Biology of the nerve growth cone

Vascular smooth muscle cell durotaxis depends on extracellular matrix composition

Mechanical compliance has been demonstrated to be a key determinant of cell behavior, directing processes such as spreading, migration, and differentiation. Durotaxis, directional migration from softer to more stiff regions of a substrate, has been observed for a variety of cell types. Recent stiffness mapping experiments have shown that local changes in tissue stiffness in disease are often accompanied by an altered ECM composition in vivo. However, the importance of ECM composition in durotaxis has not yet been explored. To address this question, we have developed and characterized a polyacrylamide hydrogel culture platform featuring highly tunable gradients in mechanical stiffness. This feature, together with the ability to control ECM composition, allows us to isolate the effects of mechanical and biological signals on cell migratory behavior. Using this system, we have tracked vascular smooth muscle cell migration in vitro and quantitatively analyzed differences in cell migration as a function of ECM composition. Our results show that vascular smooth muscle cells undergo durotaxis on mechanical gradients coated with fibronectin but not on those coated with laminin. These findings indicate that the composition of the adhesion ligand is a critical determinant of a cell's migratory response to mechanical gradients.

Rectified cell migration on saw-like micro-elastically patterned hydrogels with asymmetric gradient ratchet teeth

To control cell motility is one of the essential technologies for biomedical engineering. To establish a methodology of the surface design of elastic substrate to control the long-range cell movements, here we report a sophisticated cell culture hydrogel with a micro-elastically patterned surface that allows long-range durotaxis. This hydrogel has a saw-like pattern with asymmetric gradient ratchet teeth, and rectifies random cell movements. Durotaxis only occurs at boundaries in which the gradient strength of elasticity is above a threshold level. Consequently, in gels with unit teeth patterns, durotaxis should only occur at the sides of the teeth in which the gradient strength of elasticity is above this threshold level. Therefore, such gels are expected to support the long-range biased movement of cells via a mechanism similar to the Feynman-Smoluchowski ratchet, i.e., rectified cell migration. The present study verifies this working hypothesis by using photolithographic microelasticity patterning of photocurable gelatin gels. Gels in which each teeth unit was 100–120 µm wide with a ratio of ascending:descending elasticity gradient of 1:2 and a peak elasticity of ca. 100 kPa supported the efficient rectified migration of 3T3 fibroblast cells. In addition, long-range cell migration was most efficient when soft lanes were introduced perpendicular to the saw-like patterns. This study demonstrates that asymmetric elasticity gradient patterning of cell culture gels is a versatile means of manipulating cell motility.

Cerebellar Purkinje cells exhibit increased expression of HMGB-1 and apoptosis in congenital hydrocephalic H-Tx rats

BACKGROUND

Highly integrated anatomic and functional interactions between the cerebrum and the cerebellum during development have been reported. In our previous study, we conducted a proteome analysis to identify the proteins present in the congenital noncommunicating hydrocephalus in the cerebellum. We found higher expression of high-mobility group box-1 protein (HMGB-1) in hydrocephalic H-Tx rats.

OBJECTIVE

We studied the expression pattern of HMGB-1 in the cerebellum.

METHODS

We studied congenital hydrocephalic H-Tx rats aged 1 day and 7 days along with age-matched nonhydrocephalic H-Tx and Sprague-Dawley rats as controls. Gene and protein expressions of HMGB-1 in the cerebellum were assayed by real-time polymerase chain reaction and Western blotting, respectively; furthermore, immunohistochemical analyses were performed by using HMGB-1 (indicator of apoptosis), single-stranded DNA; adhesion factor related to cell migration, HNK-1; and the Purkinje cell-specific antibody, calbindin.

RESULTS

Cytoplasmic HMGB-1 expression observed in Purkinje cells in the 1-day-old hydrocephalic group was stronger than that in the nonhydrocephalic and Sprague-Dawley groups. Double fluorescent staining with single-stranded DNA confirmed that Purkinje cells were undergoing apoptosis. HNK-1 expression was lower in the Purkinje cell layer in the 7-day-old rats in the hydrocephalic group, and Purkinje cells were disrupted in comparison with the control groups. Morphological changes in the cerebellum were observed in the 7-day-old rats in the hydrocephalic group in comparison with the control groups.

CONCLUSION

Our results suggest that cerebellar neuronal cell damage in the early postnatal period may be related to the higher expression of HMGB-1 in the Purkinje cells.

Numerical study of axonal outgrowth in grooved nerve conduits

Nerve conduits with grooved inner texture, working as a topographical guidance cue, have been experimentally proved to play a significant role in axonal alignment. How grooved conduits guide axonal outgrowth is of particular interest for studying nerve regeneration. A viscoelastic model of axonal outgrowth in a conduit with a defined grooved geometry characterized by its width in the circumferential direction and its height in the radial direction is developed in this work. In this model, the axon is considered as an elastic beam and the axonal deformation and motion, including stretching, bending and torsion, are described using a Cosserat rod theory. The friction between axon and substrate is also considered in this model as well as the tip outgrowth. It is found that the directional outgrowth of the axon can be significantly improved by the grooved texture: when the groove width decreases or the groove height increases, the axonal elongation in the longitudinal direction of the conduit can be increased, which is in good agreement with experimental observations. This work is the first numerical model to study the effect of the substrate geometry on axonal outgrowth.

Keeping in touch with contact inhibition of locomotion

Contact inhibition of locomotion (CIL) is the process by which cells in vitro change their direction of migration upon contact with another cell. Here, we revisit the concept that CIL plays a central role in the migration of single cells and in collective migration, during both health and disease. Importantly, malignant cells exhibit a diminished CIL behaviour which allows them to invade healthy tissues. Accumulating evidence indicates that CIL occurs in vivo and that regulation of small Rho GTPases is important in the collapse of cell protrusions upon cell contact, the first step of CIL. Finally, we propose possible cell surface proteins that could be involved in the initial contact that regulates Rho GTPases during CIL.

SDF-1 alpha regulates mesendodermal cell migration during frog gastrulation

During frog gastrulation, mesendodermal cells become apposed to the blastocoel roof (BCR) by endoderm rotation, and migrate towards the animal pole. The leading edge of the mesendodermal cells (LEM) contributes to the directional migration of involuting marginal zone (IMZ) cells, but the molecular mechanism of this process is not well understood. Here we show that CXCR4/SDF-1 signaling mediates the directional movement of the LEM in Xenopus embryos. Expression of xCXCR4 was detected in the IMZ, and was complemented by xSDF-1alpha expression in the inner surface of the BCR. Over-expression of xCXCR4 and xSDF-1alpha caused gastrulation defects. An xCXCR4 N-terminus deletion construct and xSDF-1alpha-MO also inhibited gastrulation. Furthermore, explants of LEM migrate towards the dorsal BCR in the presence of xSDF-1alpha, and altered xCXCR4 expression in the LEM inhibited LEM migration. These results suggest that CXCR4/SDF-1 signaling is necessary for the migrations of massive numbers of cells during gastrulation.

Developmental imaging: Insights into the avian embryo

The study of embryonic events using different animal model systems is crucial for gaining insights into human development and birth defects. Biological imaging plays a major role in this effort by providing a spatiotemporal framework to link complex cell movements with molecular data. However, depending on the age of the embryo and the location of a morphogenetic event, visualization often requires the design of novel culture and imaging techniques. One of the primary model systems for biological imaging is the avian embryo, due to its accessibility to manipulation, relatively two-dimensional morphogenesis early on, and viability when grown in culture. Significant work in avian embryo culture and cell labeling, together with advances in imaging technology, now make it possible to monitor many developmental events within the period from egg laying to hatching. Here, we present the latest in avian developmental imaging, focusing on cell labeling, embryo culture, and imaging technologies.

Nitric oxide induces dose-dependent CA(2+) transients and causes temporal morphological hyperpolarization in human neutrophils

We exposed adherent neutrophils to the nitric oxide (NO)-radical donors S-nitroso-N-acetylpenicillamine (SNAP), S-nitrosoglutathione (GSNO), and sodium nitroprusside (SNP) to study the role of NO in morphology and Ca(2+) signaling. Parallel to video imaging of cell morphology and migration in neutrophils, changes in intracellular free Ca(2+) ([Ca(2+)](i)) were assessed by ratio imaging of Fura-2. NO induced a rapid and persistent morphological hyperpolarization followed by migrational arrest that usually lasted throughout the 10-min experiments. Addition of 0.5-800 microM SNAP caused concentration-dependent elevation of [Ca(2+)](i) with an optimal effect at 50 microM. This was probably induced by NO itself, because no change in [Ca(2+)](i) was observed after treatment with NO donor byproducts, i.e. D-penicillamine, glutathione, or potassium cyanide. Increasing doses of SNAP (>/=200 microM) attenuated the Ca(2+) response to the soluble chemotactic stimulus formyl-methionyl-leucyl-phenylalanine (fMLP), and both NO- and fMLP-induced Ca(2+) transients were abolished at 800 microM SNAP or more. In kinetic studies of fluorescently labeled actin cytoskeleton, NO markedly reduced the F-actin content and profoundly increased cell area. Immunoblotting to investigate the formation of nitrotyrosine residues in cells exposed to NO donors did not imply nitrosylation, nor could we mimic the effects of NO with the cell permeant form of cGMP, i.e., 8-Br-cGMP. Hence these processes were probably not the principal NO targets. In summary, NO donors initially increased neutrophil morphological alterations, presumably due to an increase in [Ca(2+)](i), and thereafter inhibited such shape changes. Our observations demonstrate that the effects of NO donors are important for regulation of cellular signaling, i.e., Ca(2+) homeostasis, and also affect cell migration, e.g., through effects on F-actin turnover. Our results are discussed in relation to the complex mechanisms that govern basic cell shape changes, required for migration.

Gradient in convergent cell movement during Fundulus gastrulation

This contribution represents a continuation of our studies of a gradient in convergent cell movement in the germ ring (GR) during Fundulus gastrulation (Trinkaus et al. [1992] J. Exp. Zool., 261:40-61). In our previous study, we discovered that cells in the dorsal GR nearest the embryonic shield (ES) move toward the ES at a net faster rate than those farther away and that this is due to less meandering. Those farther away meander more. These data suggest the hypothesis that there is a gradient of cues that direct cells to the ES. If so, cells in the ventral GR, farthest from the ES, should meander even more and hence show little or no convergence toward the ES. To test this hypothesis, we have traced the trajectories of individual cells in the midventral GR during midepiboly and have found that, although the general motile behavior of ventral GR cells is the same as that of dorsal cells, they do indeed meander much more and, as a result, show little or no directional movement toward the ES. Taken together, these results indicate that cells of the germ ring move up a gradient in directionality as they converge toward their target, the embryonic shield. One possible explanation for this is that the embryonic shield attracts cells to itself.

Delayed retraction of filopodia in gelsolin null mice

Growth cones extend dynamic protrusions called filopodia and lamellipodia as exploratory probes that signal the direction of neurite growth. Gelsolin, as an actin filament-severing protein, may serve an important role in the rapid shape changes associated with growth cone structures. In wild-type (wt) hippocampal neurons, antibodies against gelsolin labeled the neurite shaft and growth cone. The behavior of filopodia in cultured hippocampal neurons from embryonic day 17 wt and gelsolin null (Gsn-) mice (Witke, W., A.H. Sharpe, J.H. Hartwig, T. Azuma, T.P. Stossel, and D.J. Kwiatkowski. 1995. Cell. 81:41-51.) was recorded with time-lapse video microscopy. The number of filopodia along the neurites was significantly greater in Gsn- mice and gave the neurites a studded appearance. Dynamic studies suggested that most of these filopodia were formed from the region of the growth cone and remained as protrusions from the newly consolidated shaft after the growth cone advanced. Histories of individual filopodia in Gsn- mice revealed elongation rates that did not differ from controls but an impaired retraction phase that probably accounted for the increased number of filopodia long the neutrite shaft. Gelsolin appears to function in the initiation of filopodial retraction and in its smooth progression.

Specific induction of cell motility on laminin by alpha 7 integrin

Laminin, the major glycoprotein of basement membranes, actively supports cell migration in development, tissue repair, tumor growth, metastasis, and other pathological processes. Previously we have shown that the locomotion of murine skeletal myoblasts is specifically and significantly enhanced on laminin but not on other matrix proteins. One of the major laminin receptors of myoblasts is the alpha 7 beta 1 integrin, which was first described in human MeWo melanoma cells and Rugli glioblastoma cells. In order to investigate and directly test the role of the alpha 7 integrin in cell migration on laminin, we expressed the murine alpha 7B splice variant in human 293 kidney cells and 530 melanoma cells which cannot migrate on laminin and are devoid of endogenous alpha 7. Northern blotting of the transfected cells showed that the alpha 7 mRNA was expressed efficiently, and the protein was detected on the cell surface by immunofluorescence and fluorescence-activated cell sorter analysis. Cell motility measurements by computer-assisted time-lapse videomicroscopy of the alpha 7-transfected cells revealed an 8-10-fold increase in motility on laminin-1 and its E8 fragment, but not on fibronectin. Mock-transfected cells did not migrate significantly of alpha 7-transfected 293 cells through laminin-coated filters in a Boyden chamber assay was significantly enhanced in comparison to mock transfected cells. These findings prove that alpha 7 integrin expression confers a gain of function-motile phenotype to immobile cells and may be responsible for transduction of the laminin-induced cell motility.

A pollen tube growth stimulatory glycoprotein is deglycosylated by pollen tubes and displays a glycosylation gradient in the flower

In plant sexual reproduction, pollen tubes elongate from the stigma, through the stylar transmitting tissue, to the ovary of the pistil to deliver the male gametes for fertilization. TTS protein is a tobacco transmitting tissue glycoprotein shown to attract pollen tubes and promote their growth. Here, we show TTS proteins adhere to the pollen tube surface and tips, suggesting that they may serve as adhesive substrates for pollen tube growth. TTS proteins are also incorporated into pollen tube walls and are deglycosylated by pollen tubes, suggesting that they may provide nutrients to this process. Within the transmitting tissue, TTS proteins display a gradient of increasing glycosylation from the stigmatic end to the ovarian end of the style, coincident with the direction of pollen tube growth. These results together suggest that the TTS protein-bound sugar gradient may contribute to guiding pollen tubes from the stigma to the ovary.

Cytomechanics of axonal development

Mechanical tension is a robust regulator of axonal development of cultured neurons. We review work from our laboratory, using calibrated glass needles to measure or apply tension to chick sensory neurons, chick forebrain neurons, and rat PC12 cells. We survey direct evidence for two different regimes of tension effects on neurons, a fluid-like growth regime, and a nongrowth, elastic regime. Above a minimum tension threshold, we observe growth effects of tension regulating four phases of axonal development: 1. Initiation of process outgrowth from the cell body; 2. Growth cone-mediated elongation of the axon; 3. Elongation of the axon after synaptogenesis, which normally accommodates the skeletal growth of vertebrates; and 4. Axonal elimination by retraction. Significantly, the quantitative relationship between the force and the growth response is surprisingly similar to the simple relationship characteristic of Newtonian fluid mechanical elements: elongation rate is directly proportional to tension (above the threshold), and this robust linear relationship extends from physiological growth rates to far-above-physiological rates. Thus, tension apparently integrates the complex biochemistry of axonal elongation, including cytoskeletal and membrane dynamics, to produce a simple "force input/growth output" relationship. In addition to this fluid-like growth response, peripheral neurons show elastic behaviors at low tensions (below the threshold tension for growth), as do most cell types. Thus, neurites could exert small static forces without diminution for long periods. In addition, axons of peripheral neurons can actively generate modest tensions, presumably similar to muscle contraction, at tensions near zero. The elastic and force-generating capability of neural axons has recently been proposed to play a major role in the morphogenesis of the brain.

Ultrastructure of an identified array of growth cones and possible substrates for guidance in the embryonic medicinal leech, Hirudo medicinalis

The oblique muscle organizer (Comb- or C-cell) in the embryonic medicinal leech, Hirudo medicinalis, provides an amenable situation to examine growth cone navigation in vivo. Each of the segmentally iterated C-cells extends an array of growth cones through the body wall along oblique trajectories. C-cell growth cones undergo an early, relatively slow period of extension followed by later, protracted and rapid directed outgrowth. During such transitions in extension, guidance might be mediated by a number of factors, including intrinsic constraints on polarity, spatially and temporally regulated cell and matrix interactions, physical constraints imposed by the environment, or guidance along particular cells in advance of the growth cones. Growth cones and their environment were examined by transmission electron microscopy to define those factors that might play a significant role in migration and guidance in this system. The ultrastructural examination has made the possibility very unlikely that simple, physical constraints play a prominent role in guiding C-cell growth cones. No anatomically defined paths or obliquely aligned channels were found in advance of these growth cones, and there were no identifiable physical boundaries, which might constrain young growth cones to a particular location in the body wall before rapid extension. There were diverse associations with many matrices and basement membranes located above, below, and within the layer in which growth cones appear to extend at the light level. Additionally, a preliminary examination of myocyte assembly upon processes proximal to the growth cones further implicates a role for matrix-associated interactions in muscle histogenesis as well as process outgrowth during embryonic development.

Ductus arteriosus smooth muscle cell migration on collagen: dependence on laminin and its receptors

During permanent closure of the ductus arteriosus, smooth muscle cells migrate through the extracellular matrix (ECM) to form intimal mounds that occlude the vessel's lumen. Smooth muscle cells (SMC) migrate over surfaces coated with collagen in vitro. During the migration SMC also synthesize fibronectin (FN) and laminin (LN). Antibodies against FN and LN inhibit migration on collagen by 30% and 67%, respectively. Because of the apparent importance of LN in migration, we examined how SMC interact with LN and LN fragments (P1, E8, P1′, E1′, E3, E4, and G). Ductus SMC adhere to high concentrations of LN and two fragments of the molecule: P1 and E8. They use a unique set of integrin receptors to bind to LN (alpha 1 beta 1, alpha 6 beta 1 and alpha v beta 3), to P1 (alpha 1 beta 1, alpha v beta 3), and to E8 (alpha 6 beta 1, alpha v beta 3). The alpha v beta 3 integrin binds to the P1 fragment of LN in an RGD peptide-dependent manner, and to the E8 fragment in an RGD-independent manner; the RGD site on the P1 fragment probably is not available to the cell in intact LN. Antibodies against beta 1 integrins completely inhibit SMC adhesion to LN; antibodies against the alpha v beta 3 integrin do not block SMC adhesion to LN, but do prevent cell spreading. LN is also capable of interfering with SMC adhesion to other ECM components. The antiadhesive effect of LN is located in the E1′ domain. Both exogenous and endogenous LN increase SMC motility on collagen I. The locomotion-promoting activity of LN resides in the E1′ antiadhesive domain, and not in its adhesive (P1, E8) domains. LN causes a decrease in the number of focal contacts on collagen I. This might enable SMC to alter their mobility as they move through the extracellular matrix to occlude the ductus arteriosus lumen.

Stem cell factor regulates human melanocyte-matrix interactions

Stem cell factor (SCF) is hypothesized to play a critical role in the migration of melanocytes during embryogenesis because mutations in either the SCF gene, or its ligand, c-kit, result in defects in coat pigmentation in mice and in skin pigmentation in humans. In this report we directly show that SCF alters the adhesion and migration of human melanocytes to extracellular matrix (ECM) ligands and regulates integrin expression at the protein level. SCF decreased adhesion of neonatal and fetal cells to collagen IV, and increased attachment of fetal cells to laminin. Attachment of fetal cells to fibronectin was decreased, but was unchanged in neonatal cells. Flow cytometry analysis of neonatal melanocytes showed that SCF down-regulated the expression of the alpha 2 receptor, and up-regulated the expression of the alpha 3, alpha 5 and beta 1 integrin receptors. SCF down-regulated expression of alpha 2, alpha 5 and beta 1 integrins by fetal melanocytes, and up-regulated expression of the alpha v and alpha 3 integrin receptors. Analysis of melanocyte migration using time-lapse videomicroscopy showed that SCF significantly increased migration of neonatal, but not fetal, melanocytes on fibronectin (FN). We conclude that SCF regulates integrin expression at the protein level and that SCF has pleiotropic effects on melanocyte attachment and migration on ECM ligands. We suggest that this may be one mechanism by which SCF regulates melanocyte migration during development of the skin.

On the generation of form by the continuous interactions between cells and their extracellular matrix

The central issue of this essay is the problem of how multicellular organisms develop and maintain the complex architecture and intricate shape of tissues and organs. The concepts pattern formation, morphogenesis and differentiation are defined and discussed suggesting a distinction between processes that underlie uniformity (e.g. basic body plans) and those underlying inter- and intra-species variation. The initial stage of limb bone development--the formation of the mesenchymal condensation--is described in detail. On the basis of these data and many additional example from other developmental systems, the central role of continuous cell-ECM interactions in the generation of form is deduced. Evidence is provided as to the leading role of the mesenchymal-fibroblast-like cells in sculpturing tissue and organ architecture. It is proposed that a group of cells within their ECM, rather than the single cell, is the functional unit relevant to the generation of form. The continuous cell-ECM interactions lead to the generation of form not by a detailed obligate pathway, but rather by a process of 'selective stabilization' (Kirschner & Mitchison, 1986), i.e. a gradual organization into more stable structures, where existing structural configuration serve to increase the likelihood of certain configurations and reduce that of others. Data are quoted to support the notion that even cell division does not erase all the structural information imprinted in the cell. The role of the metazoan genome in morphogenesis is discussed in the light of the process of selective stabilization.

Tropic effects of otic epithelium on cochleo-vestibular ganglion fiber growth in vitro

Sensory nerve fibers of the cochleo-vestibular ganglion (CVG) innervate the otic epithelium in the early chick embryo by directed growth. To see if the target tissue could exert a tropic influence, we co-cultured CVGs from chick embryos (Hamburger-Hamilton stages 16-30) in a 3D collagen matrix with their normal target epithelium or with other epithelial tissues taken from the same or different stages of development. The pattern of neurite outgrowth and the viability of the CVG after five days in vitro were assessed histologically with a silver method. On the basis of the patterns of neurite outgrowth directed toward the epithelium, the cultures were classified as having slightly, mostly, exclusively, or no directed outgrowth. Of 49 cultures containing otic epithelium, 33 had mostly or exclusively directed growth patterns. This effect did not depend on any particular stage difference between co-cultures or on their viability in vitro. Cultures of non-sensory otic epithelium (endolymphatic duct) also presented directed growth patterns. Co-cultures with ectoderm from forelimb or visceral arch had little, if any, directed growth. The directed growth could not be explained simply as a result of guidance by non-neuronal cells or of the viability of the explants. The results are consistent with the hypothesis that the otic epithelium provides a tropic factor that attracts growing CVG fibers.

The oblique muscle organizer in Hirudo medicinalis, an identified embryonic cell projecting multiple parallel growth cones in an orderly array

The oblique muscle layer in the leech body wall is built upon the processes of a unique identified embryonic cell, the Comb- or C-cell. Each C-cell is composed of a spindle-shaped soma that projects approximately 70 parallel processes through the developing body wall at an angle oblique to the long axis. The morphogenesis of this cell and the navigation of its growth cones were examined by intracellular dye filling and antibody staining. At the earliest stages described each C-cell had about six processes, with those near the center of the cell oriented obliquely. As processes were added at the axial ends of the soma they often projected along previously developed longitudinal or circular muscle founder cells and then secondarily aligned themselves parallel to the older processes from the same C-cell. All growth cones initially extended to a particular location in the body wall, where they ceased growing until all 70 processes had been added (over the course of about 5 days). As adjacent segmental homologs met, their growth cones intermingled, eventually sorting out to align parallel. When one of these cells was ablated early--but not later--in development, the remaining adjacent segmental homologs expanded into the vacant territory, consistent with a hypothesis of mutual avoidance between segmental homologs. Most processes that expanded into the experimentally induced vacancy remained correctly oriented and parallel; the few exceptions projected instead along the mirror-image trajectory. Thus, expression of specific avoidance between adjacent C-cell processes is developmentally regulated and functions as a guidance mechanism in vivo, in that it serves to restrict possible trajectories. After aligning its growth cones, each cell stopped adding processes and the processes rapidly extended in concert along relatively precise trajectories. Processes of contralateral homologs cross to form the orthogonal grid used as a scaffold by myocytes to form the oblique muscles. The advancing fronts of growth cones reached the dorsal midline at about the same time as body closure occurs (at about Embryonic Day 20) at which time the C-cells became granular, lost processes, and presumably died. This sequence of developmental events is consistent with temporal and spatial regulation of different morphogenetic strategies, including--but not limited to--specific avoidance, and further suggests testable hypotheses of mechanisms of growth cone navigation in the intact embryo.

Relationship between neuronal migration and cell-substratum adhesion: laminin and merosin promote olfactory neuronal migration but are anti-adhesive

Regulation by the extracellular matrix (ECM) of migration, motility, and adhesion of olfactory neurons and their precursors was studied in vitro. Neuronal cells of the embryonic olfactory epithelium (OE), which undergo extensive migration in the central nervous system during normal development, were shown to be highly migratory in culture as well. Migration of OE neuronal cells was strongly dependent on substratum-bound ECM molecules, being specifically stimulated and guided by laminin (or the laminin-related molecule merosin) in preference to fibronectin, type I collagen, or type IV collagen. Motility of OE neuronal cells, examined by time-lapse video microscopy, was high on laminin-containing substrata, but negligible on fibronectin substrata. Quantitative assays of adhesion of OE neuronal cells to substrata treated with different ECM molecules demonstrated no correlation, either positive or negative, between the migratory preferences of cells and the strength of cell-substratum adhesion. Moreover, measurements of cell adhesion to substrata containing combinations of ECM proteins revealed that laminin and merosin are anti-adhesive for OE neuronal cells, i.e., cause these cells to adhere poorly to substrata that would otherwise be strongly adhesive. The evidence suggests that the anti-adhesive effect of laminin is not the result of interactions between laminin and other ECM molecules, but rather an effect of laminin on cells, which alters the way in which cells adhere. Consistent with this view, laminin was found to interfere strongly with the formation of focal contacts by OE neuronal cells.

Peanut agglutinin and chondroitin-6-sulfate are molecular markers for tissues that act as barriers to axon advance in the avian embryo

Axon outgrowth between the spinal cord and the hindlimb of the chick embryo is constrained by three tissues that border axon pathways. Growth cones turn to avoid the posterior sclerotome, perinotochordal mesenchyme, and pelvic girdle precursor during normal development and after experimental manipulation. We wanted to know if these functionally similar barriers to axon advance also share a common molecular composition. Since the posterior sclerotome differentially binds peanut agglutinin (PNA) and since PNA binding is also typical of prechondrogenic differentiation, we examined the pattern of expression of PNA binding sites and cartilage proteoglycan epitopes in relation to axon outgrowth. We found that all three barrier tissues preferentially express both PNA binding sites and chondroitin-6-sulfate (C-6-S) immunoreactivity at the time when growth cones avoid these tissues. Moreover, both epitopes are expressed in the roof plate of the spinal cord and in the early limb bud, two additional putative barriers to axon advance. In contrast, neither epitope is detected in peripheral axon pathways. In the somites, this dichotomous pattern of expression clearly preceded the invasion of the anterior sclerotome by either motor growth cones or neural crest cells. However, in the limb, barrier markers disappeared from presumptive axon pathways in concert with the invasion of axons. Since this coordinate pattern suggested that the absence of barrier markers in these axon pathways requires an interaction with growth cones, we analyzed the pattern of barrier marker expression following unilateral neural tube deletions. We found that PNA-negative axon pathways developed normally even in the virtual absence of axon outgrowth. We conclude that the absence of staining with carbohydrate-specific barrier markers is an independent characteristic of the cells that comprise axon pathways. These results identify two molecular markers that characterize known functional barriers to axon advance and suggest that barrier tissues may impose patterns on peripheral nerve outgrowth by virtue of their distinct molecular composition.

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.

Contact response of cells can mediate morphogenetic pattern formation

Theories of morphogenetic pattern formation have included Turing's chemical prepatterns, mechanochemical interactions, cell sorting, and other mechanisms involving guided motion or signalling of cells. Many of these theories presuppose long-range cellular communication or other controls such as chemical concentration fields. However, the possibility that direct interactions between cells can lead to order and structure has not been seriously investigated in mathematical models. In this paper we consider this possibility, with emphasis on cells that reorient and align with each other when they come into contact. We show that such contact responses can account for the formation of multicellular patterns called parallel arrays. These patterns typically occur in tissue cultures of fibroblasts, and consist of clusters of cells sharing a common axis of orientation. Using predictions of a mathematical model and computer simulations of cell motion and interactions we show that contact responses alone, in the absence of other global controls, can promote the formation of these patterns. We suggest other situations in which patterns may result from direct cellular communication. Previous theories of morphogenesis are briefly reviewed and compared with this proposed mechanism.

Growth cone behavior and production of traction force

The growth cone must push its substrate rearward via some traction force in order to propel itself forward. To determine which growth cone behaviors produce traction force, we observed chick sensory growth cones under conditions in which force production was accommodated by movement of obstacles in the environment, namely, neurites of other sensory neurons or glass fibers. The movements of these obstacles occurred via three, different, stereotyped growth cone behaviors: (a) filopodial contractions, (b) smooth rearward movement on the dorsal surface of the growth cone, and (c) interactions with ruffling lamellipodia. More than 70% of the obstacle movements were caused by filopodial contractions in which the obstacle attached at the extreme distal end of a filopodium and moved only as the filopodium changed its extension. Filopodial contractions were characterized by frequent changes of obstacle velocity and direction. Contraction of a single filopodium is estimated to exert 50-90 microdyn of force, which can account for the pull exerted by chick sensory growth cones. Importantly, all five cases of growth cones growing over the top of obstacle neurites (i.e., geometry that mimics the usual growth cone/substrate interaction), were of the filopodial contraction type. Some 25% of obstacle movements occurred by a smooth backward movement along the top surface of growth cones. Both the appearance and rate of movements were similar to that reported for retrograde flow of cortical actin near the dorsal growth cone surface. Although these retrograde flow movements also exerted enough force to account for growth cone pulling, we did not observe such movements on ventral growth cone surfaces. Occasionally obstacles were moved by interaction with ruffling lamellipodia. However, we obtained no evidence for attachment of the obstacles to ruffling lamellipodia or for directed obstacle movements by this mechanism. These data suggest that chick sensory growth cones move forward by contractile activity of filopodia, i.e., isometric contraction on a rigid substrate. Our data argue against retrograde flow of actin producing traction force.

Target control of collateral extension and directional axon growth in the mammalian brain

Individual neurons in the brain send their axons over considerable distances to multiple targets, but the mechanisms governing this process are unresolved. An amenable system for studying axon outgrowth, branching, and target selection is the mammalian corticopontine projection. This major connection develops from parent corticospinal axons that have already grown past the pons, by a delayed interstitial budding of collateral branches that then grow directly into their target, the basilar pons. When cocultured with explants of developing cortex in three-dimensional collagen matrices, the basilar pons elicits the formation and directional growth of cortical axon collaterals across the intervening matrix. This effect appears to be target-specific and selectively influences neurons in the appropriate cortical layer. These in vitro findings provide evidence that the basilar pons becomes innervated by controlling at a distance the budding and directed ingrowth of cortical axon collaterals through the release of a diffusible, chemotropic molecule.

Selective growth of sensory nerve fibers on metal oxide pattern in culture

Metal oxides were used to study how the sensory nerve fibers recognize surface properties. Neurites selectively grow on the metal oxides deposited on silica glass, being guided along the axial direction of the patterns. The guiding ability depends on the electronegativity of the metal in metal oxide. Aluminum oxide or indium oxide patterns showed a remarkable ability to guide the growth direction. Neurites recognize the differences in surface properties (which are reflected by electronegativity) between metal oxides when the metal oxide substrata are only 1 micron in width.

Direct evidence that growth cones pull

There is controversy over whether axonal elongation is the result of a pulling growth cone and the role of tension in axonal elongation. Earlier in this decade, the consensus was that axons or neurites elongated from tension generated by forward motility of the growth cone. It was presumed that contractile filopodia were the source of the tension moving the growth cone. But this view was challenged by experiments showing that neurites elongate, albeit abnormally, in the presence of cytochalasin, which inhibits growth-cone and filopodial movements. Additionally, high resolution, video-enhanced observations of growth-cone activity argued against filopodial shortening as a source of tension, suggesting instead that an extrusion of cytoplasm rather than a pulling process, is the key event in neurite elongation. Studies of slow axonal transport, however, indicate that much slower cytoskeletal pushing underlies axonal elongation. We report here direct measurements of neurite force as a function of growth-cone advance which show that they are linearly related and accompanied by apparent neurite growth. No increase in force occurs in neurites whose growth cone fails to advance.

Altered cell shapes in fibroblasts treated with 5'-deoxy-5'-methylthioadenosine: relation to morphogenesis of neural cells

In the present study, mouse 3T3 fibroblasts and primary human foreskin fibroblasts exposed to MTA (5'-deoxy-5'-methylthioadenosine) were found to achieve neural-like cell shapes and to extend long, multipolar processes rapidly and reversibly. Time lapse recordings and pharmacologic studies revealed that process formation in MTA-treated fibroblasts was mechanistically related to the rapid-onset mode of neurite formation previously characterized in neural hybrid NG108-15 cells. These data, together with evidence presented elsewhere, indicate that MTA selectively reorganizes the mode of expression of a specific cytoplasmic machinery that is active in many types of cells, and which is involved in regulating cell shape and neurite formation in developing neurons.

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.

Morphologic plasticity of rapid-onset neurites in NG108-15 cells stimulated by substratum-bound laminin

Undifferentiated NG108-15 cells, when replated onto laminin-coated substrata, extend multipolar, highly branched neurite-like extensions up to 200 microns in length within 4 h; morphologic and pharmacologic properties of these 'rapid-onset neurites' have been described recently. The present study has extended these observations, using time lapse video recordings of their dynamic behavior and additional pharmacologic studies. Rapid-onset neurites and neuronal growth cones were shown to be regulated in an identical manner in all respects examined, including inhibition of outgrowth by cytochalasin B. Of particular interest was the observation that rapid-onset neurites in contact with laminin exhibited an extremely high rate of turnover, which was inhibited by 5'-deoxy-5'-methylthioadenosine (MTA). This system provides a uniquely favorable in vitro preparation in which neuritic plasticity can be elicited, directly observed and experimentally modulated under controlled conditions.

Analysis of slow-onset neurite formation in NG108-15 cells: implications for a unified model of neurite elongation

When undifferentiated NG108-15 cells are plated onto polylysine coated Petri dishes in serum-free medium, they form neurites within 1-4 h if plated in the presence of laminin or 5'-deoxy-5'-methylthioadenosine (rapid-onset neurites). In the absence of such agents, serum-deprived NG108-15 cells extend axon-like neurites onto polylysine over several days; here we characterize the dynamic behavior of this slow-onset outgrowth pattern in detail. Individual cells plated on laminin expressed a gradual multipolar-to-unipolar transition due to rapid-onset neurites becoming remodelled into the appearance of slow-onset neurites. This phenomenon reflected the selective stabilization of certain rapid-onset neurites, along with the restriction of motility to their distal tips. Based upon the properties and interactions of both rapid- and slow-onset neurites in NG108-15 cells, a unified model for neurite formation is presented.

Axon guidance and the patterning of neuronal projections in vertebrates

Over the past decade, new insights have been obtained into the cellular strategies and molecular mechanisms that guide axons to their targets in the developing vertebrate nervous system. Axons select pathways by recognizing specific cues in their environment. These cues include cell surface and extracellular matrix molecules that mediate cell and substrate adhesion and axon fasciculation, molecules with contact-dependent inhibitory properties, and diffusible tropic factors. Several guidance cues may operate in a coordinated way to generate the distinct axonal trajectories of individual neurons.

Extracellular matrix triggers a directed cell migratory response in sea urchin primary mesenchyme cells

The role of extracellular matrix in cell migration has generally been considered in terms of a substratum. However, when thin cell processes from migrating sea urchin primary mesenchyme cells contact small latex beads coated with extracellular matrix from the blastocoel, the cells migrate directly to the coated beads. Since the beads are not anchored, this result indicates that highly localized contact with the extracellular matrix can stimulate movement independently of any change in cell adhesion.

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.

Tension and compression in the cytoskeleton of PC-12 neurites. II: Quantitative measurements

We assessed the mechanical properties of PC-12 neurites by applying a force with calibrated glass needles and measured resulting changes in neurite length and deflection of the needle. We observed a linear relationship between force and length change that was not affected by multiple distensions and were thus able to determine neurite spring constants and initial, nondistended, rest tensions. 81 out of 82 neurites showed positive rest tensions ranging over three orders of magnitude with most values clustering around 30-40 mu dynes. Treatment with cytochalasin D significantly reduced neurite rest tensions to an average compression equal to 14% of the former tension and spring constants to an average of 17% of resting values. Treatment with nocodazole increased neurite rest tensions to an average of 282% of resting values but produced no change in spring constant. These observations suggest a particular type of complementary force interaction underlying axonal shape; the neurite actin network under tension and neurite microtubules under compression. Thermodynamics suggests that microtubule (MT) assembly may be regulated by changes in compressive load. We tested this effect by releasing neurite attachment to a polylysine-coated surface with polyaspartate, thus shifting external compressive support onto internal elements, and measuring the relative change in MT polymerization using quantitative Western blotting. Neurons grown on polylysine or collagen without further treatment had a 1:2 ratio of soluble to polymerized tubulin. When neurites grown on polylysine were treated with 1% polyaspartate for 15-30 min, 80% of neurites retracted, shifting the soluble: polymerized tubulin ratio to 1:1. Polyaspartate treatment of cells grown on collagen, or grown on polylysine but treated with cytochalasin to reduce tension, caused neither retraction nor a change in the soluble:polymerized tubulin ratio. We suggest that the release of adhesion to the dish shifted the compressive load formerly borne by the dish onto Mts causing their partial depolymerization. Our observations are consistent with the possibility that alterations in MT compression during growth cone advance integrates MT assembly with the advance.

A scanning electron microscope study of preserved allograft tympanic membranes: a comparison with autogenous grafts, xenografts and the normal eardrum

Scanning electron microscopy was used to investigate the surface architecture of the human tympanic membrane. The morphology of the eardrum was compared with the surface structures of preserved tympanic membranes (allografts), fresh air-dried temporalis fascia and preserved calf jugular veins (xenografts). The role of the physical structure and the composition of the extracellular matrix in the regeneration of a tympanic graft is discussed.

Directional cell movement during early development of the teleost Blennius pholis: I. Formation of epithelial cell clusters and their pattern and mechanism of movement

Embryos of the teleost Blennius pholis provide exceptional material for observation of the formation and movement of cell clusters in vivo because the clusters are packed with melanosomes and migrate beneath the transparent enveloping layer. These clusters arise from two pigmented cell masses (PCM) which appear precociously on either side of the embryonic axis at 3/5 epiboly, at the future level of somites 1 and 2. As development proceeds, each PCM enlarges and spreads on its lateral margins to form an epithelial sheet. As spreading continues, the sheet fragments, forming small cell clusters that move in a distad direction in the yolk sac. The highly motile lateral marginal cells of the spreading PCM, as well as those of the marginal cells of each moving cluster, invariably protrude highly flattened lamellipodia, which terminate in long, fine, often branched filopodia. As cell clusters leave the PCM, they form long, taut retraction fibers. The rate of spreading of both the lateral edge of the PCM and the initial phase of cluster movement, is higher (1.0 micron/min or greater) than the later rate of cluster movement, apparently because at this phase, motile activity is confined to the distal borders of each. This directional migration ceases in 24 h at 16 degrees - 18 degrees C, when the farthest clusters have reached the ventral region of the yolk sac. By then, all clusters are spaced more or less evenly, apparently due to cessation of all cluster movement at about the same time. Once movement ceases, the clusters remain immobile for 2-4 days, depending on the temperature.

Dopamine and serotonin inhibition of neurite elongation of different identified neurons

This study demonstrates that a second classical neurotransmitter, dopamine, can act to suppress regenerative neurite outgrowth. Single identified neurons were dissected from two central ganglia of the snail Helisoma, and growth cone motility was studied as neurites regenerated in cell culture. Both dopamine and serotonin inhibited growth cone motility and elongation of neurites. Outgrowth inhibition ranged from sustained arrest to a similar but transient response. The effects of dopamine and serotonin are neuron-selective. Specific neurons affected by dopamine and serotonin represent distinct sets. One neuron was found that responds to both agents. The implications of neurotransmitter regulation of the dynamics of neuronal morphology are discussed.

The 180-kD component of the neural cell adhesion molecule N-CAM is involved in cell-cell contacts and cytoskeleton-membrane interactions

N-CAM180, the molecular form of the three neural cell adhesion molecules (N-CAM) with the largest cytoplasmic domain, is accumulated at sites of cell-cell contact (cell bodies, neurites, growth cones) in cultures of neuroblastoma and cerebellum. At these sites the cytoskeleton-membrane linker protein brain spectrin and actin are also accumulated. Brain spectrin copurifies with N-CAM180 by immunoaffinity chromatography and binds specifically to N-CAM180 but not to N-CAM140 or N-CAM120 in a solid-phase binding test. These observations indicate an association of N-CAM180 with the cytoskeleton in vivo. This association may underlie the reduced lateral mobility of N-CAM180 in the surface membrane compared to N-CAM140 (Pollerberg et al. 1986). Together with the fact that N-CAM180 is only expressed after termination of neuron migration in vivo (Persohn and Schachner, unpublished) these results suggest a role for N-CAM180 in stabilization of cell contacts.

Axonal growth cones in the developing amphibian spinal cord

Axonal growth cones in the spinal cord of embryonic and larval Xenopus (stages 24-48) were filled with the anatomical tracer horseradish peroxidase (HRP). Growth cones of lateral and ventral marginal zones, including those of descending spinal and supraspinal pathways, were labeled by application of tracer to the caudal medulla or to one of several levels of the spinal cord. Central axons of sensory neurons were filled via their peripheral processes. Growth cone configuration varied widely but fell into five general categories: complex with both filopodia and veils, filopodia only, lamellipodia only, clavate, and fusiform. Several general patterns emerged from the distribution of these various configurations. Growth cones of younger animals generally were more complex than those of older ones. Growth cones closer to the leading edge of descending fiber bundles were more elaborate than those that followed. Growth cones of the dorsolateral fascicle, which carries ascending central processes of Rohon-Beard and sensory ganglion neurons, were very simple and followed a straight course rostrally, whereas those of descending axons of the lateral fiber areas were more complex and sometimes spread over almost the entire lateral marginal zone. Growth cones of Rohon-Beard central ascending axons were fusiform or clavate, while those of sensory ganglion axons showed several fine filopodia at their tips. Growth cones of both types of sensory axons change configuration as they approached the hindbrain, becoming more complex. This study demonstrates that the configurations of growth cones belonging to the same axonal pathway vary with age and with position along their routes, and that growth cones of different neuron classes exhibit characteristic ranges of morphological variation.

Cytokine-induced pseudopodial protrusion is coupled to tumour cell migration

Pseudopodia protrusion is a prominent feature of actively motile cells in vitro and invading tumour cells in vivo; however, the function and regulation of pseudopodia are poorly understood. Tumour autocrine motility factor (AMF) represents a new class of cytokines which are secreted by tumour cells and embryonic cells and induce random motility in the producer cells or in heterologous cells with appropriate receptors. Here we report that a major effect of this factor is to induce the extension of cell pseudopodia before cell translocation. Using a new method to quantify and isolate pseudopodia, we find that human breast carcinoma cell AMF (at concentrations of 1 nM or below) stimulates random pseudopodia formation in a dose-dependent and time-dependent manner. Anti-AMF antibodies inhibit pseudopodia protrusion and cell motility, showing the importance of pseudopodia formation during locomotion. AMF-stimulated motility and pseudopodia formation occur on a wide variety of adhesive substrata which suggests that certain intrinsic motility events are independent of the attachment mechanism. Induced pseudopodia show a prominent axial actin network in the electron microscope. The number of laminin receptor and fibronectin RGD recognition sites is increased by a factor of 20 in the induced pseudopodia when compared to the average distribution in unstimulated cells. Exploratory pseudopodia regulated by cell-derived motility factors contain receptors for matrix proteins and could serve as 'senseorgans' essential to the process of cell locomotion.

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.

Kinetic analysis of 'rapid onset' neurite formation in NG108-15 cells reveals a dual role for substratum-bound laminin

Undifferentiated neural hybrid NG108-15 cells plated on laminin-coated, polylysine-treated plastic Petri dishes in minimal serum-free media formed long neurites within 1-4 h post-plating. Morphologic features and pharmacologic responses of these 'rapid onset' neurites were strikingly similar to those of neuronal growth cones. Cycloheximide (1-10 micrograms/ml) and forskolin (10(-7) to 10(-6) M) accelerated the initial formation of laminin-stimulated neurites, but did not cause rapid onset neurites to emerge upon Petri dishes coated with polylysine alone. Quantitative study of the substratum-dependent effect of cycloheximide showed that it was additive even with maximally effective amounts of laminin, independent of the magnitude of the laminin-stimulated baseline rate and not limited by an inherent ceiling in the rate at which neurites could form. The methylation inhibitor 5'-deoxy-5'-methyl thioadenosine (MTA) (3 X 10(-4) to 3 X 10(-3) M) did stimulate neurites to form on polylysine. MTA- and laminin-stimulated neurites were similar in their susceptibility to calmodulin antagonists and the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA). However, formation of MTA-stimulated neurites was not accelerated by cycloheximide. A simple two-compartment kinetic model of 'rapid onset' neurite formation is proposed: compartment A is common to both laminin- and MTA-stimulated neurites. Compartment B is affected by cycloheximide, and its access to the neurite formation machinery contained in compartment A is gated according to the nature of the substratum. In addition to its direct effects, laminin controls the relative dominance of the two kinetic compartments via modulating the effectiveness of other signals acting upon an endogenously active compartment B.

Response of sensory neurites and growth cones to patterned substrata of laminin and fibronectin in vitro

During neurite elongation in the developing peripheral nervous system, the distribution of laminin and fibronectin may provide preferred substrates for neurite elongation. In this study, the response of sensory neurites and growth cones to patterns of laminin or fibronectin applied to a background substrate of Type IV collagen was studied to determine any possible substrate preference. Neurites exhibited elongation restricted to a laminin pattern, but not a fibronectin pattern, indicating that sensory neurites prefer to elongate on laminin compared to Type IV collagen. When polylysine is included in the background substrate, neurite preference for laminin is decreased. Laminin also enhances neurite elongation and defasciculation and stabilizes growth cone protrusions. These results suggest an adhesive as well as a cytoskeletal involvement in the response to laminin, but direct adhesion estimates indicate that laminin decreases overall adhesion, arguing against an adhesive involvement. Regardless of the mechanism involved, the observed neurite preference for laminin is consistent with the hypothesis that spatial and temporal laminin distributions provide preferred pathways for peripheral neurite elongation.