Spinocerebellar projection in the meander tail mutant mouse: organization in the granular posterior lobe and the agranular anterior lobe

Refinement of Cerebellar Network Organization by Extracellular Signaling During Development

The cerebellum forms regular neural network structures consisting of a few major types of neurons, such as Purkinje cells, granule cells, and molecular layer interneurons, and receives two major inputs from climbing fibers and mossy fibers. Its regular structures consist of three well-defined layers, with each type of neuron designated to a specific location and forming specific synaptic connections. During the first few weeks of postnatal development in rodents, the cerebellum goes through dynamic changes via proliferation, migration, differentiation, synaptogenesis, and maturation, to create such a network structure. The development of this organized network structure presumably relies on the communication between developing elements in the network, including not only individual neurons, but also their dendrites, axons, and synapses. Therefore, it is reasonable that extracellular signaling via synaptic transmission, secreted molecules, and cell adhesion molecules, plays important roles in cerebellar network development. Although it is not yet clear as to how overall cerebellar development is orchestrated, there is indeed accumulating lines of evidence that extracellular signaling acts toward the development of individual elements in the cerebellar networks. In this article, we introduce what we have learned from many studies regarding the extracellular signaling required for cerebellar network development, including our recent study suggesting the importance of unbiased synaptic inputs from parallel fibers.

Insights into cerebellar development and connectivity

The cerebellum has a well-established role in controlling motor functions such coordination, balance, posture, and skilled learning. There is mounting evidence that it might also play a critical role in non-motor functions such as cognition and emotion. It is therefore not surprising that cerebellar defects are associated with a wide array of diseases including ataxia, dystonia, tremor, schizophrenia, dyslexia, and autism spectrum disorder. What is intriguing is that a seemingly uniform circuit that is often described as being "simple" should carry out all of these behaviors. Analyses of how cerebellar circuits develop have revealed that such descriptions massively underestimate the complexity of the cerebellum. The cerebellum is in fact highly patterned and organized around a series of parasagittal stripes and transverse zones. This topographic architecture partitions all cerebellar circuits into functional modules that are thought to enhance processing power during cerebellar dependent behaviors. What are arguably the most remarkable features of cerebellar topography are the developmental processes that produce them. This review is concerned with the genetic and cellular mechanisms that orchestrate cerebellar patterning. We place a major focus on how Purkinje cells control multiple aspects of cerebellar circuit assembly. Using this model, we discuss evidence for how "zebra-like" patterns in Purkinje cells sculpt the cerebellum, how specific genetic cues mediate the process, and how activity refines the patterns into an adult map that is capable of executing various functions. We also discuss how defective Purkinje cell patterning might impact the pathogenesis of neurological conditions.

Experimental neurotransplantation treatment for hereditary cerebellar ataxias

Hereditary cerebellar degenerations are a heterogeneous group of diseases often having a detrimental impact on patients’ quality of life. Unfortunately, no sufficiently effective causal therapy is available for human patients at present. There are several therapies that have been shown to affect the pathogenetic process and thereby to delay the progress of the disease in mouse models of cerebellar ataxias. The second experimental therapeutic approach for hereditary cerebellar ataxias is neurotransplantation. Grafted cells might provide an effect via delivery of a scarce neurotransmitter, substitution of lost cells if functionally integrated and rescue or trophic support of degenerating cells. The results of cerebellar transplantation research over the past 30 years are reviewed here and potential benefits and limitations of neurotransplantation therapy are discussed.

Embryonic stages in cerebellar afferent development

The cerebellum is important for motor control, cognition, and language processing. Afferent and efferent fibers are major components of cerebellar circuitry and impairment of these circuits causes severe cerebellar malfunction, such as ataxia. The cerebellum receives information from two major afferent types – climbing fibers and mossy fibers. In addition, a third set of afferents project to the cerebellum as neuromodulatory fibers. The spatiotemporal pattern of early cerebellar afferents that enter the developing embryonic cerebellum is not fully understood. In this review, we will discuss the cerebellar architecture and connectivity specifically related to afferents during development in different species. We will also consider the order of afferent fiber arrival into the developing cerebellum to establish neural connectivity.

Establishment of topographic circuit zones in the cerebellum of scrambler mutant mice

The cerebellum is organized into zonal circuits that are thought to regulate ongoing motor behavior. Recent studies suggest that neuronal birthdates, gene expression patterning, and apoptosis control zone formation. Importantly, developing Purkinje cell zones are thought to provide the framework upon which afferent circuitry is organized. Yet, it is not clear whether altering the final placement of Purkinje cells affects the assembly of circuits into topographic zones. To gain insight into this problem, we examined zonal connectivity in scrambler mice; spontaneous mutants that have severe Purkinje cell ectopia due to the loss of reelin-disabled1 signaling. We used immunohistochemistry and neural tracing to determine whether displacement of Purkinje cell zones into ectopic positions triggers defects in zonal connectivity within sensory-motor circuits. Despite the abnormal placement of more than 95% of Purkinje cells in scrambler mice, the complementary relationship between molecularly distinct Purkinje cell zones is maintained, and consequently, afferents are targeted into topographic circuits. These data suggest that although loss of disabled1 distorts the Purkinje cell map, its absence does not obstruct the formation of zonal circuits. These findings support the hypothesis that Purkinje cell zones play an essential role in establishing afferent topography.

Cerebellar zones: history, development, and function

The longitudinal and transverse zonal arrangement of axonal projections to and from the cerebellum, even more than the well-known laminar cytoarchitecture, is the hallmark of cerebellar anatomy. No model of cerebellar function, whether in motor control, cognition, or emotion, will be complete without understanding the development and function of zones. To this end, a special issue of this journal is dedicated to zones, and the purpose of this article is to summarize the research and review articles that are contained within. The special issue begins by considering some of the very first studies in the 1960s and 1970s that led to our modern understanding of this unique and defining anatomical substructure. Then, it considers the molecular analogs of longitudinal zones in the form of stripes in the cerebellar cortex and related sub-areas in the deep cerebellar nuclei, and it includes studies on the genetic underpinnings of stripes and zones. Several articles address the evolution of both embryonic clusters and adult zones across vertebrate species, and others discuss the functional and clinical relevance of zones. While we do not yet fully understand the role of zones with respect to motor behavior in all of its complexities, cerebellar function is clearly modular, and combinatorial models of complex motor movements based on multi-purpose modules are beginning to emerge. This special issue, by refocusing attention on this fundamental organization of the cerebellum, sets the stage for future studies that will more fully reveal the cellular, developmental, behavioral, and clinical relevance of zones.

The stop signal revised: immature cerebellar granule neurons in the external germinal layer arrest pontine mossy fiber growth

During the formation of neuronal circuits, afferent axons often enter target regions before their target cells are mature and then make temporary contacts with nonspecific targets before forming synapses on specific target cells. The regulation of these different steps of afferent-target interactions is poorly understood. The cerebellum is a good model for addressing these aspects, because cerebellar development is well defined and identified neurons in the circuitry can be purified and combined in vitro. Previous reports from our laboratory showed that cultured granule neurons specifically arrest the extension of their pontine mossy fiber afferents, leading us to propose that granule cells arrested growth of their afferents as a prelude to synaptogenesis. However, we knew little about the differentiation state of the cultured granule cells that mediate afferent arrest. In this study, we better define the purified granule cell fraction by marker expression and morphology, and demonstrate that only freshly plated granule cells in the precursor and premigratory state arrest mossy fiber outgrowth. Mature granule cells, in contrast, support extension, defasciculation, and synapse formation, as in vivo. In addition, axonal tracing in vivo during the first postnatal week indicates that immature mossy fibers extend into the Purkinje cell layer but never into the external germinal layer (EGL), where precursors of granule cell targets reside. We found that the stop-growing signals are dependent on heparin-binding factors, and we propose that such signals in the EGL restrict the extension of mossy fiber afferents and prevent invasion of proliferative regions.

Signaling at the vertebrate synapse: new roles for embryonic morphogens?

The formation of synapses is critical for functional neuronal connectivity. The coordinated assembly at both sides of the synapse is fundamental for the proper apposition of the neurotransmitter release machinery on the presynaptic neuron and the clustering of neurotransmitter receptors and ion channels on the receptive postsynaptic cell. This process requires bidirectional communication between the presynaptic neuron and its postsynaptic target, another neuron, or muscle fiber. Extracellular signals such as WNT, TGF-beta, and FGF factors are emerging as key target-derived signals required for the initial stages of synaptic assembly. Studies in invertebrates are also providing new insights into the function of these signals in synaptic growth and homeostasis. During early embryonic patterning, WNT, TGF-beta, and FGF factors function as typical morphogens in a concentration-dependent manner to regulate cell fate decisions. This mode of action raises the provocative idea that these same morphogens might also provide a coordinate system for axons to establish the distance to their targets during axon guidance and synapse formation.

Compartmentation of the reeler cerebellum: segregation and overlap of spinocerebellar and secondary vestibulocerebellar fibers and their target cells

The cerebellum of the reeler mutant mouse has an abnormal organization; its single lobule is composed of a severely hypogranular cortex and a central cerebellar mass (CCM) consisting of Purkinje cell clusters intermixing with the cerebellar nuclei. As such the reeler represents an excellent model in which to examine the effect of the abnormal distribution of cerebellar cells on afferent-target relationships. To this effect we studied the organization of the spinocerebellar and secondary vestibulocerebellar afferent projections in homozygous reeler mice (rl/rl) using anterograde tracing techniques. Spinal cord injections resulted in labeled spinocerebellar mossy fiber rosettes in specific anterior and posterior regions of the cerebellar cortex. Some vestiges of parasagittal organization may be present in the anterior projection area. Within the CCM, labeled fibers appeared to terminate on distinct groups of Purkinje cells. Thus, the spinocerebellar mossy fibers seem to form both normal and heterologous synapses in the reeler cerebellum. Secondary vestibular injections resulted in both retrograde and anterograde labeling. Retrograde labeling was seen in clusters of Purkinje cells and cerebellar nuclear cells; anterograde labeling was distributed in the white matter and in specific regions of the anterior and posterior cortex of the cerebellum. The labeled spinocerebellar and secondary vestibulocerebellar afferents overlapped in the anterior region but in the posterior region the vestibulocerebellar termination area was ventral to the spinocerebellar area. An area devoid of labeled terminals was also observed ventral to the posterior secondary vestibulocerebellar termination field. Using calretinin immunostaining it was determined that this area contains unipolar brush cells, a cell type found primarily in the vestibulocerebellum of normal mice. Our data indicate that despite of the lack of known landmarks (fissures, lobules) the spinocerebellar and vestibulocerebellar afferent projections in the reeler cerebellum do not distribute randomly but have specific target regions, and the position of these regions, relative to each other, appears to be conserved. Two caveats to this were the finding of overlapping terminal fields of these afferents in the anterior region, and a posteroventral region that contains unipolar brush cells yet is devoid of secondary vestibulocerebellar afferents. The distribution of Purkinje cells and cerebellar nuclear cells is not random either; those that give rise to cerebellovestibular efferents form distinct groups within the central cerebellar mass.

Selective Purkinje cell ectopia in the cerebellum of the weaver mouse

The adult mouse cerebellar vermis consists of four transverse zones, each of which is further subdivided into parasagittal stripes. In the adult weaver (wv/wv) mouse, the zebrin II expression pattern in the cerebellar vermis is abnormal, consistent with the absence of a central zone (approximately lobules VI/VII). Because the small, heat shock protein HSP25 is a constitutive marker of parasagittal bands of Purkinje cells in the caudal central zone and the nodular zone (approximately lobules IX/X), we used HSP25 immunocytochemistry to show that the patterning abnormalities in wv/wv reflect selective Purkinje cell ectopia rather than the absence of the central zone. A specific HSP25-immunopositive Purkinje cell ectopia within the central zone was identified. Symmetrical clusters of HSP25-immunopositive Purkinje cells, which presumably would have formed the parasagittal stripes in the wild type, are present ectopically on either side of the midline in wv/wv. In contrast, in the nodular zone, HSP25-immunopositive Purkinje cells form a near-monolayer and are organized into parasagittal stripes. We therefore conclude that specific Purkinje cell clusters in the wv/wv cerebellum fail to disperse and that this ectopia contributes to the topographical abnormalities.

Axonal remodeling and synaptic differentiation in the cerebellum is regulated by WNT-7a signaling

Synapse formation requires changes in cell morphology and the upregulation and localization of synaptic proteins. In the cerebellum, mossy fibers undergo extensive remodeling as they contact several granule cells and form complex, multisynaptic glomerular rosettes. Here we show that granule cells secrete factors that induce axon and growth cone remodeling in mossy fibers. This effect is blocked by the WNT antagonist, sFRP-1, and mimicked by WNT-7a, which is expressed by granule cells. WNT-7a also induces synapsin I clustering at remodeled areas of mossy fibers, a preliminary step in synaptogenesis. Wnt-7a mutant mice show a delay in the morphological maturation of glomerular rosettes and in the accumulation of synapsin I. We propose that WNT-7a can function as a synaptogenic factor.

Rostral cerebellar malformation (rcm/rcm): a murine mutant to study regionalization of the cerebellum

A recently described recessive mouse mutant, rostral cerebellar malformation (rcm/rcm), demonstrates a swaying gait at approximately 12 days of age (Lane et al. [1992] J. Hered. 83:315-318). The mutant cerebellar (Cb) phenotype consists of cerebellar tissue that extends rostrally, beyond the usual distinct anterior cerebellar boundary, into the midbrain (Lane et al. [1992] J. Hered. 83:315-318; Ackerman et al. [1997] Nature 386:838-842). Interestingly, the cerebellar ectopia occurs in the absence of any significant alterations in the distribution of nuclear groups within the brainstem. The ectopic Cb tissue is 1) adherent to the posterior and lateral aspects of the inferior colliculus and to the lateral aspect of the rostral brainstem and 2) contains acellular regions within the inner granular layer (igl) and ectopic, calbindin-immunoreactive Purkinje cells (PCs) deep to the igl. Within the Cb proper, PC organization, as revealed by zebrin II immunoreactivity, is generally normal. In the ectopic Cb tissue PCs also exhibit a banded zebrin distribution. Analysis of the spinocerebellar projection in the mutant suggests a lobular distribution similar to that seen in the normal mouse. Within the anterior region, however, the normal parasagittal banding pattern is somewhat obscured. Spinocerebellar innervation of the ectopic Cb tissue exists, but it is almost exclusively to the region adjacent to the caudal inferior colliculus. In conjunction with the recent finding that the mutation appears to affect a UNC-5-like receptor protein for netrin-1, a molecule that may be involved in axonal guidance and cell migration (Ackerman et al. [1997] Nature 386:838-842), our results suggest that this mutant is an important model for the analysis of cerebellar development and regionalization.

Novel receptor protein tyrosine phosphatase (RPTPrho) and acidic fibroblast growth factor (FGF-1) transcripts delineate a rostrocaudal boundary in the granule cell layer of the murine cerebellar cortex

We have identified a novel receptor-like protein tyrosine phosphatase (RPTPrho) transcript whose expression in the cerebellar cortex is restricted to the granule cell layer of lobules 1-6. Acidic fibroblast growth factor (FGF-1) mRNA follows a similar cerebellar expression pattern. Together, the two markers define a sharp boundary in lobule 6, slightly caudal to the primary fissure. Anterior and posterior compartments became discernible only during postnatal weeks two and six, for RPTPrho and FGF-1, respectively. A rostrocaudal boundary in lobule 6 of the murine cerebellar cortex has also been identified morphologically by the effects of the meander tail mutation. The position of the RPTPrho and FGF-1 boundary on the rostrocaudal axis of the cerebellar cortex was close to, but not coincident with, the caudal extent of the disorganized anterior lobe of meander tail and the rostral extent of Otx-2 expression. The restricted pattern of FGF-1 and RPTPrho implies that these molecules may have specific signaling roles in the tyrosine phosphorylation/dephosphorylation pathway in the anterior compartment of the adult cerebellar cortex.

Evidence of spinocerebellar mossy fiber segregation in the juvenile staggerer cerebellum

Developmental and experimental studies of climbing fiber and mossy fiber connectivity in the cerebellum have suggested that Purkinje cells are the critical organizing elements for connectivity patterns. This hypothesis is supported by evidence that spinocerebellar mossy fiber projections are abnormally diffuse in P25 sg/sg mutant mice in which the differentiation of a reduced number of sg/sg Purkinje cells is blocked due to a cell autonomous defect. However, mossy fiber distribution may be disrupted in sg/sg mutants not because of the Purkinje cell deficits, but because of the death of virtually all granule cells following the 4th postnatal week. To test this hypothesis, we have analyzed the distribution of wheat germ agglutinin-horseradish peroxidase (WGA-HRP)-labeled spinocerebellar mossy fiber terminals in sg/sg mutants at the end of the period of granule cell genesis (postnatal day [P] 12-P13) and before massive granule cell death (P16). Two percent WGA-HRP was injected into the lower thoracic/upper lumbar region of the spinal cord of eight homozygous sg/sg mutants (P12-P16) and five controls (+/sg and +/+). We have found that spinocerebellar mossy fibers segregate into distinct terminal fields in the anterior cerebellar lobules of P12 to P16 sg/sg mutants, although the medial-lateral distribution of spinocerebellar mossy fiber projections is different from controls. The results from this study and previous analysis of sg/sg mutants support the hypothesis that topographic cues are expressed in the early postnatal staggerer mutant, but mossy fiber terminals become disorganized or retract as granule cells die in the older staggerer mutant. J. Comp. Neurol. 378:354-362, 1997.

The embryonic cerebellum contains topographic cues that guide developing inferior olivary axons

The formation of the olivocerebellar projection is supposed to be regulated by positional information shared between pre- and postsynaptic neurons. However, experimental evidence to support this hypothesis is missing. In the chick, caudal neurons in the inferior olive project to the anterior cerebellum and rostral ones to the posterior cerebellum. We here report in vitro experiments that strongly support the existence of anteroposterior polarity cues in the embryonic cerebellum. We developed an in vitro system that was easily accessible to experimental manipulations. Large hindbrain explants of E7.5-E8 chick embryos, containing the cerebellum and its attached brainstem, were plated and studied using axonal tracing methods. In these cultures, we have shown that the normal anteroposterior topography of the olivocerebellar projection was acquired, even when the cerebellar lamella was detached from the brainstem and placed again in its original position. We also found that, following various experimental rotations of the anteroposterior axis of the cerebellum, the rostromedian olivary neurons still project to the posterior vermis and the caudolateral neurons to the anterior vermis, that now have inverted locations. Thus, the rotation of the target region results in the rotation of the projection. In addition, we have shown that the formation of the projection map could be due to the inability of rostromedian inferior olivary axons to grow in the anterior cerebellum. All these experiments strongly indicate that olivocerebellar fibers recognize within their target region polarity cues that organize their anteroposterior topography, and we suggest that Purkinje cells might carry these cues.

The Engrailed-2 homeobox gene and patterning of spinocerebellar mossy fiber afferents

The mouse Engrailed-2 gene, En-2, appears to be involved in cerebellar pattern formation. Homozygous null mutants for En-2 have abnormal foliation patterns in the posterior half of the cerebellum and there are changes in Purkinje and granule cell gene expression in some posterior folia, possibly reflecting changes in cell identity. We have examined the distribution of spinocerebellar mossy fiber terminals in homozygous En-2hd null mutants to determine if En-2 is involved in regulating the pattern of afferent connectivity in the cerebellum. Spinocerebellar mossy fiber terminals were labeled following WGA-HRP injections in the lumbar region of 5 homozygous En-2hd mutants and 4 heterozygous controls. The distribution of spinocerebellar mossy fiber terminals was consistently altered in lobules VIII and IX of the En-2hd mutants. The principal changes were a reduction in the number of mossy fiber terminal fields in the dorsal aspect of lobule VIII and the dorsal midline field in lobule IX was fused into a single compartment. The results suggest that the deletion of En-2 expression does not transform lobule identity, at least with respect to afferent fiber positional information cues. However, the changes in foliation and afferent connectivity in the En-2 mutant support a broad role for the En-2 gene in cerebellar patterning.

Partial ablation of the neonatal external granular layer disrupts mossy fiber topography in the adult rat cerebellum

The spinocerebellar projection in the rat is compartmentalized in an array of parasagittal bands of mossy fiber terminals. These bands align reproducibly with bands of Purkinje cells that differentially express zebrin II. To investigate whether this alignment is obligatory, Purkinje cell and mossy fiber compartmentation has been compared in the rat cerebellum where the cytoarchitecture was contorted by neonatal administration of methylazoxymethanol. Methylazoxymethanol ablates many granule cell precursors, leaving a much reduced external granular layer, and adult rats that received a single methylazoxymethanol injection at birth showed varying degrees of abnormal cerebellar foliation. Zebrin II immunocytochemistry nevertheless revealed no fundamental abnormality in the Purkinje cell compartments. However, despite the normal Purkinje cell compartmentation being retained, the spinocerebellar mossy fiber-Purkinje cell topography is disrupted by methylazoxymethanol treatment. The normal precise array of parasagittal mossy fiber terminal fields becomes blurred across the lobule, and the normal clear banding is difficult to follow. These data suggest that, despite the early topography being dependent on the Purkinje cells, the granule cell-mossy fiber interactions also regulate the topography of the spinocerebellar projection.

Development of the spinocerebellar projection in the prenatal mouse

An abundance of information is available concerning the spinocerebellar projection in adult mammals. However, only a few studies have attempted a developmental analysis of this important projection system in early postnatal and/or prenatal animals. The present study provides an analysis of the development of the projection from the spinal cord to the cerebellum in fetal mice using anterograde tracing techniques in an in vitro preparation. After applications of biocytin to the caudal cervical spinal cord, anterogradely labelled fibers were present in the brainstem of embryonic day 12 (E12/13) mice, however, there was no indication of label in the cerebellum. At E13/14, labelled fibers were evident in the rostrolateral portions of the cerebellum/isthmus region. By E15/16, labelled spinocerebellar fibers had progressed farther into the cerebellum and were seen crossing the midline in a very superficial position. At older ages, the number of crossing fibers increased, and they became more ventrally positioned within the cerebellum. At E17/18 and E18/19, labelled spinocerebellar fibers were observed to branch and invade deeper portions of the cerebellum including the cerebellar nuclei. However, at E18/19, there was no indication of the parasagittal organization characteristic of this projection in the adult animal. The results of this study indicate that spinocerebellar fibers are present within the cerebellum significantly earlier than the development and differentiation of their primary targets, the granule cells. Furthermore, these data suggest that spinocerebellar fibers may form associations with cerebellar nuclear cells during fetal development.

Developing mossy fiber terminal fields in the rat cerebellar cortex may segregate because of Purkinje cell compartmentation and not competition

Many mossy fiber afferent projections to the rat cerebellar cortex terminate in parasagittal bands. In particular, the anterior lobe vermis of the cerebellum contains alternating bands of mossy fibers from the spinal cord and external cuneate nuclei. The cerebellar cortical efferents, the Purkinje cells, are also organized in parasagittal bands. These can be revealed by immunochemical staining for the antigen zebrin II, which is selectively expressed by bands of Purkinje cells. In some cases, the boundaries between mossy fiber terminal fields align with identified transitions between zebrin+/- sets of Purkinje cells, whereas others are located within apparently homogeneous Purkinje cell compartments. Two theories can explain the terminal-field topography: In one view, mossy fiber terminals segregate during development, because growth cones from different sources compete for common territory. Alternatively, mossy fiber growth cones directly recognize chemically distinct target territories, and activity-dependent mechanisms play only minor roles. To explore these issues, two sets of experiments were performed. First, the terminal-field map of the neonatal spinocerebellar projection was compared to the Purkinje cell compartmentation as revealed by anticalbindin immunocytochemistry. Second, subsets of spinocerebellar mossy fiber afferents were ablated early in postnatal development, and the consequences for the neighboring cuneocerebellar terminal fields were mapped in the adult with reference to the zebrin II+/- compartments. These experiments revealed no evidence that competitive interactions constrain the mossy fiber terminal-field distribution but, rather, suggest that the organization of the mossy fiber projections follows the compartmentation of the Purkinje cells.

Topographic spinocerebellar mossy fiber projections are maintained in the lurcher mutant

A variety of recent studies of cerebellar development have focused attention on the role of Purkinje cells as organizing elements for the topography of afferent fiber connectivity in the cerebellum. We have investigated the involvement of Purkinje and granule cells in the maintenance of topographic spinocerebellar mossy fiber projections by analyzing the distribution of spinocerebellar mossy fiber terminals in lurcher (+/Lc) mutant mice. Purkinje cells in the +/Lc mutant degenerate starting after the first week of postnatal development because of an intrinsic genetic defect. The loss of their Purkinje cell targets also results in the death of 90% of the granule cells. We examined the distribution of spinocerebellar mossy fiber terminals in the juvenile and adult +/Lc mutant to determine how the pattern of afferent projections is affected by the loss of Purkinje cells shortly after innervation of the cerebellum. Labeling of spinocerebellar mossy fiber terminals with WGA-HRP in the P38 and adult +/Lc mutant showed that, despite the loss of almost all Purkinje cells and 90% of the granule cells, spinocerebellar mossy fibers project to the appropriate folia and segregate into relatively normal parasagittal bands. While we cannot rule out the possibility that Purkinje cells may be involved in the initial establishment of topographic maps, our results indicate that Purkinje cells are not necessary for the maintenance of the normal spinocerebellar mossy fiber topographic map.

The olivocerebellar projection in normal (+/+), heterozygous weaver (wv/+), and homozygous weaver (wv/wv) mutant mice: comparison of terminal pattern and topographic organization

Olivocerebellar organization and topography were analyzed in adult normal (+/+), heterozygous weaver (wv/+), and homozygous weaver (wv/wv) mutant mice. The two genotypes (wv/+ and wv/wv) of the weaver mutant present a gradation of abnormal cerebellar morphology. Purkinje cell (PC) ectopia ranges from mild (wv/+) to moderate (wv/wv), and regional PC loss is also graded in the two types. To determine olivocerebellar organization and topography, tritiated amino acids were placed into different regions of the inferior olivary complex (IO) in normal, heterozygous, and homozygous weaver mice. Despite some PC loss and ectopia, olivocerebellar fiber (OCF) terminals in both homozygous and heterozygous weaver mice have an orthogonal distribution and topography similar to that seen in normal mice. Differences in OCF termination, such as an increased density of OCF terminal label in the lower portion of the molecular layer, the PC, and granule cell layers, are seen in homozygous weaver mice. In some heterozygous weaver and normal cases, multiple injections labeling most IO cells on one side of the IO resulted in continuous OCF terminal labeling in many regions of the contralateral cerebellar cortex, suggesting that all PCs receive OCF input. Retrograde analysis involving injections of horseradish peroxidase conjugated to wheat germ agglutinin into different mediolateral cerebellar regions in homozygous weaver mice further demonstrates a generally normal olivocerebellar topography.

Evidence of early topographic organization in the embryonic olivocerebellar projection: a model system for the study of pattern formation processes in the central nervous system

Many projection systems within the peripheral and central nervous system are topographically organized, and it has become increasingly clear that interactions which occur during development determine the projection patterns these systems exhibit in the adult. The olivocerebellar system was chosen as a model system for this study of afferent pattern formation because it has several characteristics which lend themselves to a study of this type. Applications of horseradish peroxidase were made to both the cerebellar primordium and to the inferior olive of embryonic and neonatal mice using an in vitro perfusion system to support the tissue during the transport period. Fibers labeled after restricted olivary applications are limited to particular mediolateral regions of the cerebellum. Similarly, olivary cells retrogradely labeled after discrete cerebellar applications are restricted to particular olivary subdivisions. The results indicate that the olivocerebellar projection displays elements of topographic organization as early as E15 and that the pattern displayed is roughly comparable to that of the adult mammal. The observed trajectories of olivocerebellar fibers and their concomitant association with both Purkinje and cerebellar nuclear cells during embryonic development suggests a role for either or both cell types in the pattern formation process.

The innervation of calcitonin gene-related peptide to the Purkinje cells and granule cells in the developing mouse cerebellum

The present study analyzed the ontogeny of calcitonin gene-related peptide-like immunoreactive (CGRP-IR) structures in the mouse cerebellum. No CGRP-IR neurons were detected at any stage, but three types of CGRP-IR fibers were seen: (1) CGRP-IR dense fiber plexuses which appeared transiently in the developing cerebellum, (2) thin varicose fibers, and (3) mossy fiber-like fibers. The CGRP-IR dense fiber plexuses appeared in the developing Purkinje cell layer at postnatal day 2. From postnatal days 6 to 11, these fibers formed pericellular nests around Purkinje cells. After that stage, these fibers rapidly disappeared and no such plexuses were seen in the adult cerebellum. CGRP-IR fiber plexuses were not evenly distributed, and they had a parasagittal banded pattern in the frontal sections. These plexuses existed in the region of all vermis, crus 1 of the ansiform lobe, simplex lobule, and flocculus, while the other lobules were devoid of such fibers. Under electron microscopy, these CGRP-IR fibers were seen to make synaptic contacts with somatic spines of Purkinje cells, suggesting that CGRP-IR plexuses were closely related to the developing Purkinje cells. Mossy fiber-like CGRP-IR fibers appeared in the granular layer on postnatal day 2, and increased in number to reach a peak on postnatal day 12. Thereafter, they decreased slightly to reach a plateau on postnatal day 30. Under electron microscopy these CGRP-IR fibers were revealed to be the mossy fibers which regulated the granule cells. Thin varicose CGRP-IR fibers were rarely seen at birth, but on postnatal day 8, many fibers appeared in all layers and increased by postnatal day 30. They distributed equally throughout the cerebellar cortex with a slight predominance in density in the molecular and Purkinje cell layer. Immunoelectron microscopic analysis showed that these fibers made synaptic contacts with small dendrites in the molecular layer.

Novel developmental boundary in the cerebellum revealed by zebrin expression in the lurcher (Lc/+) mutant mouse

The cerebellar cortex contains at least two classes of Purkinje cells, which are organized into alternating arrays of parasagittal bands. The clearest demonstration of this compartmentation is the pattern of expression of a family of polypeptide antigens, the zebrins, which are expressed selectively by Purkinje cell subsets. Furthermore, anterograde tracing experiments show that the zebrin compartments are closely correlated with both afferent and efferent projection maps. The further subdivision of long parasagittal bands into smaller modules may occur through several different mechanisms, including the intrinsic cerebellar lobulation and the selective distribution of afferent terminal fields. However, while the longitudinal subdivisions are straightforwardly shown, the mediolateral boundaries are more subtle. In this report we describe a novel mediolateral and anteroposterior compartmentation boundary in mice, running across lobule VIII, that is revealed by the consequences of the lurcher (Lc/+) allele for zebrin expression. In normal mice zebrin compartmentation develops in several discrete stages: until postnatal day 5 (PD5) there is no zebrin expression; from PD5-PD7 zebrin is found only in the posterior lobe vermis, with immunoreactive Purkinje cells in lobules X, IX, and VIII but not elsewhere; from PD7-PD12 most Purkinje cells in the vermis become zebrin+; from PD12-PD15 immunoreactivity also appears in the hemispheres so that almost all Purkinje cells now are zebrin+; and finally, from PD15-PD25 zebrin is gradually suppressed in those Purkinje cells that are zebrin- in the adult until the mature pattern of parasagittal compartments is revealed. In the Lc/+ mutant the normal developmental progression is interrupted at around PD7. As a result, the pattern of zebrin expression becomes frozen at that stage when immunoreactive Purkinje cells are confined exclusively to the posterior lobe vermis. A reproducible boundary between expressing and nonexpressing zones runs mediolaterally across the dorsal surface of lobule VIII. Apart from zebrin expression itself, there are no obvious structural correlates of this transition. This mediolateral boundary identifies a developmental unit in the posterior lobe vermis of the cerebellum, and provides further evidence that the cerebellum is a highly heterogeneous structure.

Expression of the Purkinje cell specific zebrin antigens in the cerebellum of the meander tail mutant mouse

The cerebellum of the meander tail mutant mouse is characterized by normal cytoarchitecture in the posterior lobe and agranular, abnormal cytoarchitecture in the anterior lobe. The Purkinje cells form a monolayer in the posterior lobe but are dispersed throughout the cortex of the anterior lobe. Examination of these cells with the zebrin antibodies demonstrates that in spite of the morphologic and laminar disorganization of these cells in the anterior lobe, they are organized into the appropriate number of correctly positioned immunopositive zebrin clusters.