Unipolar brush cells of the cerebellum are produced in the rhombic lip and migrate through developing white matter

Expression of S100B during embryonic development of the mouse cerebellum

BACKGROUND

In the cerebellum of newborn S100B-EGFP mice, we had previously noted the presence of a large population of S100B-expressing cells, which we assumed to be immature Bergmann glial cells. In the present study, we have drawn on this observation to establish the precise spatio-temporal pattern of S100B gene expression in the embryonic cerebellum.

RESULTS

From E12.5 until E17.5, S100B was expressed in the primary radial glial scaffold involved in Purkinje progenitor exit from the ventricular zone and in the Sox9+ glial progenitors derived from it. During the same period coinciding with the primary phase of granule neuron precursor genesis, transient EGFP expression tagged the Pax6+ forerunners of granule precursors born in the cerebellar rhombic lip.

CONCLUSION

This study provides the first characterization of S100B-expressing cell types of the embryonic mouse cerebellum in a high-resolution map. The transient activation of the S100B gene distinguishes granule neuron precursors from all other types of precursors so far identified in the rhombic lip, whereas its activation in radial glial precursors is a feature of Bergmann cell gliogenesis.

Cerebellar GABAergic progenitors adopt an external granule cell-like phenotype in the absence of Ptf1a transcription factor expression

We report in this study that, in the cerebellum, the pancreatic transcription factor Ptf1a is required for the specific generation of Purkinje cells (PCs) and interneurons. Moreover, granule cell progenitors in the external GCL (EGL) appear to be unaffected by deletion of Ptf1a. Cell lineage analysis in Ptf1a(Cre/Cre) mice was used to establish that, in the absence of Ptf1a expression, ventricular zone progenitors, normally fated to produce PCs and interneurons, aberrantly migrate to the EGL and express typical markers of these cells, such as Math1, Reelin, and Zic1/2. Furthermore, these cells have a fine structure typical of EGL progenitors, indicating that they adopt an EGL-like cell phenotype. These findings indicate that Ptf1a is necessary for the specification and normal production of PCs and cerebellar interneurons. Moreover, our results suggest that Ptf1a is also required for the suppression of the granule cell specification program in cerebellar ventricular zone precursors.

Developmental increase of total cell numbers in the murine cerebellum

The cerebellum has been widely used as a paradigm to study basic mechanisms of brain development and cortical histogenesis. Its highly regular structure has always made it particularly attractive to approaches relying on, and yielding, quantitative information, which provide a cornerstone of systems-oriented integrative analyses. Astonishingly, though, a systematic quantification of cell generation during cerebellar development has so far not been provided. Here, we use the isotropic fractionator (i.e., cell counts based on tissue homogenates from anatomically defined regions; cf. Herculano-Houzel S, Lent R., J Neurosci. 2005;25:2518-21) to assess the developmental increase of total cell numbers in the murine cerebellum from embryonic day 17 into early adulthood. Our data show that the quantitative increase of cerebellar cell numbers follows a classical, S-shaped growth curve as described by the Hill-equation. The adult murine cerebellum was found to comprise a total of (44.03+/-0.42) * 10(6) cells, half of which are generated before postnatal day 12+/-0.18. Consistent results were obtained by using two approaches to cell counting, one based on manual assessment, the other on flow cytometry. These data provide a reliable quantitative description of cerebellar growth in the mouse and define a predictive model that should allow their integration with quantitative and qualitative descriptions of cerebellar development.

Retrograde infection of precerebellar nuclei neurons by injection of a recombinant adenovirus into the cerebellar cortex of normal and reeler mice

The reeler mouse is an autosomal recessive mutant mouse caused by mutation of the reelin gene and characterized by cerebellar ataxia. To determine whether the distribution pattern of precerebellar nuclei neurons in the brainstem of the reeler mouse changes, we injected a small volume of a replication-defective recombinant adenovirus carrying E. coli beta-galactosidase (lacZ) into the cerebellar cortex of normal and reeler mice. Five days later, the mice were transcardially perfused by a fixative solution. X-gal staining of coronal or sagittal sections of the brainstem revealed that many origins for reticulocerebellar, cuneocerebellar, trigeminocerebellar, and pontocerebellar projections were retrogradely labeled, but only a few olivocerebellar neurons were labeled. Retrogradely labeled neurons in the lateral reticular nucleus tended to locate more laterally and be more condensed into a small compartment in the reeler compared with their normal counterparts. Retrogradely labeled neurons in the external cuneate nucleus were more dorsally shifted in the reeler mice compared with their normal counterparts. We could not find any differences between the normal and reeler mice in the distribution patterns of their trigeminocerebellar projection neurons. Retrogradely labeled pontocerebellar neurons in the basilar pons of the reeler mouse were reduced in number compared with their normal counterparts in addition to being more ventrally and laterally shifted. These findings strongly suggest that the migration of some precerebellar nuclei neurons from the rhombic lip to their final loci may be obstructed in the reeler mice.

Different types of cerebellar GABAergic interneurons originate from a common pool of multipotent progenitor cells

Different cerebellar phenotypes are generated according to a precise spatiotemporal schedule, in which projection neurons precede local interneurons. Glutamatergic neurons develop from the rhombic lip, whereas GABAergic neurons originate from the ventricular neuroepithelium. Progenitors in these germinal layers are committed toward specific phenotypes already at early ontogenetic stages. GABAergic interneurons are thought to derive from a subset of ventricular zone cells, which migrate in the white matter and proliferate up to postnatal life. During this period, different interneuron categories are produced according to an inside-out sequence, from the deep nuclei to the molecular layer (we show here that nuclear interneurons are also born during late embryonic and early postnatal days, after glutamatergic and GABAergic projection neurons). To ask whether distinct interneuron phenotypes share common precursors or derive from multiple fate-restricted progenitors, we examined the behavior of embryonic and postnatal rat cerebellar cells heterotopically/heterochronically transplanted to syngenic hosts. In all conditions, donor cells achieved a high degree of integration in the cerebellar cortex and deep nuclei and acquired GABAergic interneuron phenotypes appropriate for the host age and engraftment site. Therefore, contrary to other cerebellar types, which derive from dedicated precursors, GABAergic interneurons are produced by a common pool of progenitors, which maintain their full developmental potentialities up to late ontogenetic stages and adopt mature identities in response to local instructive cues. In this way, the numbers and types of inhibitory interneurons can be set by spatiotemporally patterned signals to match the functional requirements of developing cerebellar circuits.