Nonapical Symmetric Divisions Underlie Horizontal Cell Layer Formation in the Developing Retina In Vivo

NGL-2 regulates pathway-specific neurite growth and lamination, synapse formation, and signal transmission in the retina

Parallel processing is an organizing principle of many neural circuits. In the retina, parallel neuronal pathways process signals from rod and cone photoreceptors and support vision over a wide range of light levels. Toward this end, rods and cones form triad synapses with dendrites of distinct bipolar cell types, and the axons or dendrites, respectively, of horizontal cells (HCs). The molecular cues that promote the formation of specific neuronal pathways remain largely unknown. Here, we discover that developing and mature HCs express the leucine-rich repeat (LRR)-containing protein netrin-G ligand 2 (NGL-2). NGL-2 localizes selectively to the tips of HC axons, which form reciprocal connections with rods. In mice with null mutations in Ngl-2 (Ngl-2⁻/⁻), many branches of HC axons fail to stratify in the outer plexiform layer (OPL) and invade the outer nuclear layer. In addition, HC axons expand lateral territories and increase coverage of the OPL, but establish fewer synapses with rods. NGL-2 can form transsynaptic adhesion complexes with netrin-G2, which we show to be expressed by photoreceptors. In Ngl-2⁻/⁻ mice, we find specific defects in the assembly of presynaptic ribbons in rods, indicating that reverse signaling of complexes involving NGL-2 regulates presynaptic maturation. The development of HC dendrites and triad synapses of cone photoreceptors proceeds normally in the absence of NGL-2 and in vivo electrophysiology reveals selective defects in rod-mediated signal transmission in Ngl-2⁻/⁻ mice. Thus, our results identify NGL-2 as a central component of pathway-specific development in the outer retina.

Otx2 and Onecut1 promote the fates of cone photoreceptors and horizontal cells and repress rod photoreceptors

Cone photoreceptors carry out phototransduction in daylight conditions and provide the critical first step in color vision. Despite their importance, little is known about the developmental mechanisms involved in their generation, particularly how they are determined relative to rod photoreceptors, the cells that initiate vision in dim light. Here, we report the identification of a cis-regulatory module (CRM) for the thyroid hormone receptor beta (Thrb) gene, an early cone marker. We found that ThrbCRM1 is active in progenitor cells biased to the production of cones and an interneuronal cell type, the horizontal cell (HC). Molecular analysis of ThrbCRM1 revealed that it is combinatorially regulated by the Otx2 and Onecut1 transcription factors. Onecut1 is sufficient to induce cells with the earliest markers of cones and HCs. Conversely, interference with Onecut1 transcriptional activity leads to precocious rod development, suggesting that Onecut1 is critically important in defining cone versus rod fates.

Analysis of gene expression in wild-type and Notch1 mutant retinal cells by single cell profiling

BACKGROUND

The vertebrate retina comprises sensory neurons, the photoreceptors, as well as many other types of neurons and one type of glial cell. These cells are generated by multipotent and restricted retinal progenitor cells (RPCs), which express Notch1. Loss of Notch1 in RPCs late during retinal development results in the overproduction of rod photoreceptors at the expense of interneurons and glia.

RESULTS

To examine the molecular underpinnings of this observation, microarray analysis of single retinal cells from wild-type or Notch1 conditional knockout retinas was performed. In situ hybridization was carried out to validate some of the findings.

CONCLUSIONS

The majority of Notch1-mutant cells lost expression of known Notch target genes. These cells also had low levels of RPC and cell cycle genes, and robustly up-regulated rod precursor genes. In addition, single wild-type cells, in which cell cycle marker genes were down-regulated, expressed markers of both rod photoreceptors and interneurons.

Cone photoreceptor types in zebrafish are generated by symmetric terminal divisions of dedicated precursors

Proper functioning of sensory systems requires the generation of appropriate numbers and proportions of neuronal subtypes that encode distinct information. Perception of color relies on signals from multiple cone photoreceptor types. In cone-dominated retinas, each cone expresses a single opsin type with peak sensitivity to UV, long (L) (red), medium (M) (green), or short (S) (blue) wavelengths. The modes of cell division generating distinct cone types are unknown. We report here a mechanism whereby zebrafish cone photoreceptors of the same type are produced by symmetric division of dedicated precursors. Transgenic fish in which the thyroid hormone receptor β2 (trβ2) promoter drives fluorescent protein expression before L-cone precursors themselves are produced permitted tracking of their division in vivo. Every L cone in a local region resulted from the terminal division of an L-cone precursor, suggesting that such divisions contribute significantly to L-cone production. Analysis of the fate of isolated pairs of cones and time-lapse observations suggest that other cone types can also arise by symmetric terminal divisions. Such divisions of dedicated precursors may help to rapidly attain the final numbers and proportions of cone types (L > M, UV > S) in zebrafish larvae. Loss- and gain-of-function experiments show that L-opsin expression requires trβ2 activity before cone differentiation. Ectopic expression of trβ2 after cone differentiation produces cones with mixed opsins. Temporal differences in the onset of trβ2 expression could explain why some species have mixed, and others have pure, cone types.

Notch1 is required in newly postmitotic cells to inhibit the rod photoreceptor fate

Several models of cell fate determination can be invoked to explain how single retinal progenitor cells (RPCs) produce different cell types in a terminal division. To gain insight into this process, the effects of the removal of a cell fate regulator, Notch1, were studied in newly postmitotic cells using a conditional allele of Notch1 (N1-CKO) in mice. Almost all newly postmitotic N1-CKO cells became rod photoreceptors, whereas wild-type (WT) cells achieved a variety of fates. Single cell profiling of wild-type and N1-CKO retinal cells transitioning from progenitor to differentiated states revealed differential expression of inhibitor of DNA binding factors Id1 and Id3, as well as Notch-regulated ankyrin repeat protein (Nrarp). Misexpression of Id1 and Id3 was found to be sufficient to drive production of Müller glial cells and/or RPCs. Moreover, Id1 and Id3 were shown to partially rescue the production of bipolar and Müller glial cells in the absence of Notch1 in mitotic and newly postmitotic cells. Misexpression of Nrarp, a downstream target gene and inhibitor of the Notch signaling pathway, resulted in the overproduction of rod photoreceptors at the expense of Müller glial cells. These data demonstrate that cell fate decisions can be made in newly postmitotic retinal cells, and reveal some of the regulators downstream of Notch1 that influence the choice of rod and non-rod fates. Taken together, our results begin to address how different signals downstream from a common pathway lead to different fate outcomes.

Multiple ribosomal proteins are expressed at high levels in developing zebrafish endoderm and are required for normal exocrine pancreas development

Ribosomal protein L (rpl) genes are essential for assembly of the 60S subunit of the eukaryotic ribosome and may also carry out additional extra-ribosomal functions. We have identified a common expression pattern for rpl genes in developing zebrafish larvae. After initially widespread expression in early embryos, the expression of multiple rpl genes becomes increasingly restricted to the endoderm. With respect to the pancreas, rpl genes are highly expressed in ptf1a-expressing pancreatic progenitors at 48 hpf, suggesting possible functional roles in pancreatic morphogenesis and/or differentiation. Utilizing two available mutant lines, rpl23a(hi2582) and rpl6(hi3655b), we found that ptf1a-expressing pancreatic progenitors fail to properly expand in embryos homozygous for either of these genes. In addition to these durable homozygous phenotypes, we also demonstrated recoverable delays in ptf1a-expressing pancreatic progenitor expansion in rpl23a(hi2582) and rpl6(hi3655b) heterozygotes. Disruptions in ribosome assembly are generally understood to initiate a p53-dependent cellular stress response. However, concomitant p53 knockdown was unable to rescue normal pancreatic progenitor expansion in either rpl23a(hi2582) or rpl6(hi3655b) mutant embryos, suggesting required and p53-independent roles for rpl23a and rpl6 in pancreas development.

Heterogenic final cell cycle by chicken retinal Lim1 horizontal progenitor cells leads to heteroploid cells with a remaining replicated genome

Retinal progenitor cells undergo apical mitoses during the process of interkinetic nuclear migration and newly generated post-mitotic neurons migrate to their prospective retinal layer. Whereas this is valid for most types of retinal neurons, chicken horizontal cells are generated by delayed non-apical mitoses from dedicated progenitors. The regulation of such final cell cycle is not well understood and we have studied how Lim1 expressing horizontal progenitor cells (HPCs) exit the cell cycle. We have used markers for S- and G2/M-phase in combination with markers for cell cycle regulators Rb1, cyclin B1, cdc25C and p27Kip1 to characterise the final cell cycle of HPCs. The results show that Lim1+ HPCs are heterogenic with regards to when and during what phase they leave the final cell cycle. Not all horizontal cells were generated by a non-apical (basal) mitosis; instead, the HPCs exhibited three different behaviours during the final cell cycle. Thirty-five percent of the Lim1+ horizontal cells was estimated to be generated by non-apical mitoses. The other horizontal cells were either generated by an interkinetic nuclear migration with an apical mitosis or by a cell cycle with an S-phase that was not followed by any mitosis. Such cells remain with replicated DNA and may be regarded as somatic heteroploids. The observed heterogeneity of the final cell cycle was also seen in the expression of Rb1, cyclin B1, cdc25C and p27Kip1. Phosphorylated Rb1-Ser608 was restricted to the Lim1+ cells that entered S-phase while cyclin B1 and cdc25C were exclusively expressed in HPCs having a basal mitosis. Only HPCs that leave the cell cycle after an apical mitosis expressed p27Kip1. We speculate that the cell cycle heterogeneity with formation of heteroploid cells may present a cellular context that contributes to the suggested propensity of these cells to generate cancer when the retinoblastoma gene is mutated.

In vivo imaging of zebrafish retina

Neuronal circuits of the vertebrate retina are organized into stereotyped laminae. This orderly arrangement makes the retina an ideal model system for imaging studies aimed at understanding how circuits assemble during development. In particular, live-cell imaging techniques are readily applied to the developing retina to monitor dynamic changes over time in cell structure and connectivity. Such imaging studies have collectively revealed novel strategies by which retinal neurons contact their presynaptic and postsynaptic partners to establish synaptic connections. We describe here the procedures developed in our laboratory for confocal and multiphoton live-cell imaging of the developing retina using in vivo preparations. Zebrafish larvae are an ideal specimen for in vivo imaging experiments as they can be made to remain transparent throughout development. Isolated retinal cells can be readily labeled by DNA injection into the one-cell staged embryo, or via transplantation of fluorescently labeled cells from stable transgenics.

Deterministic or stochastic choices in retinal neuron specification

There are two views on vertebrate retinogenesis: a deterministic model dependent on fixed lineages and a stochastic model in which choices of division modes and cell fates cannot be predicted. In this issue of Neuron, He et al. (2012) address this question in zebrafish using live imaging and mathematical modeling.

Ribosomal biogenesis genes play an essential and p53-independent role in zebrafish pancreas development

Mutations in the human Shwachman-Bodian-Diamond syndrome (SBDS) gene cause defective ribosome assembly and are associated with exocrine pancreatic insufficiency, chronic neutropenia and skeletal defects. However, the mechanism underlying these phenotypes remains unclear. Here we show that knockdown of the zebrafish sbds ortholog fully recapitulates the spectrum of developmental abnormalities observed in the human syndrome, and further implicate impaired proliferation of ptf1a-expressing pancreatic progenitor cells as the basis for the observed pancreatic phenotype. It is thought that diseases of ribosome assembly share a p53-dependent mechanism. However, loss of p53 did not rescue the developmental defects associated with loss of zebrafish sbds. To clarify the molecular mechanisms underlying the observed organogenesis defects, we performed transcriptional profiling to identify candidate downstream mediators of the sbds phenotype. Among transcripts displaying differential expression, functional group analysis revealed marked enrichment of genes related to ribosome biogenesis, rRNA processing and translational initiation. Among these, ribosomal protein L3 (rpl3) and pescadillo (pes) were selected for additional analysis. Similar to knockdown of sbds, knockdown or mutation of either rpl3 or pes resulted in impaired expansion of pancreatic progenitor cells. The pancreatic phenotypes observed in rpl3- and pes-deficient embryos were also independent of p53. Together, these data suggest novel p53-independent roles for ribosomal biogenesis genes in zebrafish pancreas development.

Direction-selective retinal ganglion cells arise from molecularly specified multipotential progenitors

Single progenitors can give rise to any and all of the main retinal cell types: photoreceptors, interneurons (horizontal, bipolar, and amacrine cells), retinal ganglion cells (RGCs), and glia. Many of these types are divisible into multiple functionally, structurally, and molecularly distinct subtypes (e.g., ~25 for RGCs). It remains unknown when and how progenitors become committed to generate such subtypes. Here, we determine the origin of RGCs that respond selectively to vertical motion and express cadherin 6 (cdh6). Using Cre recombinase-based lineage tracing, we show that these RGCs arise from progenitors that themselves express cdh6. These progenitors are capable of generating all major retinal cell types, but the RGCs they generate are predominantly of the single direction-selective subtype. In contrast, cdh6-positive progenitors retain the ability to generate multiple subtypes of amacrine and bipolar cells. Our results demonstrate that type and subtype specification are regulated in different ways and suggest that multipotential but fate-restricted progenitors contribute to subtype specification in retina.

Transcription factor Olig2 defines subpopulations of retinal progenitor cells biased toward specific cell fates

Previous lineage analyses have shown that retinal progenitor cells (RPCs) are multipotent throughout development, and expression-profiling studies have shown a great deal of molecular heterogeneity among RPCs. To determine if the molecular heterogeneity predicts that an RPC will produce particular types of progeny, clonal lineage analysis was used to investigate the progeny of a subset of RPCs, those that express the basic helix-loop-helix transcription factor, Olig2. The embryonic Olig2(+) RPCs underwent terminal divisions, producing small clones with primarily two of the five cell types being made by the pool of RPCs at that time. The later, postnatal Olig2(+) RPCs also made terminal divisions, which were biased toward production of rod photoreceptors and amacrine cell interneurons. These data indicate that the multipotent progenitor pool is made up of distinctive types of RPCs, which have biases toward producing subsets of retinal neurons in a terminal division, with the types of neurons produced varying over time. This strategy is similar to that of the developing Drosophila melanogaster ventral nerve cord, with the Olig2(+) cells behaving as ganglion mother cells.

Fast homozygosity mapping and identification of a zebrafish ENU-induced mutation by whole-genome sequencing

Forward genetics using zebrafish is a powerful tool for studying vertebrate development through large-scale mutagenesis. Nonetheless, the identification of the molecular lesion is still laborious and involves time-consuming genetic mapping. Here, we show that high-throughput sequencing of the whole zebrafish genome can directly locate the interval carrying the causative mutation and at the same time pinpoint the molecular lesion. The feasibility of this approach was validated by sequencing the m1045 mutant line that displays a severe hypoplasia of the exocrine pancreas. We generated 13 Gb of sequence, equivalent to an eightfold genomic coverage, from a pool of 50 mutant embryos obtained from a map-cross between the AB mutant carrier and the WIK polymorphic strain. The chromosomal region carrying the causal mutation was localized based on its unique property to display high levels of homozygosity among sequence reads as it derives exclusively from the initial AB mutated allele. We developed an algorithm identifying such a region by calculating a homozygosity score along all chromosomes. This highlighted an 8-Mb window on chromosome 5 with a score close to 1 in the m1045 mutants. The sequence analysis of all genes within this interval revealed a nonsense mutation in the snapc4 gene. Knockdown experiments confirmed the assertion that snapc4 is the gene whose mutation leads to exocrine pancreas hypoplasia. In conclusion, this study constitutes a proof-of-concept that whole-genome sequencing is a fast and effective alternative to the classical positional cloning strategies in zebrafish.

Cell-type specific roles for PTEN in establishing a functional retinal architecture

Background

The retina has a unique three-dimensional architecture, the precise organization of which allows for complete sampling of the visual field. Along the radial or apicobasal axis, retinal neurons and their dendritic and axonal arbors are segregated into layers, while perpendicular to this axis, in the tangential plane, four of the six neuronal types form patterned cellular arrays, or mosaics. Currently, the molecular cues that control retinal cell positioning are not well-understood, especially those that operate in the tangential plane. Here we investigated the role of the PTEN phosphatase in establishing a functional retinal architecture.

Methodology/Principal Findings

In the developing retina, PTEN was localized preferentially to ganglion, amacrine and horizontal cells, whose somata are distributed in mosaic patterns in the tangential plane. Generation of a retina-specific Pten knock-out resulted in retinal ganglion, amacrine and horizontal cell hypertrophy, and expansion of the inner plexiform layer. The spacing of Pten mutant mosaic populations was also aberrant, as were the arborization and fasciculation patterns of their processes, displaying cell type-specific defects in the radial and tangential dimensions. Irregular oscillatory potentials were also observed in Pten mutant electroretinograms, indicative of asynchronous amacrine cell firing. Furthermore, while Pten mutant RGC axons targeted appropriate brain regions, optokinetic spatial acuity was reduced in Pten mutant animals. Finally, while some features of the Pten mutant retina appeared similar to those reported in Dscam-mutant mice, PTEN expression and activity were normal in the absence of Dscam.

Conclusions/Significance

We conclude that Pten regulates somal positioning and neurite arborization patterns of a subset of retinal cells that form mosaics, likely functioning independently of Dscam, at least during the embryonic period. Our findings thus reveal an unexpected level of cellular specificity for the multi-purpose phosphatase, and identify Pten as an integral component of a novel cell positioning pathway in the retina.

Retinal histogenesis and cell differentiation in an elasmobranch species, the small-spotted catshark Scyliorhinus canicula

Here we present a detailed study of the major events in the retinal histogenesis in a slow-developing elasmobranch species, the small-spotted catshark, during embryonic, postnatal and adult stages using classical histological and immunohistological methods, providing a complete neurochemical characterization of retinal cells. We found that the retina of the small-spotted catshark was fully differentiated prior to birth. The major developmental events in retinal cell differentiation occurred during the second third of the embryonic period. Maturational features described in the present study were first detected in the central retina and, as development progressed, they spread to the rest of the retina following a central-to-peripheral gradient. While the formation of both plexiform layers occurs simultaneously in the retina of the most common fish models, in the small-spotted catshark retina the emergence of the outer plexiform layer was delayed with respect to the inner plexiform layer. According to the expression of the markers used, retinal cell differentiation followed a vitreal-to-scleral gradient, with the exception of Müller cells that were the last cell type generated during retinogenesis. This vitreal-to-scleral progression of neural differentiation seems to be specific to slow-developing fish species.

Essential roles of zebrafish bmp2a, fgf10, and fgf24 in the specification of the ventral pancreas

In vertebrates, pancreas and liver arise from bipotential progenitors located in the embryonic gut endoderm. Bone morphogenic protein (BMP) and fibroblast growth factor (FGF) signaling pathways have been shown to induce hepatic specification while repressing pancreatic fate. Here we show that BMP and FGF factors also play crucial function, at slightly later stages, in the specification of the ventral pancreas. By analyzing the pancreatic markers pdx1, ptf1a, and hlxb9la in different zebrafish models of BMP loss of function, we demonstrate that the BMP pathway is required between 20 and 24 h postfertilization to specify the ventral pancreatic bud. Knockdown experiments show that bmp2a, expressed in the lateral plate mesoderm at these stages, is essential for ventral pancreas specification. Bmp2a action is not restricted to the pancreatic domain and is also required for the proper expression of hepatic markers. By contrast, through the analysis of fgf10(-/-); fgf24(-/-) embryos, we reveal the specific role of these two FGF ligands in the induction of the ventral pancreas and in the repression of the hepatic fate. These mutants display ventral pancreas agenesis and ectopic masses of hepatocytes. Overall, these data highlight the dynamic role of BMP and FGF in the patterning of the hepatopancreatic region.

Identification of a retina-specific Otx2 enhancer element active in immature developing photoreceptors

The homeodomain protein, Otx2, is a critical regulator of vertebrate photoreceptor genesis. However, the genetic elements that define the expression of Otx2 during photoreceptor development are unknown. Therefore, we sought to identify an Otx2 enhancer element that functions in photoreceptor development in order to better understand this specification event. Using the technique of electroporation, we tested a number of evolutionarily conserved elements (ECRs) for expression in the developing retina, and identified ECR2 as having robust activity in the retina. We have characterized this element using a number of assays, including Cre-fate mapping experiments. We found that ECR2 recapitulates expression/function of Otx2 primarily in newly postmitotic photoreceptor cells (PRs), as well as in a subset of retinal progenitor cells (RPCs). ECR2 was also found to be expressed in a subset of horizontal cells (HCs), in keeping with the role of Otx2 in HC development. Furthermore, we determined that the ECR2 element is not active in other Otx2-positive cells such as retinal bipolar cells (BPs), retinal pigmented epithelium (RPE), or the tectum, suggesting that the transcriptional networks controlling Otx2 expression in these cells are unique from those of developing PRs and HCs. These results reveal a distinct molecular state in dividing retinal cells and their newly postmitotic progeny, and provide genetic access to an early and critical transcriptional node involved in the genesis of vertebrate PRs.

The vertebrate retina: a model for neuronal polarization in vivo

The vertebrate retina develops rapidly from a proliferative neuroepithelium into a highly ordered laminated structure, with five distinct neuronal cell types. Like all neurons, these cells need to polarize in appropriate orientations order integrate their neuritic connections efficiently into functional networks. Its relative simplicity, amenability to in vivo imaging and experimental manipulation, as well as the opportunity to study varied cell types within a single tissue, make the retina a powerful model to uncover how neurons polarize in vivo. Here we review the progress that has been made thus far in understanding how the different retinal neurons transform from neuroepithelial cells into mature neurons, and how the orientation of polarization may be specified by a combination of pre-established intrinsic cellular polarity set up within neuroepithelial cells, and extrinsic cues acting upon these differentiating neurons.

Ascl1 expression defines a subpopulation of lineage-restricted progenitors in the mammalian retina

The mechanisms of cell fate diversification in the retina are not fully understood. The seven principal cell types of the neural retina derive from a population of multipotent progenitors during development. These progenitors give rise to multiple cell types concurrently, suggesting that progenitors are a heterogeneous population. It is thought that differences in progenitor gene expression are responsible for differences in progenitor competence (i.e. potential) and, subsequently, fate diversification. To elucidate further the mechanisms of fate diversification, we assayed the expression of three transcription factors made by retinal progenitors: Ascl1 (Mash1), Ngn2 (Neurog2) and Olig2. We observed that progenitors were heterogeneous, expressing every possible combination of these transcription factors. To determine whether this progenitor heterogeneity correlated with different cell fate outcomes, we conducted Ascl1- and Ngn2-inducible expression fate mapping using the CreER™/LoxP system. We found that these two factors gave rise to markedly different distributions of cells. The Ngn2 lineage comprised all cell types, but retinal ganglion cells (RGCs) were exceedingly rare in the Ascl1 lineage. We next determined whether Ascl1 prevented RGC development. Ascl1-null mice had normal numbers of RGCs and, interestingly, we observed that a subset of Ascl1+ cells could give rise to cells expressing Math5 (Atoh7), a transcription factor required for RGC competence. Our results link progenitor heterogeneity to different fate outcomes. We show that Ascl1 expression defines a competence-restricted progenitor lineage in the retina, providing a new mechanism to explain fate diversification.

Time-lapse imaging of neural development: zebrafish lead the way into the fourth dimension

Time-lapse imaging is often the only way to appreciate fully the many dynamic cell movements critical to neural development. Zebrafish possess many advantages that make them the best vertebrate model organism for live imaging of dynamic development events. This review will discuss technical considerations of time-lapse imaging experiments in zebrafish, describe selected examples of imaging studies in zebrafish that revealed new features or principles of neural development, and consider the promise and challenges of future time-lapse studies of neural development in zebrafish embryos and adults.

Investigating the genetics of visual processing, function and behaviour in zebrafish

Over the past three decades, the zebrafish has been proven to be an excellent model to investigate the genetic control of vertebrate embryonic development, and it is now also increasingly used to study behaviour and adult physiology. Moreover, mutagenesis approaches have resulted in large collections of mutants with phenotypes that resemble human pathologies, suggesting that these lines can be used to model diseases and screen drug candidates. With the recent development of new methods for gene targeting and manipulating or monitoring gene expression, the range of genetic modifications now possible in zebrafish is increasing rapidly. Combined with the classical strengths of the zebrafish as a model organism, these advances are set to substantially expand the type of biological questions that can be addressed in this species. In this review, we outline how the potential of the zebrafish can be harvested in the context of eye development and visual function. We review recent technological advances used to study the formation of the eyes and visual areas of the brain, visual processing on the cellular, subcellular and molecular level, and the genetics of visual behaviour in vertebrates.

The role of cell cycle in retinal development: cyclin-dependent kinase inhibitors co-ordinate cell-cycle inhibition, cell-fate determination and differentiation in the developing retina

The mature retina is formed through multi-step developmental processes, including eye field specification, optic vesicle evagination, and cell-fate determination. Co-ordination of these developmental events with cell-proliferative activity is essential to achieve formation of proper retinal structure and function. In particular, the molecular and cellular dynamics of the final cell cycle significantly influence the identity that a cell acquires, since cell fate is largely determined at the final cell cycle for the production of postmitotic cells. This review summarizes our current understanding of the cellular mechanisms that underlie the co-ordination of cell-cycle and cell-fate determination, and also describes a molecular role of cyclin-dependent kinase inhibitors (CDKIs) as co-ordinators of cell-cycle arrest, cell-fate determination and differentiation.

In vivo development of outer retinal synapses in the absence of glial contact

Astroglia secrete factors that promote synapse formation and maintenance. In culture, glial contact has also been shown to facilitate synaptogenesis. Here, we examined whether glial contact is important for establishing circuits in vivo by simultaneously monitoring differentiation of glial cells and local synaptogenesis over time. Photoreceptor circuits of the vertebrate retina are particularly suitable for this study because of the relatively simple, laminar organization of their connectivity with their target neurons, horizontal cells and bipolar cells. Also, individual photoreceptor terminals are ensheathed within the outer plexiform layer (OPL) by the processes of one type of glia, Müller glia cells (MGs). We conducted in vivo time-lapse multiphoton imaging of the rapidly developing and relatively transparent zebrafish retina to ascertain the time course of MG development relative to OPL synaptogenesis. The emergence of synaptic triads, indicative of functional photoreceptor circuits, and structural association with glial processes were also examined across ages by electron microscopy. We first show that MG processes form territories that tile within the inner and outer synaptic layers. We then demonstrate that cone photoreceptor synapses are assembled before the elaboration of MG processes in the OPL. Using a targeted cell ablation approach, we also determined whether the maintenance of photoreceptor synapses is perturbed when local MGs are absent. We found that removal of MGs had no appreciable effect on the stability of newly formed cone synapses. Thus, in contrast to other CNS circuits, contact from glia is not necessary for the formation or immediate stabilization of outer retinal synapses.

Dachshund homologues play a conserved role in islet cell development

All metazoans use insulin to control energy metabolism, but they secrete it from different cells: neurons in the central nervous system in invertebrates and endocrine cells in the gut or pancreas in vertebrates. Despite their origins in different germ layers, all of these insulin-producing cells share common functional features and gene expression patterns. In this study, we tested the role in insulin-producing cells of the vertebrate homologues of Dachshund, a transcriptional regulator that marks the earliest committed progenitors of the neural insulin-producing cells in Drosophila. Both zebrafish and mice expressed a single dominant Dachshund homologue in the pancreatic endocrine lineage, and in both species loss of this homologue reduced the numbers of all islet cell types including the insulin-producing β-cells. In mice, Dach1 gene deletion left the pancreatic progenitor cells unaltered, but blocked the perinatal burst of proliferation of differentiated β-cells that normally generates most of the β-cell mass. In β-cells, Dach1 bound to the promoter of the cell cycle inhibitor p27Kip1, which constrains β-cell proliferation. Taken together, these data demonstrate a conserved role for Dachshund homologues in the production of insulin-producing cells.

Development of the retina and optic pathway

Our understanding of the development of the retina and visual pathways has seen enormous advances during the past 25years. New imaging technologies, coupled with advances in molecular biology, have permitted a fuller appreciation of the histotypical events associated with proliferation, fate determination, migration, differentiation, pathway navigation, target innervation, synaptogenesis and cell death, and in many instances, in understanding the genetic, molecular, cellular and activity-dependent mechanisms underlying those developmental changes. The present review considers those advances associated with the lineal relationships between retinal nerve cells, the production of retinal nerve cell diversity, the migration, patterning and differentiation of different types of retinal nerve cells, the determinants of the decussation pattern at the optic chiasm, the formation of the retinotopic map, and the establishment of ocular domains within the thalamus.

Development of light response and GABAergic excitation-to-inhibition switch in zebrafish retinal ganglion cells

The zebrafish retina has been an important model for studying morphological development of neural circuits in vivo. However, its functional development is not yet well understood. To investigate the functional development of zebrafish retina, we developed an in vivo patch-clamp whole-cell recording technique in intact zebrafish larvae. We first examined the developmental profile of light-evoked responses (LERs) in retinal ganglion cells (RGCs) from 2 to 9 days post-fertilization (dpf). Unstable LERs were first observed at 2.5 dpf. By 4 dpf, RGCs exhibited reliable light responses. As the GABAergic system is critical for retinal development, we then performed in vivo gramicidin perforated-patch whole-cell recording to characterize the developmental change of GABAergic action in RGCs. The reversal potential of GABA-induced currents (E(GABA)) in RGCs gradually shifted from depolarized to hyperpolarized levels during 2-4 dpf and the excitation-to-inhibition (E-I) switch of GABAergic action occurred at around 2.5 dpf when RGCs became light sensitive. Meanwhile, GABAergic transmission upstream to RGCs also became inhibitory by 2.5 dpf. Furthermore, down-regulation of the K(+)/Cl() co-transporter (KCC2) by the morpholino oligonucleotide-based knockdown approach, which shifted RGC E(GABA) towards a more depolarized level and thus delayed the E-I switch by one day, postponed the appearance of RGC LERs by one day. In addition, RGCs exhibited correlated giant inward current (GICs) during 2.5-3.5 dpf. The period of GICs was shifted to 3-4.5 dpf by KCC2 knockdown. Taken together, the GABAergic E-I switch occurs coincidently with the emergence of light responses and GICs in zebrafish RGCs, and may contribute to the functional development of retinal circuits.

A robust cell cycle control mechanism limits E2F-induced proliferation of terminally differentiated cells in vivo

Terminally differentiated cells in Drosophila melanogaster wings and eyes are largely resistant to proliferation upon deregulation of either E2F or cyclin E (CycE), but exogenous expression of both factors together can bypass cell cycle exit. In this study, we show this is the result of cooperation of cell cycle control mechanisms that limit E2F-CycE positive feedback and prevent cycling after terminal differentiation. Aberrant CycE activity after differentiation leads to the degradation of E2F activator complexes, which increases the proportion of CycE-resistant E2F repressor complexes, resulting in stable E2F target gene repression. If E2F-dependent repression is lost after differentiation, high anaphase-promoting complex/cyclosome (APC/C) activity degrades key E2F targets to limit cell cycle reentry. Providing both CycE and E2F activities bypasses exit by simultaneously inhibiting the APC/C and inducing a group of E2F target genes essential for cell cycle reentry after differentiation. These mechanisms are essential for proper development, as evading them leads to tissue outgrowths composed of dividing but terminally differentiated cells.

The Pax6b homeodomain is dispensable for pancreatic endocrine cell differentiation in zebrafish

Pax6 is a well conserved transcription factor that contains two DNA-binding domains, a paired domain and a homeodomain, and plays a key role in the development of eye, brain, and pancreas in vertebrates. The recent identification of the zebrafish sunrise mutant, harboring a mutation in the pax6b homeobox and presenting eye abnormalities but no obvious pancreatic defects, raised a question about the role of pax6b in zebrafish pancreas. We show here that pax6b does play an essential role in pancreatic endocrine cell differentiation, as revealed by the phenotype of a novel zebrafish pax6b null mutant and of embryos injected with pax6b morpholinos. Pax6b-depleted embryos have almost no beta cells, a strongly reduced number of delta cells, and a significant increase of epsilon cells. Through the use of various morpholinos targeting intron-exon junctions, pax6b RNA splicing was perturbed at several sites, leading either to retention of intronic sequences or to deletion of exonic sequences in the pax6b transcript. By this strategy, we show that deletion of the Pax6b homeodomain in zebrafish embryos does not disturb pancreas development, whereas lens formation is strongly affected. These data thus provide the explanation for the lack of pancreatic defects in the sunrise pax6b mutants. In addition, partial reduction of Pax6b function in zebrafish embryos performed by injection of small amounts of pax6b morpholinos caused a clear rise in alpha cell number and in glucagon expression, emphasizing the importance of the fine tuning of the Pax6b level to its biological activity.

Computational prediction of neural progenitor cell fates

Understanding how stem and progenitor cells choose between alternative cell fates is a major challenge in developmental biology. Efforts to tackle this problem have been hampered by the scarcity of markers that can be used to predict cell division outcomes. Here we present a computational method, based on algorithmic information theory, to analyze dynamic features of living cells over time. Using this method, we asked whether rat retinal progenitor cells (RPCs) display characteristic phenotypes before undergoing mitosis that could foretell their fate. We predicted whether RPCs will undergo a self-renewing or terminal division with 99% accuracy, or whether they will produce two photoreceptors or another combination of offspring with 87% accuracy. Our implementation can segment, track and generate predictions for 40 cells simultaneously on a standard computer at 5 min per frame. This method could be used to isolate cell populations with specific developmental potential, enabling previously impossible investigations.

Analysis of retinal cell development in chick embryo by immunohistochemistry and in ovo electroporation techniques

Background

Retinal cell development has been extensively investigated; however, the current knowledge of dynamic morphological and molecular changes is not yet complete.

Results

This study was aimed at revealing the dynamic morphological and molecular changes in retinal cell development during the embryonic stages using a new method of targeted retinal injection, in ovo electroporation, and immunohistochemistry techniques. A plasmid DNA that expresses the green fluorescent protein (GFP) as a marker was delivered into the sub-retinal space to transfect the chick retinal stem/progenitor cells at embryonic day 3 (E3) or E4 with the aid of pulses of electric current. The transfected retinal tissues were analyzed at various stages during chick development from near the start of neurogenesis at E4 to near the end of neurogenesis at E18. The expression of GFP allowed for clear visualization of cell morphologies and retinal laminar locations for the indication of retinal cell identity. Immunohistochemistry using cell type-specific markers (e.g., Visinin, Xap-1, Lim1+2, Pkcα, NeuN, Pax6, Brn3a, Vimentin, etc.) allowed further confirmation of retinal cell types. The composition of retinal cell types was then determined over time by counting the number of GFP-expressing cells observed with morphological characteristics specific to the various retinal cell types.

Conclusion

The new method of retinal injection and electroporation at E3 - E4 allows the visualization of all retinal cell types, including the late-born neurons, e.g., bipolar cells at a level of single cells, which has been difficult with a conventional method with injection and electroporation at E1.5. Based on data collected from analyses of cell morphology, laminar locations in the retina, immunohistochemistry, and cell counts of GFP-expressing cells, the time-line and dynamic morphological and molecular changes of retinal cell development were determined. These data provide more complete information on retinal cell development, and they can serve as a reference for the investigations in normal retinal development and diseases.

Neurogenesis

For more than a decade, the zebrafish has proven to be an excellent model organism to investigate the mechanisms of neurogenesis during development. The often cited advantages, namely external development, genetic, and optical accessibility, have permitted direct examination and experimental manipulations of neurogenesis during development. Recent studies have begun to investigate adult neurogenesis, taking advantage of its widespread occurrence in the mature zebrafish brain to investigate the mechanisms underlying neural stem cell maintenance and recruitment. Here we provide a comprehensive overview of the tools and techniques available to study neurogenesis in zebrafish both during development and in adulthood. As useful resources, we provide tables of available molecular markers, transgenic, and mutant lines. We further provide optimized protocols for studying neurogenesis in the adult zebrafish brain, including in situ hybridization, immunohistochemistry, in vivo lipofection and electroporation methods to deliver expression constructs, administration of bromodeoxyuridine (BrdU), and finally slice cultures. These currently available tools have put zebrafish on par with other model organisms used to investigate neurogenesis.

Retinal horizontal cells: challenging paradigms of neural development and cancer biology

A group of retinal interneurons known as horizontal cells has recently been shown to exhibit a variety of unique biological properties, as compared with other nerve cells, that challenge many long-standing assumptions in the fields of neural development and cancer biology. These features include their unusual migratory behavior, their unique morphological plasticity, and their propensity to divide at a relatively late stage during development. Here, we review these novel features, discuss their relevance for other cell types, outline open questions in our understanding of horizontal cell development and consider their implications.

Ptf1a is expressed transiently in all types of amacrine cells in the embryonic zebrafish retina

Background

The vertebrate retina is composed of five major types of neurons: three excitatory (photoreceptors, bipolar cells and ganglion cells) and two inhibitory (horizontal and amacrine cells). The transcription factor Ptf1a (pancreas transcription factor 1a) is important for the normal development of the inhibitory retinal neurons.

Results

Using a transgenic Ptf1a:GFP reporter and in situ hybridization in the zebrafish retina, we show that ptf1a message is transiently expressed in all amacrine and horizontal cells within hours after the terminal division of multipotent progenitors at the apical surface of the retinal neuroepithelium, and remains on as these cells migrate to their final laminar location. The message then shuts off, but we can follow the stable Ptf1a:GFP protein for up to 120 hours post-fertilization. A variety of anatomically and neurochemically distinct subtypes of amacrine cells can already be distinguished at this embryonic time point.

Conclusion

The timing of Ptf1a expression suggests that it is involved in the very early stages or steps in the differentiation of amacrine cells, which, due to the perdurance of the Ptf1a:GFP, can be seen to rapidly diversify into a large number of subtypes. This work sets the stage for future studies looking at genetic specification of amacrine subtypes.

Loss of Dnmt1 catalytic activity reveals multiple roles for DNA methylation during pancreas development and regeneration

Developmental mechanisms regulating gene expression and the stable acquisition of cell fate direct cytodifferentiation during organogenesis. Moreover, it is likely that such mechanisms could be exploited to repair or regenerate damaged organs. DNA methyltransferases (Dnmts) are enzymes critical for epigenetic regulation, and are used in concert with histone methylation and acetylation to regulate gene expression and maintain genomic integrity and chromosome structure. We carried out two forward genetic screens for regulators of endodermal organ development. In the first, we screened for altered morphology of developing digestive organs, while in the second we screed for the lack of terminally differentiated cell types in the pancreas and liver. From these screens, we identified two mutant alleles of zebrafish dnmt1. Both lesions are predicted to eliminate dnmt1 function; one is a missense mutation in the catalytic domain and the other is a nonsense mutation that eliminates the catalytic domain. In zebrafish dnmt1 mutants, the pancreas and liver form normally, but begin to degenerate after 84 h post fertilization (hpf). Acinar cells are nearly abolished through apoptosis by 100 hpf, though neither DNA replication, nor entry into mitosis is halted in the absence of detectable Dnmt1. However, endocrine cells and ducts are largely spared. Surprisingly, dnmt1 mutants and dnmt1 morpholino-injected larvae show increased capacity for pancreatic beta cell regeneration in an inducible model of pancreatic beta cell ablation. Thus, our data suggest that Dnmt1 is dispensable for pancreatic duct or endocrine cell formation, but not for acinar cell survival. In addition, Dnmt1 may influence the differentiation of pancreatic beta cell progenitors or the reprogramming of cells toward the pancreatic beta cell fate.

Distinct effects of Hedgehog signaling on neuronal fate specification and cell cycle progression in the embryonic mouse retina

Cell-extrinsic signals can profoundly influence the production of various neurons from common progenitors. Yet mechanisms by which extrinsic signals coordinate progenitor cell proliferation, cell cycle exit, and cell fate choices are not well understood. Here, we address whether Hedgehog (Hh) signals independently regulate progenitor proliferation and neuronal fate decisions in the embryonic mouse retina. Conditional ablation of the essential Hh signaling component Smoothened (Smo) in proliferating progenitors, rather than in nascent postmitotic neurons, leads to a dramatic increase of retinal ganglion cells (RGCs) and a mild increase of cone photoreceptor precursors without significantly affecting other early-born neuronal cell types. In addition, Smo-deficient progenitors exhibit aberrant expression of cell cycle regulators and delayed G(1)/S transition, especially during the late embryonic stages, resulting in a reduced progenitor pool by birth. Deficiency in Smo function also causes reduced expression of the basic helix-loop-helix transcription repressor Hes1 and preferential elevation of the proneural gene Math5. In Smo and Math5 double knock-out mutants, the enhanced RGC production observed in Smo-deficient retinas is abolished, whereas defects in the G(1)/S transition persist, suggesting that Math5 mediates the Hh effect on neuronal fate specification but not on cell proliferation. These findings demonstrate that Hh signals regulate progenitor pool expansion primarily by promoting cell cycle progression and influence cell cycle exit and neuronal fates by controlling specific proneural genes. Together, these distinct cellular effects of Hh signaling in neural progenitor cells coordinate a balanced production of diverse neuronal cell types.

The Gal4/UAS toolbox in zebrafish: new approaches for defining behavioral circuits

Over recent years, several groundbreaking techniques have been developed that allow for the anatomical description of neurons, and the observation and manipulation of their activity. Combined, these approaches should provide a great leap forward in our understanding of the structure and connectivity of the nervous system and how, as a network of individual neurons, it generates behavior. Zebrafish, given their external development and optical transparency, are an appealing system in which to employ these methods. These traits allow for direct observation of fluorescence in describing anatomy and observing neural activity, and for the manipulation of neurons using a host of light-triggered proteins. Gal4/Upstream Activating Sequence techniques, as they are based on a binary system, allow for the flexible deployment of a range of transgenes in expression patterns of interest. As such, they provide a promising approach for viewing neurons in a variety of ways, each of which can reveal something different about their structure, connectivity, or function. In this study, the author will review recent progress in the development of the Gal4/Upstream Activating Sequence system in zebrafish, feature examples of promising studies to date, and examine how various new technologies can be used in the future to untangle the complex mechanisms by which behavior is generated.

Anatomy of zebrafish cerebellum and screen for mutations affecting its development

The cerebellum is important for the integration of sensory perception and motor control, but its structure has mostly been studied in mammals. Here, we describe the cell types and neural tracts of the adult zebrafish cerebellum using molecular markers and transgenic lines. Cerebellar neurons are categorized to two major groups: GABAergic and glutamatergic neurons. The Purkinje cells, which are GABAergic neurons, express parvalbumin7, carbonic anhydrase 8, and aldolase C like (zebrin II). The glutamatergic neurons are vglut1(+) granule cells and vglut2(high) cells, which receive Purkinje cell inputs; some vglut2(high) cells are eurydendroid cells, which are equivalent to the mammalian deep cerebellar nuclei. We found olig2(+) neurons in the adult cerebellum and ascertained that at least some of them are eurydendroid cells. We identified markers for climbing and mossy afferent fibers, efferent fibers, and parallel fibers from granule cells. Furthermore, we found that the cerebellum-like structures in the optic tectum and antero-dorsal hindbrain show similar Parvalbumin7 and Vglut1 expression profiles as the cerebellum. The differentiation of GABAergic and glutamatergic neurons begins 3 days post-fertilization (dpf), and layers are first detectable 5 dpf. Using anti-Parvalbumin7 and Vglut1 antibodies to label Purkinje cells and granule cell axons, respectively, we screened for mutations affecting cerebellar neuronal development and the formation of neural tracts. Our data provide a platform for future studies of zebrafish cerebellar development.

Horizontal cell progenitors arrest in G2-phase and undergo terminal mitosis on the vitreal side of the chick retina

We have addressed the question when horizontal cells in the chick retina are generated and undergo their terminal mitosis. Horizontal cell progenitors replicate their DNA early and migrate bi-directionally to the horizontal cell layer. It was hypothesized that the cells undergo mitosis directly after replication and migrate as post-mitotic transition cells before differentiating to horizontal cells. However, our results show that cells expressing markers for the axon-bearing and the axon-less subtypes of horizontal cells undergo terminal mitosis while residing on the vitreal side of the retina. By combining horizontal cell transcription factors Lim1, Isl1 and Prox1 labeling with phospho-histone H3, a marker for mitosis, we demonstrate that all or a clear majority of vitreal mitoses are undertaken by the horizontal cell committed progenitors. The pattern of cells that incorporated the thymidine analogue EdU implied that the progenitors replicated their genome while migrating towards the vitreal side. Upon arrival to the vitreal retina they become arrested for about two days prior to mitosis. Hence, cells expressing horizontal cell markers are arrested in G2-phase on the vitreal side of the retina. These results support the existence of committed progenitors that give rise to horizontal cells and that those cells become arrested in G2-phase before undergoing terminal mitosis on the vitreal side of the retina followed by migration to the horizontal cell layer. The results also indicate that the regulation of the transition from G2-phase to mitosis is important for the development of these committed progenitor cells.

Traumatic brain injury-induced hippocampal neurogenesis requires activation of early nestin-expressing progenitors

It is becoming increasingly clear that brain injuries from a variety of causes stimulate neurogenesis within the hippocampus. It remains unclear, however, how robust this response may be and what primary cell types are involved. Here, using a controlled cortical impact model of traumatic brain injury on a previously characterized transgenic mouse line that expresses enhanced green fluorescent protein (eGFP) under the control of the nestin promoter, we demonstrate that it is the earliest type-1 quiescent progenitor cells that are induced to proliferate and migrate outside the subgranular layer of the dentate gyrus. This type-1 cell activation occurs at the same time that we observe adjacent but more differentiated doublecortin-expressing progenitors (type-2 cells) being eliminated. Also, although type-2 cells remain intact in the contralateral (uninjured) dentate gyrus, the type-1 cells there are also activated and result in increased numbers of the doublecortin-expressing type-2 cells. In addition, we have generated a novel mouse transgenic that expresses a modified version of the herpes simplex virus thymidine kinase along with eGFP that allows for the visualization and inducible ablation of early dividing progenitors by exposing them to ganciclovir. Using this transgenic in the context of traumatic brain injury, we demonstrate that these early progenitors are required for injury-induced remodeling to occur. This work suggests that injury-induced hippocampal remodeling following brain injury likely requires sustained activation of quiescent early progenitors.

Transient neurites of retinal horizontal cells exhibit columnar tiling via homotypic interactions

Sensory neurons with common function are often non-randomly arranged and form dendritic territories that exhibit little overlap or tiling. Repulsive homotypic interactions underlie such patterns in cell organization in invertebrate neurons. In mammalian retinal horizontal cells, however, it is unclear how dendro-dendritic repulsive interactions can produce a non-random distribution of cells and their spatial territories because mature horizontal cell dendrites overlap substantially. By imaging developing mouse horizontal cells, we found that upon reaching their final laminar positions, these cells transiently elaborate vertical neurites that form non-overlapping columnar territories. Targeted cell ablation revealed that the vertical neurites engage in homotypic interactions resulting in tiling of neighboring cells prior to establishment of their dendritic fields. This developmental tiling of transient neurites correlates with the emergence of a non-random distribution of the cells, and could represent a mechanism that organizes neighbor relationships and territories of neurons of the same type before circuit assembly.

Somal positioning and dendritic growth of horizontal cells are regulated by interactions with homotypic neighbors

Retinal neurons extend their dendritic fields to achieve a degree of dendritic overlap with homotypic neighbors that is cell-type specific. How these neurons regulate their dendritic growth is unclear. The dendritic field of a retinal horizontal cell varies inversely with horizontal cell density across different strains of mice, suggesting that proximity to neighboring cells regulates dendritic growth. To test this directly, we have employed the Cre-loxP conditional gene targeting strategy to achieve inactivation of Lim1 function in developing horizontal cells. Through this approach, Lim1 function was prevented within a subset of horizontal cells that in turn fail to migrate to the horizontal cell layer and differentiate normally. For those remaining horizontal cells with Lim1 intact (about half of the normal population in these mice), we show that they spread themselves out tangentially and differentiate a dendritic morphology that is essentially normal but for the fact that it has nearly doubled in area. Such larger horizontal cells, sampling from an area of retina containing twice their normal afferent number, differentiate a dendritic field with nearly double the number of higher order branches and terminal clusters. These results demonstrate directly that positioning and dendritic growth are regulated by interactions with homotypic neighbors, whereas afferents instruct the differentiation of dendritic patterning.

Axon-bearing and axon-less horizontal cell subtypes are generated consecutively during chick retinal development from progenitors that are sensitive to follistatin

BACKGROUND

Horizontal cells are retinal interneurons that modulate the output from photoreceptors. A rich literature on the morphological classification and functional properties of HCs in different animals exists, however, the understanding of the events underlying their development is still limited. In most vertebrates including chicken, two main horizontal cell (HC) subtypes are identified based on the presence or absence of an axon.

RESULTS

In this work we have molecularly characterized three HC subtypes based on Lim1, Isl1, GABA and TrkA, a classification that is consistent with three chick HC subtypes previously defined by morphology. The axon-bearing and axon-less HC subpopulations molecularly defined by Lim1 and Isl1, are born consecutively on embryonic day (E) 3-4 and E4-5, respectively, and exhibit temporally distinguishable periods of migration. Their relative numbers are not adjusted by apoptosis. A sharp decrease of high endogenous levels of the activin-inhibitor follistatin at E3 coincides with the appearance of the Lim1 positive cells. Extending the follistatin exposure of the HC retinal progenitor cells by injection of follistatin at E3 increased the number of both Lim1- and Isl1 positive HCs when analysed at E9.

CONCLUSION

The results imply that the axon-bearing and axon-less HC subgroups are defined early and are generated consecutively from a retinal progenitor cell population that is sensitive to the inhibitory action of follistatin. The results are consistent with a model wherein added follistatin causes HC-generating progenitors to proliferate beyond the normal period of HC generation, thus producing extra HCs of both types that migrate to the HC layer.

The genesis of retinal architecture: an emerging role for mechanical interactions?

Patterns in nature have always fascinated human beings. They convey the idea of order, organization and optimization, and, to the enquiring mind, the alluring promise that understanding their building rules may uncover the forces that shaped them. In the retina, two patterns are outstanding: the stacking of cells in layers and, within the layers, the prevalent arrangement of neurons of the same type in orderly arrays, often referred to as mosaics for the crystalline-like order that some can display. Layers and mosaics have been essential keys to our present understanding of retinal circuital organization and function. Now, they may also be a precious guide in our exploration of how the retina is built. Here, we will review studies addressing the mechanisms controlling the formation of retinal mosaics and layers, illustrating common themes and unsolved problems. Among the intricacies of the building process, a world of physical forces is making its appearance. Cells are extremely complex to model as "physical entities", and many aspects of cell mechanotransduction are still obscure. Yet, recent experiments, focusing on the mechanical aspects of growth and differentiation, suggest that adopting this viewpoint will open new ways of understanding retinal formation and novel possibilities to approach retinal pathologies and repair.

Expanding neuronal layers by the local division of committed precursors

A general strategy in vertebrate neural development is for neuronal precursors to stop dividing, to begin to differentiate, and then to migrate to their final destination. In this issue of Neuron, however, Godhino et al. provide evidence that some neuronal precursors undergo a terminal symmetric cell division at their final destination to form an entire neuronal layer.

Retinal progenitor cells can produce restricted subsets of horizontal cells

Retinal progenitor cells have been shown to be multipotent throughout development. Similarly, many other structures of the developing central nervous system have been found to contain multipotent progenitor cells. Previous lineage studies did not address whether these multipotent progenitor cells were biased in their production of neuronal subtypes. This question is of interest because subtypes are the basis of distinct types of circuits. Here, lentivirus-mediated gene transfer was used to mark single retinal progenitor cells in vivo, and the different subtypes of horizontal cells (HCs) in each clone were quantified. Clones with two HCs consistently contained a single HC subtype, a pair of either H1 or H3 cells. This suggests that a multipotent progenitor cell produces a mitotic cell fated to make a terminal division that produces two HCs of only one subtype. This bias in production of one HC subtype suggests a previously undescribed mechanism of cell fate determination in at least a subset of retinal cells that involves decisions made by mitotic cells that are inherited in a symmetric manner by both neuronal daughter cells.