Proximal trajectory of the brachium conjunctivum in rat fetuses and its early association with the parabrachial nucleus. A study combining in vitro HRP anterograde axonal tracing and immunocytochemistry

Principal component and cluster analysis of morphological variables reveals multiple discrete sub-phenotypes in weaver mouse mutants

The present study evaluates the usefulness of the principal component analysis-based cluster analysis in the categorization of several sub-phenotypes in the weaver mutant by using several morphological parameters from the cerebellar cortex of control, heterozygous (+/wv) and homozygous (wv/wv) weaver mice. The quantified parameters were length of the cerebellar cortex, area of the external granular layer, area of the molecular layer, number of the external granular layer cells (EGL), and number of Purkinje cells (PCs). The analysis indicated that at postnatal day 8, the genotype +/wv presented three sub-phenotypes tagged as +/wv (0), +/wv (1) and +/wv (2), whereas two sub-phenotypes designated as wv (0)/wv (1) and wv (0)/wv (2) were identified in the genotype wv/wv. The number of PCs for the genotype +/wv and the number of EGL cells for the genotype wv/wv were the variables that discriminated the best among sub-phenotypes. Each one of the sub-phenotypes showed specific abnormalities in the cytoarchitecture of the cerebellar cortex as well as in the foliar pattern. In particular, the wv (0)/wv (1) and wv (0)/wv (2) sub-phenotypes had the most altered cytoarchitectonics, followed by the +/wv (2) sub-phenotype and then by the +/wv (1) one. The sub-phenotype +/wv (0) was the less affected one. Apart from reporting for the first time the coexistence of several sub-phenotypes in the weaver mutant, our approach provides a new statistical tool that can be used to assess cerebellar morphology.

Molecular markers of neuronal progenitors in the embryonic cerebellar anlage

The cerebellum, like the cerebrum, includes a nuclear structure and an overlying cortical structure. Experiments in the past decade have expanded knowledge beyond the traditional function of the cerebellum to include critical roles in motor learning and memory and sensory discrimination. The initial steps in cerebellar development depend on inductive signaling involving FGF and Wnt proteins produced at the mesencephalic/metencephalic boundary. To address the issue of how individual cerebellar cell fates within the cerebellar territory are specified, we examined the expression of transcription factors, including mammalian homologues of LIM homeodomain-containing proteins, basic helix-loop-helix proteins, and three amino acid loop-containing proteins. The results of these studies show that combinatorial codes of transcription factors define precursors of the cerebellar nuclei, and both Purkinje cells and granule neurons of the cerebellar cortex. Examination of gene expression patterns in several hundred lines of Egfp-BAC (bacterial artificial chromosome) transgenic mice in the GENSAT Project revealed numerous genes with restricted expression in cerebellar progenitor populations, including genes specific for cerebellar nuclear precursors and Purkinje cell precursors. In addition, we identified patterns of gene expression that link granule and Purkinje cells to their precerebellar nuclei. These results identify molecular pathways that offer new insights on the development of the nuclear and cortical structures of the cerebellum, as well as components of the cerebellar circuitry.

Naturally occurring neuronal death during the postnatal development of Purkinje cells and their precerebellar afferent projections

Naturally occurring neuronal death plays a substantial developmental role in the building of the neural circuitries. The neuronal death caused by different cerebellar mutations is mostly of an apoptotic nature. Apart from the identity of the intrinsic mechanisms of the mutations, adult cerebellar mutants are a powerful tool to causally study the development of the cerebellar connectivity. Thus, studies on adult cerebellar neuronal cell death occurring in mouse mutants elucidate: (i) the dependence of the postsynaptic neurons on their partners, (ii) the 'en cascade' postsynaptic transneuronal degeneration after target-deprivation, and (iii) the close relationship between the molecular modular organization of the cerebellar cortex and dying Purkinje cells. Neuronal cell death has been extensively studied in developing olivocerebellar system. However, less data are available on the occurrence of naturally occurring neuronal death during the in vivo normal development of the Purkinje cells and the mossy fiber system neurons. The developmental role of neuronal death during the establishment and refinement of the olivocerebellar projection is currently discussed. Moreover, the occurrence of neuronal death during the development of the basilar pontine nuclei and its role in the acquisition of the adult pontocerebellar projection is still poorly understood. In the present review, we correlate the dates of Purkinje cells death with the inferior olivary and basilar pontine neuronal apoptosis, discussing their developmental relationships during the elaboration of the fine-grained maps of the cerebellar afferent connections.

Regionalization of the isthmic and cerebellar primordia

The complex migrations of neurons born in the dorsal neural tube of the isthmic and rhombomere l (rl) domains complicate the delineation of the cerebellar primordium. We show that Purkinje cells (P) are likely generated over a wide territory before gathering in the future cerebellar primordium under the developing external granular layer. Later expansion of the cerebellum over a restricted ependymal domain could rely on mutual interations between P cells and granule cell progenitors (GCP). P are attracted by GCP and in turn stimulate their proliferation, increasing the surface of the developing cortex. At later stages, regionalization of the developing and adult cerebellar cortex can be detected through regional variations in the distribution of several P cell markers. Whether and how the developmental and adult P subtypes are related is still unknown and it is unclear if they delineate the same sets of cerebellar subdivisions. We provide evidence that the early P regionalization is involved in intrinsic patterning of the cerebellar primordium, in particular it relate to the organization of the corticonuclear connection. We propose that the early P regionalization provides a scaffold to the mature P regionalization but that the development of functional afferent connections induces a period of P plasticity during which the early regional identity of P could be remodeled.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

Spatiotemporal expression patterns of slit and robo genes in the rat brain

Diffusible chemorepellents play a major role in guiding developing axons toward their correct targets by preventing them from entering or steering them away from certain regions. Genetic studies in Drosophila revealed a repulsive guidance system that prevents inappropriate axons from crossing the central nervous system midline; this repulsive system is mediated by the secreted extracellular matrix protein Slit and its receptors Roundabout (Robo). Three distinct slit genes (slit1, slit2, and slit3) and three distinct robo genes (robo1, robo2, rig-1) have been cloned in mammals. However, to date, only Robo1 and Robo2 have been shown to be receptors for Slits. In rodents, Slits have been shown to function as chemorepellents for several classes of axons and migrating neurons. In addition, Slit can also stimulate the formation of axonal branches by some sensory axons. To identify Slit-responsive neurons and to help analyze Slit function, we have studied, by in situ hybridization, the expression pattern of slits and their receptors robo1 and robo2, in the rat central nervous system from embryonic stages to adult age. We found that their expression patterns are very dynamic: in most regions, slit and robo are expressed in a complementary pattern, and their expression is up-regulated postnatally. Our study confirms the potential role of these molecules in axonal pathfinding and neuronal migration. However, the persistence of robo and slit expression suggests that the couple slit/robo may also have an important function in the adult brain.

Expression of CRABP I mRNA in fastigial cells of the developing cerebellum

The expression of the cellular retinoic acid binding protein type I (CRABP I) was examined in the early phase of cerebellar development in the mouse. The CRABP I was expressed from embryonic day (E) 10.5 to E15.5 in the cerebellar plate. The expression was diffused at E10.5-E11.5 and thereafter localized in a small rostrodorsal area of the cerebellar territory of both sides. By using in situ hybridization and both immunohistochemistry and carbocyanine tracing procedures, we identified the fastigial cells as the population that expresses CRABP I in the cerebellum. The results suggest that these cells play a critical role in the early development of the cerebellum.

Evidence that the loss of Purkinje cells and deep cerebellar nuclei neurons in homozygous weaver is not related to neurogenetic patterns

To determine whether the neurogenetic patterns of Purkinje cells and deep cerebellar nuclei neurons were normal in weaver homozygotes and whether the degeneration of those neuronal types was linked to their time of origin, [3H] thymidine autoradiography was applied on sections of homozygous weaver mice and normal controls on postnatal day 90. The experimental animals were the offspring of pregnant dams injected with [3H] thymidine on embryonic days 11-12, 12-13, 13-14 and 14-15. The results show that the onset of neurogenesis, its pattern of peaks and valleys, and its total span were similar between wild type and homozygous weaver in the cerebellar areas analyzed, indicating that the loss of Purkinje cells and deep cerebellar nuclei neurons is not related to neurogenetic patterns. In weaver homozygotes, the loss of Purkinje cells and deep cerebellar nuclei neurons followed a lateral to medial gradient of increasing severity. Thus, the vermis and the fastigial nucleus, which are medially located, presented the most important neuron loss, whereas in the lateral hemisphere and the dentate nucleus, neuron loss was spared.

Crossing the floor plate triggers sharp turning of commissural axons

During development of the vertebrate CNS, commissural axons initially grow circumferentially toward the ventral midline floor plate. After crossing the floor plate, they abruptly change their trajectory from the circumferential to the longitudinal axis. Although recent studies have unraveled the mechanisms that control navigation of these axons along the circumferential axis, those that result in the transition from circumferential to longitudinal trajectory remain unknown. Here, we examined whether an interaction with the floor plate is a prerequisite for the initiation of trajectory transition of commissural axons, using in vitro preparations of the rat metencephalon. We found that commissural axons in the metencephalon, once having crossed the floor plate, turned sharply to grow longitudinally. In contrast, axons extending in floor plate-deleted preparations, continued to grow circumferentially, ignoring the hypothetical turning point. These results suggest that a prior interaction of commissural axons with floor plate cells is a key step for these axons to activate a navigation program required for their change in axonal trajectory from the circumferential to the longitudinal axis.

Guidance of circumferentially growing axons by netrin-dependent and -independent floor plate chemotropism in the vertebrate brain

Netrin-1, a diffusible signal secreted by floor plate cells at the ventral midline of the vertebrate CNS, can attract ventrally migrating axons and repel a subset of dorsally migrating axons in the spinal cord and rostral hindbrain in vitro. Whether netrin-1 can act as a global cue to guide all circumferentially migrating axons is, however, unknown. Here, we show that netrin-1 can attract alar plate axons that cross the floor plate along its entire rostrocaudal axis. Dorsally directed axons forming the posterior commissure are, however, repelled by the floor plate by a netrin-independent mechanism. These results suggest that netrin-1 functions as a global guidance cue for attraction to the midline. Moreover, floor plate-mediated chemorepulsion may also operate generally to direct dorsal migrations, but its molecular basis may involve both netrin-dependent and -independent mechanisms.

Floor plate chemoattracts crossed axons and chemorepels uncrossed axons in the vertebrate brain

In the bilaterally symmetrical vertebrate CNS, all developing axons must choose between remaining on the same side of the midline or growing across it. The mechanism underlying this axonal pathfinding is, however, poorly understood. Here we demonstrate that the ventral midline floor plate (FP) chemorepels two types of ipsilaterally projecting axons, one from the alar plate and another from the basal plate in the mesencephalon. We further demonstrate that the FP chemoattracts contralaterally projecting myelencephalic as well as metencephalic axons. The FP at all axial levels displayed both chemoattractive and chemorepellent activities, suggesting that FP chemoattraction and chemorepulsion may be at work throughout the neuraxis. Chemotropic guidance by the FP may therefore play a key role in the establishment of neuronal projection laterality.

Guidance of cerebellofugal axons in the rat embryo: directed growth toward the floor plate and subsequent elongation along the longitudinal axis

To elucidate guidance mechanisms of brain commissural axons, we examined the navigation of cerebellofugal axons. Axons were labeled by implantation of the fluorescent tracer Dil into the cerebellar plate (CP) of fixed, flat whole-mount embryonic rat brain. Axons initially grew straight toward the ventral midline floor plate (FP) in the rostral hindbrain and then, after crossing it, made a right-angled turn to grow either caudally or rostrally along the longitudinal axis. In collagen gel culture, CP axons showed directed growth toward both FP explants and heterologous cells expressing netrin-1, a FP-derived chemoattractant for spinal commissural axons. These results suggest that CP axons are guided to the midline by FP-derived chemoattractant(s) and then reoriented, possibly by another guidance cue, for longitudinal extension. Considering that the basic structures of the neural tube, including the FP, extend up to the caudal diencephalon, these results suggest that common guidance mechanisms operate for ventrally decussating commissural axons in both the brain and spinal cord.

Anatomical and physiological relationships between the anterior cerebellar vermis and the pontine parabrachial nucleus in the rabbit

The anatomical connections between the midline cerebellum and the pontine parabrachial nucleus (PBN) were investigated in the rabbit using anterograde and retrograde axonal transport techniques. Small injections (20-50 nl) of cholera toxin conjugated to horseradish peroxidase (CT-HRP) or wheat germ agglutinin conjugated HRP (WGA-HRP) into the cortex of the anterior cerebellar vermis resulted in retrograde and anterograde-like label in the PBN. Focal injections of tracer into the PBN resulted in anterogradely labeled processes in the ACV and retrogradely labeled a small, but distinct group of Purkinje cells within the anterior vermis. Focal injections into the rostral fastigial nuclei (FN) resulted in anterograde-like label in the PBN, and PBN injections labeled FN neurons. Furthermore, the projection from the PBN to ACV is effective in driving cerebellar neurons as electrical microstimulation of the PBN evoked short-latency, phasic responses in ACV Purkinje cells. These experiments generated anatomical and physiological evidence for the existence of a neuroanatomical circuit connecting the midline cerebellum (ACV, FN) with the PBN, that may serve as a functional interface between the midline cerebellum and other brain stem nuclei with cardiovascular function, particularly with respect to the midline cerebellar role in classically conditioned cardiovascular responses.

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 descent of rubrospinal fibers through the spinal cord of the rat

This study is the first description of the descent of rubrospinal fibers through the spinal cord of the rat fetus. Either horseradish peroxidase or wheat germ agglutinin-horseradish peroxidase conjugate was injected into the spinal cord, at different levels and at different gestational ages. At embryonic day 17 (E17) fibers from all subdivisions of the nucleus ruber (NR) started their descent towards the spinal cord. At E18 fibers from the ventrolateral NR reached the lower cervical spinal cord, and those from the caudal NR reached the lower thoracic spinal cord. At E19 fibers from the dorsomedial NR and from the parvicellular NR had just reached the cervical spinal cord, while fibers from the ventrolateral and caudal NR descended to lower thoracic levels. At E21 fibers from the dorsomedial NR reached the lower cervical spinal cord. Fibers from the ventrolateral and caudal NR completed their descent through the lumbosacral spinal cord during the first three postnatal days. During their descent the rubrospinal fibers were confined to the white matter of the spinal cord. The earliest descending fibers originated in the caudal NR. Fibers from the caudal part of each magnocellular subdivision of the NR descended before their rostral counterparts. Fibers from the dorsomedial NR only reached the cervical enlargement as the fibers from the ventrolateral NR descended through the cervical enlargement. The somatotopy of the adult rubrospinal projection reflects this sequence; the dorsomedial NR (dmNR) projects to the cervical spinal cord, and the ventrolateral NR (vlNR) projects to the lumbosacral spinal cord. In general, early descending fibers originated from neurons located caudally and ventrolaterally, while later descending fibers originated from neurons located progressively more rostrally and dorsomedially in the magnocellular NR.

Relationships between neuronal birthdates and cytoarchitecture in the rat inferior olivary complex

The correlation between birthdates of neurons and their ultimate location within the inferior olivary nucleus (ION) was investigated in the rat by the 5-bromodeoxyuridine (BrdU) method. We performed injections every 4 hours throughout the ION generation period, and were thus able to demonstrate that 1) neurons are distributed in the adult ION following characteristic gradients that define subdivisions identical to those established by hodological studies; and 2) ION neurons born at the same time tend to be arrayed in small clusters in the adult structure. Implications of these findings for the mechanisms of olivary neuron migration, selective aggregation, and elaboration of projectional topography are discussed. This study provides direct evidence that one of the factors governing the elaboration of the cytoarchitecture of a neuronal nucleus is the temporal sequence of generation of its neurons.