The generation of granule cells during the development and evolution of the cerebellum

The cerebellum coordinates vestibular input into the hindbrain to control balance and movement, and its anatomical complexity is increasingly viewed as a high-throughput processing center for sensory and cognitive functions. Cerebellum development however is relatively simple, and arises from a specialized structure in the anterior hindbrain called the rhombic lip, which along with the ventricular zone of the rostral-most dorsal hindbrain region, give rise to the distinct cell types that constitute the cerebellum. Granule cells, being the most numerous cell types, arise from the rhombic lip and form a dense and distinct layer of the cerebellar cortex. In this short review, we describe the various strategies used by amniotes and anamniotes to generate and diversify granule cell types during cerebellar development.

Congenital hypoplasia of the cerebellum: developmental causes and behavioral consequences

Over the last 60 years, the spotlight of research has periodically returned to the cerebellum as new techniques and insights have emerged. Because of its simple homogeneous structure, limited diversity of cell types and characteristic behavioral pathologies, the cerebellum is a natural home for studies of cell specification, patterning, and neuronal migration. However, recent evidence has extended the traditional range of perceived cerebellar function to include modulation of cognitive processes and implicated cerebellar hypoplasia and Purkinje neuron hypo-cellularity with autistic spectrum disorder. In the light of this emerging frontier, we review the key stages and genetic mechanisms behind cerebellum development. In particular, we discuss the role of the midbrain hindbrain isthmic organizer in the development of the cerebellar vermis and the specification and differentiation of Purkinje cells and granule neurons. These developmental processes are then considered in relation to recent insights into selected human developmental cerebellar defects: Joubert syndrome, Dandy–Walker malformation, and pontocerebellar hypoplasia. Finally, we review current research that opens up the possibility of using the mouse as a genetic model to study the role of the cerebellum in cognitive function.

The rhombic lip and early cerebellar development

Recent studies have transformed our understanding of the embryonic rhombic lip by revealing the inductive cues, regional origins and guidance molecules that pattern the development of this important structure and its derivatives. In the cerebellum, a precise combination of anteroposterior and dorsalising cues induces a stream of migratory progenitors that give rise to the external granule cell layer, while more caudally, Netrin orchestrates the migration of hindbrain rhombic lip derivatives to form the precerebellar nuclei. The rhombic lip is thus emerging as a spatiotemporally distinct epithelium whose late appearance in both development and evolution is instrumental in generating a complex, functionally related but spatially distributed neural system.

Intravitreal triamcinolone and elevated intraocular pressure

PURPOSE

To ascertain whether intravitreal triamcinolone given for subretinal neovascularization is associated with an ocular pressure rise.

METHODS

A total of 113 patients with angiographically proven subretinal neovascularization were enrolled into a prospective study of the effects of intravitrea triamcinolone. Intraocular pressure was one of the parameters studied.

RESULTS

Approximately 30% of the study group developed a significant rise (> or =5 mm Hg) in intraocular pressure above baseline during the first 3 months.

CONCLUSIONS

Patients considering this form of treatment should be fully informed of the known risks of intraocular injections of steroids.

The role of the rhombic lip in avian cerebellum development

We have used a combination of quail-chick fate-mapping techniques and dye labelling to investigate the development of the avian cerebellum. Using Hoxa2 as a guide for the microsurgical construction of quail-chick chimaeras, we show that the caudal boundary of the presumptive cerebellum at E6 maps to the caudal boundary of rhombomere 1. By fate mapping the dorsoventral axis of rhombomere 1, we demonstrate that granule cell precursors are generated at the rhombic lip together with neurons of the lateral pontine nucleus. DiI-labelling of cerebellum explants reveals that external germinal layer precursors have a characteristic unipolar morphology and undergo an orientated, active migration away from the rhombic lip, which is apparently independent of either glial or axon guidance or ‘chain’ formation.

Neurogenetics of the cerebellar system

The development of the cerebellum occurs in four basic steps. During the first epoch, genes that mark the cerebellar territory are expressed in a restricted pattern along the anterioposterior axis of the embryo. In the second, an embryonic region termed the rhombic lip generates precursors of the granule cell population of the cerebellar cortex, and the lateral pontine nucleus and olivary nucleus of the brain stem. In the third period, the program of neurogenesis of the granule neuron gives rise to the formation of the fundamental layers of the cerebellum and to the pattern of foliation. Concomitantly, programs of gene expression define the principal neuronal classes, the granule cell and Purkinje cell, that will establish the cerebellar circuitry in the postnatal period. Understanding the molecular mechanisms underlying these steps of development is likely to yield important insights into malformations such as Joubert syndrome.

Homeotic transformation of rhombomere identity after localized Hoxb1 misexpression

Segmentation of the hindbrain and branchial region is a conserved feature of head development, involving the nested expression of Hox genes. Although it is presumed that vertebrate Hox genes function as segment identifiers, responsible for mediating registration between elements of diverse embryonic origin, this assumption has remained untested. To assess this, retroviral misexpression was combined with orthotopic grafting in chick embryos to generate a mismatch in Hox coding between a specific rhombomere and its corresponding branchial arch. Rhombomere-restricted misexpression of a single gene, Hoxb1, resulted in the homeotic transformation of the rhombomere, revealed by reorganization of motor axon projections.

Tlx-1 and Tlx-3 homeobox gene expression in cranial sensory ganglia and hindbrain of the chick embryo: markers of patterned connectivity

Recent evidence suggests that in vertebrates the formation of distinct neuronal cell types is controlled by specific families of homeodomain transcription factors. Furthermore, the expression domains of a number of these genes correlates with functionally integrated neuronal populations. We have isolated two members of the divergent T-cell leukemia translocation (HOX11/Tlx) homeobox gene family from chick, Tlx-1 and Tlx-3, and show that they are expressed in differentiating neurons of both the peripheral and central nervous systems. In the peripheral nervous system, Tlx-1 and Tlx-3 are expressed in overlapping domains within the placodally derived components of a number of cranial sensory ganglia. Tlx-3, unlike Tlx-1, is also expressed in neural crest-derived dorsal root and sympathetic ganglia. In the CNS, both genes are expressed in longitudinal columns of neurons at specific dorsoventral levels of the hindbrain. Each column has distinct anterior and/or posterior limits that respect inter-rhombomeric boundaries. Tlx-3 is also expressed in D2 and D3 neurons of the spinal cord. Tlx-1 and Tlx-3 expression patterns within the peripheral and central nervous systems suggest that Tlx proteins may be involved not only in the differentiation and/or survival of specific neuronal populations but also in the establishment of neuronal circuitry. Furthermore, by analogy with the LIM genes, Tlx family members potentially define sensory columns early within the developing hindbrain in a combinatorial manner.

Persistence of rhombomeric organisation in the postsegmental hindbrain

Rhombomeres are morphological varicosities of the neural tube that are present between embryonic day (E) 1.5 and E5 and are characterised by compartment organisation, segmentally neuronal organisation and spatially restricted patterns of gene expression. After E5, the segmented origins of the hindbrain become indistinct, while the adult hindbrain has an longitudinal columnar nuclear organisation. In order to assess the impact of the early transverse pattern on later longitudinal organisation, we have used orthotopic quail grafts and in situ hybridisation to investigate the long-term fate of rhombomeres in the embryonic chick hindbrain. The uniformity of mixing between quail and chick cells was first verified using short-term aggregation cultures. The dispersal of the progeny of individual rhombomeres (r) was then assessed by the unilateral, isochronic and orthotopic transplantation of either r2, r3, r4, r5 or r6 from quail to chick at embryonic day E2. In addition, orthotopic, partial rhombomere grafts, encompassing an inter-rhombomere boundary and adjacent rhombomere bodies were used to assess cell mixing within rhombomeres. Operated embryos were incubated to either E7 or E10 when chimaeric brains were removed. Quail cells were identified in whole mounts or serial sections using the quail-specific antibody QCPN. Subsequently, radial glia morphology was assessed either by immunohistochemistry or DiI labelling. A series of fixed hindbrains between E6 and E9 were probed for transcripts of Hoxa-2 and Hoxb-1. Fate-mapping reveals that the progeny of individual rhombomeres form stripes of cells running dorsoventrally through the hindbrain. This pattern of dispersal precisely parallels the array of radial glia. Although the postmitotic progeny of adjacent rhombomeres spread to some extent into each others' territory in intermediate and marginal zones, there is little or no mixing between rhombomeres in the ventricular zone, which thus remains compartmentalised long after the rhombomeric morphology disappears. Segmental gene expression within this layer is also maintained after E5. A more detailed analysis of mixing between proliferating cells, using partial rhombomere grafts, reveals that both mixing and growth are non-uniform within the ventricular layer, suggesting, in particular, that longitudinal expansion within this layer is restricted. Together, these observations suggest that rhombomeres do not disappear at E5, as has previously been supposed, rather they persist in the ventricular zone to at least E9, ensuring a continuity in the presumed segmental cues that specify neuroepithelial cells in the hindbrain.

Soma and axon diameter distributions and central projections of ferret retinal ganglion cells

Using a combination of retrograde horseradish peroxidase (HRP) labelling, silver staining, and electron microscopy, we have assessed the relationship between retinal ganglion cell soma size and axon diameter in the adult ferret (Mustela putorius furo). Retinal ganglion cells were labelled following injections of HRP into the lateral geniculate nucleus (LGN), superior colliculus (SC), or LGN+SC. The soma size distributions following LGN, SC, or LGN+SC injections were all unimodal showing considerable overlap between different cell classes. This was confirmed for alpha cells identified on the basis of dendritic filling or from neurofibrillar-stained retinae. Analysis of the soma size and axon diameters of a population of heavily labelled retinal ganglion cells showed a significant correlation between the two. However, the overall distribution of intraretinal axon diameter was bimodal with an extended tail. Analysis of the ganglion cell distributions in the adult ferret indicates that beta cells comprise about 50.5-55%, gamma 42.5-47%, and alpha 2.5% of the ganglion cell population. This implies that the proportion of gamma, beta, alpha cells in both cat and ferret retina is highly conserved despite differences in visual specialization in the two species.

Retinal ganglion cell dendritic development and its control. Filling the gaps

The way in which central neurons acquire their complex and precise dendrite arbors is of considerable developmental interest. Using retinal ganglion cells (RGCs) as a model, the mechanisms that pattern dendritic development are beginning to emerge. As in other systems, final dendrite phenotype is achieved by a mixture of intrinsic and extrinsic determinants. The extrinsic determinants of RGC dendrite shape reflect the anatomical constraints of producing a paracrystalline mosaic of arbors that laminates the inner plexiform layer of the retina. In this article, the key features of RGC dendrite development are reviewed. The emerging molecular mechanisms behind dendritic laminar segregation and "dendritic competition" are described. The role of afferent extrinsic influences are contrasted with those of retrograde, activity-dependent target influences that may regulate the final maturational phase of dendrite remodeling.

Axonal target choice and dendritic development of ferret beta retinal ganglion cells

We have investigated the relationship between axon targeting and dendritic morphology in beta retinal ganglion cells in the postnatal ferret. Axonal projections were assessed by making separate injections of different fluorescent retrograde tracers into either the superior colliculus or lateral geniculate nucleus in vivo. The dendritic morphology of retrogradely labelled cells was revealed by the in vitro intracellular injection of lucifer yellow in fixed retina. In this way, 405 retinal ganglion cells were triple- or double-labelled and characterized by their dendritic branching styles. Both the distinct dendritic morphology of beta cells and the characteristic restriction of their adult axonal terminals to the lateral geniculate nucleus emerge postnatally. Beta cell dendritic morphology is established between postnatal days 5 and 9. As in the cat (Ramoa et al., 1989), beta cells extend and then retract a projection to the superior colliculus as part of their normal development. Transient beta axonal collaterals to the superior colliculus persist beyond the period of cell death, but nearly all are withdrawn by postnatal day 15. No dendritically distinct beta cell projects to the superior colliculus alone, at any age. Heterochronic injections of different colours of retrograde tracer into the superior colliculus were used to study changes in the complement of the retinocollicular projection over time. A significant proportion of cells (58%) labelled at postnatal day 0 from the superior colliculus, which subsequently survived the period of cell death, were found to be beta cells that could no longer be demonstrated to have a retinocollicular axon.(ABSTRACT TRUNCATED AT 250 WORDS)

Targeting and activity-related dendritic modification in mammalian retinal ganglion cells

We have studied factors that influence the development of dendritic morphology in hamster retinal ganglion cells. By combining fluorescent retrograde tracing with in vitro Lucifer yellow injection into fixed retina, cells with appropriate and inappropriate visuotopic projections have been compared. In adult hamsters, cells with an aberrant ipsilateral projection from the nasal retina display a uniformly sparse dendritic morphology. However, following monocular enucleation at postnatal day 0 (P0), this population displays a significantly enhanced dendritic complexity in the adult. By contrast, removal of one eye at P6 or at P12 produces progressively less effect. These results suggest that dendritic complement of the adult aberrant projection can be regulated by altering the early postnatal axonal environment. The development of aberrant ganglion cells was investigated to determine the relative influences of cell death and dendritic remodeling in shaping the composition of the adult aberrant population. Aberrant cells were found to be indistinguishable from other cells in nasal retina throughout early development. After ganglion cell death (P1-P12) is over, aberrant cells still display a full range of cell types. However, at eye opening (P16) they undergo a rapid loss of dendritic complexity by remodeling. By P22, aberrant cells display a uniformly sparse dendritic morphology. When hamsters were raised in the dark between P12 (the end of ganglion cell death) and P22, this severe remodeling was blocked. This block was maintained when hamsters were dark reared to P42. Hence, both dark rearing and monocular enucleation at P0 produce similar effects on the development of visuotopically inappropriate hamster retinal ganglion cells. We speculate that the patterns of dendritic sculpting that we have observed reflect activity-mediated modulation of dendritic form via retrograde signals from the terminal arbors. This has implications for retinal ganglion cell morphological classification and, more generally, for mechanisms that influence the dendritic development of other neurons in the CNS.

Lucifer yellow, retrograde tracers, and fractal analysis characterise adult ferret retinal ganglion cells

The dendritic morphology of retinal ganglion cells in the ferret was studied by the intracellular injection of lucifer yellow in fixed tissue. Ganglion cells were identified by the retrograde transport of red or green fluorescent microspheres that had been injected into different target nuclei, usually the lateral geniculate nucleus or superior colliculus. This approach allows the comparison of dendritic morphologies of ganglion cells in the same retina with different central projections and also identifies cells with branching axons. The digitised images of dendritic arbors were analysed quantitatively by a variety of measures. Dendritic complexity was assessed by calculating the fractal dimension of each arbor. The ferret has distinct alpha, beta, and gamma morphological classes of cells similar to those found in the cat. The gamma cell class was morphologically diverse and could be subdivided into "sparse," "loose," and "tight" groups, reflecting increasing dendritic complexity. Whereas the beta cell projection was limited to the lateral geniculate nucleus alone, alpha and gamma cells could project to either or both nuclei. Retinal ganglion cells labelled from the pretectal nuclei formed a morphologically distinct class of retinal ganglion cells. The ipsilateral projection lacked alpha cells and the most complex, "tight" gamma cells. However, ipsilaterally projecting "loose" gamma cells overlapped alpha cells in both soma and dendritic dimensions. Different morphological classes of retinal ganglion cells hence show characteristic axon behaviour both in their decussation at the chiasm and in which targets they innervate. Fractal measures were used to contrast variation within and between these identified classes.