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

Bilaterally symmetric populations of chicken dI1 (commissural) axons cross the floor plate independently of each other

Axons use temporal and directional guidance cues at intermediate targets to set the rate and direction of growth towards their synaptic targets. Our recent studies have shown that disrupting the temporal guidance process, by unilaterally accelerating the rate at which spinal dI1 (commissural) axons grow, resulted in turning errors both in the ventral spinal cord and after crossing the floor plate. Here we investigate a mechanistic explanation for these defects: the accelerated dI1 axons arrive in the ventral spinal cord before necessary fasciculation cues from incoming dI1 axons from the opposite side of the spinal cord. The identification of such an interaction would support a model of selective fasciculation whereby the pioneering dI1 axons serve as guides for the processes of the bilaterally symmetrical population of dI1 neurons. To test this model, we first developed the ability to “double” in ovo electroporate the embryonic chicken spinal cord to independently manipulate the rate of growth of the two bilateral populations of dI1 axons. Second, we examined the requirement for a putative bilateral interaction by unilaterally ablating the dI1 population in cultured explants of chicken embryonic spinal cord. Surprisingly, we find no evidence for a bilateral dI1 axon interaction, rather dI1 axons appear to project independently of each other.

Inhibitory Smads differentially regulate cell fate specification and axon dynamics in the dorsal spinal cord

The roof plate resident BMPs have sequential functions in the developing spinal cord, establishing cell fate and orienting axonal trajectories. These activities are, however, restricted to the dI1-dI3 neurons in the most dorsal region of the spinal cord. What limits the extent of the action of the BMPs to these neurons? To address this question, we have examined both the distribution of the inhibitory Smads (I-Smads), Smad6 and Smad7 in the spinal cord and the consequence of ectopically expressing the I-Smads in chicken embryos. Our studies suggest that the I-Smads function in vivo to restrict the action of BMP signaling in the dorsal spinal cord. Moreover, the I-Smads have distinct roles in regulating the diverse activities of the BMPs. Thus, the ectopic expression of Smad7 suppresses the dI1 and dI3 neural fates and concomitantly increases the number of dI4-dI6 spinal neurons. In contrast, Smad6 most potently functions to block dI1 axon outgrowth. Taken together, these experiments suggest that the I-Smads have distinct roles in spatially limiting the response of cells to BMP signaling.

L1 and GAD65 are expressed on dorsal commissural axons in embryonic rat spinal cord

Using immunocytochemical methods, the cell adhesion molecule L1 was detected on axons crossing in the dorsal commissure of developing rat spinal cord. Immunoreactive axons were found in locations similar to fiber bundles illustrated by Ramón y Cajal and designated the anterior, middle and posterior bundles of the dorsal commissure. L1-immunoreactive dorsal commissural axons were first observed on embryonic day 17 (E17), appeared more numerous by E19, and remained detectable in early postnatal ages. The massive middle axon bundles extended bilaterally from the dorsolateral funiculi towards the midline and crossed in the central part of the commissure. In horizontal sections, bundles of L1-labeled middle axons were observed to traverse the dorsal commissure in a periodic pattern along the entire rostrocaudal extent of the spinal cord. Bundles of glutamic acid decarboxylase (GAD65)-positive axons were detected crossing in the middle and posterior regions of the dorsal commissure between E17 and E20. Results from double-labeling experiments demonstrated that GAD65-positive fibers were embedded in larger bundles of L1-labeled axons and that some dorsal commissural axons were double-labeled. To determine if there were axons crossing in the dorsal commissure that did not express L1, double-labeling experiments were conducted using neurofilament and L1 antibodies. Results indicated that bundles of axons identified with anti-neurofilament antibodies were also L1-positive, whereas individually coursing axons within the commissure were L1-negative. The predominance of L1 on fiber bundles traversing the dorsal commissure adds to the growing evidence that this molecule may play a role in axon outgrowth and fasciculation.

BMPs as mediators of roof plate repulsion of commissural neurons

During spinal cord development, commissural (C) neurons, located near the dorsal midline, send axons ventrally and across the floor plate (FP). The trajectory of these axons toward the FP is guided in part by netrins. The mechanisms that guide the early phase of C axon extension, however, have not been resolved. We show that the roof plate (RP) expresses a diffusible activity that repels C axons and orients their growth within the dorsal spinal cord. Bone morphogenetic proteins (BMPs) appear to act as RP-derived chemorepellents that guide the early trajectory of the axons of C neurons in the developing spinal cord: BMP7 mimics the RP repellent activity for C axons in vitro, can act directly to collapse C growth cones, and appears to serve an essential function in RP repulsion of C axons.

Development of interneurons with ipsilateral projections in embryonic rat spinal cord

Considerable progress has been made in recent years in identifying molecules with restricted expression in mammalian spinal cord at early developmental stages. However, the significance of the different expression patterns for most of these molecules is unclear because so little is known about the development of various classes of spinal interneurons. Recently, we have characterized the development of rat spinal cord interneurons with an axon that crosses in the ventral commissure (Silos-Santiago and Snider, J. Comp. Neurol., 325:514, 1992). In the current study, we describe the morphological development of ipsilaterally projecting spinal interneurons in laminae V-VIII of the thoracic spinal cord. These neurons were labelled by retrograde lateral diffusion of DiI after crystals were placed in various locations in the embryonic thoracic cord. By E14, approximately 48 hours after the first interneurons are generated, eight different groups of ipsilateral interneurons are present in the spinal cord. By E15, these groups of ipsilateral interneurons have reached distinct locations within the gray matter. Even at this early stage, different groups of cells have elaborated characteristic dendritic arborizations. By E19, at least 17 different types of ipsilateral interneurons can be identified on the basis of location and dendritic morphology. In general, ipsilateral interneurons are located more dorsally and laterally than commissural interneurons at all stages of embryonic development. Furthermore, in comparison with commissural neurons, fewer ipsilateral interneurons have dendritic arbors with a mediolateral orientation in the transverse plane. This work demonstrates that rat embryonic spinal cord contains a large number of morphologically distinct classes of interneurons that extend axons into the ipsilateral lateral funiculus. These neurons can be distinguished from commissural neurons on the basis of location and morphology. These results, taken together with those from our previous study, provide a framework for the localization of gene expression to different classes of spinal interneurons at early developmental stages.

Immunolocalization studies of putative guidance molecules used by axons and growth cones of intersegemental interneurons in the chick embryo spinal cord

The earliest developing interneurons in the chick spinal cord can be divided into two groups: neurons in the ventral region whose axons pioneer the primitive longitudinal pathway (PL-cells) and neurons whose axons project circumferentially (C-cells) along the lateral marginal zone and join the ipsilateral or contralateral ventrolateral longitudinal pathways. To begin to examine the molecular cues for axonal pathway formation of these interneurons, we screened a variety of molecules from embryonic day (E) 2 to E6.5 [stage 14-30 of Hamburger and Hamilton (1951) J. Morphol. 88:49-92]. These include cell adhesion and related molecules (G4, F11, neurofascin, N-cadherin, TAG-1-like molecule), extracellular matrix (ECM) molecules (laminin, fibronectin, heparan sulfate proteoglycan, laminin-heparan sulfate proteoglycan complex, and collagen type IV), and receptors for ECM molecules (beta 1-class integrin). PL-cells first expressed neurofascin at stage 14+ before the onset of axonogenesis. When the PL-cells began to extend their axons at stage 15, they expressed G4 and avian TAG-1-like molecules, as well as neurofascin, on both cell bodies and longitudinal axons. In the following stages, PL-cells continued to strongly express neurofascin and G4 on their fasciculating axons, suggesting the involvement of these glycoproteins in growth and fasciculation. C-cells began to express G4 and TAG-1-like molecules on cell bodies and axons at stage 15-16 shortly after axonal growth. In the following stages, C-cells expressed several cell adhesion molecules differentially on their axonal segments. The proximal segment of C-axons in the circumferential pathway strongly expressed a TAG-1-like molecule, whereas the distal segment in the longitudinal pathway strongly expressed G4 and neurofascin. The commissural axonal segment in the floor plate expressed TAG-1-like molecule, neurofascin, N-cadherin, and beta 1-class integrin. The basement membrane around the spinal cord was enriched with ECM glycoproteins (laminin, fibronectin, heparan sulfate proteoglycan, and collagen type IV) during the stages examined (stage 15-27), and commissural C-cell axons became strongly integrin positive in the floor plate where they contacted the basement membrane. These data indicate that interneurons may use multiple molecules during axonal pathway formation, depending on cell type, pathway position, and developmental stage.

Pathfinding by growth cones of commissural interneurons in the chick embryo spinal cord: a light and electron microscopic study

To investigate putative axonal guidance mechanisms used by commissural interneurons in the chick embryo spinal cord, we have examined growth cone morphology, the microenvironment through which the growth cones advance, and interactions between growth cones and their surroundings. Growth cones of both early and late developing commissural interneurons were examined. The growth cones were visualized by injection of either horseradish peroxidase (HRP) or the fluorescent dye Di-I. Unlabelled growth cones as well as HRP-labelled growth cones were also examined by electron microscopy. The early developing growth cones project circumferentially without fasciculation until they reach the region of the longitudinal pathway in the contralateral ventral funiculus (CVF). In their trajectory towards the floor plate, axons exhibited elaborate growth cones with filopodia and lamellipodia. They projected between processes of neuroepithelial cells within abundant extracellular spaces. Upon arrival at the ipsilateral ventral funiculus, growth cones did not appear to contact preexisting longitudinal axons. Within the floor plate, the growth cones were less complex and lacked long filopodia and exhibited bulbous or varicose shapes with short processes. Electron microscopic observations of the floor plate at this stage revealed that there was only a small amount of extracellular space and that the basal portion of the floor plate cells were directionally oriented (polarized) in the transverse plane. It is of particular interest that contacts between growth cones and the basement membrane in the floor plate were often observed. When the growth cones reached the contralateral ventrolateral region, they again exhibited an elaborate morphology. Close contacts between growth cones and the preexisting contralateral longitudinal axons were observed. Growth cones advancing in the contralateral longitudinal pathway exhibited various shapes and were observed to contact other axons and processes of neuroepithelial cells. Most of the later developing growth cones of commissural cells exhibited lamellipodial shapes irrespective of their location along the circumferential trajectory. Electron microscopic observations revealed that these late developing growth cones always contacted or fasciculated with preexisting axons and that the cellular environment through which they grow is oriented in such a way that the growth cones appear to be guided in specific directions. Growth cones entering the CVF exhibited more elaborated shapes with ramified lamellipodia that made multiple contacts with preexisting longitudinal axons. The present results indicate that differential axonal guidance mechanisms may be employed along the pathway followed by spinal commissural interneurons and that axons and growth cones projecting along this pathway at different developmental stages employ different mechanisms for pathfinding and guidance.(ABSTRACT TRUNCATED AT 400 WORDS)

Migratory patterns of clonally related cells in the developing central nervous system

Neurons and glioblasts that arise in the ventricular zone migrate to form discrete nuclei and laminae as the central nervous system develops. By stably labeling precursor cells in the ventricular zone, pathways taken by different cells within an individual clone can be described. We have used recombinant retroviruses to label precursor cells with a heritable marker, the E. coli lacZ gene; clones of lacZ-positive cells are later mapped histochemically. Here we review results from three regions of the chicken central nervous system--the optic tectum, spinal cord, and forebrain--and compare them with previous results from mammalian cortex and other regions of the vertebrate CNS. In particular, we consider the relationship between migratory patterns and functional organization, the existence of multiple cellular sources of migratory guidance, and the issue of whether a cell's choice of migratory pathway influences its ultimate phenotype.

Spatial regulation of axonal glycoprotein expression on subsets of embryonic spinal neurons

The identification of surface proteins restricted to subsets of embryonic axons and growth cones may provide information on the mechanisms underlying axon fasciculation and pathway selection in the vertebrate nervous system. We describe here the characterization of a 135 kd cell surface glycoprotein, TAG-1, that is expressed transiently on subsets of embryonic spinal cord axons and growth cones. TAG-1 is immunochemically distinct from the cell adhesion molecules N-CAM and L1 (NILE) and is expressed on commissural and motor neurons over the period of initial axon extension. Moreover, TAG-1 and L1 appear to be segregated on different segments of the same embryonic spinal axons. These observations provide evidence that axonal guidance and pathway selection in vertebrates may be regulated in part by the transient and selective expression of distinct surface glycoproteins on subsets of developing neurons.

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.

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

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