Invited review: guidance cue patterns and cell migration in multicellular organisms

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

Rapid Quantification of 3D Collagen Fiber Alignment and Fiber Intersection Correlations with High Sensitivity

Metastatic cancers aggressively reorganize collagen in their microenvironment. For example, radially orientated collagen fibers have been observed surrounding tumor cell clusters in vivo. The degree of fiber alignment, as a consequence of this remodeling, has often been difficult to quantify. In this paper, we present an easy to implement algorithm for accurate detection of collagen fiber orientation in a rapid pixel-wise manner. This algorithm quantifies the alignment of both computer generated and actual collagen fiber networks of varying degrees of alignment within 5°°. We also present an alternative easy method to calculate the alignment index directly from the standard deviation of fiber orientation. Using this quantitative method for determining collagen alignment, we demonstrate that the number of collagen fiber intersections has a negative correlation with the degree of fiber alignment. This decrease in intersections of aligned fibers could explain why cells move more rapidly along aligned fibers than unaligned fibers, as previously reported. Overall, our paper provides an easier, more quantitative and quicker way to quantify fiber orientation and alignment, and presents a platform in studying effects of matrix and cellular properties on fiber alignment in complex 3D environments.

An epigenetic model for pigment patterning based on mechanical and cellular interactions

Pigment patterning in animals generally occurs during early developmental stages and has ecological, physiological, ethological, and evolutionary significance. Despite the relative simplicity of color patterns, their emergence depends upon multilevel complex processes. Thus, theoretical models have become necessary tools to further understand how such patterns emerge. Recent studies have reevaluated the importance of epigenetic, as well as genetic factors in developmental pattern formation. Yet epigenetic phenomena, specially those related to physical constraints that might be involved in the emergence of color patterns, have not been fully studied. In this article, we propose a model of color patterning in which epigenetic aspects such as cell migration, cell-tissue interactions, and physical and mechanical phenomena are central. This model considers that motile cells embedded in a fibrous, viscoelastic matrix-mesenchyme-can deform it in such a way that tension tracks are formed. We postulate that these tracks act, in turn, as guides for subsequent cell migration and establishment, generating long-range phenomenological interactions. We aim to describe some general aspects of this developmental phenomenon with a rather simple mathematical model. Then we discuss our model in the context of available experimental and morphological evidence for reptiles, amphibians, and fishes, and compare it with other patterning models. We also put forward novel testable predictions derived from our model, regarding, for instance, the localization of the postulated tension tracks, and we propose new experiments. Finally, we discuss how the proposed mechanism could constitute a dynamic patterning module accounting for pattern formation in many animal lineages.

Adhesion, alignment, and migration of cultured Schwann cells on ultrathin fibronectin fibres

Individual fibres of fibronectin (Fn-fibres), an extracellular matrix cell adhesion glycoprotein, were produced from a purified solution of fibronectin. These fibres range from 0.5-7 microm in width and have been engineered to produce mats (Fn-mats) by using a unidirectional shear force to orientate the fibres. Fn-fibres have been shown to promote alignment by contact guidance of human dermal fibroblasts, neurites, macrophages, and epitenon fibroblasts. Fn-mats have been used to orientate and enhance the regeneration of peripheral nerve components. We investigated cell spreading, orientation, formation of focal contacts, and the speed of cell movement on individual Fn-fibres, glass-covered with poly-L-lysine and poly-L-lysine/laminin/Fn. Fibronectin fibres significantly promoted cell spreading and the speed of cell migration with alignment of focal contacts and F-actin filaments to the axis of the fibres. The study reveals the potential of Fn-fibres to guide and direct cellular behaviour by contact guidance. The increase in migration and other behaviour exhibited by Schwann cells on Fn-fibres justifies the use of Fn-mats for peripheral nerve repair and is clinically important in that atrophy of the target organ, which is the most common failure of nerve repair, may be minimised.

The early neuronal organization predicts the path followed by some major axonal bundles in the embryonic brainstem

In the embryonic CNS, preformed pathways precede the growth of axonal fasciculi [Katz M. J. and Lasek R. J. (1980) Cell Motil. 1, 141-157; Katz M. J. et al. (1980) Neuroscience 5, 821-833]. What are the developmental events that lead to the elaboration of these preformed pathways? To answer this question, we investigated the organization of the primitive neural tube and more particularly the arrangement of the early-generated cells using [3H]thymidine autoradiography or bromodeoxyuridine. Our data suggest that the position of early-generated cells might be involved in the setting of such pathways. In the brain stem of E12(0) (12 days and 0 h) and E12(15) rat embryos, the first-generated cells were organized into three longitudinal columns associated with glycoconjugate-rich extracellular spaces in the adjacent primitive marginal layer. Also, axons traced with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) were contiguous to the early-generated cellular columns and represented the primordium of the medial longitudinal fasciculus, the lateral longitudinal tract and the mesencephalic trigeminal tract. Our results show a correlation between the organization of early-generated cells, likely neurons, and the pattern of extracellular spaces in the marginal layer where axons grow. It has been reported in the literature that neurons produce elements of the extracellular matrix such as growth-modulating molecules or space-creating molecules. We therefore suggest that the position of early-generated neurons could be involved in the elaboration of a template for the setting of some major longitudinal tracts during embryonic development of the brainstem.

Isolation and characterization of human malignant glioma cells from histologically normal brain

Brain invasion prevents complete surgical extirpation of malignant gliomas; however, invasive cells from distant, histologically normal brain previously have not been isolated, cultured, and characterized. To evaluate invasive human malignant glioma cells, the authors established cultures from gross tumor and histologically normal brain. Three men and one woman, with a mean age of 67 years, underwent two frontal and two temporal lobectomies for tumors, which yielded specimens of both gross tumor and histologically normal brain. Each specimen was acquired a minimum of 4 cm from the gross tumor. The specimens were split: a portion was sent for neuropathological evaluation (three glioblastomas multiforme and one oligodendroglioma) and a portion was used to establish cell lines. Morphologically, the specimens of gross tumor and histologically normal brain were identical in three of the four cell culture pairs. Histochemical staining characteristics were consistent both within each pair and when compared with the specimens sent for neuropathological evaluation. Cultures demonstrated anchorage-independent growth in soft agarose and neoplastic karyotypes. Growth rates in culture were greater for histologically normal brain than for gross tumor in three of the four culture pairs. Although the observed increases in growth rates of histologically normal brain cultures do not correlate with in vivo behavior, these findings corroborate the previously reported stem cell potential of invasive glioma cells. Using the radial dish assay, no significant differences in motility between cultures of gross tumor and histologically normal brain were found. In summary, tumor cells were cultured from histologically normal brain acquired from a distance greater than 4 cm from the gross tumor, indicating the relative insensitivity of standard histopathological identification of invasive glioma cells (and hence the inadequacy of frozen-section evaluation of resection margins). Cell lines derived from gross tumor and histologically normal brain were usually histologically identical and demonstrated equivalent motility, but had different growth rates.

A methodology for the systematic and quantitative study of cell contact guidance in oriented collagen gels. Correlation of fibroblast orientation and gel birefringence

Despite the likely role of contact guidance in every physiological process involving cell migration, its study in a three-dimensional tissue-equivalent environment has been precluded, heretofore, by inherent difficulties in systematically preparing well-defined contact guidance fields and quantifying the resultant contact guidance. Here, we describe a novel use of a magnetic field to orient collagen fibrils during fibrillogenesis, entrapping cells dispersed in the collagen solution. Using computer-controlled staging and image analysis, we show from automated birefringence measurements of the resultant slab of cell-populated gel contained in a specially designed observation chamber that the fibril orientation is biased along the long axis of the chamber uniformly throughout the chamber. Further, we show that the degree of fibril orientation, and consequently the elicited contact guidance, can be controlled by independently varying the magnetic field strength or temperature during fibrillogenesis. We characterize the contact guidance response to the imposed contact guidance field by measuring cell orientation relative to the axis of fibril orientation from still images obtained in time-lapse via automated image analysis. We present the first quantitative correlation of contact guidance (based on cell orientation) with collagen fibril orientation (based on birefringence) for human foreskin fibroblasts cultured in a collagen gel, by using gels of varying orientation resulting from different magnetic field strengths and temperatures during fibrillogenesis, and by using sufficiently low cell concentrations and early observation times.

Neurite outgrowth on immobilized axonin-1 is mediated by a heterophilic interaction with L1(G4)

Axonin-1 is an axon-associated cell adhesion molecule with dualistic expression, one form being glycophosphatidylinositol-anchored to the axonal membrane, the other secreted from axons in a soluble form. When presented as a substratum for neuronal cultures it strongly promotes neurite outgrowth from chicken embryonic dorsal root ganglia neurons. In this study, the axon-associated cell adhesion molecule G4, which is identical with Ng-CAM and 8D9, and homologous or closely related to L1 of the mouse and NILE of the rat, was investigated with respect to a receptor function for axonin-1. Using fluorescent microspheres with covalently coupled axonin-1 or L1(G4) at their surface we showed that these proteins bind to each other. Within the sensitivity of this microsphere assay, no interaction of axonin-1 with itself could be detected. Axonin-1-coated microspheres also bound to the neurites of cultured dorsal root ganglia neurons. This interaction was exclusively mediated by L1(G4), as indicated by complete binding suppression by monovalent anti-L1(G4) antibodies. The interaction between neuritic L1(G4) and immobilized axonin-1 was found to mediate the promotion of neurite growth on axonin-1, as evidenced by the virtually complete arrest of neurite outgrowth in the presence of anti-L1(G4) antibodies. Convincing evidence has recently been presented that neurite growth on L1(8D9) is mediated by the homophilic binding of neuritic L1(G4) (1989. Neuron. 2: 1597-1603). Thus, both L1(G4)- and axonin-1-expressing axons may serve as "substrate pathways" for the guidance of following axons expressing L1(G4) into their target area. Conceivably, differences in the concentration of axonin-1 and L1(G4), and/or modulatory influences on their specific binding parameters in leading pathways and following axons could represent elements in the control of axonal pathway selection.

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.

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.

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.

Age and cell type influences on nerve--non-nerve contact interactions

Eight-day ciliary ganglion neurons respond in a significantly different fashion to contact with dorsal root ganglia non-neurons than do 8-day dorsal root neurons. The ciliary neuron-dorsal root non-neuron interactions result in contact inhibition of both cells, whereas in the dorsal root neuron-dorsal root non-neuron case no such inhibition is observed. In addition, contact of 8- and 14-day ciliary neurons with heart fibroblasts results in inhibition of locomotion. However, the response of the fibroblast to contact with these neurons of different ages varies in a predictable fashion.

Early axon patterns of the spinal cord: experiments with a computer

In the vertebrate central nervous system, most axons appear in one of two elemental patterns--sheets or bundles. Many developmental mechanisms are involved in the formation of the elemental axon patterns, and these mechanisms often act simultaneously. The major axon-patterning mechanisms include differential adhesivity, internal growth constraints on axons, and initial orientation of axonal outgrowth. To evaluate the effects of these mechanisms on the formation of axon patterns, a computer was used to model axonal growth. Experiments with the computer model suggest that axon sheets are produced by the cooperative action of more than one mechanism. Furthermore, in the appropriate combination, these mechanisms produce orderly axon sheets even on patternless substrates. On the other hand, to transform the sheets into axon bundles, the substrate must be patterned.

The control of cell motility during embryogenesis

Morphogenesis is the establishment during development of the complex organization of tissues and organs that characterizes the adult. In multicellular animals, one of the most important processes is morphogenetic movement, the translocation of individual cells or whole tissue rudiments from one site in the body to another. Active cellular locomotion is important in many situations of morphogenetic movement. Characteristically, cell migration in the embryo displays impressive precision: cells at defined sites in the embryo begin migration at particular stages of development, traverse precisely-characterized pathways during migration, and localize finally at particular sites in the body, in specific association with other tissues. One of the most challenging problems of experimental biology is the definition of the mechanisms that regulate the active migration of embryonic cells and tissues. Recent years have seen gratifying progress in this direction, with the definition and characterization of a number of processes of potential importance. This review describes selected instances of morphogenetic movement and contains a discussion of our current understanding of the problem of regulation of cell motility in the embryo.

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.

Neurotrophic interactions during in vitro development of the inner ear

The objective of this study was to explore the hypothesis that developing labyrinthine sensory receptors attract ingrowing neurites by chemotaxis. Cocultured otic explants which shared a single statoacoustic nerve (VIIIn) ganglion were explanted from 11-, 12.5-, and 14-day-old mouse embryos. Heterotypic ganglion explants consisted of 12-day-old otic explants which had their VIIIn replaced by a 10.5-day-old trigeminal nerve (Vn) ganglion. All cultures were grown to the equivalent of 20 days of gestation. Neurites of the (+) VIIIn explants grew into the sensory areas of both the (+) and (-) VIIIn cocultured explants. Neurites of the 14-day-old cocultured otic explants were only found in association with sensory areas within the (+) VIIIn explant. Neurites of nerve V of the heterotypic ganglion explants were found in association with the sensory areas of these otic explants. These results support the hypothesis that a limited period of chemotaxis (nonspecific in nature) is a possible mechanism for the establishment of the pattern of neurite ingrowth to the areas of the inner ear sensory receptors.

An SEM examination of granule cell migration in the mouse cerebellum

The inward migration of external granule cells (EGC) from the pial surface of the developing cerebellum to form the (internal) granule cell layer was examined using SEM. Cerebella from male mice ranging in age from days 1-20 were fixed, then fractured through the developing pyramid region. EGC were initially unspecialized cells, forming 2-3 layers at the pial surface. EGC layers increased to 6-8, granule cells in the deeper regions elongated, and a prominent space formed between superficial and deep (premigratory) strata. During peak migration (days 8-12), nests of 4-6 EGC were associated with Bergmann glial fibers (BF) of the Golgi epithelial cells, which crossed molecular and EGC layers to terminate as spiny endfeet at the pial surface. Fibrils of extracellular material (ECM) often linked both premigratory and migrating EGC with a nearby BF. The molecular layer thickened considerably and the parallel fibers were traversed by an increasing number of Bergmann fibers and Purkinje cell processes during this period. As active migration slowed (days 13-20) and EGC reached their destination below the Purkinje cell layer, they lost their polarity and were enmeshed in ECM. The role of the Bergmann fibers and extracellular material in granule cell migration is considered.

Ontophyletics of the nervous system: development of the corpus callosum and evolution of axon tracts

The evolution of nervous systems has included significant changes in the axon tracts of the central nervous system. These evolutionary changes required changes in axonal growth in embryos. During development, many axons reach their targets by following guidance cues that are organized as pathways in the embryonic substrate, and the overall pattern of the major axon tracts in the adult can be traced back to the fundamental pattern of such substrate pathways. Embryological and comparative anatomical studies suggest that most axon tracts, such as the anterior commissure, have evolved by the modified use of preexisting substrate pathways. On the other hand, recent developmental studies suggest that a few entirely new substrate pathways have arisen during evolution; these apparently provided opportunities for the formation of completely new axon tracts. The corpus callosum, which is found only in placental mammals, may be such a truly new axon tract. We propose that the evolution of the corpus callosum is founded on the emergence of a new preaxonal substrate pathway, the "glial sling," which bridges the two halves of the embryonic forebrain only in placental mammals.

Development of the nematocyte junctional complex in hydra tentacles in relation to cellular recognition and positioning

Formation of the nematocyte-battery cell-mesoglea (NBM) junctional complex of hydra was studied. Normal animals were grafted to nematocyte-free animals and the tentacles of the repopulating host were examined by transmission electron microscopy. Migrating nematocytes extend cytoplasmic processes between battery cell myonemes to contact the mesoglea. Tufts of extracellular filaments radiate from the base of the battery cell adjacent to some of these regions of contact. The fascial desmosome of the NBM complex develops from a lateral fusion of macular desmosomes which often lie near a condensation of extracellular filaments. Microtubules within the intervening battery cell process become oriented perpendicularly to form the apposing half of the desmosomal junction and connect it with the hemidesmosomal portion of the NBM complex. These findings suggest that a migrating nematocyte receives environmental cues associated with the mesoglea-battery cell interface which may serve to direct the nematocyte to its definitive position and induce the subsequent formation of the complete NBM complex.

A possible embryonic mechanism for the establishment of innervation of inner ear sensory structures

The objective of this study was to explore the hypothesis (Van De Water, 1976) that differentiating sensory receptors of the inner ear may attract ingrowing neurites of the statoacoustic (VIIIn) ganglion by chemotaxis. Co-cultured embryonic inner ears which shared a single VIIIn ganglion were grown "in vitro" to the equivalent of 20 days gestation and then processed histologically to show both cytodifferentiation of sensory structures and the presence of neural elements. Specimens of both 11- and 12.5-day-old co-cultured otocysts showed that VIIIn ganglion neurites grew into sensory receptors of both(+) with and (-) without ganglion inner ear explants. Fourteen-day-old co-cultured inner ears revealed that only the (+) ganglion inner ear explants received VIIIn ganglion neurites into the sensory areas, and that neurites were not attracted into the (-) ganglion explants. The results were found to support the hypothesis of a limited period of chemotaxis as being a possible mechanism for the establishment of the pattern of innervation of inner ear sensory receptors by its VIIIn ganglion.

Developmental relationships between trigeminal ganglia and trigeminal motoneurons in chick embryos. III. Ganglion perikarya direct motor axon growth in the periphery

The previous study in this series demonstrated that the ingrowth of the central axons of the trigeminal (V) ganglion is prerequisite to V motor axon outgrowth and somatic translocation. In the present experiment we determined whether further interactions with V ganglion cell bodies were required by V motoneurons after the V ganglion innervates the brainstem. Soon after the ganglion axons had penetrated the brainstem they were severed, and a barrier, either permeable or impermeable, was placed between the ganglion cell bodies and the metencephalon. V motor axons grew along aberrant pathways to circumvent the impermeable barriers, many rerouting to reach the V ganglion. Only those V motor nerves which contacted the V ganglion distal to the barrier reached their target musculature in the mandible. The pattern of migration of V motoneurons was normal regardless of the V motor nerve trajectory, but the cell bodies of those axons which did not reach a muscle were not fully differentiated. When permeable barriers (Millipore filters) were implanted, the nerves followed two types of trajectories. If the pore size of the filter was small (0.45 and 0.025 microns), the V motor nerves grew identically to those observed in embryos in which impermeable barriers had been implanted. If the pore size of the filter was large (8.0 and 0.08 microns), the V motor nerve grew along its normal path directly to the barrier. Small axonal bundles from these nerves frequently grew into the filter toward the distal V ganglion. These results indicate that V motor axons preferentially grow to the V ganglion perikarya after exiting from the brainstem. Contact with the V ganglion always results in V motor nerve growth to the mandible while growth of the V motor axons to aberrant target sites only occurs when the axons fail to contact the V ganglion cells distal to the barrier.

Differential response to contact during embryonic nerve-nonnerve cell interactions

The outcome of contact interactions involving neurons and nonneurons varies depending on the cell types involved. When neuronal growth cones from either ciliary (motor) or dorsal root (sensory) ganglia directly contact the lamellipodium of an embryonic heart fibroblast, both neurite elongation and fibroblast locomotion are inhibited. This occurs in spite of the fact that cell-surface activity in both cells continues unabated. Such contact inhibition is not observed when homologous ganglionic nonneurons are involved in the interaction. In fact, these cells become intimately associated with growth cones and/or neuritic shafts as a result of the contact. The detailed nature of the response to contact exhibited by nerves and nonnerves varies not only with cell type but also with the portion of the cell involved in the contact. Growth cone filopodia tend to actively palpate the fibroblast surface, whereas spread regions, termed "veils," form areas of apposition with fibroblast lamellipodia. This latter situation resembles the "typical" contact inhibition of locomotion that occurs following embryonic heart fibroblast-fibroblast interactions. Growth cones also frequently exhibit contact guidance when interacting with nonruffling lateral surfaces of heart fibroblasts.

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

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

Translocation of the neuronal cytoskeleton and axonal locomotion

Recent studies of axonal transport indicate that cytoskeletal proteins are assembled into polymers in the neuron cell body and that these polymers move from the cell body toward the end of the axon. On the other hand, membranous elements appear to be inserted into the axonal plasma membrane preferentially at the end of the axon. These new observations are explored in relation to our current understanding of axonal elongation.

Guidance of neuritic growth in the transverse plane of embryonic mouse spinal cord

The development of cytoarchitecture in the lumbar spinal cord of mouse embryo between embryonic day 13 (E13) and E15 was studied by scanning and transmission electron microscopy. A pattern of two perpendicularly oriented sets of cellular elements was found in the intermediate zone at both E13 and E14. One set consisted of radially oriented processes that often originated from cell bodies in the ventricular zone. The second set consisted of cell bodies and processes that were oriented predominantly parallel to the neural tube margin and in the transverse plane. This set was termed circumferential elements and included most of the immature neurons. At E13, the circumferentially oriented cells were arranged into rostrocaudally compressed sheets or layers that were partially segregated one from another by flattened bundles of radially oriented processes. This pattern of orthogonally arranged circumferential and radial elements remained evident through E14 when the profuse growth of cellular processes began of obscure the identity of individual cells seen on the scanned surface. The exposed intermediate zone surface at E14 and E15 often has relatively flat transverse areas separated by ledges of broken tissues, indicating that most neurites grow in the transverse plane. These observations indicate that the sequential patterned development of an organized morphological substratum is an important factor in orienting neurite growth.

Substrate pathways demonstrated by transplanted Mauthner axons

A substrate pathway is a set of aligned guidance cues. (Such cues may be either cells or molecules.) CNS substrate pathways can be demonstrated by transplanting axons to different starting locations. The stereotyped routes of transplanted axons will then demonstrate the locations of effective substrate pathways. To map CNS substrate pathways, Mauthner axons were transplanted to various unnatural locations along the CNS of Xenopus embryos. The routes of 24 experimental Mauthner axons were traced. Twenty-one of these axons grew along parts of a stereotyped route extending in the ventral marginal zone from the caudal diencephalon through the spinal cord. This ventral substrate pathway ran the length of the basal plate; thus, we call it a basal substrate pathway. One experimental Mauthner axon grew along the alar substrate pathway previously demonstrated by transplanted optic axons. The demonstrations of the alar and the basal substrate pathways suggest that during development a few long substrate pathways organize the overall layout of the long tracts of the CNS. We propose that the pattern of the earliest CNS substrate pathways is established in the neural plate and is topologically preserved as the neural plate rolls into a neural tube. This pattern may be manifest as the three-dimensional organization of the early marginal zones formed by the peripheral processes (the endfeet) of certain developing ependyma and radial glia. Subsequently, the detailed anatomy of the axon tracts and the specific terminations of individual axons are probably determined by other local chemical cues.