Differential addition of cells to the retina in Rana pipiens tadpoles

Tadpoles rely on mechanosensory stimuli for communication when visual capabilities are poor

The ways in which animals sense the world changes throughout development. For example, young of many species have limited visual capabilities, but still make social decisions, likely based on information gathered through other sensory modalities. Poison frog tadpoles display complex social behaviors that have been suggested to rely on vision despite a century of research indicating tadpoles have poorly-developed visual systems relative to adults. Alternatively, other sensory modalities, such as the lateral line system, are functional at hatching in frogs and may guide social decisions while other sensory systems mature. Here, we examined development of the mechanosensory lateral line and visual systems in tadpoles of the mimic poison frog (Ranitomeya imitator) that use vibrational begging displays to stimulate egg feeding from their mothers. We found that tadpoles hatch with a fully developed lateral line system. While begging behavior increases with development, ablating the lateral line system inhibited begging in pre-metamorphic tadpoles, but not in metamorphic tadpoles. We also found that the increase in begging and decrease in reliance on the lateral line co-occurs with increased retinal neural activity and gene expression associated with eye development. Using the neural tracer neurobiotin, we found that axonal innervations from the eye to the brain proliferate during metamorphosis, with few retinotectal connections in recently-hatched tadpoles. We then tested visual function in a phototaxis assay and found tadpoles prefer darker environments. The strength of this preference increased with developmental stage, but eyes were not required for this behavior, possibly indicating a role for the pineal gland. Together, these data suggest that tadpoles rely on different sensory modalities for social interactions across development and that the development of sensory systems in socially complex poison frog tadpoles is similar to that of other frog species.

Comparative Biology of Vertebrate Retinal Regeneration: Restoration of Vision through Cellular Reprogramming

The regenerative capacity of the vertebrate retina varies substantially across species. Whereas fish and amphibians can regenerate functional retina, mammals do not. In this perspective piece, we outline the various strategies nonmammalian vertebrates use to achieve functional regeneration of vision. We review key differences underlying the regenerative potential across species including the cellular source of postnatal progenitors, the diversity of cell fates regenerated, and the level of functional vision that can be achieved. Finally, we provide an outlook on the field of engineering the mammalian retina to replace neurons lost to injury or disease.

Retinal Stem/Progenitor Cells Derived From Adult Müller Glia for the Treatment of Retinal Degeneration

Over the past two decades, progress in our understanding of glial function has been revolutionary. Within the retina, a subset of glial cells termed the “Müller glia (MG),” have been demonstrated to play key roles in retinal homeostasis, structure and metabolism. Additionally, MG have also been shown to possess the regenerative capacity that varies across species. In teleost fish, MG respond to injury by reprogramming into stem-like cells capable of regenerating lost tissue. The expression of stem/progenitor cell markers has been demonstrated broadly in mammalian MG, including human MG, but their in vivo regenerative capacity appears evolutionarily limited. Advances in stem cell therapy have progressively elucidated critical mechanisms underlying innate MG reprogramming in teleost fish, which have shown promising results when applied to rodents. Furthermore, when cultured ex vivo, MG from mammals can differentiate into several retina cell types. In this review, we will explore the reparative and regenerative potential of MG in cellular therapy approaches, and outline our current understanding of embryonic retinal development, the stem-cell potential of MG in adult vertebrate retina (including human), and microenvironmental cues that guide MG reprogramming.

Prospects for the application of Müller glia and their derivatives in retinal regenerative therapies

Neural cell death is the main feature of all retinal degenerative disorders that lead to blindness. Despite therapeutic advances, progression of retinal disease cannot always be prevented, and once neuronal cell damage occurs, visual loss cannot be reversed. Recent research in the stem cell field, and the identification of Müller glia with stem cell characteristics in the human eye, have provided hope for the use of these cells in retinal therapies to restore vision. Müller glial cells, which are the major structural cells of the retina, play a very important role in retinal homeostasis during health and disease. They are responsible for the spontaneous retinal regeneration observed in zebrafish and lower vertebrates during early postnatal life, and despite the presence of Müller glia with stem cell characteristics in the adult mammalian retina, there is no evidence that they promote regeneration in humans. Like many other stem cells and neurons derived from pluripotent stem cells, Müller glia with stem cell potential do not differentiate into retinal neurons or integrate into the retina when transplanted into the vitreous of experimental animals with retinal degeneration. However, despite their lack of integration, grafted Müller glia have been shown to induce partial restoration of visual function in spontaneous or induced experimental models of photoreceptor or retinal ganglion cell damage. This improvement in visual function observed after Müller cell transplantation has been ascribed to the release of neuroprotective factors that promote the repair and survival of damaged neurons. Due to the development and availability of pluripotent stem cell lines for therapeutic uses, derivation of Müller cells from retinal organoids formed by iPSC and ESC has provided more realistic prospects for the application of these cells to retinal therapies. Several opportunities for research in the regenerative field have also been unlocked in recent years due to a better understanding of the genomic and proteomic profiles of the developing and regenerating retina in zebrafish, providing the basis for further studies of the human retina. In addition, the increased interest on the nature and function of cellular organelle release and the characterization of molecular components of exosomes released by Müller glia, may help us to design new approaches that could be applied to the development of more effective treatments for retinal degenerative diseases.

Potential Endogenous Cell Sources for Retinal Regeneration in Vertebrates and Humans: Progenitor Traits and Specialization

Retinal diseases often cause the loss of photoreceptor cells and, consequently, impairment of vision. To date, several cell populations are known as potential endogenous retinal regeneration cell sources (RRCSs): the eye ciliary zone, the retinal pigment epithelium, the iris, and Müller glia. Factors that can activate the regenerative responses of RRCSs are currently under investigation. The present review considers accumulated data on the relationship between the progenitor properties of RRCSs and the features determining their differentiation. Specialized RRCSs (all except the ciliary zone in low vertebrates), despite their differences, appear to be partially "prepared" to exhibit their plasticity and be reprogrammed into retinal neurons due to the specific gene expression and epigenetic landscape. The "developmental" characteristics of RRCS gene expression are predefined by the pathway by which these cell populations form during eye morphogenesis; the epigenetic features responsible for chromatin organization in RRCSs are under intracellular regulation. Such genetic and epigenetic readiness is manifested in vivo in lower vertebrates and in vitro in higher ones under conditions permissive for cell phenotype transformation. Current studies on gene expression in RRCSs and changes in their epigenetic landscape help find experimental approaches to replacing dead cells through recruiting cells from endogenous resources in vertebrates and humans.

Comparing Sensory Organs to Define the Path for Hair Cell Regeneration

Deafness or hearing deficits are debilitating conditions. They are often caused by loss of sensory hair cells or defects in their function. In contrast to mammals, nonmammalian vertebrates robustly regenerate hair cells after injury. Studying the molecular and cellular basis of nonmammalian vertebrate hair cell regeneration provides valuable insights into developing cures for human deafness. In this review, we discuss the current literature on hair cell regeneration in the context of other models for sensory cell regeneration, such as the retina and the olfactory epithelium. This comparison reveals commonalities with, as well as differences between, the different regenerating systems, which begin to define a cellular and molecular blueprint of regeneration. In addition, we propose how new technical advances can address outstanding questions in the field.

Distribution and density of mixed-input ON bipolar cells of the goldfish (Carassius auratus) during growth

Neurons are continuously produced at different rates and locations in the teleost retina. Goldfish rods are homogeneously distributed and maintain a stable density throughout growth, whereas little is known about their postsynaptic partners. We examined the distribution and density of mixed-input ON bipolar cells (ON mBCs) in 57 goldfish of various sizes by immunolabeling their retinas with an antibody against PKCα and counting PKCα-positive neurons in wholemounts. Cell densities were correlated with morphometric data for the same animals, and the spatial resolution of the ON mBC mosaic was calculated in each case. The distribution of ON mBCs is homogeneous throughout growth. For a 10-fold change in body size (i.e., from 20 to 200 mm), the total number of ON mBCs increases 2.8 times, while retinal area expands around 10 times. As a consequence, the density of ON mBCs in large fish falls to ∼1/3 of that of small animals, and intercellular spacing doubles. The eye and the lens become around three times larger from small to large fish. This causes the retinal magnification factor (and thereby the image projected onto retina) to augment by the same amount. Because the retinal magnification factor rises more than the intercellular spacing in the same animals, the spatial resolution of the ON mBC mosaic improves from 0.8 to 1.4 cycles/degree as the body size increases from 20 to 200 mm. As ON mBCs are mostly rod-driven, our results suggest that the scotopic acuity of the goldfish may improve as the animal grows.

Variations in the proliferative activity of the peripheral retina correlate with postnatal ocular growth in squamate reptiles

The retina is a complex, multilayered tissue responsible for the perception of visual stimuli from the environment. Contrary to mammals, the capacity for postnatal eye growth in fish and amphibians, and to a lower extent in birds, is coordinated with a progenitor population residing in the ciliary marginal zone (CMZ) at the retinal peripheral margin. However, little is known about embryonic retinogenesis and postnatal retinal growth in squamates (lizards, snakes), despite their exceptional array of ecologies and ocular morphologies. Here, we address this gap by performing the first large‐scale study assessing both ontogenetic and adult changes in the stem/progenitor activity of the squamate peripheral retina. Our study reveals for the first time that squamates exhibit a source of proliferating progenitors persisting post embryogenesis in a newly identified retinociliary junction anteriorly adjacent to the retina. This region is strikingly similar to the vertebrate CMZ by its peripheral location and pseudostratified nature, and shares a common pattern of slow‐cycling cells, spatial differentiation gradient, and response to postnatal ocular growth. Additionally, its proliferative activity varies considerably among squamate species, in correlation with embryonic and postnatal differences in eye size and growth. Together our data indicate that squamates possess a proliferative peripheral retina that acts as a source of progenitors to compensate, at least in part, for postnatal ocular growth. Our findings also highlight the remarkable variation in activity and location of vertebrate retinal progenitors, indicating that the currently accepted scenario of reduced CMZ activity over the course of evolution is too simplistic.

Müller Cells Derived from Adult Chicken and Mouse Retina Neurospheres Acquire the Dopaminergic Phenotype

Neurospheres prepared from multipotent progenitors in the retina obtained from postnatal mice differentiate into neurons and Müller glia (De Melo Reis et al., in Cell Mol Neurobiol 31:835-846, 2011). Here, we investigated whether neurospheres prepared from adult chickens (ciliary marginal zone, CMZ) or (ciliary body) retina could also lead to differentiated neurons and glia. Neurospheres were prepared from post-hatched chickens or from adult mice after 7 days in the presence of mitogenic factors (FGFb, insulin, and EGF), generating neurons and glial cells. In addition, Müller (2M6 or glutamine synthetase positive cells) derived from post-hatch chicken CMZ neurospheres displayed the dopaminergic phenotype. Furthermore, we observed that Müller cells derived from adult chickens and mice retina neurospheres released significant amounts of dopamine as well as of its metabolites. Taken together, our data lead us to conclude that as for embryonic (chick) or newborn (mouse), the dopaminergic phenotype is a default condition of Müller glial cells obtained from neurospheres prepared from mature retina. Our data raise the possibility that Müller cells from differentiated tissue could be used to ameliorate neurodegenerative diseases involving dopaminergic dysfunction as in Parkinson's disease as shown previously (Stutz et al., in J Neurochem 128:829-840, 2014).

Notch signalling patterns retinal composition by regulating atoh7 during post-embryonic growth

Patterning of a continuously growing naive field in the context of a life-long growing organ such as the teleost eye is of high functional relevance. Intrinsic and extrinsic signals have been proposed to regulate lineage specification in progenitors that exit the stem cell niche in the ciliary marginal zone (CMZ). The proper cell-type composition arising from those progenitors is a prerequisite for retinal function. Our findings in the teleost medaka (Oryzias latipes) uncover that the Notch-Atoh7 axis continuously patterns the CMZ. The complement of cell types originating from the two juxtaposed progenitors marked by Notch or Atoh7 activity contains all constituents of a retinal column. Modulation of Notch signalling specifically in Atoh7-expressing cells demonstrates the crucial role of this axis in generating the correct cell-type proportions. After transiently blocking Notch signalling, retinal patterning and differentiation is re-initiated de novo. Taken together, our data show that Notch activity in the CMZ continuously structures the growing retina by juxtaposing Notch and Atoh7 progenitors that give rise to distinct complementary lineages, revealing coupling of de novo patterning and cell-type specification in the respective lineages.

Summary: Mutually exclusive activity of Notch and Atoh7 in the ciliary marginal zone gives rise to two distinct lineages resulting in specification of the full complement of cell types in medaka retina.

Identification of retinal homeobox (rax) gene-dependent genes by a microarray approach: The DNA endoglycosylase neil3 is a major downstream component of the rax genetic pathway

BACKGROUND

The retinal homeobox (rx/rax) gene is a transcription factor expressed in the developing eye field that is necessary for normal eye development. rax is necessary for retinal specification and stem cell development. The genetic program of early retinal development, including rax expression, can be induced in naïve ectoderm by activation of insulin-like growth factor (IGF) signaling. We have undertaken a microarray-based approach to identify rax-dependent IGF-induced genes.

RESULTS

We identified 21 IGF-induced genes that exhibit at least a two-fold decrease in expression when rax expression is knocked down. Ten of these genes were expressed in the developing eye, eight were expressed in the ciliary marginal zone of the mature tadpole retina, and four could significantly rescue the rax knockdown phenotype. One of these, the nei endonuclease VIII-like 3 (neil3) gene, rescued the rax knockdown phenotype to a remarkable degree. We found that neil3 is necessary for normal retinal lamination and retinal neuron differentiation.

CONCLUSIONS

We have identified neil3 as a component of the rax genetic pathway necessary for normal retinal progenitor cell development. neil3 is involved in the base excision DNA repair pathway, suggesting that this pathway is essential for normal rax-dependent progenitor cell development in the mature retina. Developmental Dynamics 247:1199-1210, 2018. © 2018 Wiley Periodicals, Inc.

Spatio-temporal pattern of neuronal differentiation in the Drosophila visual system: A user's guide to the dynamic morphology of the developing optic lobe

Visual information processing in animals with large image forming eyes is carried out in highly structured retinotopically ordered neuropils. Visual neuropils in Drosophila form the optic lobe, which consists of four serially arranged major subdivisions; the lamina, medulla, lobula and lobula plate; the latter three of these are further subdivided into multiple layers. The visual neuropils are formed by more than 100 different cell types, distributed and interconnected in an invariant highly regular pattern. This pattern relies on a protracted sequence of developmental steps, whereby different cell types are born at specific time points and nerve connections are formed in a tightly controlled sequence that has to be coordinated among the different visual neuropils. The developing fly visual system has become a highly regarded and widely studied paradigm to investigate the genetic mechanisms that control the formation of neural circuits. However, these studies are often made difficult by the complex and shifting patterns in which different types of neurons and their connections are distributed throughout development. In the present paper we have reconstructed the three-dimensional architecture of the Drosophila optic lobe from the early larva to the adult. Based on specific markers, we were able to distinguish the populations of progenitors of the four optic neuropils and map the neurons and their connections. Our paper presents sets of annotated confocal z-projections and animated 3D digital models of these structures for representative stages. The data reveal the temporally coordinated growth of the optic neuropils, and clarify how the position and orientation of the neuropils and interconnecting tracts (inner and outer optic chiasm) changes over time. Finally, we have analyzed the emergence of the discrete layers of the medulla and lobula complex using the same markers (DN-cadherin, Brp) employed to systematically explore the structure and development of the central brain neuropil. Our work will facilitate experimental studies of the molecular mechanisms regulating neuronal fate and connectivity in the fly visual system, which bears many fundamental similarities with the retina of vertebrates.

Retinal Degeneration and Regeneration-Lessons From Fishes and Amphibians

PURPOSE OF REVIEW

Retinal degenerative diseases have immense socio-economic impact. Studying animal models that recapitulate human eye pathologies aids in understanding the pathogenesis of diseases and allows for the discovery of novel therapeutic strategies. Some non-mammalian species are known to have remarkable regenerative abilities and may provide the basis to develop strategies to stimulate self-repair in patients suffering from these retinal diseases.

RECENT FINDINGS

Non-mammalian organisms, such as zebrafish and Xenopus, have become attractive model systems to study retinal diseases. Additionally, many fish and amphibian models of retinal cell ablation and cell lineage analysis have been developed to study regeneration. These investigations highlighted several cellular sources for retinal repair in different fish and amphibian species. Moreover, major differences in repair mechanisms have been reported in these animal models.

SUMMARY

This review aims to emphasize first on the importance of zebrafish and Xenopus models in studying the pathogenesis of retinal diseases and, second, on the different modes of regeneration processes in these model organisms.

Comparative analysis of glucagonergic cells, glia, and the circumferential marginal zone in the reptilian retina

Retinal progenitors in the circumferential marginal zone (CMZ) and Müller glia-derived progenitors have been well described for the eyes of fish, amphibians, and birds. However, there is no information regarding a CMZ and the nature of retinal glia in species phylogenetically bridging amphibians and birds. The purpose of this study was to examine the retinal glia and investigate whether a CMZ is present in the eyes of reptilian species. We used immunohistochemical analyses to study retinal glia, neurons that could influence CMZ progenitors, the retinal margin, and the nonpigmented epithelium of ciliary body of garter snakes, queen snakes, anole lizards, snapping turtles, and painted turtles. We compare our observations on reptile eyes to the CMZ and glia of fish, amphibians, and birds. In all species, Sox9, Pax6, and the glucocorticoid receptor are expressed by Müller glia and cells at the retinal margin. However, proliferating cells were found only in the CMZ of turtles and not in the eyes of anoles and snakes. Similar to eyes of chickens, the retinal margin in turtles contains accumulations of GLP1/glucagonergic neurites. We find that filamentous proteins, vimentin and GFAP, are expressed by Müller glia, but have different patterns of subcellular localization in the different species of reptiles. We provide evidence that the reptile retina may contain nonastrocytic inner retinal glial cells, similar to those described in the avian retina. We conclude that the retinal glia, glucagonergic neurons, and CMZ of turtles appear to be most similar to those of fish, amphibians, and birds.

Early divergence of central and peripheral neural retina precursors during vertebrate eye development

BACKGROUND

During development of the vertebrate eye, optic tissue is progressively compartmentalized into functionally distinct tissues. From the central to the peripheral optic cup, the original optic neuroepithelial tissue compartmentalizes, forming retina, ciliary body, and iris. The retina can be further sub-divided into peripheral and central compartments, where the central domain is specialized for higher visual acuity, having a higher ratio and density of cone photoreceptors in most species.

RESULTS

Classically, models depict a segregation of the early optic cup into only two domains, neural and non-neural. Recent studies, however, uncovered discrete precursors for central and peripheral retina in the optic vesicle, indicating that the neural retina cannot be considered as a single unit with homogeneous specification and development. Instead, central and peripheral retina may be subject to distinct developmental pathways that underlie their specialization.

CONCLUSIONS

This review focuses on lineage relationships in the retina and revisits the historical context for segregation of central and peripheral retina precursors before overt eye morphogenesis.

Stem cell therapy for glaucoma: science or snake oil?

In recent years there has been substantial progress in developing stem cell treatments for glaucoma. As a downstream approach that targets the underlying susceptibility of retinal ganglion and trabecular meshwork cells, stem cell therapy has the potential to both replace lost, and protect damaged, cells by secreting neurotrophic factors. A variety of sources, including embryonic cells, adult cells derived from the central nervous system, and induced pluripotent stem cells show promise as therapeutic approaches. Even though safety concerns and ethical controversies have limited clinical implementation, some institutions have already commercialized stem cell therapy and are using direct-to-consumer advertising to attract patients with glaucoma. We review the progress of stem cell therapy and its current commercial availability.

Bone morphogenetic proteins and secreted frizzled related protein 2 maintain the quiescence of adult mammalian retinal stem cells

Rare retinal stem cells (RSCs) within the ciliary epithelium at the retinal margin of the adult mouse and human eyes can divide in vitro in the absence of growth factors to generate clonal, self-renewing spheres which can generate all the retinal cell types. Since no regenerative properties are seen in situ in the adult mammalian eye, we sought to determine the factors that are involved in the repression of endogenous RSCs. We discovered that factors secreted by the adult lens and cornea block the proliferation of adult RSCs in vitro. Bone morphogenetic protein (BMP)2, BMP4, and secreted frizzled related protein 2 were identified as principal effectors of the anti-proliferative effects on RSCs. As a similar induced quiescence was observed in vitro on both mouse and human RSCs, targeting these molecules in vivo may reactivate RSCs directly in situ in the eyes of the blind.

Müller glia: Stem cells for generation and regeneration of retinal neurons in teleost fish

Adult zebrafish generate new neurons in the brain and retina throughout life. Growth-related neurogenesis allows a vigorous regenerative response to damage, and fish can regenerate retinal neurons, including photoreceptors, and restore functional vision following photic, chemical, or mechanical destruction of the retina. Müller glial cells in fish function as radial-glial-like neural stem cells. During adult growth, Müller glial nuclei undergo sporadic, asymmetric, self-renewing mitotic divisions in the inner nuclear layer to generate a rod progenitor that migrates along the radial fiber of the Müller glia into the outer nuclear layer, proliferates, and differentiates exclusively into rod photoreceptors. When retinal neurons are destroyed, Müller glia in the immediate vicinity of the damage partially and transiently dedifferentiate, re-express retinal progenitor and stem cell markers, re-enter the cell cycle, undergo interkinetic nuclear migration (characteristic of neuroepithelial cells), and divide once in an asymmetric, self-renewing division to generate a retinal progenitor. This daughter cell proliferates rapidly to form a compact neurogenic cluster surrounding the Müller glia; these multipotent retinal progenitors then migrate along the radial fiber to the appropriate lamina to replace missing retinal neurons. Some aspects of the injury-response in fish Müller glia resemble gliosis as observed in mammals, and mammalian Müller glia exhibit some neurogenic properties, indicative of a latent ability to regenerate retinal neurons. Understanding the specific properties of fish Müller glia that facilitate their robust capacity to generate retinal neurons will inform and inspire new clinical approaches for treating blindness and visual loss with regenerative medicine.

Lens and retina regeneration: new perspectives from model organisms

Comparative studies of lens and retina regeneration have been conducted within a wide variety of animals over the last 100 years. Although amphibians, fish, birds and mammals have all been noted to possess lens- or retina-regenerative properties at specific developmental stages, lens or retina regeneration in adult animals is limited to lower vertebrates. The present review covers the newest perspectives on lens and retina regeneration from these different model organisms with a focus on future trends in regeneration research.

Distinct neurogenic potential in the retinal margin and the pars plana of mammalian eye

Unlike many other vertebrates, a healthy mammalian retina does not grow throughout life and lacks a ciliary margin zone capable of actively generating new neurons. The isolation of stem-like cells from the ciliary epithelium has led to speculation that the mammalian retina and/or surrounding tissues may retain neurogenic potential capable of responding to retinal damage. Using genetically altered mouse lines with varying degrees of retinal ganglion cell loss, we show that the retinal margin responds to ganglion cell loss by prolonging specific neurogenic activity, as characterized by increased numbers of Atoh7(LacZ)-expressing cells. The extent of neurogenic activity correlated with the degree of ganglion cell deficiency. In the pars plana, but not the retinal margin, cells remain proliferative into adulthood, marking the junction of pars plana and retinal margin as a niche capable of producing proliferative cells in the mammalian retina and a potential cellular source for retinal regeneration.

Using myc genes to search for stem cells in the ciliary margin of the Xenopus retina

The ciliary marginal zone (CMZ) of fish and frog retinas contains cells that proliferate throughout postembryonic development as the retina grows with increasing body size, indicating the presence of stem cells in this region. However, neither the location nor the molecular identity of retinal stem cells has been identified. Here, we show in Xenopus that c-myc and n-myc are sequentially expressed both during development and in the post-embryonic retina. The c-myc+/n-myc- cells near the extreme periphery of the CMZ cycle more slowly and preferentially retain DNA label compared to their more central cmyc+/n-myc+ neighbors which cycle rapidly and preferentially dilute DNA label. During retinal development c-myc is functionally required earlier than n-myc, and n-myc expression depends on earlier c-myc expression. The expression of c-myc but not n-myc in the CMZ depends on growth factor signaling. Our results suggest that c-myc+/n-myc- cells in the far peripheral CMZ are candidates for a niche-dependent population of retinal stem cells that give rise to more centrally located and rapidly dividing n-myc+ progenitors of more limited proliferative potential. Analysis of homologues of these genes in the zebrafish CMZ suggests that the transition from c-myc to n-myc expression might be conserved in other lower vertebrates whose retinas growth throughout life.

Promises of stem cell therapy for retinal degenerative diseases

With the development of stem cell technology, stem cell-based therapy for retinal degeneration has been proposed to restore the visual function. Many animal studies and some clinical trials have shown encouraging results of stem cell-based therapy in retinal degenerative diseases. While stem cell-based therapy is a promising strategy to replace damaged retinal cells and ultimately cure retinal degeneration, there are several important challenges which need to be overcome before stem cell technology can be applied widely in clinical settings. In this review, different types of donor cell origins used in retinal treatments, potential target cell types for therapy, methods of stem cell delivery to the eye, assessments of potential risks in stem cell therapy, as well as future developments of retinal stem cells therapy, will be discussed.

Cath6, a bHLH atonal family proneural gene, negatively regulates neuronal differentiation in the retina

Basic helix-loop-helix (bHLH) transcription factors play important roles in cell type specification and differentiation during the development of the nervous system. In this study, we identified a chicken homolog of Atonal 8/ath6 (Cath6) and examined its role in the developing retina. Unlike other Atonal-family proneural genes that induce neuronal differentiation, Cath6 was expressed in stem cell-like progenitor cells in the marginal region of the retina, and its overexpression inhibited neuronal differentiation. A Cath6 fused with a VP16 transactivation domain recapitulated the inhibitory effect of Cath6 on neuronal differentiation, indicating that Cath6 functions as a transcription activator. These results demonstrate that Cath6 constitutes a unique member of the Atonal-family of genes in that it acts as a negative regulator of neuronal differentiation.

Loss of adenomatous polyposis coli (apc) results in an expanded ciliary marginal zone in the zebrafish eye

The distal region of neural retina (ciliary marginal zone [CMZ]) contains stem cells that produce non-neural and neuronal progenitors. We provide a detailed gene expression analysis of the eyes of apc mutant zebrafish where the Wnt/beta-catenin pathway is constitutively active. Wnt/beta-catenin signaling leads to an expansion of the CMZ accompanied by a central shift of the retinal identity gene sox2 and the proneural gene atoh7. This suggests an important role for peripheral Wnt/beta-catenin signaling in regulating the expression and localization of neurogenic genes in the central retina. Retinal identity genes rx1 and vsx2, as well as meis1 and pax6a act upstream of Wnt/beta-catenin pathway activation. Peripheral cells that likely contain stem cells can be identified by the expression of follistatin, otx1, and axin2 and the lack of expression of myca and cyclinD1. Our results introduce the zebrafish apc mutation as a new model to study signaling pathways regulating the CMZ.

Adult ciliary epithelial cells, previously identified as retinal stem cells with potential for retinal repair, fail to differentiate into new rod photoreceptors

The ciliary margin in lower vertebrates is a site of continual retinal neurogenesis and a stem cell niche. By contrast, the human eye ceases retinal neuron production before birth and loss of photoreceptors during life is permanent and a major cause of blindness. The discovery of a proliferative cell population in the ciliary epithelium (CE) of the adult mammalian eye, designated retinal stem cells, raised the possibility that these cells could help to restore sight by replacing lost photoreceptors. We previously demonstrated the feasibility of photoreceptor transplantation using cells from the developing retina. CE cells could provide a renewable source of photoreceptors for transplantation. Several laboratories reported that these cells generate new photoreceptors, whereas a recent report questioned the existence of retinal stem cells. We used Nrl.gfp transgenic mice that express green fluorescent protein in rod photoreceptors to assess definitively the ability of CE cells to generate new photoreceptors. We report that CE cells expanded in monolayer cultures, lose pigmentation, and express a subset of eye field and retinal progenitor cell markers. Simultaneously, they continue to express some markers characteristic of differentiated CE and typically lack a neuronal morphology. Previously reported photoreceptor differentiation conditions used for CE cells, as well as conditions used to differentiate embryonic retinal progenitor cells (RPCs) and embryonic stem cell-derived RPCs, do not effectively activate the Nrl-regulated photoreceptor differentiation program. Therefore, we conclude that CE cells lack potential for photoreceptor differentiation and would require reprogramming to be useful as a source of new photoreceptors.

Adult retinal stem cells revisited

Recent advances in retinal stem cell research have raised the possibility that these cells have the potential to be used to repair or regenerate diseased retina. Various cell sources for replacement of retinal neurons have been identified, including embryonic stem cells, the adult ciliary epithelium, adult Müller stem cells and induced pluripotent stem cells (iPS). However, the true stem cell nature of the ciliary epithelium and its possible application in cell therapies has now been questioned, leaving other cell sources to be carefully examined as potential candidates for such therapies. The need for identification of the ontogenetic state of grafted stem cells in order to achieve their successful integration into the murine retina has been recognized. However, it is not known whether the same requirements may apply to achieve transplant cell integration into the adult human eye. In addition, the existence of natural barriers for stem cell transplantation, including microglial accumulation and abnormal extracellular matrix deposition have been demonstrated, suggesting that several obstacles need to be overcome before such therapies may be implemented. This review addresses recent scientific developments in the field and discusses various strategies that may be potentially used to design cell based therapies to treat human retinal disease.

Development and neurogenic potential of Müller glial cells in the vertebrate retina

Considerable research on normal and diseased states within the retina has focused on neurons. Recent research on glia throughout the central nervous system, including within the retina where Müller glia are the main type of glia, has provided a more in depth view of glial functions in health and disease. Glial cells have been recognized as being vital for the maintenance of a healthy tissue environment, where they actively participate in neuronal activity. More recently, Müller glia have been recognized as being very similar to retinal progenitor cells, particularly when compared at the molecular level using comprehensive expression profiling techniques. The molecular similarities, as well as the developmental events that occur at the end of the genesis period of retinal cells, have led us to propose that Müller glia are a form of late stage retinal progenitor cells. These late stage progenitor cells acquire some specialized glial functions, but do not irreversibly leave the progenitor state. Indeed, Müller glia appear to be able to behave as a progenitor in that they have been shown to proliferate and produce neurons in several instances when an acute injury has been applied to the retina. Enhancement of this response is thus an exciting strategy for retinal repair.

Neural regeneration and cell replacement: a view from the eye

Neuronal degenerations in the retina are leading causes of blindness. Like most other areas of the CNS, the neurons of the mammalian retina are not replaced following degeneration. However, in nonmammalian vertebrates, endogenous repair processes restore neurons very efficiently, even after complete loss of the retina. We describe the phenomenon of retinal regeneration in nonmammalian vertebrates and attempts made in recent years to stimulate similar regenerative processes in the mammalian retina. In addition, we review the various strategies employed to replace lost neurons in the retina and the recent use of stem cell technologies to address problems of retinal repair.

Hairy1 acts as a node downstream of Wnt signaling to maintain retinal stem cell-like progenitor cells in the chick ciliary marginal zone

In the vertebrate retina, stem cell-like progenitor cells are maintained in a distinct region called the ciliary marginal zone (CMZ). Canonical Wnt signaling regulates the maintenance of the progenitor cells in the CMZ. However, its downstream molecular mechanisms have remained largely unclear. Here, we show that chick Hairy1, an established Notch signaling effector,mediates the Wnt-dependent maintenance of CMZ progenitor cells in chicken. Interestingly, unlike other developmental contexts in which Hes gene expression is regulated by Notch signaling, Hairy1 expression in the CMZ is regulated by Wnt signaling. Hairy1 is necessary and sufficient for the expression of a set of molecular markers characteristic of the CMZ, and Wnt2b fails to induce CMZ markers when Hairy1 activity is inhibited. Furthermore,microarray analysis identifies multiple Wnt-responsive transcription factors that activate Hairy1 expression. We thus propose that Hairy1 functions as a node downstream of Wnt signaling to maintain progenitor cells in the chick CMZ.

The development of retinal ganglion cells in a tetraploid strain of Xenopus laevis: a morphological study utilizing intracellular dye injection

The morphological development of retinal ganglion cells was examined in a tetraploid strain of Xenopus frogs. The enlarged cells of the tetraploid strain facilitate the application of intracellular techniques. Using an in vitro retinal preparation and Nomarski optics, intracellular recording and dye injection were carried out under visual control on ganglion cells in central retina from 2 days of development (stage 24) to metamorphosis (stage 64). We identified three phases in the morphological differentiation of ganglion cells. During the first phase (stages 24-30), all cells were neuroepitheliallike in form and possessed robust resting potentials in the range of -35 to -60 mV, and dye-coupling was occasionally observed between neighboring cells. During the second phase of ganglion cell development (stages 31-45) the neurons had begun to elaborate axons and dendrites. These cells possessing neurites had resting potentials between -15 and -30 mV, and no dye-coupling was observed between neighbors. During the third and final phase of maturation, from stage 46 onward, three distinct morphological types of ganglion cells could be identified. Type I cells had the smallest somata and the smallest-diameter dendritic arborizations. The profusely branched dendrites of these cells ramify extensively throughout the inner plexiform layer. Type II cells had large somata, intermediate-diameter dendritic fields, and a highly elaborate dendritic branching pattern. These cells were seen to arborize within two sublamina in the inner plexiform layer. Type III cells had large somata, the largest-diameter dendritic fields, and a dendritic arbor with long primary branches but little higher-order branching. These large dendritic fields were confined to a single sublamina of the inner plexiform layer, abutting the inner nuclear layer. While most phase 3 cells showed radial axon trajectories from the soma to the optic disc, a minority of cells (1-5%) with erratic and nonradial axon trajectories were also observed. Our data provide a morphological description of ganglion cell maturation in the central retina of Xenopus. We show that very early in development (as early as stage 46) three distinct morphological types of retinal ganglion cells are present, which correspond to the three classes of ganglion cells previously described in adult Xenopus (Chung et al., '75).

Development of the retinotectal system in the direct-developing frog Eleutherodactylus coqui in comparison with other anurans

BACKGROUND

Frogs primitively have a biphasic life history with an aquatic larva (tadpole) and a usually terrestrial adult. However, direct developing frogs of the genus Eleutherodactylus have lost a free living larval stage. Many larval structures never form during development of Eleutherodactylus, while limbs, spinal cord, and an adult-like cranial musculoskeletal system develop precociously.

RESULTS

Here, I compare growth and differentiation of the retina and tectum and development of early axon tracts in the brain between Eleutherodactylus coqui and the biphasically developing frogs Discoglossus pictus, Physalaemus pustulosus, and Xenopus laevis using morphometry, immunohistochemical detection of proliferating cell nuclear antigen (PCNA) and acetylated tubulin, biocytin tracing, and in situ hybridization for NeuroD. Findings of the present study indicate that retinotectal development was greatly altered during evolution of Eleutherodactlyus mostly due to acceleration of cell proliferation and growth in retina and tectum. However, differentiation of retina, tectum, and fiber tracts in the embryonic brain proceed along a conserved slower schedule and remain temporally coordinated with each other in E. coqui.

CONCLUSION

These findings reveal a mosaic pattern of changes in the development of the central nervous system (CNS) during evolution of the direct developing genus Eleutherodactylus. Whereas differentiation events in directly interconnected parts of the CNS such as retina, tectum, and brain tracts remained coordinated presumably due to their interdependent development, they were dissociated from proliferation control and from differentiation events in other parts of the CNS such as the spinal cord. This suggests that mosaic evolutionary changes reflect the modular character of CNS development.

Wnt signaling in retinal stem cells and regeneration

In the vertebrate retina, stem cells with prolonged proliferative capacities reside in the most peripheral region, the ciliary marginal zone (CMZ), and they persist even after the functional eye has formed. These stem cells contribute to the formation of the retinal structures during the postnatal period in vivo, or can expand as neurospheres in vitro. Despite the wealth of anatomical descriptions of the characteristics of CMZ cells, molecular mechanisms for their specification or maintenance have long been uncharacterized. Recent studies provide evidence that certain secreted signaling molecules act as key regulators at multiple steps during these processes. In this review, we discuss the molecular basis for the regulation of retinal stem cells and their related cell types, especially focusing on the role of Wnt signaling.

Retinal stem cells

During the embryonic development of the eye, a group of founder cells in the optic vesicle gives rise to multipotent progenitor cells that generate all the neurons and the Müller glia of the mature retina. In most vertebrates, a small group of retinal stem cells persists at the margin of the retina, near the junction with the ciliary epithelium. In fish and amphibians, the retinal stem cells continue to produce progenitors throughout life, adding new retina to the periphery of the existing retina as the eye grows. In birds the new retinal addition is more limited, and it is absent in those mammals that have been analyzed. Nevertheless, cells from the retinal periphery and ciliary body of mammals can be isolated and grown in vitro for extended periods. Methods for the study of both embryonic progenitors and adult retinal stem cells in vitro and in vivo have led to a better understanding of retinal development, allowed for the screening of factors important in retinal growth and differentiation, and enabled the development of methods to direct stem and progenitor cells to specific fates. These methods may ultimately lead to the development of strategies for retinal repair.

Comparative Analysis of Progenitor Cells Isolated from the Iris, Pars Plana, and Ciliary Body of the Adult Porcine Eye

Photoreceptor loss causes irreversible blindness in many retinal diseases. The identification of suitable donor cell populations is of considerable interest because of their potential use to replace the photoreceptors lost in disease. Stem or progenitor cells that give rise to neurons and glia have been identified in several regions of the brain, including the embryonic retina and the ciliary epithelium of the adult eye, raising the possibility of autologous transplantation. However, there has been little systematic investigation into precisely which regions of the large mammalian adult eye give rise to such cells. Here, we show for the first time using the porcine eye the presence of progenitor cells in additional regions of the adult eye, including the pars plana and iris, regions that, in the human, are readily accessible during routine eye surgery. When cultured in the presence of growth factors, these cells proliferate to form neurospheres comprised of cells expressing retinal progenitor markers. Using an adherent monolayer culture system, these cells could be readily expanded to increase their number more than 1 million-fold and maintain a progenitor phenotype. When grown on the substrate laminin in the presence of serum, cells derived from both spheres and monolayer cultures differentiated into neurons and glia. These results suggest that a population of cells derived from the adult iris, pars plana, and ciliary body of a large mammalian species, the pig, has progenitor properties and neurogenic potential, thereby providing novel sources of donor cells for transplantation studies. Disclosure of potential conflicts of interest is found at the end of this article.

Patterning of the circumferential marginal zone of progenitors in the chicken retina

A circumferential marginal zone (CMZ) of retinal progenitors has been identified in most vertebrate classes, with the exception of mammals. Little is known about the formation of the CMZ during late stages of embryonic retinal histogenesis. Thus, the purpose of this study was to characterize the formation and patterning of the CMZ in the embryonic chicken retina. We identified progenitors by assaying for the expression of proliferating cell nuclear antigen (PCNA), N-cadherin and the nestin-related filament transitin, and newly generated cells by using BrdU-birthdating. We found that there is a gradual spatial restriction of progenitors into a discreet CMZ during late stages of embryonic development between E16 and hatching, at about E21. In addition, we found that retinal neurons remain immature for prolonged periods of time in far peripheral regions of the retina. Early markers of neuronal differentiation (such as HuC/D, calretinin and visinin) are expressed by neurons that are found directly adjacent to the CMZ. By contrast, genes (protein kinase C, calbindin, red/green opsin) that are expressed with a delay (7-10 days) after terminal mitosis in the central retina are not expressed until as many as 30 days after terminal mitosis in the far peripheral retina. We conclude that the neurons that are generated by late-stage CMZ progenitors differentiate much more slowly than neurons generated during early stages of retinal development. We propose that the microenvironment within the far peripheral retina at late stages of development permits the maintenance of a zone of progenitors and slows the differentiation of neurons.

Activation of canonical Wnt pathway promotes proliferation of retinal stem cells derived from adult mouse ciliary margin

Adult retinal stem cells represent a possible cell source for the treatment of retinal degeneration. However, only a small number of stem cells reside in the ciliary margin. The present study aimed to promote the proliferation of adult retinal stem cells via the Wnt signaling pathway. Ciliary margin cells from 8-week-old mice were dissociated and cultured to allow sphere colony formation. Wnt3a, a glycogen synthase kinase (GSK) 3 inhibitor, fibroblast growth factor (FGF) 2, and a FGF receptor inhibitor were then applied in the culture media. The primary spheres were dissociated to prepare either monolayer or secondary sphere cultures. Wnt3a increased the size of the primary spheres and the number of Ki-67-positive proliferating cells in monolayer culture. The Wnt3a-treated primary sphere cells were capable of self-renewal and gave rise to fourfold the number of secondary spheres compared with nontreated sphere cells. These cells also retained their multilineage potential to express several retinal markers under differentiating culture conditions. The Wnt3a-treated cells showed nuclear accumulation of beta-catenin, and a GSK3 inhibitor, SB216763, mimicked the mitogenic activity of Wnt3a. The proliferative effect of SB216763 was attenuated by an FGF receptor inhibitor but was enhanced by FGF2, with Ki-67-positive cells reaching over 70% of the total cells. Wnt3a and SB216763 promoted the proliferation of retinal stem cells, and this was partly dependent on FGF2 signaling. A combination of Wnt and FGF signaling may provide a therapeutic strategy for in vitro expansion or in vivo activation of adult retinal stem cells.

Wnt2b inhibits differentiation of retinal progenitor cells in the absence of Notch activity by downregulating the expression of proneural genes

During the development of the central nervous system, cell proliferation and differentiation are precisely regulated. In the vertebrate eye, progenitor cells located in the marginal-most region of the neural retina continue to proliferate for a much longer period compared to the ones in the central retina, thus showing stem-cell-like properties. Wnt2b is expressed in the anterior rim of the optic vesicles, and has been shown to control differentiation of the progenitor cells in the marginal retina. In this paper,we show that stable overexpression of Wnt2b in retinal explants inhibited cellular differentiation and induced continuous growth of the tissue. Notably,Wnt2b maintained the undifferentiated progenitor cells in the explants even under the conditions where Notch signaling was blocked. Wnt2b downregulated the expression of multiple proneural bHLH genes as well as Notch. In addition,expression of Cath5 under the control of an exogenous promoter suppressed the negative effect of Wnt2b on neuronal differentiation. Importantly, Wnt2b inhibited neuronal differentiation independently of cell cycle progression. We propose that Wnt2b maintains the naive state of marginal progenitor cells by attenuating the expression of both proneural and neurogenic genes, thus preventing those cells from launching out into the differentiation cascade regulated by proneural genes and Notch.

Neural regeneration in the chick retina

In warm-blooded vertebrates, possibilities for retinal regeneration have recently become reality with the discovery of neural stem cells in the mature eye. A number of different cellular sources of neural stem cells have been identified. These sources include stem cells at the retinal margin, pigmented cells in the ciliary body and iris, non-pigmented cells in the ciliary body and Müller glia within the retina. This review focuses on recent reports of neural stem cells and regeneration in the postnatal chicken retina. In the chicken eye sources of neurogensis and regeneration include: (1) retinal stem cells at the peripheral edge of the retina; (2) Müller glia in central regions of the retina; (3) non-pigmented epithelial cells in the posterior portion of the ciliary body; and (4) possibly pigmented cells in the pars plana of the ciliary body. This review discusses the similarities between the retinal progenitor cells in the postnatal eye and those found in the embryo. In addition, I discuss combinations of growth factors, (insulin, IGF-I, EGF and FGF2) that are capable of stimulating the proliferation and production of neurons from neural progenitors, non-neural epithelial cells, and postmitotic support cells in the avian eye. In summary, the mechanisms that regulate the proliferation and differentiation of cells with neurogenic potential are beginning to be understood and the postnatal chicken eye has proven to be a useful model system to study retinal regeneration.

Persistent and injury-induced neurogenesis in the vertebrate retina

The brains of all vertebrates are persistently neurogenic. However, this is not true for the neural retinas. Only three extant classes of vertebrates show significant posthatch/postnatal retinal neurogenesis: amphibians, birds and fish. The retinas of these animals contain an annulus of progenitors at the margin, from which differentiated neurons emerge. In posthatch amphibians and fish the vast majority of the adult retina is added from the margin and neurogenesis is lifelong, whereas in posthatch birds neurogenesis is limited. Unique to fish, rod photoreceptors are added in situ from stem cells within the mature retina. Strikingly, for each class of animal retinal lesions stimulate neuronal regeneration, however the cellular source differs for each: the retinal pigmented epithelium in amphibians and embryonic birds, Müller glia in posthatch birds and intrinsic stem cells in fish. The molecular events surrounding injury-induced neuronal regeneration are beginning to be identified.

Persistent progenitors at the retinal margin of ptc+/- mice

The hedgehog signaling pathway is a key regulator of neural development, affecting both proliferation and differentiation of neural progenitors. Sonic hedgehog (Shh) is a mitogenic factor for retinal progenitors in vitro. To determine whether this signaling system is important in vivo for regulating retinal progenitor proliferation, we analyzed mice with a single functional allele of the Shh receptor patched (ptc). We found that ptc+/- mice had increased numbers of neural progenitors at every stage of retinal development that we examined. In addition, these mice had persistent progenitors at the retinal margin for up to 3 months of age, reminiscent of the ciliary marginal zone of lower vertebrates. To test whether the progenitors at the retinal margin of ptc+/- mice could be induced to regenerate retinal neurons in response to damage, we bred ptc+/- mice onto a retinal degeneration background (pro23his rhodopsin transgenic) and labeled newly generated cells with combined immunohistochemistry for bromodeoxyuridine and retinal neuron and photoreceptor-specific markers. We found newly generated neurons and photoreceptors at the retinal margin in ptc+/-;pro23his mice. We propose that the Shh pathway may act as a regulator of both prenatal and postnatal retinal growth.

Persistence of graded EphA/Ephrin-A expression in the adult frog visual system

Many studies have demonstrated the involvement of the EphA family of receptor tyrosine kinases and their ligands, ephrin-A2 and -A5, in the development of the temporonasal axis of the retinotectal/collicular map, but the role of these molecules in optic nerve regeneration has not been well studied. Noting that the characteristic gradients of the EphA/ephrin-A family that are expressed topographically in the retina and tectum of embryonic chicks and mice tend to disappear after birth, we took as our starting point an analysis of EphA and ephrin-A expression in leopard frogs (Rana pipiens and utricularia), species capable of regenerating the retinotectal map as adults. For the EphA family to be involved in the regeneration, one would expect these topographic gradients to persist in the adult or, if downregulated after metamorphosis, to be reexpressed after optic nerve injury. Using EphA3 receptor and ephrin-A5 ligand alkaline phosphatase in situ affinity probes (RAP and LAP, respectively) in whole-mount applications, we report that reciprocally complementary gradients of RAP and LAP binding persist in the optic tract and optic tectum of postmetamorphic frogs, including mature adults. EphA expression in temporal retinal axons in the optic tract was significantly reduced after nerve section but returned during regeneration. However, ephrin-A expression in the tectal parenchyma was not significantly elevated by either eye removal, with degeneration of optic axons, or during regeneration of the retinotectal projection. Thus, the present study has demonstrated a persisting expression of EphA/ephrin-A family members in the retinal axons and tectal parenchyma that may help guide regenerating fibers, but we can offer no evidence for an upregulation of ephrin-A expression in conjunction with optic nerve injury.

Wnt2b controls retinal cell differentiation at the ciliary marginal zone

The ciliary marginal zone of the vertebrate retina contains undifferentiated progenitor cells that continue to proliferate and add new neurons and glia peripherally during the embryonic stages - even after the formation of a functional retina. To understand the molecular mechanism that controls the prolonged progenitor cell proliferation in the ciliary marginal zone, we employed a candidate molecule approach, focusing on Wnt2b (formerly know as Wnt13), which is expressed in the marginal most tip of the retina. Frizzled 4 and 5, seven-pass transmembrane Wnt receptors, were expressed in the peripheral and central part of the retina, respectively. LEF1, a downstream Wnt signaling component, was expressed at high levels in the ciliary marginal zone with expression gradually decreasing towards the central retina. The LEF1-expressing region, which is where Wnt signaling is supposedly activated, expressed a set of molecular markers that are characteristic of the progenitor cells in the ciliary marginal zone. Overexpression of Wnt2b by use of in ovo electroporation in the central retina inhibited neuronal differentiation and induced the progenitor cell markers. Blocking of the Wnt downstream signaling pathway by a dominant-negative LEF1 inhibited proliferation of the cells in the marginal area, which resulted in their premature neuronal differentiation. The progenitor cells in the ciliary marginal zone differentiated into all the neuronal and glial cell types when cultured in vitro, and they proliferated for a longer period than did centrally located progenitor cells that underwent a limited number of cell divisions. In addition, the proliferation of these progenitor cells was promoted in the presence of Wnt2b. These results suggest that Wnt2b functions to maintain undifferentiated progenitor cells in the ciliary marginal zone, and thus serves as a putative stem cell factor in the retina.

Using heterochrony plots to detect the dissociated coevolution of characters

The comparison of developmental sequences among species is notoriously difficult. Here, heterochrony plots are introduced as a new graphic method to detect temporal shifts in the development of characters in pair-wise species comparisons. Plotting the timing of character development in one species against the timing of character development in another species allows us to compare a principally unlimited number of characters simultaneously and can detect whether suites of characters are dissociated from one another or not. Such heterochrony plots can be embedded into a comparative phylogenetic analysis in order to establish whether observed patterns of character codissociation are indeed due to their dissociated coevolution. Comparative phylogenetic analysis may also reveal multiple independent events of dissociated coevolution of the same suite of characters in a certain lineage, suggesting that the characters of this suite reciprocally constrain their evolutionary modifiability, thereby forming a unit of evolution. This ability to identify units of evolution is a prerequisite for assessing the validity of recently proposed scenarios, suggesting that modules of development and/or function tend to act as units of evolution. Starting from a detailed heterochrony plot comparing development in the direct developing frog Eleutherodactylus coqui and in the biphasically developing frog Discoglossus pictus, this comparative approach is illustrated focusing on the evolution of development of limbs, the nervous system and the pharyngeal arches in amphibians.

From stem cells towards neural layers: a lesson from re-aggregated embryonic retinal cells

Cells from dissociated embryonic avian retinae have the capacity to re-aggregate in rotation culture and form cellular spheres reconstituting a complete arrangement of all retinal layers. This exquisite phenomenon is based upon in vitro proliferation of multipotent precursor stem cells and spatial organization of their differentiating descendants. The addition of soluble factors from cultured retinal pigmented epithelial (RPE) or radial glial cells is essential to revert inside-out spheres (rosetted retinal spheres) into correctly laminated outside-out spheres (stratified spheres). Such complete restoration of a laminated brain tissue by cell re-aggregation has been achieved only for the embryonic avian retina, but not the mammalian retina, nor for other brain parts. This review summarises the history of the re-aggregation approach, presents avian retinal re-aggregate models, and analyses roles of the RPE and Müller cells for successful retinal tissue regeneration. It is predicted that these results will become biomedically relevant, as stem cell biology will soon open ways to produce large amounts of human retinal precursors.