An eye on eye development

MECHANISMS OF DEVELOPMENT

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Review

An eye on eye development Rebecca Sinn a b

a,b

, Joachim Wittbrodt

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a,*

Centre for Organismal Studies, COS Heidelberg, University of Heidelberg, Im Neuenheimer Feld 230, 69120 Heidelberg, Germany Hartmut Hoffmann-Berling International Graduate School HBIGS, University of Heidelberg, Germany

A R T I C L E I N F O

A B S T R A C T

Article history:

The vertebrate eye is composed of both surface ectodermal and neuroectodermal deriva-

Available online 15 May 2013

tives that evaginate laterally from an epithelial anlage of the forming diencephalon. The retina is composed of a limited number of neuronal and non-neuronal cell types and is

Keywords: Eye development Retinal differentiation Eye morphogenesis Retinal patterning

seen as a model for the brain with reduced complexity. The eye develops in a stereotypic manner building on evolutionarily conserved molecular networks. Eye formation is initiated at the onset of gastrulation by the determination of the eye field in the anterior neuroectoderm. Homeobox transcription factors, in particular Six3 are crucially involved in the establishment and maintenance of retinal identity. The eye field expands by proliferation as gastrulation proceeds and is initially confined to a single retinal primordium by the differential activity of specifying transcription factors. This central field is subsequently split in response to secreted factors emanating from the ventral midline. Concomitant with medio-lateral patterning at the onset of neurulation, morphogenesis sets in and laterally evaginates the optic vesicle. Strikingly during this process the neuroectoderm in the eye field transiently loses epithelial features and cells migrate individually. In a second morphogenetic event, the vesicle is transformed into the optic cup, concomitant with onset and progression of retinal differentiation. Accompanying optic cup morphogenesis, neural differentiation is initiated from a retinal signalling centre in a stereotypic and species specific manner by secreted signalling factors. Here we will give an overview of key events during vertebrate eye formation and highlight key players in the respective processes. Ó 2013 Published by Elsevier Ireland Ltd.

Contents 1. 2. 3. 4.

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Establishment of the eye field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sub-patterning of the anterior neural plate . . . . . . . . . . . . . . . . . . . . . Transcription factors involved in the establishment of retinal identity Split of the eye field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: +49 6221 546499; fax: +49 6221 546439. E-mail address: [email protected] (J. Wittbrodt). 0925-4773/$ - see front matter Ó 2013 Published by Elsevier Ireland Ltd. http://dx.doi.org/10.1016/j.mod.2013.05.001

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5. 6. 7. 8. 9.

1.

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Balance between proliferation and differentiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optic vesicle evagination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optic cup formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Initiation of retinal differentiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Perspectives and challenges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Establishment of the eye field

Eye formation in vertebrates is tightly interconnected with the patterning of the forming neuroectoderm at the onset of gastrulation. Controlled by factors patterning the neural plate, the eye field is determined in the anterior neuroectoderm soon after neural induction (Chow and Lang, 2001) by a series of inductive signals. These are sub-divided into three major steps: neural induction in the presumptive ectoderm, anterior–posterior subdivision of the neural plate and specification of the eye field in the diencephalic territories (Fig. 1). Neural induction follows a molecular mechanism that likely applies to all vertebrates (Wilson and Houart, 2004). Factors of the fibroblast growth factor family (Fgf) are secreted prior to the onset of gastrulation (Gamse and Sive, 2001; Itoh et al., 2002; Wessely et al., 2001; Wilson et al., 2000) and trigger neural fate, when Wnt-signalling is suppressed (Niehrs, 1999; Stern, 2006). To establish and maintain neural fate, bone mor-

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phogenetic proteins (BMPs) must be repressed by antagonists such as Follistatin, Noggin, Chordin and Cerberus (Bouwmeester et al., 1996; Hemmati-Brivanlou et al., 1994; Niehrs, 2001) which are specifically expressed in organising centres such as the Spemann organiser (amphibia), Hensen’s node (birds) or the mouse node. Fates along the anterior–posterior axis of the neural plate are determined by posteriorising factors such as retinoic acid, or secreted proteins of the Wnt, Fgf, BMP or Nodal family (Agathon et al., 2003; Munoz-Sanjuan and Brivanlou, 2001). The role of Wnt-signalling becomes obvious when members of the Wnt-pathway are mutated. Enhancement of Wnt-signalling results in the absence of head structures in the zebrafish mutant headless (hdl/Tcf3), a negative Wnt-modulator (Kim et al., 2000). The opposite phenotype is observed in mutants affected in Nodal-signalling. Here the telencephalon is massively expanded as observed in the zebrafish mutant one-eyed pinhead (oep) (Gritsman et al., 1999; Kim et al., 2000)

Fig. 1 – Christmas tree scheme of vertebrate eye development. The central trunk of the tree represents both, temporal axis as well as ventral midline. Orange indicates neuroectodermal parts fated to become retina during stages prior to neurulation. Concomitant with neurulation optic vesicles undergo the first morphogenetic transition: they evaginate and get subpatterned into optic stalk (blue), pigmented retinal epithelium (yellow) and prospective neuroretina (red). In the subsequent second morphogenetic transition, the formation of the optic cup, retinal differentiation triggered by a Fgf signalling centre, spreads over the entire retina. The lens (green) forms from surface ectodermal derivatives. Adapted from (Wittbrodt et al., 2002) with permission.

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in which a co-receptor for Nodal is affected. In its absence, posteriorising signals, relayed by functional Nodal-signalling, are lacking (Schier, 2003). Thus the induction of the eye anlage in the anteriormost neuroectoderm requires the delicate balance between neural inducers, posteriorising factors and their secreted antagonists expressed in the anterior neuroectoderm (Esteve and Bovolenta, 2006).

2.

Sub-patterning of the anterior neural plate

To define the eye anlage, telencephalon and diencephalon, the anterior neural plate is subsequently further subdivided rostro-caudally by a graded Wnt-activity (Wilson and Houart, 2004) as illustrated by the phenotype observed in the Axin1 mutant masterblind (mbl) in zebrafish. mbl/Axin1 acts as a scaffolding protein and is required for the down-regulation of Wnt-signalling (Heisenberg et al., 2001). mbl mutant embryos exhibit a local increase in Wnt-signalling activity due to the absence of Axin1 and consequently fail to subdivide the anterior neural plate. Instead, telencephalon and the eyes are respecified to a more posterior diencephalic fate (Heisenberg et al., 2001; van de Water et al., 2001). In addition, secreted frizzled related proteins (FRPs) such as Tlc (Houart et al., 2002) and sFRP1 (Esteve et al., 2004) contribute to a sharpening of the local Wnt-activity. Secreted by the anterior neural plate they act as Wnt-antagonists to eventually establish well-defined, sharp boundaries.

3. Transcription factors involved establishment of retinal identity

in

the

Wnt needs to be repressed in the eye anlage to allow the subsequent development of the optic vesicles. This is in part mediated through the transcription factor Six3, one of the key factors involved in early eye field specification (Lagutin et al., 2003; Liu et al., 2010; Loosli et al., 1998, 1999; Oliver et al., 1995). It is initially expressed in the eye anlage and encodes a homeobox containing transcription factor of the six/sine oculis family (Loosli et al., 1998; Oliver et al., 1995). It was identified by its homology to the Drosophila gene sine oculis (Cheyette et al., 1994) and is considered to be the ortholog of the Drosophila gene, Optix (Seimiya and Gehring, 2000; Toy et al., 1998). Ectopic overexpression of Six3 results in the formation of ectopic eye cups in fish and amphibia (Loosli et al., 1999; Zuber et al., 1999). Strikingly, this only occurs in a competence domain in the midbrain (and in the eye), where exogenous Six3 triggers a feedback loop that results in the persistent activation of endogenous Six3 (Loosli et al., 1999). Clonal expression of Six3 in contrast results in the transformation of the otic placode to ectopic lenses replacing the ear (Oliver et al., 1996). Loss of function data highlight the crucial role of Six3 in the specification of the vertebrate eye anlage and the patterning of the anterior neural plate. Both in medaka morphant embryos as well as in mouse knock-out (KO) mutants, structures anterior to the midbrain are not established in the absence of Six3 function (Carl et al., 2002; Lagutin et al., 2003). In mouse KO mutants the absence of Six3 results in the anterior expansion of posterior markers, hinting at a direct role for Six3 in shaping the Wnt-activity gradient in the anterior

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neural plate (Lagutin et al., 2003; Liu et al., 2010). In line with that, Six3 overexpression rescues forebrain patterning and thus telencephalic and eye fate in hdl mutant embryos, lacking the Wnt-repressor Tcf3 (Lagutin et al., 2003). Conversely, increased Wnt-signalling prevents Six3 expression (and eye field specification) (Braun et al., 2003; Kiecker and Niehrs, 2001; Nordstrom et al., 2002). Taken together, Six3 is active in regions of low Wnt-activity in the anterior neural plate, where it acts as a negative modulator of Wnt-signalling in a self-reinforcing pathway (Wilson and Houart, 2004). Interestingly this function of Six3 in patterning the anterior neural plate appears evolutionarily ancient and conserved (Arendt and Wittbrodt, 2001; Sinigaglia et al., 2013). Following the establishment of the eye anlage, a family of retinal specific transcription factors, named retinal homeobox transcription factors (Rx), is crucially involved in optic cup morphogenesis and photoreceptor specification. Rx genes are found in all vertebrates studied so far; while the mouse genome contains only one Rx orthologue (Rax), three Rx paralogues have been uncovered in fish (Rx1, Rx2 and Rx3) (Chuang et al., 1999; Deschet et al., 1999; Mathers et al., 1997). The targeted inactivation of Rx/Rax in mouse results in the complete absence of eyes, both on the level of morphology as well as on the level of retina specific markers (Mathers et al., 1997) indicating an early role of Rx genes in the specification of retinal progenitor cells (RPCs) (Zhang et al., 2000). Mutations in the Rx3 gene in fish medaka (eyeless/el), zebrafish (chokh/chk) both result in the establishment of an eye field that fails to laterally evaginate optic vesicles (Loosli et al., 2001, 2003; Rojas-Mun˜oz et al., 2005; Stigloher et al., 2006; Winkler et al., 2000). At this stage of development, Rx genes are involved in proliferation control within the eye field facilitated through the regulation by Six3/Optx2 (Six6) (Del Bene and Wittbrodt, 2005; Loosli et al., 1999; Zuber et al., 1999, 2003). The ‘most prominent eye gene’ is probably Pax6, a transcription factor containing two DNA binding domains, a paired-type homeodomain and a paired domain (Czerny and Busslinger, 1995; Glaser et al., 1994; Mikkola et al., 1999; Tang et al., 1998). It is highly conserved between vertebrates and invertebrates and mis-expression of Drosophila ey/Pax6 or the mouse orthologue smalleye/Pax6 in Drosophila imaginal discs can trigger the development of ectopic eyes in competent tissue at the site of mis-expression (Halder et al., 1995a). These results, together with the early expression of vertebrate Pax6 genes in the anterior neural plate (Grindley et al., 1995; Hirsch and Harris, 1997; Li et al., 1994; Loosli et al., 1998; Pu¨schel et al., 1992; Walther and Gruss, 1991) has led to the suggestive ‘master control gene hypothesis’ for Pax6, as key regulator of eye development conserved throughout bilaterian evolution (Gehring and Ikeo, 1999; Halder et al., 1995b; Quiring et al., 1994). Recent data however indicate that this function is rather taken by Six3 as an evolutionarily conserved anterior neural plate/eye determinant (Sinigaglia et al., 2013). While in absence of Six3 the eyes do not form, Pax6 mutant mice or rats initially develop eyes that eventually fail to be maintained (Hill et al., 1991; Hogan et al., 1986; Philips et al., 2005). Since Pax6 can activate the expression of Six3, it has been discussed that ectopic retinal structures induced with low efficiency by the ectopic expression of Pax6 in vertebrates, are due to the

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activation of Six3 expression in response to ectopic Pax6 (Chow et al., 1999; Kobayashi et al., 1998; Loosli et al., 1999). Another prerequisite for the establishment of retinal identity is the repression of the transcription factor Otx2, the vertebrate orthologue of the Drosophila transcription factor orthodenticle (Finkelstein et al., 1990). It is expressed throughout the anterior forebrain early in embryogenesis (Mori et al., 1994) where it precedes any other eye field marker (Simeone et al., 1993). Otx2 is required for the establishment of the eye field specific transcription factor network formed by Six3, Rx3 and Pax6 (Andreazzoli et al., 1999; Chow and Lang, 2001; Chuang and Raymond, 2002; Lupo et al., 2000; Mathers and Jamrich, 2000). With the onset of the expression of Six3, Rx3 and Pax6 in the eye field, Otx2 expression is down-regulated (Andreazzoli et al., 1999; Loosli et al., 1998) and Six3 and Rx3 can efficiently restrict Otx2 if expressed ectopically (Andreazzoli et al., 1999). Strikingly, the potential of Six3 to induce ectopic eyes is restricted to the Otx2 expression domain in the midbrain (Loosli et al., 1999) and the potency of a cocktail of eye specification genes to induce ectopic eye structures in Xenopus is strongly enhanced by the addition of Otx2 (Zuber et al., 2003). Thus Otx2 plays a permissive role in the initiation of the eye field and confers eye field competence to the neural plate, to eventually be down-regulated during eye field specification.

4.

Split of the eye field

After retinal identity is established in the eye anlage, indicated by the overlapping expression of Six3, Rx3 and Pax6 in the single, centrally positioned eye field, secreted factors of the Tgf-ß-, Fgf- and Shh-families are emanating from the underlying axial mesoderm to split the eye anlage into two bilateral symmetric retinal primordia. If this process fails, one centrally positioned (cyclopic) eye is formed. Genes affected in cyclopic zebrafish mutants (e. g. cyclops/cyc or squint/sqt) encode Nodal-related factors of the Tgf-ß family (Feldman et al., 1998; Hatta et al., 1991; Rebagliati et al., 1998; Sampath et al., 1998). In response to the activity of these secreted factors, prospective hypothalamic cells migrate from a position posterior to the eye field. This medial stream of cells separates the single, central eye field and establishes two eye primordia (England et al., 2006; Hirose et al., 2004; Varga et al., 1999). This occurs concomitant with the expression of Shh in the ventral midline in the underlying mesendoderm. The absence or blocking of Shh-signalling also results in cyclopia. Therefore Shh acts as a crucial patterning factor in the abutting neuroectodermal domain. This domain is sensitised by midline secreted Fgf signals (Carl and Wittbrodt, 1999) and is rendered competent to respond to Shh. High concentrations of Shh induce more proximal identities (e.g. optic stalk) at the expense of distal structures that form in response to low concentrations of Shh. A number of fate determining homeobox transcription factors (e.g. Vax1, Vax2 and Pax2) (Barbieri et al., 1999; Carl and Wittbrodt, 1999; Dressler et al., 1990; Hallonet et al., 1999; Schulte et al., 1999) indicate the response of the tissue to the fate induction by Shh. Strikingly the graded response to Shh is mimicked morphologically by a graded loss of Six3, uncovering an unexpected role for this transcription factor also in prox-

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imo-distal patterning of the eye primordia. The central part of the eye field is most sensitive to the loss of Six3 and disappears first in a graded knock-down resulting in cyclopia phenotypes (Carl and Wittbrodt, 1999). The more Six3 activity is repressed, the further the loss of diencephalon extends distally to eventually result in the complete absence of eye and forebrain in a complete knock-down (Carl and Wittbrodt, 1999).

5. Balance differentiation

between

proliferation

and

The role of Six3 in balancing proliferation and differentiation is of particular interest. Even though Six3 acts already during eye field specification, its activity is still required at all subsequent steps of eye formation. To control cell proliferation Six3 has entered an additional route: rather than influencing the cell cycle machinery on the transcriptional level by controlling the expression of its components, Six3 interacts directly with the machinery. On the one hand, Six3 and its paralogue Six6 function as transcriptional repressors in interaction with co-repressors of the Groucho family to repress target genes (Kobayashi et al., 2001; Lopez-Rios et al., 2003; Zhu et al., 2002). A direct transcriptional regulation of the cell cycle inhibitor p27kip1 by Six6 has been demonstrated (Li et al., 2002). Here Six6 stimulates proliferation by repressing the transcription of a cell cycle inhibitor. On the other hand, Six3 also employs an immediate route by directly interacting with the cell cycle machinery. It controls the proliferation of RPCs by binding to and thus directly inhibiting Geminin, an inhibitor of the initiation of replication (Del Bene et al., 2004). This unusual interaction of a transcription factor with a key component of the cell cycle machinery (Tessmar et al., 2002), has independently been reported for Hox genes in a different context (Luo et al., 2004). By binding to Geminin, Six3 inhibits an inhibitor and allows Cdt1 (Tada et al., 2001; Wohlschlegel et al., 2000) to be loaded to the initiation complex on the origins of replication and thus to permit the licensing of the origins for G1/S transition (Li and Rosenfeld, 2004). During the formation of the eye field, Six3 and Geminin antagonise each other to tune cell proliferation. Consequently gain-of-function phenotypes of Geminin closely resemble the loss-of-function phenotypes of Six3 and vice versa (Del Bene et al., 2004). Geminin is inhibited by the binding of Six3 and this permits cell cycle progression and proliferation during the establishment of the eye field. On the other hand, there is a direct influence of Geminin on the regulatory potential of Six3 as a transcription factor, now favouring transcriptional activation and neuronal differentiation. In this delicate interaction, Six3 acts as a bimodal switch that, depending on its developmental context, stimulates proliferation during the initiation of retinal identity and later during retinal differentiation promotes RPCs to adopt their terminal fate as retinal neurons or glia (Del Bene et al., 2004).

6.

Optic vesicle evagination

While the eye field is patterned and split, it also expands massively in size. Even though morphogenesis and

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proliferation are intimately linked, proliferation does not appear to be the driving force of optic vesicle evagination (Harris and Hartenstein, 1991). Also the change of cell shape which has been proposed to contribute to the optic vesicle formation in mouse (Svoboda and O’Shea, 1987) does not seem to drive the process in fish. Here the enlargement of the area of the optic vesicle rather appears to be driven by a specific migratory behaviour of RPCs (Brown et al., 2010; Rembold et al., 2006). Medaka or zebrafish embryos carrying a mutation in Rx3 fail to laterally evaginate optic vesicles (Loosli et al., 2001, 2003; Winkler et al., 2000) (Fig. 2). The detailed in vivo analysis of individual cell behaviour during optic vesicle evagination by 4D microscopy allowed description of the process with cellular resolution (Keller et al., 2008; Rembold et al., 2006). The enlargement of the forming optic vesicles is orchestrated by a particular sequence of events that involve the specific softening of the epithelialized neural tube at the site of evagination (Rembold et al., 2006). This is followed by the immigration of individual cells

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to this area in the lateral diencephalon, where they intercalate, epithelialize, and thus contribute to the enlargement of the optic vesicle. The vesicle ‘‘rolls out’’ from its anterior origin by the continuous intercalation of newly arriving RPCs (Rembold et al., 2006). The migrating cells follow a specific migratory pattern and their behaviour is determined by cell adhesion and signalling molecules and corresponding receptors, in particular NLCAM (Brown et al., 2010) and SDF/CXCR4 (Weisswange and Wittbrodt unpublished data, (Bielen and Houart, 2012)). Optic vesicle evagination is initiated at the onset of neurulation by a retarded migration of the Rx3 positive RPCs of the eye field in their approach toward the midline. Consequently these RPCs are still in a lateral position when the tube is closed since they have been passed by their non RPC neighbour cells (anterior, posterior and lateral/dorsal). In wild-type embryos, these RPCs have transiently given up their epithelialisation. In the absence of Rx3 activity however, all cells approach the midline with the same pace, the lateral

Fig. 2 – Rx3 mutant phenotypes – loss of eyes. Left panel medaka eyeless (el) mutant reprinted from (Winkler et al., 2000), right panel zebrafish chokh (chk) mutant reprinted from (Loosli et al., 2003) with permission. (A, B) At mid neurula stage (stage 17/ 25hpf) evagination of optic vesicles starts in wild-type embryos but not in el mutants. (C, D) At 26hpf (stage 19) optic vesicles in wild-type embryos are fully evaginated while they are not visible in mutants. (E, F) Onset of pigmentation of PRE in wildtype embryos in contrast to el mutants where no eyes are found. (G, H) Adult fish completely lack eye structures in the eyeless mutant. hyp, hypothalamus; lp, lens placode; ls, lens; no, nose; nr, neuroretina; ov, optiv vesicle; pe, prosencephalon; pre, pigmented retinal epithelium. (I–R) Comparison of zebrafish Rx3 (chk) mutants with wild-type. Mutant embryos display absence of eyes trough all developmental stages. Overall body and head structures of mutant and wild-type embryos do not show gross morphological differences. While in the medaka mutant eyeless lens structures do not develop, zebrafish chokh mutant embryos from a small lens at 48 hpf (white arrow). fb, forebrain; hb, hindbrain; mb, midbrain.

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diencephalic region of the neural retains its epithelialisation and even though RPCs are formed, they do not migrate outwards (Rembold et al., 2006) (Fig. 3). The relative contribution of migration and loss of epithelialisation is illustrated by clonal 4D time lapse analysis.

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Transplantation of wild type cells into Rx3 mutant hosts indicates that the migratory potential of wild-type cells is sufficient to rescue optic vesicle formation. Strikingly, in this condition vesicle formation occurs while the neural tube remains epithelialised. Individual cells are penetrating the

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epithelium and converge to rescue optic vesicles (Rembold et al., 2006). The coordination of cell behaviour during vesicle formation seems of crucial relevance for their future function (Picker et al., 2009) since morphogenesis and pattern formation in the optic vesicle are intimately linked (Picker et al., 2009) by Fgf-signalling.

7.

Optic cup formation

The morphogenetic challenge in optic cup formation is the same for all vertebrates, with variations of the theme. At the beginning of optic cup morphogenesis the optic vesicle represents an epithelial, bi-layered ball-like structure that eventually is transformed into the eye cup with a multi-layered neuro-retina surrounded by the retinal pigmented epithelium. The combination of light sheet microscopy (Ho¨ckendorf et al., 2012; Huisken et al., 2004; Keller et al., 2008) and transgenic fish expressing GFP in the forming eyes allowed to follow and delineate the entire process of optic cup morphogenesis (Martinez-Morales et al., 2009) in vivo. Once the optic vesicles have evaginated they undergo a series of morphogenetic transformations to establish a hemispheric optic cup from a ball like vesicle. In fish this process is subdivided into three major steps: the bending of the epithelial sheet of RPCs from anterior to posterior, the bending along its dorso-ventral axis and finally the closure of the optic fissure to form a regular optic cup. The anterior–posterior bending occurs concomitant with the dorso-ventral bending while the closure of the optic fissure is concluded subsequently (Martinez-Morales and Wittbrodt, 2009; Martinez-Morales et al., 2009). While the described requirement of interactions of the early lens placode with the retina for retinal initiation was suggestive for a contribution of the lens also in initiating

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and guiding optic cup folding, recent data indicate that these processes are not interdependent (reviewed in Martinez-Morales and Wittbrodt, 2009) (Fig. 4). Optic cup formation rather depends on cell intrinsic factors and can even occur in vitro (Eiraku et al., 2011). The cell biology of optic cup formation involves epithelial bending by basal constriction. This was uncovered in the medaka mutant ojoplano (opo/c11c3) that fails to undergo optic cup morphogenesis, while retinal patterning and differentiation occur normally (Loosli et al., 2004; Martinez-Morales et al., 2009). The gene affected in opo encodes a novel, vertebrate specific transmembrane protein involved in vesicular transport. The mutation affects basal constriction of RPCs in the prospective neuro-retinal epithelium. In the absence of opo, cells retain a columnar shape, while in wild-type embryos they become triangular, basally restricted. To achieve this basal constriction of RPCs, the opo protein controls the polarised, basal localisation of focal adhesion components (Martinez-Morales et al., 2009). opo interacts with the PTB-domain of the clathrin adaptors, Numb and Numbl, and opo and Numb/Numbl antagonise each other in regulating integrin-ß trafficking to control optic cup morphogenesis. opo facilitates the pointed basal anchoring through integrin-ß by negatively regulating integrin endocytosis specifically at the basal surface of the retinal progenitor cells. This way, opo locally stabilises the pointed focal contact at the basal side, resulting in the triangular cells shape required to bend the optic vesicle (Bogdanovic´ et al., 2012).

8.

Initiation of retinal differentiation

In the vertebrate retina the differentiation of retinal progenitor cells follows a stereotypic order that is conserved among different vertebrate species (Cepko et al., 1996; Livesey

b Fig. 3 – Optic vesicle evagination. Confocal sections taken from time-lapse analysis of wild-type medaka embryos, dorsal view with anterior to the top. Rx3 expressing, prospective eye cells labelled by GFP expression (rx3::GFP) in a ubiquitous nuclear RFP (H2B-RFP mRNA) background. (A, B) During gastrulation the early eye field is established and cells are condense to a single domain in the developing forebrain. They emigrate laterally to form the optic vesicles (C). Reprinted from (Martinez-Morales et al., 2009) with permission. (D, F) Cell shape analysis of wild-type and eyeless (el) mutants embryos in single confocal planes of rx3::mYFP expressing embryos, anterior top. In el mutants vesicles do not evaginate from the neural keel (compare white arrows in D to brackets in E). (F, G) Anti-tubulin (green) and anti-GFP stain on transverse section of wildtype and el mutant embryos, dorsal top. While in wild-type embryos the epithelialisation is transiently loosened at the onset of optic vesicle evagination, the cells in the el mutant maintain their epithelial organisation (brackets). (H–K) Mosaic analysis of wild-type cells and el cells. Confocal sections of the 4D sequence (neurula to somitogenesis stages) are shown; anterior is to the top. (H, I) Red wild-type cells (membrane-mRFP) contribute and migrate with the green labelled eye field cells of the wild-type host (rx3::mYFP). (J, K) The optic vesicle is formed exclusively by transplanted red wild-type cells penetrating the epithelial lateral wall of the diencephalon formed by el mutant green host cells, which stay in the neural keel. Asterisk, mutant tissue remaining in the forebrain; dashed white line, anterior border of the forebrain. Time is indicated in hh:mm:ss (h, hours; m, minutes; s, seconds) Reprinted from (Rembold et al., 2006) with permission. Effects of loss of Rx3 on cell behaviour mediated by Nlcam. Upper panel: left side indicates the migration of wild-type (blue) and Nlcam + (red) transplanted cells during optic vesicle morphogenesis. Arrow length corresponds to degree of migration. The differential behaviour result in a final distribution where Nlcam overexpressing cells occupy more medial positions in the evaginated vesicles. Right side represents the endogenous expression pattern of Nlcam (maroon), and the normal movements of forebrain and retinal (green) cells. Telencephalic cells, expressing high levels of Nlcam, migrate rapidly inwards and then epithelialise to form the neural keel. RPCs, with low Nlcam levels, converge less and then migrate outwards into the optic vesicles. Lower panel: summary of the effects of Rx3 on Nlcam expression and hence on midline convergence of RPCs and forebrain cells. Reprinted from (Brown et al., 2010) with permission.

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Fig. 4 – Shaping the medaka eye. Schematic representation of the cellular movements involved in optic vesicle evagination. Red and green arrows indicate the differences in the speed of convergence of diencephalic cells and retinal precursors. The epithelialized forebrain cells and the trajectories of migratory retinal precursors are represented by red stripes and blue arrows, respectively. Uniform cell division rate throughout the eye field is indicated by the yellow asterisks. Threedimensional reconstruction of optic cup morphogenesis in medaka embryos. The renderings show the forming optic cup at four different time points through morphogenesis (stage 22 –stage 27+). The renderings are obtained from SPIM time-lapse analyses on rx2::Gfp transgenic lines. Blue and brown arrows indicate antero-posterior and dorso-ventral contribution to the morphogenetic movement. Adapted from (Martinez-Morales and Wittbrodt, 2009) with permission.

and Cepko, 2001). Retinal ganglion cells are the first neurones to be born in the retina of all vertebrates analysed. From the site of initial neuronal differentiation a wave of neurogenesis sweeps over the retina (Hu and Easter, 1999; Laessing and Stuermer, 1996; McCabe et al., 1999; Young, 1985) and by analogy to Drosophila, Shh has been proposed to drive this differentiation wave (Neumann and Nu¨sslein-Volhard, 2000). To achieve this, Shh modulates intrinsic timers that, once trigged, run independently (Kay et al., 2005). In clear contrast to Drosophila, the initiation of retinal differentiation however is independent of Shh (Neumann, 2001). Interestingly, the precise spot where the first neurones will be born is tightly regulated, both spatially and temporally. The ‘‘organising centre’’ for neurogenesis is located at different positions depending on the vertebrate species analysed. Initial data published for zebrafish hint at the optic stalk as the source for the initiating signal (Masai et al., 2000). Here the establishment of the optic stalk territory is a prerequisite to trigger retinal differentiation in its close vicinity (Masai et al., 2000). In chicken embryos, in contrast, retinal differentiation is initiated in a central retinal domain, at a distance to the optic stalk. It was shown that in both chicken and fish embryos, the direct activity of Fgf, emanating from species-specific local organisers, triggers retinal ganglion cell differentiation (Martinez-Morales et al., 2005). In an elegant combination of gene expression analysis, experimental embryology and tissue culture studies, using wild type and mutant zebrafish and chicken embryos it was demonstrated that the combined activity

of Fgf8 and Fgf3 directly triggers the differentiation of retinal ganglion cells (Martinez-Morales et al., 2005). Similarly Fgf8 rescues neuronal differentiation in the retina of oep embryos (Martinez-Morales et al., 2005), which normally fail to initiate this process due to the mis-specification of the optic stalk territory (Masai et al., 2000) indicating a conserved mechanism involving secreted Fgf ligands to initiate retinal differentiation at a defined, species specific organising centre (Esteve and Bovolenta, 2006).

9.

Perspectives and challenges

The teleost eye has always been an ideal model for neurobiology that touches key aspects of neural development as well as neuronal network formation and function, and facilitates functional in vivo studies in an experimentally accessible system. Among the most striking features of the eye of anamniotes is their life-long growth and unique regenerative capacity. During both processes functionality has to be ensured and homeostasis of the entire visual system has to be maintained, not only of the growing or regenerating retina, but also homeostasis of the (growing or regenerating) processing areas in the brain. The challenging complexity of this connection in the eyes of anamniotes and in particular of teleosts can now be tackled following new functional avenues, advanced genetics, lineage analysis and optogenetic approaches. The eye that has triggered far reaching thoughts on evolution long ago, has now entered a new functional

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era with exciting perspectives in basic research and applied biomedicine.

Acknowledgements We would like to thank Felix Loosli and the Wittbrodt lab for helpful comments and critically reading the manuscript. This work was supported by the Collaborative Research Center CRC 488 ‘‘Molecular and cellular bases of neural development’’ (JW). RS is a fellow of the Harmut Hoffmann-Berling International Graduate School of the University of Heidelberg.

R E F E R E N C E S

Agathon, A., Thisse, C., Thisse, B., 2003. The molecular nature of the zebrafish tail organizer. Nature 424, 448–452. Andreazzoli, M., Gestri, G., Angeloni, D., Menna, E., Barsacchi, G., 1999. Role of Xrx1 in Xenopus eye and anterior brain development. Development 126, 2451–2460. Arendt, D., Wittbrodt, J., 2001. Reconstructing the eyes of Urbilateria. Philos. Trans. R. Soc. Lond. B Biol. Sci. 356, 1545– 1563. Barbieri, A., Lupo, G., Bulfone, A., Andreazzoli, M., Mariani, M., Fougerousse, F., Consalez, G., Borsani, G., Beckmann, J., Barsacchi, G., et al, 1999. A homeobox gene, vax2, controls the patterning of the eye dorsoventral axis. Proc. Natl. Acad. Sci. U.S.A. 96, 10729–10734. Bielen, H., Houart, C., 2012. BMP signaling protects telencephalic fate by repressing eye identity and its Cxcr4-dependent morphogenesis. Dev. Cell 23, 812–822. Bogdanovic´, O., Delfino-Machı´n, M., Nicola´s-Pe´rez, M., Gavila´n, M.P., Gago-Rodrigues, I., Ferna´ndez-Min˜a´n, A., Lillo, C., Rı´os, R.M., Wittbrodt, J., Martı´nez-Morales, J.R., 2012. Numb/Numblopo antagonism controls retinal epithelium morphogenesis by regulating integrin endocytosis. Dev. Cell 23, 782–795. Bouwmeester, T., Kim, S., Sasai, Y., Lu, B., De Robertis, E., 1996. Cerberus is a head-inducing secreted factor expressed in the anterior endoderm of Spemann’s organizer. Nature 382, 595–601. Braun, M.M., Etheridge, A., Bernard, A., Robertson, C.P., Roelink, H., 2003. Wnt signaling is required at distinct stages of development for the induction of the posterior forebrain. Development 130, 5579–5587. Brown, K.E., Keller, P.J., Ramialison, M., Rembold, M., Stelzer, E.H.K., Loosli, F., Wittbrodt, J., 2010. Nlcam modulates midline convergence during anterior neural plate morphogenesis. Dev. Biol. 339, 14–25. Carl, M., Wittbrodt, J., 1999. Graded interference with FGF signalling reveals its dorsoventral asymmetry at the midhindbrain boundary. Development 126, 5659–5667. Carl, M., Loosli, F., Wittbrodt, J., 2002. Six3 inactivation reveals its essential role for the formation and patterning of the vertebrate eye. Development 129, 4057–4063. Cepko, C., Austin, C., Yang, X., Alexiades, M., 1996. Cell fate determination in the vertebrate retina. Proc. Natl. Acad. Sci. U.S.A. 93, 589–595. Cheyette, B., Green, P., Martin, K., Garren, H., Hartenstein, V., Zipursky, S., 1994. The Drosophila sine oculis locus encodes a homeodomain-containing protein required for the development of the entire visual system. Neuron 12, 977–996. Chow, R., Lang, R., 2001. Early eye development in vertebrates. Annu. Rev. Cell Dev. Biol. 17, 255–296. Chow, R., Altmann, C., Lang, R., Hemmati-Brivanlou, A., 1999. Pax6 induces ectopic eyes in a vertebrate. Development 126, 4213–4222.

1 3 0 ( 2 0 1 3 ) 3 4 7 –3 5 8

355

Chuang, J., Raymond, P., 2002. Embryonic origin of the eyes in teleost fish. BioEssays 24, 519–529. Chuang, J., Mathers, P., Raymond, P., 1999. Expression of three Rx homeobox genes in embryonic and adult zebrafish. Mech. Dev. 84, 195–198. Czerny, T., Busslinger, M., 1995. DNA-binding and transactivation properties of Pax-6: three amino acids in the paired domain are responsible for the different sequence recognition of Pax-6 and BSAP (Pax-5. Mol. Cell. Biol. 15, 2858–2871. Del Bene, F., Wittbrodt, J., 2005. Cell cycle control by homeobox genes in development and disease. Semin. Cell Dev. Biol. 16, 449–460. Del Bene, F., Tessmar-Raible, K., Wittbrodt, J., 2004. Direct interaction of geminin and Six3 in eye development. Nature 427, 745–749. Deschet, K., Bourrat, F., Ristoratore, F., Chourrout, D., Joly, J.-S., 1999. Expression of the medaka (Oryzias latipes) Ol-Rx3 pairedlike gene in two diencephalic derivatives, the eye and the hypothalamus. Mech. Dev. 83, 179–182. Dressler, G., Deutsch, U., Chowdhury, K., Nornes, H., Gruss, P., 1990. Pax2, a new murine paired-box-containing gene and its expression in the developing excretory system. Development 109, 787–795. Eiraku, M., Takata, N., Ishibashi, H., Kawada, M., Sakakura, E., Okuda, S., Sekiguchi, K., Adachi, T., Sasai, Y., 2011. Selforganizing optic-cup morphogenesis in three-dimensional culture. Nature 472, 51–56. England, S.J., Blanchard, G.B., Mahadevan, L., Adams, R.J., 2006. A dynamic fate map of the forebrain shows how vertebrate eyes form and explains two causes of cyclopia. Development 133, 4613–4617. Esteve, P., Lopez-Rios, J., Bovolenta, P., 2004. SFRP1 is required for the proper establishment of the eye field in the medaka fish. Mech. Dev. 121, 687–701. Esteve, P., Bovolenta, P., 2006. Secreted inducers in vertebrate eye development: more functions for old morphogens. Curr. Opin. Neurobiol. 16, 13–19. Feldman, B., Gates, M., Egan, E., Dougan, S., Rennebeck, G., Sirotkin, H., Schier, A., Talbot, W., 1998. Zebrafish organizer development and germ-layer formation require nodal-related signals. Nature 395, 181–185. Finkelstein, R., Smouse, D., Capaci, T., Spradling, A., Perrimon, N., 1990. The orthodenticle gene encodes a novel homeo domain protein involved in the development of the Drosophila nervous system and ocellar visual structures. Genes Dev. 4, 1516–1527. Gamse, J., Sive, H., 2001. Early anteroposterior division of the presumptive neurectoderm in Xenopus. Mech. Dev. 104, 21–36. Gehring, W., Ikeo, K., 1999. Pax 6: mastering eye morphogenesis and eye evolution. Trends Genet. 15, 371–377. Glaser, T., Ton, C., Mueller, R., Petzlerler, M., Oliver, C., Nevin, N., Housman, D., Maas, R., 1994. Absence of Pax6 gene mutations in Gillespie syndrome (partial aniridia, cerebellar ataxia, and mental retardation). Genomics 19, 145–148. Grindley, J., Davidson, D., Hill, R., 1995. The role of PAX6 in eye and nasal development. Development 121, 1433–1442. Gritsman, K., Zhang, J., Cheng, S., Heckscher, E., Talbot, W., Schier, A., 1999. The EGF-CFC protein one-eyed pinhead is essential for nodal signaling. Cell 97, 121–132. Halder, G., Callaerts, P., Gehring, W., 1995a. Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. Science 267, 1788–1792. Halder, G., Callaerts, P., Gehring, W., 1995b. New perspectives on eye evolution. Curr. Opin. Genet. Dev. 5, 602–609. Hallonet, M., Hollemann, T., Pieler, T., Gruss, P., 1999. Vax1, a novel homeobox-containing gene, directs development of the basal forebrain and visual system. Genes Dev. 13, 3106–3114.

356

MECHANISMS OF DEVELOPMENT

Harris, W., Hartenstein, V., 1991. Neuronal determination without cell division in Xenopus embryos. Neuron 6, 499– 515. Hatta, K., Kimmel, C., Ho, R., Walker, C., 1991. The cyclops mutation blocks specification of the floor plate of the zebrafish central nervous system. Nature 350, 339–341. Heisenberg, C.-P., Houart, C., Take-uchi, M., Rauch, G.J., Young, N., Coutinho, P., Masai, I., Caneparo, L., Concha, M.L., Geisler, R., et al, 2001. A mutation in the Gsk3-binding domain of zebrafish Masterblind/Axin1 leads to a fate transformation of telencephalon and eyes to diencephalon. Genes Dev. 15, 1427– 1434. Hemmati-Brivanlou, A., Kelly, O., Melton, D., 1994. Follistatin, an antagonist of activin, is expressed in the Spemann organizer and displays direct neuralizing activity. Cell 77, 283–295. Hill, R.E., Favor, J., Hogan, B.L., Ton, C.C., Saunders, G.F., Hanson, I.M., Prosser, J., Jordan, T., Hastie, N.D., van Heyningen, V., 1991. Mouse small eye results from mutations in a paired-like homeobox-containing gene. Nature 354, 522–525. Hirose, Y., Varga, Z., Kondoh, H., Furutani-Seiki, M., 2004. Single cell lineage and regionalization of cell populations during Medaka neurulation. Development 131, 2553–2563. Hirsch, N., Harris, W., 1997. Xenopus Pax-6 and retinal development. J. Neurobiol. 32, 45–61. Hogan, B., Horsburgh, G., Cohen, J., Hetherington, C., Fisher, G., Lyon, M., 1986. Small eyes (Sey): a homozygous lethal mutation on chromosome 2 which affects the differentiation of both lens and nasal placodes in the mouse. J. Embryol. Exp. Morph. 97, 95–110. Houart, C., Caneparo, L., Heisenberg, C., Barth, K., Take-uchi, M., Wilson, S., 2002. Establishment of the telencephalon during gastrulation by local antagonism of Wnt signaling. Neuron 35, 255–265. Ho¨ckendorf, B., Thumberger, T., Wittbrodt, J., 2012. Quantitative analysis of embryogenesis: a perspective for light sheet microscopy. Dev. Cell 23, 1111–1120. Hu, M., Easter, S., 1999. Retinal neurogenesis: the formation of the initial central patch of postmitotic cells. Dev. Biol. 207, 309–321. Huisken, J., Swoger, J., Del Bene, F., Wittbrodt, J., Stelzer, E.H.K., 2004. Optical sectioning deep inside live embryos by selective plane illumination microscopy. Science 305, 1007–1009. Itoh, M., Kudoh, T., Dedekian, M., Kim, C., Chitnis, A., 2002. A role for iro1 and iro7 in the establishment of an anteroposterior compartment of the ectoderm adjacent to the midbrainhindbrain boundary. Development 129, 2317–2327. Kay, J.N., Link, B.A., Baier, H., 2005. Staggered cell-intrinsic timing of ath5 expression underlies the wave of ganglion cell neurogenesis in the zebrafish retina. Development 132, 2573– 2585. Keller, P.J., Schmidt, A.D., Wittbrodt, J., Stelzer, E.H.K., 2008. Reconstruction of zebrafish early embryonic development by scanned light sheet microscopy. Science 322, 1065–1069. Kiecker, C., Niehrs, C., 2001. A morphogen gradient of Wnt/betacatenin signalling regulates anteroposterior neural patterning in Xenopus. Development 128, 4189–4201. Kim, C., Oda, T., Itoh, M., Jiang, D., Artinger, K., Chandrasekharappa, S., Driever, W., Chitnis, A., 2000. Repressor activity of Headless/Tcf3 is essential for vertebrate head formation. Nature 407, 913–916. Kobayashi, M., Nishikawa, K., Suzuki, T., Yamamoto, M., 2001. The homeobox protein Six3 interacts with the Groucho corepressor and acts as a transcriptional repressor in eye and forebrain formation. Dev. Biol. 232, 315–326. Kobayashi, M., Toyama, R., Takeda, H., Dawid, I., Kawakami, K., 1998. Overexpression of the forebrain-specific homeobox gene six3 induces rostral forebrain enlargement in zebrafish. Development 125, 2973–2982.

1 3 0 ( 2 0 1 3 ) 3 4 7 –3 5 8

Laessing, U., Stuermer, C., 1996. Spatiotemporal pattern of retinal ganglion cell differentiation revealed by the expression of neurolin in embryonic zebrafish. J. Neurobiol. 29, 65–74. Lagutin, O.V., Zhu, C.C., Kobayashi, D., Topczewski, J., Shimamura, K., Puelles, L., Russell, H.R.C., McKinnon, P.J., Solnica-Krezel, L., Oliver, G., 2003. Six3 repression of Wnt signaling in the anterior neuroectoderm is essential for vertebrate forebrain development. Genes Dev. 17, 368–379. Li, H., Yang, J., Jacobson, R., Pasko, D., Sundin, O., 1994. Pax-6 is first expressed in a region of ectoderm anterior to the earlyneural plate: implications for stepwise determination of the lens. Dev. Biol. 162, 181–194. Li, X., Rosenfeld, M.G., 2004. Transcription: origins of licensing control. Nature 427, 687–688. Li, X., Perissi, V., Liu, F., Rose, D.W., Rosenfeld, M.G., 2002. Tissuespecific regulation of retinal and pituitary precursor cell proliferation. Science 297, 1180–1183. Liu, W., Lagutin, O., Swindell, E., Jamrich, M., Oliver, G., 2010. Neuroretina specification in mouse embryos requires Six3mediated suppression of Wnt8b in the anterior neural plate. J. Clin. Invest. 120, 3568–3577. Livesey, F.J., Cepko, C.L., 2001. Vertebrate neural cell-fate determination: lessons from the retina. Nat. Rev. Neurosci. 2, 109–118. Loosli, F., Ko¨ster, R.W., Carl, M., Krone, A., Wittbrodt, J., 1998. Six3, a medaka homologue of the Drosophila homeobox gene sine oculis is expressed in the anterior embryonic shield and the developing eye. Mech. Dev. 74, 159–164. Loosli, F., Winkler, S., Wittbrodt, J., 1999. Six3 overexpression initiates the formation of ectopic retina. Genes Dev. 13, 649– 654. Loosli, F., Winkler, S., Burgtorf, C., Wurmbach, E., Ansorge, W., Henrich, T., Grabher, C., Arendt, D., Carl, M., Krone, A., et al, 2001. Medaka eyeless is the key factor linking retinal determination and eye growth. Development 128, 4035–4044. Loosli, F., Del Bene, F., Quiring, R., Rembold, M., Martinez-Morales, J.-R., Carl, M., Grabher, C., Iquel, C., Krone, A., Wittbrodt, B., et al, 2004. Mutations affecting retina development in Medaka. Mech. Dev. 121, 703–714. Loosli, F., Staub, W., Finger-Baier, K.C., Ober, E.A., Verkade, H., Wittbrodt, J., Baier, H., 2003. Loss of eyes in zebrafish caused by mutation of chokh/rx3. EMBO Rep. 4, 894–899. Lopez-Rios, J., Tessmar, K., Loosli, F., Wittbrodt, J., Bovolenta, P., 2003. Six3 and Six6 activity is modulated by members of the groucho family. Development 130, 185–195. Luo, L., Yang, X., Takihara, Y., Knoetgen, H., Kessel, M., 2004. The cell-cycle regulator geminin inhibits Hox function through direct and polycomb-mediated interactions. Nature 427, 749– 753. Lupo, G., Reazzoli, M., Gestri, G., Liu, Y., He, R.Q., Barsacchi, G., 2000. Homeobox genes in the genetic control of eye development. Int. J. Dev. Biol. 44, 627–636. Martinez-Morales, J.-R., Wittbrodt, J., 2009. Shaping the vertebrate eye. Curr. Opin. Genet. Dev. 19, 511–517. Martinez-Morales, J.-R., Del Bene, F., Nica, G., Hammerschmidt, M., Bovolenta, P., Wittbrodt, J., 2005. Differentiation of the vertebrate retina is coordinated by an FGF signaling center. Dev. Cell 8, 565–574. Martinez-Morales, J.-R., Rembold, M., Greger, K., Simpson, J.C., Brown, K.E., Quiring, R., Pepperkok, R., Martin-Bermudo, M.D., Himmelbauer, H., Wittbrodt, J., 2009. Ojoplano-mediated basal constriction is essential for optic cup morphogenesis. Development 136, 2165–2175. Masai, I., Stemple, D., Okamoto, H., Wilson, S., 2000. Midline signals regulate retinal neurogenesis in zebrafish. Neuron 27, 251–263. Mathers, P., Jamrich, M., 2000. Regulation of eye formation by the Rx and pax6 homeobox genes. Cell. Mol. Life Sci. 57, 186–194.

MECHANISMS OF DEVELOPMENT

Mathers, P., Grinberg, A., Mahon, K., Jamrich, M., 1997. The Rx homeobox gene is essential for vertebrate eye development. Nature 387, 603–607. McCabe, K., Gunther, E., Reh, T., 1999. The development of the pattern of retinal ganglion cells in the chick retina: mechanisms that control differentiation. Development 126, 5713–5724. Mikkola, I., Bruun, J., Bjorkoy, G., Holm, T., Johansen, T., 1999. Phosphorylation of the transactivation domain of Pax6 by extracellular signal-regulated kinase and p38 mitogenactivated protein kinase. J. Biol. Chem. 274, 15115–15126. Mori, H., Miyazaki, Y., Morita, T., Nitta, H., Mishina, M., 1994. Different spatio-temporal expressions of three otx homeoprotein transcripts during zebrafish embryogenesis. Brain Res. Mol. Brain Res. 27, 221–231. Munoz-Sanjuan, I., H-Brivanlou, A., 2001. Early posterior/ventral fate specification in the vertebrate embryo. Dev. Biol. 237, 1–17. Neumann, C., 2001. Pattern formation in the zebrafish retina. Semin. Cell Dev. Biol. 12, 485–490. Neumann, C., Nu¨sslein-Volhard, C., 2000. Patterning of the zebrafish retina by a wave of sonic hedgehog activity. Science 289, 2137–2139. Niehrs, C., 1999. Head in the WNT: the molecular nature of Spemann’s head organizer. Trends Genet. 15, 314–319. Niehrs, C., 2001. The Spemann organizer and embryonic head induction. EMBO J. 20, 631–637. Nordstrom, U., Jessell, T., Edlund, T., 2002. Progressive induction of caudal neural character by graded Wnt signaling. Nat. Neurosci. 5, 525–532. Oliver, G., Loosli, F., Ko¨ster, R., Wittbrodt, J., Gruss, P., 1996. Ectopic lens induction in fish in response to the murine homeobox gene Six3. Mech. Dev. 60, 233–239. Oliver, G., Mailhos, A., Wehr, R., Copeland, N., Jenkins, N., Gruss, P., 1995. Six3, a murine homologue of the sine oculis gene, demarcates the most anterior border of the developing neural plate and is expressed during eye development. Development 121, 4045–4055. Philips, G.T., Stair, C.N., Young Lee, H., Wroblewski, E., Berberoglu, M.A., Brown, N.L., Mastick, G.S., 2005. Precocious retinal neurons: Pax6 controls timing of differentiation and determination of cell type. Dev. Biol. 279, 308–321. Picker, A., Cavodeassi, F., Machate, A., Bernauer, S., Hans, S., Abe, G., Kawakami, K., Wilson, S.W., Brand, M., 2009. Dynamic coupling of pattern formation and morphogenesis in the developing vertebrate retina. PLoS Biol. 7, e1000214. Pu¨schel, A., Gruss, P., Westerfield, M., 1992. Sequence and expression pattern of pax-6 are highly conserved between zebrafish and mice. Development 114, 643–651. Quiring, R., Walldorf, U., Kloter, U., Gehring, W.J., 1994. Homology of the eyeless gene of Drosophila to the Small eye gene in mice and Aniridia in humans. Science 265, 785–789. Rebagliati, M., Toyama, R., Haffter, P., Dawid, I., 1998. Cyclops encodes a nodal-related factor involved in midline signaling. Proc. Natl. Acad. Sci. U.S.A. 95, 9932–9937. Rembold, M., Loosli, F., Adams, R.J., Wittbrodt, J., 2006. Individual cell migration serves as the driving force for optic vesicle evagination. Science 313, 1130–1134. Rojas-Mun˜oz, A., Dahm, R., Nu¨sslein-Volhard, C., 2005. Chokh/rx3 specifies the retinal pigment epithelium fate independently of eye morphogenesis. Dev. Biol. 288, 348–362. Sampath, K., Rubinstein, A., Cheng, A., Liang, J., Fekany, K., Solnica-Krezel, L., Korzh, V., Halpern, M., Wright, C., 1998. Induction of the zebrafish ventral brain and floorplate requires cyclops/nodal signalling. Nature 395, 185–189. Schier, A., 2003. Nodal signaling in vertebrate development. Annu. Rev. Cell Dev. Biol. 19, 589–621. Schulte, D., Furukawa, T., Peters, M., Kozak, C., Cepko, C., 1999. Misexpression of the Emx-related homeobox genes cVax and

1 3 0 ( 2 0 1 3 ) 3 4 7 –3 5 8

357

mVax2 ventralizes the retina and perturbs the retinotectal map. Neuron 24, 541–553. Seimiya, M., Gehring, W., 2000. The Drosophila homeobox gene opitx is capable of inducing ectopic eyes by an eyeless-independent mechanism. Development 127, 1879– 1886. Simeone, A., Acampora, D., Mallamaci, A., Stornaiuolo, A., Dapice, M., Nigro, V., Boncinelli, E., 1993. A vertebrate gene related to orthodenticle contains a homeodomain of the bicoid class and demarcates anterior neuroectoderm in the gastrulating mouse embryo. EMBO J. 12, 2735–2747. Sinigaglia, C., Busengdal, H., Lecle`re, L., Technau, U., Rentzsch, F., 2013. The bilaterian head patterning gene six3/6 controls aboral domain development in a cnidarian. PLoS Biol. 11, e1001488. Stern, C.D., 2006. Neural induction: 10 years on since the ‘default model’. Curr. Opin. Cell Biol. 18, 692–697. Stigloher, C., Ninkovic, J., Laplante, M., Geling, A., Tannha¨user, B., Topp, S., Kikuta, H., Becker, T.S., Houart, C., Bally-Cuif, L., 2006. Segregation of telencephalic and eye-field identities inside the zebrafish forebrain territory is controlled by Rx3. Development 133, 2925–2935. Svoboda, K., O’Shea, K., 1987. An analysis of cell shape and the neuroepithelial basal lamina during optic vesicle formation in the mouse embryo. Development 100, 185–200. Tada, S., Li, A., Maiorano, D., Mechali, M., Blow, J., 2001. Repression of origin assembly in metaphase depends on inhibition of RLFB/Cdt1 by geminin. Nat. Cell Biol. 3, 107–113. Tang, H., Singh, S., Saunders, G., 1998. Dissection of the transactivation function of the transcription factor encoded by the eye developmental gene PAX6. J. Biol. Chem. 273, 7210– 7221. Tessmar, K., Loosli, F., Wittbrodt, J., 2002. A screen for co-factors of Six3. Mech. Dev. 117, 103–113. Toy, J., Yang, J.-M., Leppert, G., Sundin, O., 1998. The Optx2 homeobox gene is expressed in early precursors of the eye and activates retina-specific genes. Proc. Natl. Acad. Sci. U.S.A. 95, 10643–10648. van de Water, S., van de Wetering, M., Joore, J., Esseling, J., Bink, R., Clevers, H., Zivkovic, D., 2001. Ectopic Wnt signal determines the eyeless phenotype of zebrafish masterblind mutant. Development 128, 3877–3888. Varga, Z., Wegner, J., Westerfield, M., 1999. Anterior movement of ventral diencephalic precursors separates the primordial eye field in the neural plate and requires cyclops. Development 126, 5533–5546. Walther, C., Gruss, P., 1991. Pax-6, a murine paired box gene, is expressed in the developing CNS. Development 113, 1435–1449. Wessely, O., Agius, E., Oelgeschlager, M., Pera, E., De Robertis, E., 2001. Neural induction in the absence of mesoderm: betacatenin-dependent expression of secreted BMP antagonists at the blastula stage in Xenopus. Dev. Biol. 234, 161–173. Wilson, S., Graziano, E., Harland, R., Jessell, T., Edlund, T., 2000. An early requirement for FGF signalling in the acquisition of neural cell fate in the chick embryo. Curr. Biol. 10, 421– 429. Wilson, S.W., Houart, C., 2004. Early steps in the development of the forebrain. Dev. Cell 6, 167–181. Winkler, S., Loosli, F., Henrich, T., Wakamatsu, Y., Wittbrodt, J., 2000. The conditional medaka mutation eyeless uncouples patterning and morphogenesis of the eye. Development 127, 1911–1919. Wittbrodt, J., Shima, A., Schartl, M., 2002. Medaka – a model organism from the far East. Nat. Rev. Genet. 3, 53–64. Wohlschlegel, J., Dwyer, B., Dhar, S., Cvetic, C., Walter, J., Dutta, A., 2000. Inhibition of eukaryotic DNA replication by geminin binding to Cdt1. Science 290, 2309–2312.

358

MECHANISMS OF DEVELOPMENT

Young, R., 1985. Cell differentiation in the retina of the mouse. Anat. Rec. 212, 199–205. Zhang, L., Mathers, P., Jamrich, M., 2000. Function of Rx, but not Pax6, is essential for the formation of retinal progenitor cells in mice. Genesis 28, 135–142. Zhu, C., Dyer, M., Uchikawa, M., Kondoh, H., Lagutin, O., Oliver, G., 2002. Six3-mediated auto repression and eye development requires its interaction with members of the

1 3 0 ( 2 0 1 3 ) 3 4 7 –3 5 8

Groucho-related family of co-repressors. Development 129, 2835–2849. Zuber, M.E., Perron, M., Philpott, A., Bang, A., Harris, W.A., 1999. Giant eyes in Xenopus laevis by overexpression of XOptx2. Cell 98, 341–352. Zuber, M.E., Gestri, G., Viczian, A.S., Barsacchi, G., Harris, W.A., 2003. Specification of the vertebrate eye by a network of eye field transcription factors. Development 130, 5155–5167.