: influence of laminin, schwannoma polyornithine-binding neurite promoting factor and ciliary neuronotrophic factor

Ciliary ganglion neurons extend neuritic processes when cultured for 24 h in medium containing ciliary neuronotrophic factor (CNTF) and on a polyornithine substratum precoated with either laminin or a Schwannoma-derived neurite promoting factor (PNPF). We have examined the roles of laminin, PNPF and CNTF for each of four parameters of neuritic growth, including: initiation time, neuronal polarity, neuritic branching and average neurite output (lengths) with time. Increasing laminin and PNPF levels were found to advance the time of neurite initiation as well as shift the majority (70-80%) of the neurons from a unipolar to multipolar neuritic morphology. The polarity imposed by any given concentration of either neurite promoting factor remained constant over the 24 h culture period examined. The average lengths from the longest neurites per neuron over a 10-28 h culture interval were not affected by increasing levels of laminin or PNPF, but total neuritic output per neuron was increased. This increased total neuritic output could be attributed to a combination of earlier neuritic initiation time and an increased neuronal polarity at high laminin or PNPF levels. CNTF at threshold survival levels did not promote initiation time, neuronal polarity or total neuritic output. However, cultures receiving less CNTF than that required for maximal neuronal survival displayed an increased neuronal polarity and a reduced neuritic output before any apparent loss of neurons. Neuritic branching was not affected by either the neurite promoting or trophic factors after 24 h of culture. Laminin and PNPF were found to be indistinguishable in their effects on the ciliary ganglion neurons in each of the four parameters studied.

Preferential outgrowth of central nervous system neurites on astrocytes and Schwann cells as compared with nonglial cells in vitro

I have compared central nervous system (CNS) neurite outgrowth on glial and nonglial cells. Monolayers of glial cells (astrocytes and Schwann cells) or nonglial cells (e.g., fibroblasts) were prepared and were shown to be greater than 95% pure as judged by cell type-specific markers. These monolayers were then tested for their ability to support neurite outgrowth from various CNS explants. While CNS neurites grew vigorously on the glial cells, most showed little growth on nonglial cell monolayers. Neurites grew singly or in fine fascicles on the glial cells at rates greater than 0.5 mm/d. The neurite outgrowth on astrocytes was investigated in detail. Scanning and transmission electron microscopy showed that the neurites were closely apposed to the astrocyte surface and that the growth cones were well spread with long filopodia. There was no evidence of significant numbers of explant-derived cells migrating onto the monolayers. Two types of experiments indicated that factors associated with the astrocyte surface were primarily responsible for the vigorous neurite outgrowth seen on these cells: (a) Conditioned media from either astrocytes or fibroblasts had no effect on the pattern of outgrowth on fibroblasts and astrocytes, and conditioned media factors from either cell type did not promote neurite outgrowth when bound to polylysine-coated dishes. (b) When growing CNS neurites encountered a boundary between astrocytes and fibroblasts, they stayed on the astrocytes and did not encroach onto the fibroblasts. These experiments strongly suggest that molecules specific to the surfaces of astrocytes make these cells particularly attractive substrates for CNS neurite outgrowth, and they raise the possibility that similar molecules on embryonic glial cells may play a role in guiding axonal growth during normal CNS development.

Steady growth cone currents revealed by a novel circularly vibrating probe: a possible mechanism underlying neurite growth

The rate and direction of neurite growth have been shown in a number of studies to be determined by the distribution of adhesive sites on the growth cone. Recent evidence showing that the application of extrinsic electric fields can redistribute membrane molecules and alter both the rate and direction of neurite growth have raised the question whether endogenous electric fields might be produced by steady currents in growth cones. To investigate this question, we have devised a novel circularly vibrating microprobe capable of measuring current densities in the range of 5 nA/cm2 (near the theorectical limit of sensitivity), with a spatial resolution of 2 micron. The design of this device and the development of a novel algorithm for computing current vectors on-line is described. Using this probe we have found that cultured goldfish retinal ganglion cell growth cones generate steady inward currents at their tips. The measured currents, in the range of 10-100 nA/cm2, appear to flow into the filopodia at their tips and back outward near the junctures of the filopodia and the growth cone. The currents appear to be produced only during active growth. Ion substitution experiments support the conclusion that the majority of this current is carried by Ca2+ ions, which we postulate flow through a population of activated voltage-sensitive Ca2+ channels located on the filopodial tips. Calculation of the transmembrane current density (4 X 10(-6) nA/cm2) leads to an estimate of channel density (10 channels/micron2) in close agreement with the measured density of Ca2+ channels in other systems. The assumption that calcium channel proteins are conveyed to nerve terminals by active transport, whereas sodium channel proteins are conveyed passively by a slower somatofugal diffusion process [Strichartz et al, 1984], would explain why developing neurons tend to display Ca2+-sensitive electrogenesis at their growing tips, and Na+-sensitive action potentials later in development. In order to gain some insight into the possible role of these steady growth currents, we estimated the membrane depolarization and axial voltage gradient they produce. It is likely that the currents produce sufficient membrane depolarization (approximately equal to 4 mV) to cause autogenous activation of ion channel permeabilities. Similarly, the axial voltage gradient (approximately equal to 4 mV/cm) would be expected to move intracytoplasmic vesicles by electrophoresis at a rate (20-40 microns/hr) very close to that at which the filopodia are observed to grow.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth cone-target interactions in the frog retinotectal pathway

The growth cones of retinal ganglion cell axons were studied in the optic tract and tectum with horseradish peroxidase (HRP) histochemistry and electron microscopy. The ganglion cell growth cones has many morphological features similar to those described in vitro and in other in vivo systems. However, we found that some processes formed highly differentiated terminal arborizations, while retaining growth cones on many of their branches. In addition, ultrastructural examination of the tectal neuropil revealed that many ganglion cell axonal processes had characteristics of both growth cones and presynaptic endings. These findings are discussed in the context of the hypothesis of shifting connections and the evidence that retinotectal map formation involves several mechanisms, including a process that depends on the action potential activity in the optic fibers.

Biology of the nerve growth cone

This essay discusses the directional movements of metazoan tissue cells generally, with special emphasis on neurons, in an attempt to show that the directional movements of all share fundamental similarities.