Extrinsic cytological determinants of basket and stellate cell dendritic pattern in the cerebellar molecular layer

Morphological pseudotime ordering and fate mapping reveal diversification of cerebellar inhibitory interneurons

Understanding how diverse neurons are assembled into circuits requires a framework for describing cell types and their developmental trajectories. Here we combine genetic fate-mapping, pseudotemporal profiling of morphogenesis, and dual morphology and RNA labeling to resolve the diversification of mouse cerebellar inhibitory interneurons. Molecular layer interneurons (MLIs) derive from a common progenitor population but comprise diverse dendritic-, somatic-, and axon initial segment-targeting interneurons. Using quantitative morphology from 79 mature MLIs, we identify two discrete morphological types and presence of extensive within-class heterogeneity. Pseudotime trajectory inference using 732 developmental morphologies indicate the emergence of distinct MLI types during migration, before reaching their final positions. By comparing MLI identities from morphological and transcriptomic signatures, we demonstrate the dissociation between these modalities and that subtype divergence can be resolved from axonal morphogenesis prior to marker gene expression. Our study illustrates the utility of applying single-cell methods to quantify morphology for defining neuronal diversification.

A Golgi study of neurons in the camel cerebellum (Camelus dromedarius)

Neurons in the cerebellar cortex of camels were studied using modified Golgi impregnation methods. Neurons were classified according to their position, morphology of their soma, density and distribution of dendrites, and the course of their axons. Accordingly, eight types of neurons were identified. Three types were found in the molecular layer: upper and lower stellate cells and basket cells, and four types were found in the granular layer: granule cells, Golgi Type II cells, Lugaro cells, and unipolar brush cells. Only the somata of Purkinje cells were found in the Purkinje cell layer. The molecular layer is characterized by the presence of more dendrites, dendritic spines, and transverse fibers. Golgi cells also show extensive dendritic branching and spines. The results illustrate the neuronal features of the camel cerebellum as a large mammal living in harsh environmental conditions. These findings should contribute to advancing our understanding of species-comparative anatomy in achieving better coordination of motor activity.

Stellate cell computational modeling predicts signal filtering in the molecular layer circuit of cerebellum

The functional properties of cerebellar stellate cells and the way they regulate molecular layer activity are still unclear. We have measured stellate cells electroresponsiveness and their activation by parallel fiber bursts. Stellate cells showed intrinsic pacemaking, along with characteristic responses to depolarization and hyperpolarization, and showed a marked short-term facilitation during repetitive parallel fiber transmission. Spikes were emitted after a lag and only at high frequency, making stellate cells to operate as delay-high-pass filters. A detailed computational model summarizing these physiological properties allowed to explore different functional configurations of the parallel fiber-stellate cell-Purkinje cell circuit. Simulations showed that, following parallel fiber stimulation, Purkinje cells almost linearly increased their response with input frequency, but such an increase was inhibited by stellate cells, which leveled the Purkinje cell gain curve to its 4 Hz value. When reciprocal inhibitory connections between stellate cells were activated, the control of stellate cells over Purkinje cell discharge was maintained only at very high frequencies. These simulations thus predict a new role for stellate cells, which could endow the molecular layer with low-pass and band-pass filtering properties regulating Purkinje cell gain and, along with this, also burst delay and the burst-pause responses pattern.

Rethinking organoid technology through bioengineering

In recent years considerable progress has been made in the development of faithful procedures for the differentiation of human pluripotent stem cells (hPSCs). An important step in this direction has also been the derivation of organoids. This technology generally relies on traditional three-dimensional culture techniques that exploit cell-autonomous self-organization responses of hPSCs with minimal control over the external inputs supplied to the system. The convergence of stem cell biology and bioengineering offers the possibility to provide these stimuli in a controlled fashion, resulting in the development of naturally inspired approaches to overcome major limitations of this nascent technology. Based on the current developments, we emphasize the achievements and ongoing challenges of bringing together hPSC organoid differentiation, bioengineering and ethics. This Review underlines the need for providing engineering solutions to gain control of self-organization and functionality of hPSC-derived organoids. We expect that this knowledge will guide the community to generate higher-grade hPSC-derived organoids for further applications in developmental biology, drug screening, disease modelling and personalized medicine.

Gradients in the mammalian cerebellar cortex enable Fourier-like transformation and improve storing capacity

Cerebellar granule cells (GCs) make up the majority of all neurons in the vertebrate brain, but heterogeneities among GCs and potential functional consequences are poorly understood. Here, we identified unexpected gradients in the biophysical properties of GCs in mice. GCs closer to the white matter (inner-zone GCs) had higher firing thresholds and could sustain firing with larger current inputs than GCs closer to the Purkinje cell layer (outer-zone GCs). Dynamic Clamp experiments showed that inner- and outer-zone GCs preferentially respond to high- and low-frequency mossy fiber inputs, respectively, enabling dispersion of the mossy fiber input into its frequency components as performed by a Fourier transformation. Furthermore, inner-zone GCs have faster axonal conduction velocity and elicit faster synaptic potentials in Purkinje cells. Neuronal network modeling revealed that these gradients improve spike-timing precision of Purkinje cells and decrease the number of GCs required to learn spike-sequences. Thus, our study uncovers biophysical gradients in the cerebellar cortex enabling a Fourier-like transformation of mossy fiber inputs.

Short-term plasticity at cerebellar granule cell to molecular layer interneuron synapses expands information processing

Information processing by cerebellar molecular layer interneurons (MLIs) plays a crucial role in motor behavior. MLI recruitment is tightly controlled by the profile of short-term plasticity (STP) at granule cell (GC)-MLI synapses. While GCs are the most numerous neurons in the brain, STP diversity at GC-MLI synapses is poorly documented. Here, we studied how single MLIs are recruited by their distinct GC inputs during burst firing. Using slice recordings at individual GC-MLI synapses of mice, we revealed four classes of connections segregated by their STP profile. Each class differentially drives MLI recruitment. We show that GC synaptic diversity is underlain by heterogeneous expression of synapsin II, a key actor of STP and that GC terminals devoid of synapsin II are associated with slow MLI recruitment. Our study reveals that molecular, structural and functional diversity across GC terminals provides a mechanism to expand the coding range of MLIs.

NeuroD2 controls inhibitory circuit formation in the molecular layer of the cerebellum

The cerebellar cortex is involved in the control of diverse motor and non-motor functions. Its principal circuit elements are the Purkinje cells that integrate incoming excitatory and local inhibitory inputs and provide the sole output of the cerebellar cortex. However, the transcriptional control of circuit assembly in the cerebellar cortex is not well understood. Here, we show that NeuroD2, a neuronal basic helix-loop-helix (bHLH) transcription factor, promotes the postnatal survival of both granule cells and molecular layer interneurons (basket and stellate cells). However, while NeuroD2 is not essential for the integration of surviving granule cells into the excitatory circuit, it is required for the terminal differentiation of basket cells. Axons of surviving NeuroD2-deficient basket cells follow irregular trajectories and their inhibitory terminals are virtually absent from Purkinje cells in Neurod2 mutants. As a result inhibitory, but not excitatory, input to Purkinje cells is strongly reduced in the absence of NeuroD2. Together, we conclude that NeuroD2 is necessary to instruct a terminal differentiation program in basket cells that regulates targeted axon growth and inhibitory synapse formation. An imbalance of excitation and inhibition in the cerebellar cortex affecting Purkinje cell output may underlay impaired adaptive motor learning observed in Neurod2 mutants.

Caspase-3 Mediated Cell Death in the Normal Development of the Mammalian Cerebellum

Caspase-3, onto which there is a convergence of the intrinsic and extrinsic apoptotic pathways, is the main executioner of apoptosis. We here review the current literature on the intervention of the protease in the execution of naturally occurring neuronal death (NOND) during cerebellar development. We will consider data on the most common altricial species (rat, mouse and rabbit), as well as humans. Among the different types of neurons and glia in cerebellum, there is ample evidence for an intervention of caspase-3 in the regulation of NOND of the post-mitotic cerebellar granule cells (CGCs) and Purkinje neurons, as a consequence of failure to establish proper synaptic contacts with target (secondary cell death). It seems possible that the GABAergic interneurons also undergo a similar type of secondary cell death, but the intervention of caspase-3 in this case still remains to be clarified in full. Remarkably, CGCs also undergo primary cell death at the precursor/pre-migratory stage of differentiation, in this instance without the intervention of caspase-3. Glial cells, as well, undergo a process of regulated cell death, but it seems possible that expression of caspase-3, at least in the Bergmann glia, is related to differentiation rather than death.

Synaptic input as a directional cue for migrating interneuron precursors

During CNS development, interneuron precursors have to migrate extensively before they integrate in specific microcircuits. Known regulators of neuronal motility include classical neurotransmitters, yet the mechanisms that assure interneuron dispersal and interneuron/projection neuron matching during histogenesis remain largely elusive. We combined time-lapse video microscopy and electrophysiological analysis of the nascent cerebellum of transgenic Pax2-EGFP mice to address this issue. We found that cerebellar interneuronal precursors regularly show spontaneous postsynaptic currents, indicative of synaptic innervation, well before settling in the molecular layer. In keeping with the sensitivity of these cells to neurotransmitters, ablation of synaptic communication by blocking vesicular release in acute slices of developing cerebella slows migration. Significantly, abrogation of exocytosis primarily impedes the directional persistence of migratory interneuronal precursors. These results establish an unprecedented function of the early synaptic innervation of migrating neuronal precursors and demonstrate a role for synapses in the regulation of migration and pathfinding.

Consensus Paper: Cerebellar Development

The development of the mammalian cerebellum is orchestrated by both cell-autonomous programs and inductive environmental influences. Here, we describe the main processes of cerebellar ontogenesis, highlighting the neurogenic strategies used by developing progenitors, the genetic programs involved in cell fate specification, the progressive changes of structural organization, and some of the better-known abnormalities associated with developmental disorders of the cerebellum.

Developmental delay in motor skill acquisition in Niemann-Pick C1 mice reveals abnormal cerebellar morphogenesis

Niemann-Pick type C1 (NPC1) disease is a lysosomal storage disorder caused by defective intracellular trafficking of exogenous cholesterol. Purkinje cell (PC) degeneration is the main sign of cerebellar dysfunction in both NPC1 patients and animal models. It has been recently shown that a significant decrease in Sonic hedgehog (Shh) expression reduces the proliferative potential of granule neuron precursors in the developing cerebellum of Npc1^−/− mice. Pursuing the hypothesis that this developmental defect translates into functional impairments, we have assayed Npc1-deficient pups belonging to the milder mutant mouse strain Npc1^nmf164 for sensorimotor development from postnatal day (PN) 3 to PN21. Npc1^nmf164/ Npc1^nmf164 pups displayed a 2.5-day delay in the acquisition of complex motor abilities compared to wild-type (wt) littermates, in agreement with the significant disorganization of cerebellar cortex cytoarchitecture observed between PN11 and PN15. Compared to wt, Npc1^nmf164 homozygous mice exhibited a poorer morphological differentiation of Bergmann glia (BG), as indicated by thicker radial shafts and less elaborate reticular pattern of lateral processes. Also BG functional development was defective, as indicated by the significant reduction in GLAST and Glutamine synthetase expression. A reduced VGluT2 and GAD65 expression also indicated an overall derangement of the glutamatergic/GABAergic stimulation that PCs receive by climbing/parallel fibers and basket/stellate cells, respectively. Lastly, Npc1-deficiency also affected oligodendrocyte differentiation as indicated by the strong reduction of myelin basic protein. Two sequential 2-hydroxypropyl-β-cyclodextrin administrations at PN4 and PN7 counteract these defects, partially preventing functional impairment of BG and fully restoring the normal patterns of glutamatergic/GABAergic stimulation to PCs.

These findings indicate that in Npc1^nmf164 homozygous mice the derangement of synaptic connectivity and dysmyelination during cerebellar morphogenesis largely anticipate motor deficits that are typically observed during adulthood.

Laminar Differences in Dendritic Structure of Pyramidal Neurons in the Juvenile Rat Somatosensory Cortex

Pyramidal cell structure varies between different cortical areas and species, indicating that the cortical circuits that these cells participate in are likely to be characterized by different functional capabilities. Structural differences between cortical layers have been traditionally reported using either the Golgi method or intracellular labeling, but the structure of pyramidal cells has not previously been systematically analyzed across all cortical layers at a particular age. In the present study, we investigated the dendritic architecture of complete basal arbors of pyramidal neurons in layers II, III, IV, Va, Vb, and VI of the hindlimb somatosensory cortical region of postnatal day 14 rats. We found that the characteristics of basal dendritic morphologies are statistically different in each cortical layer. The variations in size and branching pattern that exist between pyramidal cells of different cortical layers probably reflect the particular functional properties that are characteristic of the cortical circuit in which they participate. This new set of complete basal dendritic arbors of 3D-reconstructed pyramidal cell morphologies across each cortical layer will provide new insights into interlaminar information processing in the cerebral cortex.

Switching modes in corticogenesis: mechanisms of neuronal subtype transitions and integration in the cerebral cortex

Information processing in the cerebral cortex requires the activation of diverse neurons across layers and columns, which are established through the coordinated production of distinct neuronal subtypes and their placement along the three-dimensional axis. Over recent years, our knowledge of the regulatory mechanisms of the specification and integration of neuronal subtypes in the cerebral cortex has progressed rapidly. In this review, we address how the unique cytoarchitecture of the neocortex is established from a limited number of progenitors featuring neuronal identity transitions during development. We further illuminate the molecular mechanisms of the subtype-specific integration of these neurons into the cerebral cortex along the radial and tangential axis, and we discuss these key features to exemplify how neocortical circuit formation accomplishes economical connectivity while maintaining plasticity and evolvability to adapt to environmental changes.

Cellular commitment in the developing cerebellum

The mammalian cerebellum is located in the posterior cranial fossa and is critical for motor coordination and non-motor functions including cognitive and emotional processes. The anatomical structure of cerebellum is distinct with a three-layered cortex. During development, neurogenesis and fate decisions of cerebellar primordium cells are orchestrated through tightly controlled molecular events involving multiple genetic pathways. In this review, we will highlight the anatomical structure of human and mouse cerebellum, the cellular composition of developing cerebellum, and the underlying gene expression programs involved in cell fate commitments in the cerebellum. A critical evaluation of the cell death literature suggests that apoptosis occurs in ~5% of cerebellar cells, most shortly after mitosis. Apoptosis and cellular autophagy likely play significant roles in cerebellar development, we provide a comprehensive discussion of their role in cerebellar development and organization. We also address the possible function of unfolded protein response in regulation of cerebellar neurogenesis. We discuss recent advancements in understanding the epigenetic signature of cerebellar compartments and possible connections between DNA methylation, microRNAs and cerebellar neurodegeneration. Finally, we discuss genetic diseases associated with cerebellar dysfunction and their role in the aging cerebellum.

Reelin Induces Branching of Neurons and Radial Glial Cells during Corticogenesis

Newborn neurons migrate along the processes of radial glial cells (RGCs) to reach their final positions in the cortex. Here, we visualized individual migrating neurons and RGCs using in utero electroporation. We show that branching of migrating neurons and RGCs is closely correlated spatiotemporally with the distribution of Reelin. Time-lapse imaging revealed that the leading processes of migrating neurons gave rise to increasingly more branches once their growth cones contacted the Reelin-containing marginal zone. This was accompanied by translocation of the nucleus and gradual shortening of the leading process. Absence of Reelin in reeler mice altered these processes resulting in misorientation, loss of bipolarity, and aberrant migration of cortical neurons. Moreover, in reeler, the branching of the basal processes of RGCs in the marginal zone was severely disrupted. Consistent with previous reports, we show that in dissociated reeler cortical cultures, exposure to recombinant Reelin enhanced dendritic complexity and glial branching. Our results suggest that Reelin induces branching of the leading processes of migrating neurons and that of basal processes of RGCs when they arrive at the Reelin-containing marginal zone. Branching of these processes may be crucial for the termination of nuclear translocation during the migratory process and for correct neuronal positioning.

Comparative neuronal morphology of the cerebellar cortex in afrotherians, carnivores, cetartiodactyls, and primates

Although the basic morphological characteristics of neurons in the cerebellar cortex have been documented in several species, virtually nothing is known about the quantitative morphological characteristics of these neurons across different taxa. To that end, the present study investigated cerebellar neuronal morphology among eight different, large-brained mammalian species comprising a broad phylogenetic range: afrotherians (African elephant, Florida manatee), carnivores (Siberian tiger, clouded leopard), cetartiodactyls (humpback whale, giraffe) and primates (human, common chimpanzee). Specifically, several neuron types (e.g., stellate, basket, Lugaro, Golgi, and granule neurons; N = 317) of the cerebellar cortex were stained with a modified rapid Golgi technique and quantified on a computer-assisted microscopy system. There was a 64-fold variation in brain mass across species in our sample (from clouded leopard to the elephant) and a 103-fold variation in cerebellar volume. Most dendritic measures tended to increase with cerebellar volume. The cerebellar cortex in these species exhibited the trilaminate pattern common to all mammals. Morphologically, neuron types in the cerebellar cortex were generally consistent with those described in primates (Fox et al., 1967) and rodents (Palay and Chan-Palay, 1974), although there was substantial quantitative variation across species. In particular, Lugaro neurons in the elephant appeared to be disproportionately larger than those in other species. To explore potential quantitative differences in dendritic measures across species, MARSplines analyses were used to evaluate whether species could be differentiated from each other based on dendritic characteristics alone. Results of these analyses indicated that there were significant differences among all species in dendritic measures.

Structured connectivity in cerebellar inhibitory networks

Defining the rules governing synaptic connectivity is key to formulating theories of neural circuit function. Interneurons can be connected by both electrical and chemical synapses, but the organization and interaction of these two complementary microcircuits is unknown. By recording from multiple molecular layer interneurons in the cerebellar cortex, we reveal specific, nonrandom connectivity patterns in both GABAergic chemical and electrical interneuron networks. Both networks contain clustered motifs and show specific overlap between them. Chemical connections exhibit a preference for transitive patterns, such as feedforward triplet motifs. This structured connectivity is supported by a characteristic spatial organization: transitivity of chemical connectivity is directed vertically in the sagittal plane, and electrical synapses appear strictly confined to the sagittal plane. The specific, highly structured connectivity rules suggest that these motifs are essential for the function of the cerebellar network.

Cerebellar granule cells are generated postnatally in humans

How many cerebellar granule cells are generated pre- or postnatally in human is unknown. Using a rigorous design-based stereologic approach we investigated postmortem cerebella from 14 children who died between the first postnatal day (P1) and 11 months of age (M11). We found a statistically significant (p < 0.05) age-related increase in the total number of granule cells from 5.9 × 10(9) at M1 to 37.6 × 10(9) at M10/11 per cerebellar half but not in the total number of Purkinje cells (12.1 × 10(6) at M1 vs. 13.9 × 10(6) at M10/11 per cerebellar half). Accordingly, approximately 85 % of the cerebellar granule cells are generated postnatally in human, and the number of granule cells per Purkinje cell in the human cerebellum increases from 485 at M1 to 2,700 at M10/11, approximately. These data indicate that the human cerebellum has a much higher functional plasticity during the first year of life than previously thought, and may respond very sensitively to internal and external influences during this time. This has important implications for several neuropsychiatric conditions in which cerebellar involvement has been demonstrated.

The compartmental restriction of cerebellar interneurons

The Purkinje cells (PC's) of the cerebellar cortex are subdivided into multiple different molecular phenotypes that form an elaborate array of parasagittal stripes. This array serves as a scaffold around which afferent topography is organized. The ways in which cerebellar interneurons may be restricted by this scaffolding are less well-understood. This review begins with a brief survey of cerebellar topography. Next, it reviews the development of stripes in the cerebellum with a particular emphasis on the embryological origins of cerebellar interneurons. These data serve as a foundation to discuss the hypothesis that cerebellar compartment boundaries also restrict cerebellar interneurons, both excitatory [granule cells, unipolar brush cells (UBCs)] and inhibitory (e.g., Golgi cells, basket cells). Finally, it is proposed that the same PC scaffold that restricts afferent terminal fields to stripes may also act to organize cerebellar interneurons.

The genesis of cerebellar GABAergic neurons: fate potential and specification mechanisms

All cerebellar neurons derive from progenitors that proliferate in two germinal neuroepithelia: the ventricular zone (VZ) generates GABAergic neurons, whereas the rhombic lip is the origin of glutamatergic types. Among VZ-derivatives, GABAergic projection neurons, and interneurons are generated according to distinct strategies. Projection neurons (Purkinje cells and nucleo-olivary neurons) are produced at the onset of cerebellar neurogenesis by discrete progenitor pools located in distinct VZ microdomains. These cells are specified within the VZ and acquire mature phenotypes according to cell-autonomous developmental programs. On the other hand, the different categories of inhibitory interneurons derive from a single population of Pax-2-positive precursors that delaminate into the prospective white matter (PWM), where they continue to divide up to postnatal development. Heterotopic/heterochronic transplantation experiments indicate that interneuron progenitors maintain full developmental potentialities up to the end of cerebellar development and acquire mature phenotypes under the influence of environmental cues present in the PWM. Furthermore, the final fate choice occurs in postmitotic cells, rather than dividing progenitors. Extracerebellar cells grafted to the prospective cerebellar white matter are not responsive to local neurogenic cues and fail to adopt clear cerebellar identities. Conversely, cerebellar cells grafted to extracerebellar regions retain typical phenotypes of cerebellar GABAergic interneurons, but acquire type-specific traits under the influence of local cues. These findings indicate that interneuron progenitors are multipotent and sensitive to spatio-temporally patterned environmental signals that regulate the genesis of different categories of interneurons, in precise quantities and at defined times and places.

Astrocyte-secreted factors modulate a gradient of primary dendritic arbors in nucleus laminaris of the avian auditory brainstem

Neurons in nucleus laminaris (NL) receive binaural, tonotopically matched input from nucleus magnocelluaris (NM) onto bitufted dendrites that display a gradient of dendritic arbor size. These features improve computation of interaural time differences, which are used to determine the locations of sound sources. The dendritic gradient emerges following a period of significant reorganization at embryonic day 15 (E15), which coincides with the emergence of astrocytes that express glial fibrillary acidic protein (GFAP) in the auditory brainstem. The major changes include a loss of total dendritic length, a systematic loss of primary dendrites along the tonotopic axis, and lengthening of primary dendrites on caudolateral NL neurons. Here we have tested whether astrocyte-derived molecules contribute to these changes in dendritic morphology. We used an organotypic brainstem slice preparation to perform repeated imaging of individual dye-filled NL neurons to determine the effects of astrocyte-conditioned medium (ACM) on dendritic morphology. We found that treatment with ACM induced a decrease in the number of primary dendrites in a tonotopically graded manner similar to that observed during normal development. Our data introduce a new interaction between astrocytes and neurons in the auditory brainstem and suggest that these astrocytes influence multiple aspects of auditory brainstem maturation.

Modulation of cell-cycle dynamics is required to regulate the number of cerebellar GABAergic interneurons and their rhythm of maturation

The progenitors of cerebellar GABAergic interneurons proliferate up to postnatal development in the prospective white matter, where they give rise to different neuronal subtypes, in defined quantities and according to precise spatiotemporal sequences. To investigate the mechanisms that regulate the specification of distinct interneuron phenotypes, we examined mice lacking the G1 phase-active cyclin D2. It has been reported that these mice show severe reduction of stellate cells, the last generated interneuron subtype. We found that loss of cyclin D2 actually impairs the whole process of interneuron genesis. In the mutant cerebella, progenitors of the prospective white matter show reduced proliferation rates and enhanced tendency to leave the cycle, whereas young postmitotic interneurons undergo severe delay of their maturation and migration. As a consequence, the progenitor pool is precociously exhausted and the number of interneurons is significantly reduced, although molecular layer interneurons are more affected than those of granular layer or deep nuclei. The characteristic inside-out sequence of interneuron placement in the cortical layers is also reversed, so that later born cells occupy deeper positions than earlier generated ones. Transplantation experiments show that the abnormalities of cyclin D2(-/-) interneurons are largely caused by cell-autonomous mechanisms. Therefore, cyclin D2 is not required for the specification of particular interneuron subtypes. Loss of this protein, however, disrupts regulatory mechanisms of cell cycle dynamics that are required to determine the numbers of interneurons of different types and impairs their rhythm of maturation and integration in the cerebellar circuitry.

Maturational trajectories of cortical brain development through the pubertal transition: unique species and sex differences in the monkey revealed through structural magnetic resonance imaging

Characterizing normal brain development in the rhesus macaque is a necessary prerequisite for establishing better nonhuman primate models of neuropathology. Structural magnetic resonance imaging scans were obtained on 37 rhesus monkeys (20 Male, 17 Female) between 10 and 64 months of age. Effects of age and sex were analyzed with a cross-sectional design. Gray matter (GM) and white matter (WM) volumes were determined for total brain and major cortical regions using an automatic segmentation and parcellation pipeline. Volumes of major subcortical structures were evaluated. Unlike neural maturation in humans, GM volumes did not show a postpubertal decline in most cortical regions, with the notable exception of the prefrontal cortex. Similar to humans, WM volumes increased through puberty with less change thereafter. Caudate, putamen, amygdala, and hippocampus increased linearly as did the corpus callosum. Males and females showed similar maturational patterns, although males had significantly larger brain volumes. Females had a proportionately larger caudate, putamen, and hippocampus, whereas males had both an absolute and relatively larger corpus callosum. The authors discuss the possible implications of these findings for research using the rhesus macaque as a model for neurodevelopmental disorders.

Laminar fate and phenotype specification of cerebellar GABAergic interneurons

In most CNS regions, the variety of inhibitory interneurons originates from separate pools of progenitors residing in discrete germinal domains, where they become committed to specific phenotypes and positions during their last mitosis. We show here that GABAergic interneurons of the rodent cerebellum are generated through a different mechanism. Progenitors for these interneurons delaminate from the ventricular neuroepithelium of the embryonic cerebellar primordium and continue to proliferate in the prospective white matter during late embryonic and postnatal development. Young postmitotic interneurons do not migrate immediately to their final destination, but remain in the prospective white matter for several days. The different interneuron categories are produced according to a continuous inside-out positional sequence, and cell identity and laminar placement in the cerebellar cortex are temporally related to birth date. However, terminal commitment does not occur while precursors are still proliferating, and postmitotic cells heterochronically transplanted to developing cerebella consistently adopt host-specific phenotypes and positions. However, solid grafts of prospective white matter implanted into the adult cerebellum, when interneuron genesis has ceased, produce interneuron types characteristic of the donor age. Therefore, specification of cerebellar GABAergic interneurons occurs through a hitherto unknown process, in which postmitotic neurons maintain broad developmental potentialities and their phenotypic choices are dictated by instructive cues provided by the microenvironment of the prospective white matter. Whereas in most CNS regions the repertoire of inhibitory interneurons is produced by recruiting precursors from different origins, in the cerebellum it is achieved by creating phenotypic diversity from a single source.

The specificity of interactions between the cortex and the thalamus

The functioning of the adult mammalian cerebral cortex depends critically upon precise interconnections between specific thalamic nuclei and distinct cortical regions. Therefore, one central issue in understanding cortical development is determining the cellular and molecular strategies underlying the specification of thalamocortical projections. We address the role of axon-axon interactions and membrane-bound guidance molecules in the establishment of the development of layer-specific patterns of afferent and efferent cortical connections does not depend upon neuronal activity. We present evidence that activity conveyed by thalamic afferents is required for the elaboration of the columnar specificity of cortical circuits.

GABAergic synaptogenesis marks the onset of differentiation of basket and stellate cells in mouse cerebellum

Type 2 glycine transporter (GlyT2) mediates intracellular glycine transport and is expressed selectively in glycinergic neurons. Expression of GlyT2 gene promoter-driven enhanced green fluorescent protein (eGFP) in BAC transgenic mice allows selective visualization of glycinergic neurons by fluorescence microscopy. Here, we show that cerebellar interneuron precursors identified by the transcription factor Pax2, including gamma-aminobutyric acid (GABA)ergic interneurons of the molecular layer (ML; basket and stellate cells), transiently express GlyT2-eGFP during development. In contrast, expression of endogenous GlyT2 is restricted to glycinergic Golgi cells. Comparison with knock-in mice expressing eGFP in GABAergic neurons [glutamic acid decarboxylase (GAD)67-eGFP] revealed that GlyT2-eGFP expression often precedes GAD67-eGFP and is therefore a marker of immature GABAergic interneurons. In the internal granule cell layer, GABAergic Golgi cells differentiated shortly after birth, prior to glycinergic Golgi cells. In the ML, GlyT2-eGFP-positive precursor cells migrated until the boundary with the external granule cell layer, forming an inside-out maturation gradient that determined the final position of interneurons in the ML. After migration, GlyT2-eGFP gradually disappeared, while interneurons differentiated morphologically and became immunoreactive for parvalbumin, the GABA(A) receptor alpha1 subunit, and the K(+)Cl(-) exchanger KCC2 (K(+)Cl(-) cotransporter type 2). Numerous presumptive GABAergic synaptic terminals were seen on immature ML interneurons as early as P4, preceding the expression of these neurochemical markers. These results suggest that GABAergic synaptogenesis marks the onset of differentiation of basket and stellate cells in the mouse cerebellum, and that GABAergic synaptic function might contribute to the differentiation of interneurons in the cerebellar cortex.

Postnatal growth and morphological development of the brain: a species comparison

The objective of this report is to summarize the available literature regarding the postnatal growth and morphological development of the brain and compare the timelines for these events between humans and experimental species. While not the primary focus of this report, in acknowledgement of the evident role of maturation of neurotransmitter systems in development, a brief description of the comparative development of the NMDA receptor is included. To illustrate the challenges faced in estimating developmental toxicity potential in humans, the importance of postnatal experience in CNS development is also briefly reviewed. This review is part of the initial phase of a project undertaken by the Developmental and Reproductive Toxicology Technical Committee of the ILSI Health and Environmental Sciences Institute (HESI) to bring together information on a selected number of organ systems and compare their postnatal development across several species (Hurtt and Sandler: Birth Defects Res Part B 68:307-308, 2003).

Aligned neurite bundles of granule cells regulate orientation of Purkinje cell dendrites by perpendicular contact guidance in two-dimensional and three-dimensional mouse cerebellar cultures

To identify structures that determine the 90 degree orientation of thin espalier dendritic trees of Purkinje cells with respect to parallel fibers (axonal neurite bundles of granule cells) in the cerebellar cortex, we designed five types of two-dimensional and three-dimensional cell and tissue cultures of cerebella from postnatal mice and analyzed the orientation of Purkinje cell dendrites with respect to neurite bundles and astrocyte fibers by immunofluorescence double or triple staining. We cultured dissociated cerebellar cells on micropatterned substrates and preformed neurite bundles of a microexplant culture two-dimensionally and in matrix gels three-dimensionally. Dendrites, but not axons, of Purkinje cells extended toward the neurites of granule cells and oriented at right angles two-dimensionally to aligned neurite bundles in the three cultures. In a more organized explant proper of the microexplant culture, Purkinje cell dendrites extended toward thin aligned neurite bundles not only consistently at right angles but also two-dimensionally. However, in the "organotypic microexplant culture," in which three-dimensionally aligned thick neurite bundles mimicking parallel fibers were produced, Purkinje cell dendrites often oriented perpendicular to the thick bundles three-dimensionally. Astrocytes were abundant in all cultures, and there was no definite correlation between the presence of and orientation to Purkinje cell dendrites, although their fibers were frequently associated in parallel with dendrites in the organotypic microexplant culture. Therefore, Purkinje cells may grow their dendrites to the newly produced neurite bundles of parallel fibers in the cerebellar cortex and be oriented at right angles three-dimensionally mainly via "perpendicular contact guidance."

Constructing circuits: neurogenesis and migration in the developing neocortex

Our knowledge of the proliferation, migration, and differentiation of neurons has changed dramatically over the last 10 years. Whereas traditionally it was thought that glial and neuronal cells were separate cell lines with different lineages, we now know that this is not true. Radial glia are a type of neural stem cell that generate excitatory pyramidal neurons directly through asymmetric cell division in the ventricular zone (VZ) of the telencephalon and indirectly through the symmetric division of daughter intermediate precursor cells that divide in the subventricular zone (SVZ). Moreover, pyramidal neurons, once thought to migrate only along radial guide fibers to the developing layers of the cortex, have been shown to proceed through four distinct stages of migration during which they change shape, direction, and speed. Gamma-aminobutyric acid (GABAergic) inhibitory interneurons, on the other hand, are generated not in the cortex, but in the medial ganglionic eminence and migrate tangentially to their final cortical destinations. Evidence suggests that GABA activation may play a role in coordinating the generation and migration of both pyramidal and interneuron populations. At the end of neurogenesis, radial glial cells translocate to the cortex and transform into astrocytes. Although they do not actively divide in the adult brain, astrocytes may retain the potential to generate new neurons. These new findings have increased our understanding of the mechanisms underlying certain developmental disorders and, in doing so, reveal potentially useful modes of therapeutic intervention.

GDNF and GFRalpha1 promote differentiation and tangential migration of cortical GABAergic neurons

Cortical GABAergic neurons are generated in the ventral telencephalon and migrate dorsally into the cortex following a tangential path. GDNF signaling via GFRalpha1 was found to promote the differentiation of ventral precursors into GABAergic cells, enhancing their neuronal morphology and motility. GDNF stimulated axonal growth in cortical GABAergic neurons and acted as a potent chemoattractant of GABAergic cells. These effects required GFRalpha1 but neither RET nor NCAM, the two transmembrane signaling receptors known for GDNF. Mutant mice lacking GDNF or GFRalpha1, but neither RET nor NCAM, showed reduced numbers of GABAergic cells in the cerebral cortex and hippocampus. We conclude that one of the normal functions of GDNF signaling via GFRalpha1 in the developing brain is to promote the differentiation and migration of cortical GABAergic neurons. The lack of involvement of RET or NCAM in these processes suggests the existence of additional transmembrane effectors for GDNF.

Cell migration in the forebrain

The forebrain comprises an intricate set of structures that are required for some of the most complex and evolved functions of the mammalian brain. As a reflection of its complexity, cell migration in the forebrain is extremely elaborated, with widespread dispersion of cells across multiple functionally distinct areas. Two general modes of migration are distinguished in the forebrain: radial migration, which establishes the general cytoarchitectonical framework of the different forebrain subdivisions; and tangential migration, which increases the cellular complexity of forebrain circuits by allowing the dispersion of multiple neuronal types. Here, we review the cellular and molecular mechanisms underlying each of these types of migrations and discuss how emerging concepts in neuronal migration are reshaping our understanding of forebrain development in normal and pathological situations.

Morphological development and neurochemical differentiation of cerebellar inhibitory interneurons in microexplant cultures

The cerebellar cortex comprises a rather limited variety of interneurons, prominently among them inhibitory basket and stellate cells and Golgi neurons. To identify mechanisms subserving the positioning, morphogenesis, and neurochemical maturation of these inhibitory interneurons, we analyzed their development in primary microexplant cultures of the early postnatal cerebellar cortex. These provide a well-defined, patterned lattice within which the development of individual cells is readily accessible to experimental manipulation and observation. Pax-2-positive precursors of inhibitory interneurons were found to effectively segregate from granule cell perikarya. They emigrate from the core explant and avoid the vicinity of granule cells, which also emigrate and aggregate into small clusters around the explant proper. This contrasts with the behavior of Purkinje neurons, which remain within the explant proper. During migration, a subset of Pax-2-positive cells gradually acquires a GABAergic phenotype, and subsequently also expresses the type 2 metabotropic receptor for glutamate, or parvalbumin, markers for Golgi neurons and basket or stellate cells, respectively. The latter eventually orient their dendrites such that they take a preferentially perpendicular orientation relative to granule cell axons. Both the neurochemical maturation of basket/stellate cells and the specific orientation of their dendrites are independent of their continuous contact with radially oriented glia or Purkinje cell dendrites projecting from the core explant. Numbers of parvalbumin-positive basket/stellate cells and the prevalence of glutamate-positive neurites, which form a dense network preferentially within cell clusters containing granule cell perikarya and their dendrites, are subject to regulation by chronic depolarization. In contrast, brain-derived neurotrophic factor results in a drastic decrease of numbers of basket/stellate cells. These findings document that granule cell axons (parallel fibers) are the major determinant of basket/stellate cell dendritic orientation. They also show that the neurochemical maturation of cerebellar interneurons is sensitive to regulation by activity and neurotrophic factors.

Age-related changes in rat cerebellar basket cells: a quantitative study using unbiased stereological methods

Cortical cerebellar basket cells are stable postmitotic cells; hence, they are liable to endure age-related changes. Since the cerebellum is a vital organ for the postural control, equilibrium and motor coordination, we aimed to determine the quantitative morphological changes in those interneurons with the ageing process, using unbiased techniques. Material from the cerebellar cortex (Crus I and Crus II) was collected from female rats aged 2, 6, 9, 12, 15, 18, 21 and 24 mo (5 animals per each age group), fixed by intracardiac perfusion, and processed for transmission electron microscopy, using conventional techniques. Serial semithin sections were obtained (5 blocks from each rat), enabling the determination of the number-weighted mean nuclear volume (by the nucleator method). On ultrathin sections, 25 cell profiles from each animal were photographed. The volume density of the nucleus, ground substance, mitochondria, Golgi apparatus (Golgi) and dense bodies (DB), and the mean surface density of the rough endoplasmic reticulum (RER) were determined, by point counting, using a morphometric grid. The mean total volumes of the soma and organelles and the mean total surface area of the RER [SN (RER)] were then calculated. The results were analysed with 1-way ANOVA; posthoc pairwise comparisons of group means were performed using the Newman-Keuls test. The relation between age and each of the parameters was studied by regression analysis. Significant age-related changes were observed for the mean volumes of the soma, ground substance, Golgi, DB, and SN (RER). Positive linear trends were found for the mean volumes of the ground substance, Golgi, and DB; a negative linear trend was found for the SN (RER). These results indicate that rat cerebellar basket cells endure important age-related changes. The significant decrease in the SN (RER) may be responsible for a reduction in the rate of protein synthesis. Additionally, it may be implicated in a cascade of events leading to cell damage due to the excitotoxic activity of glutamate, which could interfere in the functioning of the complex cerebellar neuronal network.

Mode and tempo of tangential cell migration in the cerebellar external granular layer

After their final mitosis, cerebellar granule cells remain in the external granular layer (EGL) for 20-48 hr before initiating their radial migration across the molecular layer (ML), but the significance of this latent period is not well understood. In the present study, we used a confocal microscope to examine morphogenetic changes and behavior of postmitotic granule cells restricted to the EGL in slice preparations of the postnatal mouse cerebellum. We found that, coincident with the extension of two uneven horizontal processes oriented parallel to the longitudinal axis of the folium, postmitotic granule cells start to migrate tangentially in the direction of the larger process. Interestingly, their morphology and the speed of cell movement change systematically with their position within the EGL. The rate of tangential cell movement is fastest (approximately 14.8 micrometer/hr) in the middle of the EGL, when cells have two short horizontal processes. As granule cells elongate their somata and extend longer horizontal processes at the bottom of the EGL, they move at a reduced rate (approximately 12.6 micrometer/hr). At the interface of the EGL and ML where cells migrate tangentially at the slowest rate (approximately 4.1 micrometer/hr), their somata round and then begin to extend couples of the descending processes into the ML. After the stationary period, granule cells abruptly extend a single vertical process and initiate the transition from tangential to radial migration, reshaping their rounded somata into a vertically elongated spindle. These observations suggest that tangential migration of granule cells within the EGL may provide the developmental mechanisms for their appropriate allocation across parasagittal compartments of the expanding cerebellar cortex.

Molecular signals for development of neuronal circuitry in the retina

In this review article, we summarize recently accumulated knowledge regarding the molecular mechanisms, which control retinal development. Retinal neurons are born in two waves of cytogenesis. In the first wave, neurons of cone circuitry are generated, whereas in the second wave, rod circuitry is added. Neurons generated in these two waves of cytogenesis differ in many respects, including the molecular cues used for migrational guidance. The neurons generated in the second phase of proliferation are arranged in radial columns associated with Müller cells, whereas those of the first phase are often found outside the radial columns. Certain early born cone photoreceptors may form templates for the arrangement of additional mosaics of other cell types. These mosaic arrangements of cell bodies are subsequently refined by lateral displacement of cells and apoptosis. Müller cells may play an important role in directing migration of second phase neurons within groups of radial columns and also in guiding the projections of these neurons so that specific connections are formed. The Müller cell's ability to exert these influences perhaps resides in a variety of cell adhesion molecules such as L1/NgCAM, F11, and 5A11, which are expressed on the surface of Müller cells and retinal neurons. CAMs also promote neurite outgrowth through second messenger pathways.

Contact-dependent inhibition of cortical neurite growth mediated by notch signaling

The exuberant growth of neurites during development becomes markedly reduced as cortical neurons mature. In vitro studies of neurons from mouse cerebral cortex revealed that contact-mediated Notch signaling regulates the capacity of neurons to extend and elaborate neurites. Up-regulation of Notch activity was concomitant with an increase in the number of interneuronal contacts and cessation of neurite growth. In neurons with low Notch activity, which readily extend neurites, up-regulation of Notch activity either inhibited extension or caused retraction of neurites. Conversely, in more mature neurons that had ceased their growth after establishing numerous connections and displayed high Notch activity, inhibition of Notch signaling promoted neurite extension. Thus, the formation of neuronal contacts results in activation of Notch receptors, leading to restriction of neuronal growth and a subsequent arrest in maturity.

Cerebellar and brainstem development: an overview in relation to Joubert syndrome

An overview of cerebellar and brainstem development is provided as a foundation for suggesting hypotheses about developmental defects in Joubert syndrome. Although neuropathologic studies of Joubert syndrome are rare, and the spectrum of brain pathology is not yet known, consistent findings include agenesis of the cerebellar vermis and hypoplasia or fragmentation of several brainstem nuclei (including dentate nuclei, inferior olives, and basis pontis), nuclei and tracts of cranial nerve V, solitary nuclei and tracts, and nuclei gracilis and cuneatus. Two aspects of cerebellar development might be important in the pathogenesis of Joubert syndrome: First, cerebellar development is regulated by a critical region of the embryo called the "midbrain-hindbrain organizer," and both mesencephalic and metencephalic elements take part in normal cerebellar development. While the metencephalon gives rise to the cerebellar hemispheres, the vermis is derived almost exclusively from the mesencephalon. This suggests that Joubert syndrome could involve an abnormality in formation of the pontomesencephalic junction (rhombomere 1). Second, the histogenesis of cranial nerve nuclei and brainstem structures derived from the embryonic rhombic lip (such as the inferior olives, neurons of the basis pontis, and arcuate nuclei) involves the formation, migration, and reorganization of nuclei and tracts during a critical period of development (6 to 8 weeks' gestation). Because these structures are abnormal in Joubert syndrome, an understanding of factors that regulate the proper formation and migration of cells that give rise to them could provide important clues about the pathogenesis of this disorder.

Dynamic behavior of the ends of growing parallel fibers in early postnatal rat cerebellum

The molecular layer of the cerebellum contains parallel fibers, the axons of granule neurons. We have examined the morphology and behavior of parallel fiber growth cones in the early postnatal rat cerebellum using the fluorescent tracer DiI. Parallel fiber growth cones distributed into three categories based on size and shape: short torpedo-like, long torpedo-like, and lamellopodial in form. The torpedo-like growth cones were modified by the addition of lamellopodia and/or filopodia, and the lamellopodial growth cones were often decorated with a filopodium. These three different growth cone morphologies were found throughout the growing region of the molecular layer. The nascent axons elaborated by premigratory granule neurons differed form the longer axons of more developed neurons in that they often had forked growth cones and extensive lamellopodial decoration along the axon shaft. Growth cones in living slices closely resembled those observed in the fixed preparations. The living growth cones exhibited frequent lamellopodial rearrangement and a side-to-side headwaving movement. The axon proximal to the growth cone was also dynamic. The axons curved and undulated, and mobile swellings formed along the axon shaft. These observations show that the growth cones of parallel fibers are similar to growth cones described for axons in other developing systems in terms of size, morphological characteristics, and dynamic behavior.

Thyroid hormone-responsive genes in developing cerebellum include a novel synaptotagmin and a hairless homolog

Proper development of the mammalian CNS requires sufficient thyroid hormone; thyroid hormone deficiency during a brief perinatal period produces severe neurological defects in humans and experimental animals. Thyroid hormone exerts its effects through nuclear receptors, which modulate the transcription of downstream genes in response to hormone binding. Surprisingly, few genes that are regulated by thyroid hormone receptors in the CNS have been described. Here, I report the isolation and characterization of genes that are expressed in response to thyroid hormone in developing rat brain. One such gene (Srg1) encodes a novel protein related to synaptotagmin, a protein involved in regulating neurotransmitter release; another (hr) encodes a putative zinc finger protein related to the product of a recently identified mouse gene, hairless. Both Srg1 and hr are induced rapidly (<4 hr), suggesting that they are regulated directly by thyroid hormone. The temporal and spatial expression of both Srg1 and hr is characteristic of genes important to nervous system development. Srg1 and hr are likely part of a cascade of gene activation induced by thyroid hormone that is critical for CNS organization and development.

Glycinergic transmission regulates dendrite size in organotypic culture

We previously demonstrated that inhibitory synaptic transmission influences dendrite development in vivo. We now report an analogous finding in an organotypic culture of a glycinergic projection nucleus, the medial nucleus of the trapezoid body (MNTB), and its postsynaptic target, the lateral superior olive (LSO) of gerbils. Cultures were generated at 6-7 days postnatal and grown in serum containing medium with or without the glycine receptor antagonist, strychnine (SN), at 2 microM. LSO neurons were then labeled with biocytin, and the dendritic arbors were analyzed morphometrically. Compared to neurons form age-matched in vivo tissue, the neurons cultured in control media were somewhat atrophic, including decreases in dendritic branching and length. Incubation in strychnine led to a dramatic increase in dendritic branching and total dendritic length. Control neurons averaged 6.3 branches compared to 18 branches/neuron in SN-treated cultures. There was a similar increase in primary dendrites and total dendritic length. The physical elimination of MNTB cells did not mimic SN treatment, presumably because glycinergic LSO neurons generated intrinsic connections. In fact, the LSO soma area was significantly greater following MNTB removal, suggesting that these afferents provide a second signal to postsynaptic neurons. These results suggest that spontaneous glycinergic transmission regulates the growth of postsynaptic processes.

Developmental fates and migratory pathways of dividing progenitors in the postnatal rat cerebellum

To investigate the developmental fates and the migratory pathways of dividing progenitors in both the white matter (WM) and the external granule layer (EGL) in the early postnatal rat cerebellum, a replication-deficient retrovirus carrying the beta-galactosidase gene (BAG) was injected into the deep cerebellar tissue or the EGL of postnatal rats to label dividing progenitors. After 1-3 days post-injection (1-3 dpi) of BAG into the deep cerebellar tissue of postnatal day 4/5 (P4/5) rats, labeled immature, unipolar cells were found mainly in the WM. From 4 to 6 dpi, similar cells appeared in the internal granule (IGL), Purkinje cell, and molecular layers, although about half of the labeled cells still resided in the WM and appeared immature. The first morphologically definable Bergmann glia, astrocytes, and oligodendrocytes were also observed. From 14 to 20 dpi, most labeled cells had developed into Bergmann glia, astrocytes, oligodendrocytes, and interneurons in their appropriate layers. When BAG injections were performed at P14, unipolar cells were initially observed, but the majority of these differentiated into myelinating oligodendrocytes in the WM and IGL by 17 dpi. Few immature cells were labeled by injections administered at P20, and these did not develop into mature glia, but into cells with lacy, fine processes, possible representing immature oligodendrocytes. In contrast, BAG-labeled progenitors of EGL produced only granule neurons. Thus, within the first 2 postnatal weeks, dividing progenitors in the WM migrate as immature cells into the cortex before differentiating into a variety of glia and interneurons. The genesis of oligodendrocytes continues through the 2nd postnatal week and largely ceases by P20. EGL cells do not produce glia, but only granule cells.

Multiple factors shape development of olfactory glomeruli: insights from an insect model system

The antennal system of the moth Manduca sexta is a useful model for studies of the development of olfactory glomeruli, the complex synaptic structures that typically underlie the initial processing of olfactory input in vertebrates and invertebrates. In this review, we summarize cellular events in the construction of glomeruli in Manduca and highlight experiments that reveal factors that influence glomerulus development. By methodically manipulating each of various cell types, both neuronal and glial, that contribute to glomerular architecture, we have found that: olfactory receptor axons lay a template for developing glomeruli, stabilization of the template by glial cells is necessary to permit subsequent steps in development of the glomeruli, and the hormone that regulates adult development causes production of adequate numbers of glial cells. Neither electrical activity nor the presence of a serotonin-containing neuron that persists throughout development is required for a glomerular pattern to develop; these factors might, however, influence the synaptic organization of individual glomeruli.

Generation of cerebellar interneurons from dividing progenitors in white matter

The traditional view of the external granular layer of the cerebellar cortex giving rise to interneurons has been challenged by recent studies with quail-chick chimeras. To clarify the time and site of origins of interneurons, a retrovirus carrying the beta-galactosidase gene was injected into the deep cerebellar tissue or external granular layer of postnatal day 4/5 rats to label dividing progenitors. After deep cerebellar tissue injection, unipolar cells were found initially in white matter at 2 days postinjection and subsequently in the internal granule and molecular layers 4-6 days postinjection. Morphologically defined basket, stellate, and Golgi neurons were clearly identified by 20 days postinjection. In contrast, retroviral labeling of cells in the external granular layer produced only granule neurons in the internal granule layer. Thus, dividing progenitors in the cerebellar white matter migrate through the white matter into the cortex before differentiating into a variety of cortical interneurons.