Morphology and cellular interactions of growth cones in the developing corpus callosum

Neuroembryology and brain malformations: an overview

Modern neuroembryology integrates descriptive morphogenesis with more recent insight into molecular genetic programing and data enabled by cell-specific tissue markers that further define histogenesis. Maturation of individual neurons involves the development of energy pumps to maintain membrane excitability, ion channels, and membrane receptors. Most malformations of the nervous system are best understood in the context of aberrations of normal developmental processes that result in abnormal structure and function. Early malformations usually are disorders of genetic expression along gradients of the three axes of the neural tube, defective segmentation, or mixed lineages of individual cells. Later disorders mainly involve cellular migrations, axonal pathfinding, synaptogenesis, and myelination. Advances in neuroimaging now enable the diagnosis of many malformations in utero, at birth, or in early infancy in the living patient by abnormal macroscopic form of the brain. These images are complimented by modern neuropathological methods that disclose microscopic, immunocytochemical, and subcellular details beyond the resolution of MRI. Correlations may be made of both normal and abnormal ontogenesis with clinical neurological and EEG maturation in the preterm or term neonate for a better understanding of perinatal neurological disease. Precision in terminology is a key to scientific communication.

Cellular strategies of axonal pathfinding

Axons follow highly stereotyped and reproducible trajectories to their targets. In this review we address the properties of the first pioneer neurons to grow in the developing nervous system and what has been learned over the past several decades about the extracellular and cell surface substrata on which axons grow. We then discuss the types of guidance cues and their receptors that influence axon extension, what determines where cues are expressed, and how axons respond to the cues they encounter in their environment.

Recombinant inbreeding in mice reveals thresholds in embryonic corpus callosum development

The inbred strains BALB/cWah1 and 129P1/ReJ both show incomplete penetrance for absent corpus callosum (CC); about 14% of adult mice have no CC at all. Their F(1) hybrid offspring are normal, which proves that the strains differ at two or more loci pertinent to absent CC. Twenty-three recombinant inbred lines were bred from the F(2) cross of BALB/c and 129, and several of these expressed a novel and severe phenotype after only three or four generations of inbreeding - total absence of the CC and severe reduction of the hippocampal commissure (HC) in every adult animal. As inbreeding progressed, intermediate sizes of the CC and the HC remained quite rare. This striking phenotypic distribution in adults arose from developmental thresholds in the embryo. CC axons normally cross to the opposite hemisphere via a tissue bridge in the septal region at midline, where the HC forms before CC axons arrive. The primary defect in callosal agenesis in the BALB/c and 129 strains is severe retardation of fusion of the hemispheres in the septal region, and failure to form a CC is secondary to this defect. The putative CC axons arrive at midline at the correct time and place in all groups, but in certain genotypes, the bridge is not yet present. The relative timing of axon growth and delay of the septal bridge create a narrow critical period for forming a normal brain.

Abnormalities in neuronal process extension, hippocampal development, and the ventricular system of L1 knockout mice

In humans, mutations in the L1 cell adhesion molecule are associated with a neurological syndrome termed CRASH, which includes corpus callosum agenesis, mental retardation, adducted thumbs, spasticity, and hydrocephalus. A mouse model with a null mutation in the L1 gene (Cohen et al., 1997) was analyzed for brain abnormalities by Nissl and Golgi staining and immunocytochemistry. In the motor, somatosensory, and visual cortex, many pyramidal neurons in layer V exhibited undulating apical dendrites that did not reach layer I. The hippocampus of L1 mutant mice was smaller than normal, with fewer pyramidal and granule cells. The corpus callosum of L1-minus mice was reduced in size because of the failure of many callosal axons to cross the midline. Enlarged ventricles and septal abnormalities were also features of the mutant mouse brain. Immunoperoxidase staining showed that L1 was abundant in developing neurons at embryonic day 18 (E18) in wild-type cerebral cortex, hippocampus, and corpus callosum and then declined to low levels with maturation. In the E18 cortex, L1 colocalized with microtubule-associated protein 2, a marker of dendrites and somata. These new findings suggest new roles for L1 in the mechanism of cortical dendrite differentiation, as well as in guidance of callosal axons and regulation of hippocampal development. The phenotype of the L1 mutant mouse indicates that it is a potentially valuable model for the human CRASH syndrome.

Timing and origin of the first cortical axons to project through the corpus callosum and the subsequent emergence of callosal projection cells in mouse

A precise knowledge of the timing and origin of the first cortical axons to project through the corpus callosum (CC) and of the subsequent emergence of callosal projection cells is essential for understanding the early ontogeny of this commissure. By using a series of mouse embryos and fetuses of the hybrid cross B6D2F2/J weighing from 0.36 g to 1.0 g (embryonic day E15.75-E17.25), we examined the spatial and temporal distribution of callosal projection cells by inserting crystals of the lipophilic dye (DiI: 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) into the contralateral white matter just lateral to the midsagittal plane. Around 0.4 g or E15.8, retrogradely labeled cells were found restricted to a discrete cluster continuously distributed from the most ventral part of presumptive cingulate cortex to the hippocampus. During subsequent development, however, the tangential distribution of these labeled cells in ventromedial cortex did not extend further dorsally, and in fetuses where the CC became distinct from the hippocampal commissure (HC), labeled axons of cells in the ventral cingulate cortex were observed to intersect the callosal pathway and merge with labeled axons of the HC derived from cells in the hippocampus. The first cortical axons through the CC crossed the midline at about 0.64 g or E16.4, and these axons originated from a scattered neuronal population in the dorsal to lateral part of the presumptive frontal cortex. The earliest callosal cells were consistently located in the cortical plate and showed an immature bipolar appearance, displaying an ovoid- or pearl-shaped perikaryon with an apical dendrite coursing in a zig-zagging manner toward the pial surface and a slender axon directed toward the underlying white matter. Callosal projection cells spread progressively with development across the tangential extent of the cerebral cortex in both lateral-to-medial and rostral-to-caudal directions. In any cortical region, the first labeled cells appeared in the cortical plate and their number in the subplate was insignificant compared to that in the cortical plate. Thus, these results clarify that the CC is pioneered by frontal cortical plate cells, and the subsequent ontogeny of callosal projection cells proceeds according to the gradient of cortical maturation.

Retarded formation of the hippocampal commissure in embryos from mouse strains lacking a corpus callosum

A precise description of the timing and route traveled by axons traversing the telencephalic midline through the ventral hippocampal commissure (HC) is essential for understanding the role it plays in the formation of the corpus callosum (CC). A normal baseline of HC development was described in B6D2F2 hybrid mice and then compared with two inbred strains of mice displaying callosal agenesis, BALB/cWah1 (50% CC defect) and 129/J (70% CC defect), their F2 hybrid (C129F2-33% CC defect), and a recombinant inbred strain (RI-1-100% CC defect) derived from pairs of C129F2 mice. Embryos weighing from 0.25 g to 0.70 g (E14.5-E17) were collected and fixed by perfusion. Axon tracts were labeled using crystals of the lipophilic dyes DiI and DiA inserted into the hippocampal fimbria and cerebral cortex. HC axons in B6D2F2 mice first cross the midline at about 0.350 g body weight (E14.8) by traveling over the dorsal septum and along the pia membrane lining the longitudinal fissure. Earlier crossing was prevented by the presence of a deep cleft formed by the longitudinal fissure extending down into the septal region. Subsequent axons fasciculated along existing axons, gradually building the dorsoventral height of the HC to about 200 microns by 0.600 g. The earliest callosal axons from frontal cortex crossed the midline at 0.620 g and were clearly seen fasciculating along and between existing hippocampal axons at the dorsal surface of the HC as they crossed. In the acallosal strains, HC formation was delayed by the continued presence of the cleft deep in the septal region. This delay in time of crossing was correlated with later CC defect expression. Initial HC crossing occurred at about 0.470 g (E16.25) in BALB mice and about 0.520 g (E16.5) in 129 mice. In the RI-1 embryos, first HC crossing was estimated at about 0.750 g (E17.5), although several older embryos showed no crossing. These results show the importance of the HC for successful CC formation and suggest that absent CC arises as a consequence of a developmental defect which affects the formation of the hippocampal commissure prior to arrival of CC axons at midplane.

The optic tract in embryonic hamsters: fasciculation, defasciculation, and other rearrangements of retinal axons

The early development of the optic tract in hamsters was studied by labeling retinal axons with Dil applied to the eye, and then examining the labeled axons in flatmount preparations of the rostral brain stem. This technique permits a panoramic view of the entire retinal projection, from the chiasm to the caudal end of the superior colliculus. In the E11 embryo, retinal axons have reached the chiasm. They defasciculate as they emerge from the nerve, prior to reaching the ventral midline of the diencephalon, then converge again as they pass over to the opposite side. At the midline, many axonal trajectories crisscross, implying some shuffling of relative positions. Retinal axons are tightly bundled within the optic tract. Upon reaching the ventral border of the lateral geniculate body (LGB), they splay out over the nucleus, revealing a wavefront of pioneer axons individually distributed across the rostro-caudal extent of the LGB. Later-emerging retinal axons course over the surface of the thalamus in waves; subsequent waves of axons interdigitate between the lead fibers without fasciculating along them. Past the LGB, the axons undergo a second change in relative positions as the ribbon of fibers swerves caudally, prior to entering the superior colliculus. Retinal axons are tipped with growth cones of varying morphologies. No strong correlation is evident between the structural complexity of the growth cone and its position within the tract. In the majority of cases, ipsilaterally and contralaterally directed axons follow a similar developmental course along the optic tract, without any indication of a temporal lag in the ipsilateral projection as claimed in earlier reports. Understanding the changes in spatial distribution of embryonic retinal axons as they navigate along the optic tract provides a further step towards elucidating how point-to-point projections form in developing sensory systems.

Effects of neonatal enucleation on the organization of callosal linkages in striate cortex of the rat

Lewis and Olavarria ([1995] J. Comp. Neurol. 361:119-137) showed that the mediolateral organization of callosal linkages differs markedly between medial and lateral regions of striate cortex in the rat. Thus, callosal fibers originating from medial regions of striate cortex interconnect loci that are mirror-symmetric with respect to the midsagittal plane. In contrast, fibers from lateral regions of striate cortex show a reversed pattern of connections: tracer injections into the 17/18a border produce retrograde cell labeling in regions medial to the contralateral 17/18a border, whereas injections placed somewhat medial to the 17/18a border label cells located at the contralateral 17/18a border. Based on the interpretation that callosal fibers from lateral striate cortex connect retinotopically corresponding loci (Lewis and Olavarria [1995] J. Comp. Neurol. 361:119-137) we propose here that the development of the reversed pattern of connections in lateral portions of striate cortex is guided by activity-dependent cues originating from spontaneously active ganglion cells in temporal retina. In the present study we have attempted to falsify this hypothesis by investigating the effects of neonatal bilateral enucleation on the organization of callosal linkages in striate cortex of the rat. Once enucleated rats reached adulthood, we studied the mediolateral organization of callosal connections by placing small injections of different fluorescent tracers into different loci within medial and lateral striate cortex. The analysis of the distribution of retrogradely labeled callosal cells indicated that connections from lateral portions of striate cortex were no longer organized in a reversed fashion, rather, they resembled the mirror image pattern normally found in the medial callosal region, i.e., injections at the 17/18a border produced labeled cells at the opposite 17/18a border, whereas injections into slightly more medial regions produced labeled cells in the opposite, mirror-symmetric location. In addition, we found that enucleation does not alter the organization of callosal linkages in medial portions of striate cortex. Thus, by showing that enucleation significantly changes the pattern of connections from lateral portions of striate cortex, the present study does not falsify, but rather strengthens the hypothesis that interhemispheric correlated activity driven from the temporal retinal crescent guides the normal development of reversed callosal linkages in lateral portions of rat striate cortex. Furthermore, the present study shows that, in the absence of the eyes, the pattern of callosal linkages in lateral portions of striate cortex resembles the mirror image pattern normally found only in medial striate cortex.

Chick wing innervation. II. Morphology of motor and sensory axons and their growth cones during early development

The development and distribution of neuronal projections to the developing chick wing was studied using anterograde transport of horseradish peroxidase (HRP). Small injections of HRP were made into motor or sensory neuronal populations in order to visualize individual axons and their associated growth cones. Motor growth cones were observed in different regions of the embryo at different stages, in a proximal-to-distal pattern of distribution which paralleled the process of axon outgrowth and nerve formation. Different growth cone morphologies were associated with differing regions of the developing projection. In the spinal nerves, axons destined for the limb were unbranched and terminated in simply shaped growth cones. As axons approached the developing limb and entered the plexus region, their growth cones became more complex and larger primarily because of widening, and they sometimes branched, producing processes which could extend tens of microns from a tricorne branch point on the parent axon. Both motor and sensory fibers showed similar morphological changes in the plexus region. A distinctively shaped growth cone expanded on its leading edge was observed, sequentially apparent in the distal spinal nerves, in the plexus region, in the loosely organized axonal sheets projecting to the uncleaved dorsal or ventral muscle masses, and where muscle nerves diverged from nerve trunks and within muscle nerves. It is likely that some of these are transitional growth cones preparing to branch, because complex and branched growth cones were also observed in these regions. Branched axons oriented along the anteroposterior axis were similarly observed in the plexus region and distal to the plexus when axons first projected to the limb bud. At somewhat older stages when the basic peripheral nerve branching pattern had formed, motor growth cones were observed in common nerve trunks and in individual muscle nerves, but they were no longer found in the plexus region. Branched axons were likewise restricted to these peripheral locations. Taken together, these observations suggest that one of the ways in which axons navigate is by exploration in the form of growth cone widening, and in some cases terminal bifurcation which may produce axon branches. Selection of the most appropriately directed growth cone process and/or precocious axonal branches may be one of the ways in which axons respond to specific growth cues which guide axons into the limb bud. Alternatively, this precocious branching may be an early neurotrophic response to developing muscle and play no significant role in axon navigation.

Neuropeptide Y immunoreactive axons in the corpus callosum of the cat during postnatal development

Many immunocytochemical studies have identified different types of neurotransmitters localized in the corpus callosum (CC) axons in the adult mammal. Few studies have looked at the development of different neurochemically identified CC systems. Previous studies on the development of cat CC axons have indicated that a large number of transitory CC axons project to the cortex during early postnatal development. The present study focuses on the development of one neurochemically identified group of CC axons in the cat, labeled with an antibody against neuropeptide Y (NPY), to determine if this group participates in transitory CC axonal growth. Cats at specified ages from birth to adulthood were studied with a routine method of immunocytochemistry for antiserum to NPY. NPY-immunoreactive (ir) CC axons were detected at all stages examined, from newborn to adult; the peak density occurred during postnatal weeks (PNW) 3-4. During PNW 1-2, the density of NPY-ir CC axons increased gradually; some NPY-ir axons at this age had growth cones located within the CC bundle between the cerebral hemispheres. The density of the NPY-ir CC axons decreased gradually during PNW 5-7, and from PNW 8 to maturity only a few NPY-ir CC axons were observed. These results indicate that at least two types of NPY-ir CC axons (i.e., transitory and permanent) exist during development, and that most of these axons are eliminated or only express NPY-ir for a short period during development. The results also indicate that neurochemical subsets of CC axons participate in the extensive transitory growth observed by means of the membrane tracer DiI but they may follow unique developmental timetables.

Development of the rat septohippocampal projection: tracing with DiI and electron microscopy of identified growth cones

The factors determining the development of specific fiber tracts in the central nervous system as well as the interactions of growth cones with the surrounding micromilieu are largely unknown. Here we investigated the ontogenetic development of the septohippocampal projection in the rat with the lipophilic carbocyanine dye DiI which is transported anterogradely and retrogradely in neurons and can be applied to fixed embryonic tissue. Photoconversion of anterogradely labeled fibers allowed us to study individual growth cones by electron microscopy. The first axons originating from the septal complex were found in the hippocampus as early as on embryonic day (ED) 19, reaching the fimbrial pole of the hippocampus on ED 18. However, on ED 17 we consistently found retrogradely labeled cells in the hippocampus, indicating that the development of the hippocamposeptal projection precedes that of the septohippocampal projection. On ED 19, the majority of the axons directed toward the hippocampal formation passed the hippocampus and grew further into the subicular complex and entorhinal cortex. These axons gave off collaterals that invaded the hippocampus proper. A fairly adult pattern of the septohippocampal projection was reached on postnatal day 10, although may growth cones were still found. A comparative analysis of individual growth cones found in the fimbria and the hippocampus proper revealed no striking differences in their morphology. Electron microscopic analysis showed that growth cones in the fimbria were mainly contacted by other axons, whereas growth cones in the hippocampus had contact with all available elements. This may indicate that growing septohippocampal fibers are guided by axons of the earlier formed hippocamposeptal projection. In the hippocampus proper, other cues, probably derived from the target itself, may guide the septohippocampal axons to their appropriate target cells.

The prenatal development of the anterior commissure in hamsters: pioneer fibers lead the way

The prenatal development of the anterior commissure (AC) was studied in 130 hamster embryos with ages varying from E12 to E16 (E1 = day of conception and E16 = P1 = day of birth) by use of carbocyanine crystals (DiI, DiA and/or DiO) implanted into different rostrocaudal segments of the paleocortex. On E12 and E13, many AC axons were seen with tortuous trajectories pointing towards the midline (precrossing stage). On E13.5 and E14, most AC fibers abutted the midsagittal plane, led by a few pioneer axons that grew as far as 500 microns ahead into the opposite hemisphere (crossing stage). Pioneers were present in most brains at these ages irrespective of the rostrocaudal position of the carbocyanine crystal. Somata of pioneer axons could be identified by retrograde labelling. They were characteristically immature neurons, located either in the olfactory peduncle or in the superficial layers of the olfactory cortex. On E14.5 and E15, pioneers and followers were seen close to the targets and on E15.5 and E16 interstitial budding occurred, and arborization started within the olfactory peduncle and the paleocortex (postcrossing stage). If the existence of pioneer fibers represents something more than a stochastic phenomenon, their appearance in the developing AC may reflect the operation of signals at the midline and/or in the contralateral hemisphere that either accelerate the growth of pioneers, or decelerate the growth of followers.

Guidance and topographic stabilization of nasal chick retinal axons on target-derived components in vitro

We studied mechanisms underlying the generation of topographic order within the developing chick retinotectal connection by combining the recently introduced stripe assay with a novel membrane protein fractionation technique. Our experiments show a preference of temporal and nasal retinal fibers for growing on cell membranes prepared from their proper target area. In addition, membrane preparations from posterior tectum were found to prolong substantially the survival of nasal neurites in vitro. We conclude that tropic as well as trophic interactions contribute to the generation of topographic maps during embryogenesis, in our case to the homing of nasal axons within the posterior tectum.

Sensitivity of neurite outgrowth to microfilament disruption varies with adhesion molecule substrate

Interactions between the cytoskeleton and cell adhesion molecules are presumed responsible for neurite extension. We have examined the role of microfilaments in neurite outgrowth on the cell adhesion molecules L1, P84, N-CAM, and on laminin. Cerebellar neurons growing on each substrate exhibited differing growth cone morphologies and rates of neurite extension. Growth of neurites in the presence of cytochalasin B (CB) was not inhibited on substrates of L1 or P84 but was markedly inhibited on N-CAM. Neurons on laminin were initially unable to extend neurites in the presence of CB but recovered this ability within 9 h. These studies suggest that neurite outgrowth mediated by different cell adhesion molecules proceeds via involvement of distinct cytoskeletal interactions.

The ontogenesis of the forebrain commissures and the determination of brain asymmetries

We have reviewed the organization and development of the interhemispheric projections through the forebrain commissures, especially those of the CC, in connection with the development of brain asymmetries. Analyzing the available data, we conclude that the developing CC plays an important role in the ontogenesis of brain asymmetries. We have extended a previous hypothesis that the rodent CC may exert a stabilizing effect over the unstable populational asymmetries of cortical size and shape, and that it participates in the developmental stabilization of lateralized motor behaviors.

Fate of severed cortical projection axons

Corticospinal neurons show a primarily degenerative response to axotomy in adult mammals. The long remaining proximal axon with its extensive synaptic contacts may contribute to the lack of initial regenerative response in this cell type. We examined a related group of cortical axons after lesions in the subcortical white matter close to their cell bodies of origin. With cholera B chain conjugated to horseradish peroxidase (CTB-HRP), transcallosal axons projecting into areas of a lesion were labeled. Animals surviving between 2 days and 4 months were examined with both light microscopic and ultrastructural techniques. During the first several days after injury, many of the axon terminals projecting into the lesion site had the appearance of axonal sprouts, although the majority of endings had the appearance of degenerating terminal swellings. By 2 weeks after injury some axonal sprouts had extended a short distance along the margins of the lesions, into overlying cortex. Four weeks after injury there is a reduction in the number of axons extending toward the lesion. This loss of axons appeared progressive and resulted in not only a loss of labeled axons, but also eventually in atrophy of the subcortical white matter near the lesion. In comparison to corticospinal axon lesions in the spinal cord or medullary pyramids, there is more extensive axonal sprouting and elongation after subcortical lesions. Degenerative morphological features still predominate after subcortical lesions and no successful trans-lesion axonal regeneration occurs. Axonal retraction and loss are both accelerated and more extensive after proximal subcortical axotomy than after corticospinal tract lesions.

Development of callosal connections in the sensorimotor cortex of the hamster

To investigate the development of corpus callosal connectivity in the hamster sensorimotor cortex, we have used the sensitive axonal tracer 1,1 dioctadecyl-3,3,3',3', tetramethylindocarbocyanine perchlorate (DiI), which was injected either in vivo or in fixed brains of animals 3-6 days postnatal. First, to study changes in the overall distribution of developing callosal afferents we made large injections of DiI into the corpus callosal tract. We found that the anterogradely labeled callosal axons formed a patchy distribution in the contralateral sensorimotor cortex, which was similar to the pattern of adult connectivity described in earlier studies of the rodent corpus callosum. This result stands in contrast to previous retrograde studies of developing callosal connectivity which showed that the distribution of callosal neurons early in development is homogeneous and that the mature, patchy distribution arises later, primarily as a result of the retraction of exuberant axons. The initial patchy distribution of callosal axon growth into the sensorimotor cortex described in the present study suggests that exuberant axons destined to be eliminated do not enter the cortex. In addition, small injections of DiI into developing cortex resulted in homotopic patterns of callosal topography in which reciprocal regions of sensorimotor cortex are connected, as has been shown in the adult. Second, to study the radial growth of callosal afferents we followed the extension of individual callosal axons into the developing cortex. We found that callosal axons began to invade the contralateral cortex on about postnatal day 3, with little or no waiting period in the callosal tract. Callosal afferents then advanced steadily through the cortex, never actually invading the cortical plate but extending into layers on the first day that they could be distinguished from the cortical plate. The majority of callosal axons grew radially through the cortex and did not exhibit substantial branching until postnatal day 8, the age when the cortical plate disappears and callosal afferents reach the outer layer of cortex. This mode of radial growth through cortex prior to axon branching could serve to align callosal afferents with their radial or columnar targets before arborizing laterally.

Early Development of Olivocerebellar Projections in the Fetal Rat Using CGRP Immunocytochemistry

The expression of calcitonin gene-related peptide (CGRP) immunoreactivity in certain inferior olivary neurons is transient and developmentally regulated. Labelled neurons begin to appear at embryonic day 16 (E16), and reach their maximal extent by postnatal day 2 (P2). The extinction of the labelling occurs between P13 and P16. Expression of CGRP immunoreactivity is also observed in a few cerebellar fibres from E17, when axons in the restiform bundle begin to enter medially the cerebellar parenchyma. Their maximal extent is reached by P6, and thereafter they slowly disappear following a precise pattern, although fibre extinction is not complete. The spatio-temporal changes in the olivary distribution of the labelled neurons and the changes in the cerebellar labelled fibres follow the known pattern of topographic arrangement of the olivocerebellar system in adult rats. Moreover, the developmental phases of the CGRP-labelled fibres in postnatal rats correspond to those known for climbing fibre phenotypic acquisition. Thus, CGRP immunocytochemistry identifies in the fetal rat a subset of inferior olivary neurons and their corresponding cerebellar climbing fibres. Using this approach, we have analysed some of the initial events leading to the formation of the olivocerebellar projection, and obtained the following information: (i) Olivocerebellar axons are not randomly distributed in the restiform bundle before they enter the cerebellum. (ii) In the presence of a large spectrum of choices at the surface of the rostral half of the cerebellar plate the labelled olivary axons begin to enter the cerebellum at a precise medial point to abut a region composed solely of migrating Purkinje cells, and establish contacts with their targets before these neurons reach their final cortical location. (iii) From E18 to E19, the bundle of labelled fibres loses its superficial location, being bypassed by migrating Purkinje cells, to occupy a region corresponding to the prospective white matter. This translocation is coincident with the occurrence of a second axonal entry point, somewhat more lateral than the previous one, and with the appearance of a new lateral stripe of labelled fibres. (iv) Both the early and the late appearing labelled stripes remain confined from the time of their formation in precise cerebellar territories, indicating that only some clusters of Purkinje cells are contacted by the CGRP fibres. The results obtained imply that there is neither a waiting period nor an initial phase of randomness in the formation of the olivocerebellar projection map. This absence of chaotic cerebellar invasion, and the high selectivity of the entry points, suggest that the orientation of CGRP-positive olivocerebellar fibres towards their targets is regulated by positional information shared between subsets of olivary neurons and clusters of Purkinje cells. The result of this process would be the formation of a precocious coarse topography that would need further refinement.

Prenatal formation of the normal mouse corpus callosum: a quantitative study with carbocyanine dyes

Judgment of abnormalities in fetal cortical axon development is more sensitive when a good standard of normal ontogeny is established. The recent availability of postmortem tract tracing methods has greatly improved the observation of axon extension and growth cone morphology in mouse fetuses, which allows much stronger statements about the timing of crucial steps in the formation of the corpus callosum in particular. The first outgrowth and crossing of midplane by axons of the corpus callosum (CC) were examined in 153 normal mouse embryos and fetuses of the hybrid cross B6D2F2/J with carbocyanine dyes applied to brains fixed by perfusion. In most brains a crystal of DiI was inserted into either frontal, parietal, temporal, or occipital cortex in one hemisphere, and a crystal of DiA was placed into a different site in the opposite hemisphere. Although dye diffusion obscured the emergence of axons, linear regression analysis revealed that the first callosal axons emerged from their cortical cells of origin at about 0.4 g body weight or 15.5 days after conception for all four sites. Subsequent axon growth rate was substantially faster for those from frontal cortex (3.2 mm/day) than occipital cortex (1.8 mm/day). Axons from frontal cortex crossed the cerebral midplane first (0.69 g, E16.3), followed by those from parietal (0.74 g), temporal (0.77 g) and occipital cortex (0.92 g, E16.9). Prior to crossing midplane, the pioneering CC axons were usually 200 microns or less in advance of the main bundle, but when they crossed midplane and encountered CC axons growing from homotopic sites in the opposite hemisphere, the pioneering axons were often 0.5 to 2.5 mm ahead of the main bundle. Growth cones were usually large and complex until they had crossed midplane and were thereafter smaller with simple and flat morphologies. The topography of axons in the CC at midplane was organized according to cortical region of origin from the very beginning, when the CC was only a small cap over the hippocampal commissure and dorsal septum. The quantitative results provide a convenient standard for normal callosal development in mice and should facilitate comparative studies.

Specificity of a target cell-derived stop signal for afferent axonal growth

With a novel model culture system in which afferents are co-cultured with purified populations of target neurons, we have demonstrated that a target cell within the central nervous system (CNS), the cerebellar granule neuron, poses a "stop-growing signal" for its appropriate afferents, the mossy fibers. To ask whether this stop signal is afferent specific, we co-cultured granule neurons with another cerebellar afferent system, the climbing fibers from the inferior olivary nuclei, which normally contact Purkinje neurons, and with retinal ganglion cell afferents, which never enter the cerebellum. Granule neurons do not pose a stop signal to either of these afferents. In contrast to pontine mossy afferents that grow well on laminin and showed reduced outgrowth on granule neurons, both olivary and retinal fibers displayed similar growth on laminin alone or on granule neurons. In addition, each afferent showed different degrees of fasciculation and growth cone morphology on laminin. Thus, the growth arrest signal sent by granule neurons is specifically recognized by their appropriate afferents. Moreover, these three types of afferents exhibit varying growth patterns on the same noncellular and cellular substrates, implicating distinct molecular characteristics of growth regulation for different classes of neurons that would contribute to specificity of synapse formation.

Myelination of the cerebral commissures of the hamster, as revealed by a monoclonal antibody specific for oligodendrocytes

Myelination of the cerebral commissures of the hamster was studied by immunostaining with a monoclonal antibody (Rip) specific for oligodendrocytes. Immunostained, preensheathing cells were first observed in the anterior commissure on P6 (P1 = day of birth). By P8, immunopositive oligodendrocytes and myelinated fibers clustered around some of them were detected within the posterior limb of the anterior commissure, ventrally at the rostral half of the callosum, and in the hippocampal commissure. On P12, all the commissures had myelinated fibers throughout their extent, but the callosum and the hippocampal commissure exhibited higher densities of myelinated fibers rostrally. Between P15 and P22, the pattern of myelination approached that of the adult. In the context of other developmental events, myelination of the corpus callosum and of the anterior commissure is a late event, occurring predominantly after stabilization of axon number, either at the end of the progressive accretion of axons, as in the anterior commissure, or after the selective elimination of callosal projections.

Development of the main efferent cells of the olfactory bulb and of the bulbar component of the anterior commissure

The development of the efferent cells of the main olfactory bulb and the development of the bulbar part of the anterior commissure were studied in the rat from E16 to P7. DiI was used in fixed tissues as a neuronal tracer. From E16 onwards cells located in the olfactory bulb anlage were stained in a Golgi-like appearance. The morphological changes of these cells were: from E16 to P4, re-orientation from a tangential position to a radial position, elongation of the principal dendrite and spreading out of the secondary dendrites. From P4 onwards, there was a lack of migrating mitral cells in the inner part of the bulb. At E16 some fibers of the anterior commissure reached the midline, the number of fibers increased slowly until P0/P1. At P2 there was an explosive increase in the number of fibers crossing the midline and reaching the contralateral bulb. The development in two stages is hypothesized.

Relationship of glia to GnRH axonal outgrowth from third ventricular grafts in hpg hosts

The homozygous mutant hypogonadal (hpg) mouse lacks a functional gene for the neuropeptide gonadotropin releasing hormone (GnRH). The consequence of this defect is an infantile reproductive tract in adulthood. This condition can be reversed by the implantation of normal fetal preoptic area tissue that contains GnRH neurons. Reversal is always preceded by the outgrowth of GnRH axons into the host target tissue, the median eminence, by a stereotyped pathway. In the current experiments we investigated the cellular nature of the path taken by early emerging GnRH axons focusing on their relationship with astrocytic components and with the specialized ependymal population of this area, the tanycytes. In control tissue glial fibrillary acid protein (GFAP) immunoreactivity was confined to the exterior of cerebral blood vessels and glial limitans. Both GFAP and vimentin, another intermediate filament protein, marked the specialized ependymal cells of this region, the tanycytes. There was a robust reactive astrocytic response to the injury of transplantation in both the donor and host tissue within 5 days of implantation and the reactive astrocytes persisted for 60 days. These cells were GFAP-positive and were present in many areas of the host along the cannula tract and not confined to the area of GnRH axonal outgrowth. Vimentin, another intermediate filament, marked only the specialized ependymal cells of this region, the tanycytes, in both control and grafted tissue. Despite the profound reactive gliosis, GnRH axons were shown to exit the implant as early as 5 days after grafting suggesting that the gliotic process did not constitute a barrier to this phenomenon. At the light microscopic level, double label immunocytochemical studies did not reveal any specific association between GFAP or vimentin-positive cellular processes and these pioneer GnRH fibers. However, since normal GnRH axons had been reported to travel in tanycytic channels through the medial basal hypothalamus we reinvestigated the pattern of early emerging GnRH axons at the ultrastructural level. With this higher resolution, GnRH axons were found adjacent to glial elements along their entire traverse from the graft-host interface, through the host basal hypothalamus to their termination on the hypophysial portal capillaries. At the interface, GnRH-positive axons appeared to exit via glial channels similar to those described in other developing and regenerating systems. In the host, GnRH immunoreactive axonal profiles were surrounded by glial processes though the latter could not be further defined as tanycytic or astroglial. Other, immunonegative, axons were frequently seen in axonal bundles or fascicles and not necessarily in contact with glia.(ABSTRACT TRUNCATED AT 400 WORDS)

Lesion-induced establishment of the crossed corticorubral projections in kittens is associated with axonal proliferation and topographic refinement

The aberrant crossed corticorubral projection of the cat, which is very weak compared to the uncrossed one at about 1 month postnatal, becomes pronounced following unilateral lesions of the sensorimotor cortex. In order to determine whether or not terminal proliferation of pre-existing axons underlie this enlargement, the morphological changes of the crossed axons were examined, using the anterograde tracer Phaseolus vulgar- is leukoagglutinin (PHA-L). The crossed corticorubral axons in normal kittens were mostly simple in morphology with infrequent branching and did not often exhibit growth-cone-like axonal endings at 1 month postnatal. Two to 5 days after unilateral lesions of the sensorimotor cortex placed at this age, the axons were as simple as those in normal animals but ended in growth cones more frequently. Seven to 10 days post-lesion, the axons often bore side-branches which ended in growth cones. Two to 3 weeks post-lesion axons with sprays of finger-like fine sprouts occurred throughout the projection zone. There was no clear topography for the crossed projection in normal animals, but at 1-2 weeks post-lesion the axons started to show a certain amount of localization in the regions of the red nucleus which corresponded to the densely innervated region on the ipsilateral side. The topography of the crossed projections roughly mirrors that of the ipsilateral projection at about 1 month post-lesion. Thus, the lesions of the sensorimotor cortex induce substantial growth and proliferation of the crossed corticorubral axons. The post-lesion changes in axonal morphology and topographic refinement are reminiscent of developmental events. It is likely that the lesions permit the crossed axons, which normally fail to develop, to develop like the uncrossed ones.

Morphology of pioneer and follower growth cones in the developing cerebral cortex

In the developing nervous systems of both invertebrates and vertebrates, neurons must develop precise sets of axonal connections. One strategy used by both orders of animals is to generate a special class of neurons whose axons "pioneer" the first pathways between these cells and their targets. In the developing mammalian telencephalon, the subplate neurons (which are among the first neurons to be generated in development) extend axons to long-distance subcortical targets before the neurons of the deep cortical layers 5 and 6 have been generated. The axons of layer 5 and 6 neurons later follow a similar pathway to form permanent subcortical projections to the thalamus and tectum, and thereafter the vast majority of subplate neurons die. These results have generated the hypothesis that subplate axons may actually be required for the axons of layer 5 and 6 neurons to innervate their appropriate subcortical targets. The complexity of growth cones has previously been correlated with axonal decision making: differences in growth cone morphologies have been noted in comparisons of leading versus following axons (LoPresti, Macagno, and Levinthal, 1973; Nordlander, 1987; Yaginuma, Homma, Kunzi, and Oppenheim, 1991), and at choice points along axon pathways (Raper, Bastiani, and Goodman, 1983; Tosney and Landmesser, 1985; Caudy and Bentley, 1986a,b; Bovolenta and Mason, 1987; Holt, 1989; Bovolenta and Dodd, 1990; Yaginuma et al., 1991). Thus, as a first step toward addressing the question of whether the axons of deep-layer neurons simply follow subplate axons to their targets, we have studied the morphology of cortical growth cones at various points along the corticothalamic pathway and at different stages of development. We examined the brains of fetal ferrets and cats at ages ranging from embryonic days (E) 24 to E50, using the fluorescent lipophilic tracer 1,1-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI) to reveal the axons and growth cones of cortical neurons. Growth cones were drawn, and quantitative measurements of their complexity were made by counting filopodia and calculating their surface area. No morphological differences were found among growth cones at different points along the corticothalamic pathway at a given age. However, growth cones belonging to early-generated cells (likely to be subplate neurons) are significantly larger and more complex than are the growth cones of later-generated cortical neurons. This evidence is consistent with the suggestion that subplate growth cones actively pioneer the corticothalamic pathway, and that the axons of layer 5 and 6 neurons follow it.

Growth cones and axon trajectories of a sensory pathway in the amphibian spinal cord

Central axons of sensory ganglion (SG) neurons of the Xenopus tail enter the spinal cord via the ventral roots and travel dorsally and rostrally following a diagonal course within the lateral marginal zone (LMZ) to reach the dorsolateral fasciculus (DLF) (Nordlander et al.: Brain Res., 440:391-395, 1988). Axons are dispersed as they cross the cord. At the DLF they turn and travel together rostrally, sharing the fascicle with axons of primary sensory neurons (Rohon-Beard cells) already present in the tract. In this paper we analyze the growth patterns of the central projections of SG axons in the tail by using HRP applied to proximal branches of tail spinal nerves. Growth cones of the diagonal route are variable in configuration, often bearing processes that spread within the LMZ. Once the DLF, growth cones change shape, becoming distinctly linear. While growth cones navigating the diagonal part of the route never contact or fasciculate with other diagonal SG axons, SG growth cones and axons of the DLF are more closely associated with their fellows. Measurements of the slopes of SG axons in the diagonal route indicated a limited range with a mean of 23 degrees with respect to the cord axis. On the basis of these observations, we conclude that 1) navigational patterns for growth cones of this pathway differ for the diagonal versus the DLF part of its course, and 2) fasciculation is not a mechanism used by SG axons to reach the DLF, but that instead, each axon is able to find its way independently.

Developmental changes in B-50 (GAP-43) in primary cultures of cerebral cortex: B-50 immunolocalization, axonal elongation rate and growth cone morphology

Changes in neurite outgrowth parameters and in the immunolocalization of the neuronal growth-associated protein B-50 (GAP-43) were studied in cultured neocortex as a function of development. In addition, we studied the effects of chronic blockade of bioelectric activity (BEA) with tetrodotoxin (TTX) on these parameters. Axonal outgrowth rate in control cultures reached a maximum at 8 days in vitro (DIV) and declined to a low level at 21 DIV. B-50 staining shifted from the perikaryon to the axons and growth cones during the first 3 DIV. In axons the intensity of B-50 staining increased towards the growth cone. Within growth cones, the central/basal region and filopodia were intensely stained, whereas lamellipodia showed only marginal staining. Growth cone size gradually decreased after 3 DIV, due to the successive loss of lamellipodia and filopodia, and became club-shaped during the second week, until by 21 DIV growth cones were completely lost, and axons started retracting and degenerated. In the central area of the cultures, growth cones also decreased in size with time, but became stabilized as presynaptic elements onto other neurons. Acute addition of TTX did not affect the outgrowth rate at 6 DIV. Chronic TTX treatment led to an earlier retraction and degeneration of axons than in control cultures and to a loss of B-50-stained cells and varicosities during the third week, but did not affect growth cone morphology or B-50 staining. The regressive phenomena are probably due to an increased neuronal cell death shown to occur after chronic TTX treatment. The developmental changes in axonal elongation rate and growth cone morphology may be related to developmental changes in the content and/or phosphorylation of B-50 (GAP-43, which are studied in the same cultures in the following paper (Ramakers et al. (1991) Int. J. Devl Neurosci. 9, 231-241].

Developmental changes in B-50 (GAP-43) in primary cultures of cerebral cortex: content and phosphorylation of B-50

The content and phosphorylation of the neuronal growth-associated protein B-50 (GAP-43) were studied in cultured neocortex as a function of normal development and development in the presence of tetrodotoxin (TTX), a blocker of bioelectric activity (BEA). The observations were correlated with previous morphological findings on neurite outgrowth and B-50 immunolocalization in the same cultures. In control cultures, the concentration of B-50 reached a maximum at 7 days in vitro (DIV) and decreased thereafter, whereas the concentration of neuron specific enolase (NSE), which was used as a neuronal reference marker, rose till 28 DIV and leveled off towards 42 DIV. The degree of basal phosphorylation of B-50 (relative to that of total protein) decreased after the first week in vitro. Stimulation of B-50 phosphorylation by phorbol ester also decreased with age in vitro, indicating that changes in B-50 phosphorylation were mainly due to changes in protein kinase C (PKC) activity. The chronic presence of TTX led to a reduced content of B-50 and NSE after 14 DIV. The basal phosphorylation of B-50 was neither affected by acute nor chronic TTX treatment. However, upon stimulation of PKC with phorbol esters, some alterations of B-50 phosphorylation were revealed in cultures grown in TTX. These biochemical observations are in line with the absence of effects of TTX on neurite outgrowth during the first 2 weeks in culture, and later effects of TTX on neuronal survival. The developmental changes in B-50 concentration and phosphorylation largely correlate with previous morphological observations on axonal outgrowth and growth cone shape in the same cultures. We suggest that B-50 phosphorylation plays an important role in transducing extracellular signals into directed neurite outgrowth.