Metamorphosis of the amphibian eye

Adeno-associated viral tools to trace neural development and connectivity across amphibians

Amphibians, by virtue of their phylogenetic position, provide invaluable insights on nervous system evolution, development, and remodeling. The genetic toolkit for amphibians, however, remains limited. Recombinant adeno-associated viral vectors (AAVs) are a powerful alternative to transgenesis for labeling and manipulating neurons. Although successful in mammals, AAVs have never been shown to transduce amphibian cells efficiently. We screened AAVs in three amphibian species-the frogs Xenopus laevis and Pelophylax bedriagae and the salamander Pleurodeles waltl-and identified at least two AAV serotypes per species that transduce neurons. In developing amphibians, AAVs labeled groups of neurons generated at the same time during development. In the mature brain, AAVrg retrogradely traced long-range projections. Our study introduces AAVs as a tool for amphibian research, establishes a generalizable workflow for AAV screening in new species, and expands opportunities for cross-species comparisons of nervous system development, function, and evolution.

Diversity and evolution of amphibian pupil shapes

Pupil constriction has important functional consequences for animal vision, yet the evolutionary mechanisms underlying diverse pupil sizes and shapes are poorly understood. We aimed to quantify the diversity and evolution of pupil shapes among amphibians and to test for potential correlations to ecology based on functional hypotheses. Using photographs, we surveyed pupil shape across adults of 1294 amphibian species, 74 families and three orders, and additionally for larval stages for all families of frogs and salamanders with a biphasic ontogeny. For amphibians with a biphasic life history, pupil shape changed in many species that occupy distinct habitats before and after metamorphosis. In addition, non-elongated (circular or diamond) constricted pupils were associated with species inhabiting aquatic or underground environments, and elongated pupils (with vertical or horizontal long axes) were more common in species with larger absolute eye sizes. We propose that amphibians provide a valuable group within which to explore the anatomical, physiological, optical and ecological mechanisms underlying the evolution of pupil shape.

Transcriptomic evidence for visual adaptation during the aquatic to terrestrial metamorphosis in leopard frogs

BACKGROUND

Differences in morphology, ecology, and behavior through ontogeny can result in opposing selective pressures at different life stages. Most animals, however, transition through two or more distinct phenotypic phases, which is hypothesized to allow each life stage to adapt more freely to its ecological niche. How this applies to sensory systems, and in particular how sensory systems adapt across life stages at the molecular level, is not well understood. Here, we used whole-eye transcriptomes to investigate differences in gene expression between tadpole and juvenile southern leopard frogs (Lithobates sphenocephalus), which rely on vision in aquatic and terrestrial light environments, respectively. Because visual physiology changes with light levels, we also tested the effect of light and dark exposure.

RESULTS

We found 42% of genes were differentially expressed in the eyes of tadpoles versus juveniles and 5% for light/dark exposure. Analyses targeting a curated subset of visual genes revealed significant differential expression of genes that control aspects of visual function and development, including spectral sensitivity and lens composition. Finally, microspectrophotometry of photoreceptors confirmed shifts in spectral sensitivity predicted by the expression results, consistent with adaptation to distinct light environments.

CONCLUSIONS

Overall, we identified extensive expression-level differences in the eyes of tadpoles and juveniles related to observed morphological and physiological changes through metamorphosis and corresponding adaptive shifts to improve vision in the distinct aquatic and terrestrial light environments these frogs inhabit during their life cycle. More broadly, these results suggest that decoupling of gene expression can mediate the opposing selection pressures experienced by organisms with complex life cycles that inhabit different environmental conditions throughout ontogeny.

Eye‐body allometry across biphasic ontogeny in anuran amphibians

Animals with biphasic lifecycles often inhabit different visual environments across ontogeny. Many frogs and toads (Amphibia: Anura) have free-living aquatic larvae (tadpoles) that metamorphose into adults that inhabit a range of aquatic and terrestrial environments. Ecological differences influence eye size across species, but these relationships have not yet been explored across life stages in an ontogenetic allometric context. We examined eye-body size scaling in a species with aquatic larvae and terrestrial adults, the common frog Rana temporaria, using a well-sampled developmental series. We found a shift in ontogenetic allometric trajectory near metamorphosis indicating prioritized growth in tadpole eyes. To explore the effects of different tadpole and adult ecologies on eye-body scaling, we expanded our taxonomic sampling to include developmental series of eleven additional anuran species. Intraspecific eye-body scaling was variable among species, with 8/12 species exhibiting a significant change in allometric slope between tadpoles and adults. Traits categorizing both tadpole ecology (microhabitat, eye position, mouth position) and adult ecology (habitat, activity pattern) across species had significant effects on allometric slopes among tadpoles, but only tadpole eye position had a significant effect among adults. Our study suggests that relative eye growth in the preliminary stages of biphasic anuran ontogenies is somewhat decoupled and may be shaped by both immediate ecological need (i.e. tadpole visual requirements) and what will be advantageous during later adult stages.

Synchrotron microtomography applied to the volumetric analysis of internal structures of Thoropa miliaris tadpoles

Amphibians are models for studying applied ecological issues such as habitat loss, pollution, disease, and global climate change due to their sensitivity and vulnerability to changes in the environment. Developmental series of amphibians are informative about their biology, and X-ray based 3D reconstruction holds promise for quantifying morphological changes during growth—some with a direct impact on the possibility of an experimental investigation on several of the ecological topics listed above. However, 3D resolution and discrimination of their soft tissues have been difficult with traditional X-ray computed tomography, without time-consuming contrast staining. Tomographic data were initially performed (pre-processing and reconstruction) using the open-source software tool SYRMEP Tomo Project. Data processing and analysis of the reconstructed tomography volumes were conducted using the segmentation semi-automatic settings of the software Avizo Fire 8, which provide information about each investigated tissues, organs or bone elements. Hence, volumetric analyses were carried out to quantify the development of structures in different tadpole developmental stages. Our work shows that synchrotron X-ray microtomography using phase-contrast mode resolves the edges of the internal tissues (as well as overall tadpole morphology), facilitating the segmentation of the investigated tissues. Reconstruction algorithms and segmentation software played an important role in the qualitative and quantitative analysis of each target structure of the Thoropa miliaris tadpole at different stages of development, providing information on volume, shape and length. The use of the synchrotron X-ray microtomography setup of the SYRMEP beamline of Elettra Synchrotron, in phase-contrast mode, allows access to volumetric data for bone formation, eye development, nervous system and notochordal changes during the development (ontogeny) of tadpoles of a cycloramphid frog Thoropa miliaris. As key elements in the normal development of these and any other frog tadpole, the application of such a comparative ontogenetic study, may hold interest to researchers in experimental and environmental disciplines.

Dynamics of perineuronal nets over amphibian metamorphosis

Extracellular matrix materials known as perineuronal nets (PNNs) have been shown to have remarkable consequences for the maturation of neural circuits and stabilization of behavior. It has been proposed that, due to the possibly long-lived biochemical nature of their components, PNNs may be an important substrate by which long-term memories are stored in the central nervous system. However, little empirical evidence exists that shows that PNNs are themselves stable once established. Thus, the question of their temporal dynamics remains unresolved. We leverage the dramatic morphological and behavioral transformations that occur during amphibian metamorphosis to show that PNNs can be highly dynamic in nature. We used established lectin histochemistry to show that PNNs undergo drastic reconstruction during the metamorphic transition. Pre-metamorphic tadpoles have abundant lectin-labeled pericellular material, which we interpret to be PNNs, surrounding neurons throughout the central nervous system. During the metamorphic transition, these structures degrade, and begin to reform in the months following metamorphosis. We show that PNN sizes and staining intensity further change over metamorphosis, suggesting compositional rearrangement. We found PNNs in brain regions with putative homology to regions in mammals with known PNN function, and in other shared regions where PNN function is unknown. Our results suggest that PNNs are susceptible to remodeling by endogenous mechanisms during development. Interpreting the roles of PNNs in circuit maturation and stability requires understanding their temporal relationship with the neurons and synapses they surround. Our work provides further impetus to investigate this relationship in tandem with developmental and behavioral studies.

A frog's eye view: Foundational revelations and future promises

From the mid-19th century until the 1980's, frogs and toads provided important research models for many fundamental questions in visual neuroscience. In the present century, they have been largely neglected. Yet they are animals with highly developed vision, a complex retina built on the basic vertebrate plan, an accessible brain, and an experimentally useful behavioural repertoire. They also offer a rich diversity of species and life histories on a reasonably restricted physiological and evolutionary background. We suggest that important insights may be gained from revisiting classical questions in anurans with state-of-the-art methods. At the input to the system, this especially concerns the molecular evolution of visual pigments and photoreceptors, at the output, the relation between retinal signals, brain processing and behavioural decision-making.

Bony labyrinth morphometry reveals hidden diversity in lungless salamanders (Family Plethodontidae): Structural correlates of ecology, development, and vision in the inner ear

Lungless salamanders (Family Plethodontidae) form a highly speciose group that has undergone spectacular adaptive radiation to colonize a multitude of habitats. Substantial morphological variation in the otic region coupled with great ecological diversity within this clade make plethodontids an excellent model for exploring the ecomorphology of the amphibian ear. We examined the influence of habitat, development, and vision on inner ear morphology in 52 plethodontid species. We collected traditional and 3D geometric morphometric measurements to characterize variation in size and shape of the otic endocast and peripheral structures of the salamander ear. Phylogenetic comparative analyses demonstrate structural convergence in the inner ear across ecologically similar species. Species that dwell in spatially complex microhabitats exhibit robust, highly curved semicircular canals suggesting enhanced vestibular sense, whereas species with reduced visual systems demonstrate reduced canal curvature indicative of relaxed selection on the vestibulo-ocular reflex. Cave specialists show parallel enlargement of auditory-associated structures. The morphological correlates of ecology among diverse species reveal underlying evidence of habitat specialization in the inner ear and suggest that there exists physiological variation in the function of the salamander ear even in the apparent absence of selective pressures on the auditory system to support acoustic behavior.

Ocular Surface Biology and Disease in Amphibians

Vision is essential for amphibians, so a healthy ocular surface is critically important. There are ocular surface abnormalities that occur predominantly in captive animals, such as corneal lipidosis, whereas others, such as UV-induced trauma or infectious and parasitic conditions, may be critical to survival for animals in the wild. It is believed that inherited defects are going to be seen in small captive populations but it may be that confined wild groups of amphibians can be just as severely affected. Anything that blinds an animal severely affects its life changes.

Dissecting the role of regulators of thyroid hormone availability in early brain development: Merits and potential of the chicken embryo model

Thyroid hormones (THs) are important mediators of vertebrate central nervous system (CNS) development, thereby regulating the expression of a wide variety of genes by binding to nuclear TH receptors. TH transporters and deiodinases are both needed to ensure appropriate intracellular TH availability, but the precise function of each of these regulators and their coaction during brain development is only partially understood. Rodent knockout models already provided some crucial insights, but their in utero development severely hampers research regarding the role of TH regulators during early embryonic stages. The establishment of novel gain- and loss-of-function techniques has boosted the position of externally developing non-mammalian vertebrates as research models in developmental endocrinology. Here, we elaborate on the chicken as a model organism to elucidate the function of TH regulators during embryonic CNS development. The fast-developing, relatively big and accessible embryo allows easy experimental manipulation, especially at early stages of brain development. Recent data on the characterisation and spatiotemporal expression pattern of different TH regulators in embryonic chicken CNS have provided the necessary background to dissect the function of each of them in more detail. We highlight some recent advances and important strategies to investigate the role of TH transporters and deiodinases in various CNS structures like the brain barriers, the cerebellum, the retina and the hypothalamus. Exploiting the advantages of this non-classical model can greatly contribute to complete our understanding of the regulation of TH bioavailability throughout embryonic CNS development.

Metamorphosis in a frog that does not have a tadpole

The evolutionary removal of the tadpole from the frog life history is a very successful strategy, particularly in the tropics. These direct developers form limbs and a frog-like head early in embryogenesis, and they have reduced or lost tadpole-specific structures, like gills, a long, coiled intestine, and tadpole teeth and jaws. Despite the apparently continuous development to the frog morphology, the direct developer, Eleutherodactylus coqui, undergoes a cryptic metamorphosis requiring thyroid hormone. As in Xenopus laevis, there is a stimulation by corticotrophin-releasing factor (CRF) and an upregulation of thyroid hormone receptor β (thrb). In addition to changes in skin and muscle, thyroid hormone stimulates yolk utilization for froglet growth from a novel tissue, the nutritional endoderm. The activities of CRF and corticosterone (CORT) in metamorphosis may provide the basis for the multiple evolutionary origins of direct development in anuran amphibians. Potential roles for maternally supplied thyroid hormone and its receptor and for deiodinases in regulating tissue sensitivity to thyroid hormone should be the subjects of future investigations.

Thyroid hormone's action on progenitor/stem cell biology: new challenge for a classic hormone?

BACKGROUND

Thyroid hormones are involved in developmental and homeostatic processes in several tissues. Their action results in different outcomes depending on the developmental stage, tissue and/or cellular context. Interestingly, their pleiotropic roles are conserved across vertebrates. It is largely documented that thyroid hormones act via nuclear receptors, the TRs, which are transcription factors and whose activity can be modulated by the local availability of the hormone T3. In the "classical view", the T3-induced physiological response depends on the expression of specific TR isoforms and the iodothyronine deiodinase selenoenzymes that control the local level of T3, thus TR activity.

SCOPE OF THE REVIEW

Recent data have clearly established that the functionality of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and propose a new and intriguing role for thyroid hormones in two selected examples.

MAJOR CONCLUSIONS

In the intestinal epithelium and the retina, TRα1 and TRβ2 are expressed at the level of the precursors where they induce cell proliferation and differentiation, respectively. Moreover, these different functions result from the integration of the hormone signal with other intrinsic pathways, which play a fundamental role in progenitor/stem cell physiology.

GENERAL SIGNIFICANCE

Taken together, the interaction of TRs with other signaling pathways, specifically in stem/progenitor cells, is a new concept that may have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Onset and dynamic expression of S100 proteins in the olfactory organ and the lateral line system in zebrafish development

In the zebrafish olfactory epithelium, three morphologically distinct olfactory neurons express different marker proteins. We utilize this feature to access developmental dynamics of one of the neuron types, the crypt cells, to determine whether they are differentiated at a stage similar to other olfactory neurons. Immunohistochemical studies using an S100 antibody that specifically recognizes crypt cells showed that S100-positive cells appear in olfactory rosettes as early as at 2day postfertilization (dpf). However, some of the rosettes did not have any S100-positive cells until 4 dpf. The number of S100-positive cells in individual rosettes increased steadily over the next 3days before it decreased significantly. There were 7.8 S100-positive cells per rosettes on average in larvae at 7 dpf. The number reduced to 2.2 at 9 dpf. A recovery to a pre-reduction level was detected in 12 dpf larvae. We also observed S100-positive cells in neuromasts of the lateral line system in 2 dpf larvae, suggesting that the crypt cells and sensory cells in the neuromasts have similar onsets of differentiation. Our data have provided a time line of differentiation of crypt cells in development of the olfactory system and demonstrated that this type of cell is differentiated at a stage similar to ciliated and microvillous olfactory neurons. A nonlinear growth trajectory of the crypt cell population in the first nine days of zebrafish development implicates a possible functional significance of crypt cells in early life stages of zebrafish.

Dynamically tunable hemispherical electronic eye camera system with adjustable zoom capability

Imaging systems that exploit arrays of photodetectors in curvilinear layouts are attractive due to their ability to match the strongly nonplanar image surfaces (i.e., Petzval surfaces) that form with simple lenses, thereby creating new design options. Recent work has yielded significant progress in the realization of such "eyeball" cameras, including examples of fully functional silicon devices capable of collecting realistic images. Although these systems provide advantages compared to those with conventional, planar designs, their fixed detector curvature renders them incompatible with changes in the Petzval surface that accompany variable zoom achieved with simple lenses. This paper describes a class of digital imaging device that overcomes this limitation, through the use of photodetector arrays on thin elastomeric membranes, capable of reversible deformation into hemispherical shapes with radii of curvature that can be adjusted dynamically, via hydraulics. Combining this type of detector with a similarly tunable, fluidic plano-convex lens yields a hemispherical camera with variable zoom and excellent imaging characteristics. Systematic experimental and theoretical studies of the mechanics and optics reveal all underlying principles of operation. This type of technology could be useful for night-vision surveillance, endoscopic imaging, and other areas that require compact cameras with simple zoom optics and wide-angle fields of view.

Thyroid hormone receptor subtype specificity for hormone-dependent neurogenesis in Xenopus laevis

Thyroid hormone (T(3)) influences cell proliferation, death and differentiation during development of the central nervous system (CNS). Hormone action is mediated by T(3) receptors (TR) of which there are two subtypes, TRalpha and TRbeta. Specific roles for TR subtypes in CNS development are poorly understood. We analyzed involvement of TRalpha and TRbeta in neural cell proliferation during metamorphosis of Xenopus laevis. Cell proliferation in the ventricular/subventricular neurogenic zones of the tadpole brain increased dramatically during metamorphosis. This increase was dependent on T(3) until mid-prometamorphosis, after which cell proliferation decreased and became refractory to T(3). Using double labeling fluorescent histochemistry with confocal microscopy we found TRalpha expressed throughout the tadpole brain, with strongest expression in proliferating cells. By contrast, TRbeta was expressed predominantly outside of neurogenic zones. To corroborate the histochemical results we transfected living tadpole brain with a Xenopus TRbeta promoter-EGFP plasmid and found that most EGFP expressing cells were not dividing. Lastly, treatment with the TRalpha selective agonist CO23 increased brain cell proliferation; whereas, treatment with the TRbeta-selective agonists GC1 or GC24 did not. Our findings support the view that T(3) acts to induce cell proliferation in the tadpole brain predominantly, if not exclusively, via TRalpha.

Thyroid hormone components are expressed in three sequential waves during development of the chick retina

BACKGROUND

Thyroid hormone (TH) is an important developmental regulator in many tissues, including the retina. TH is activated locally via deiodinase 2 (Dio2), and it is destroyed by deiodinase 3 (Dio3). The TH receptors, TRa and TRb, mediate TH activity through hormone and DNA binding, and interactions with transcription regulators.

RESULTS

In the current work, the expression of these TH components was examined in the chick retina over time. Three waves of expression were characterized and found to be correlated with critical developmental events. The first wave occurred as progenitor cells began to make photoreceptors, the second as some cell types adopted a more mature location and differentiation state, and the third as Müller glia were generated. The cell types expressing the components, as well as the kinetics of expression within the cell cycle, were defined. TRb expression initiated during G2 in progenitor cells, concomitant with NeuroD and Otx2, which are expressed in early photoreceptor cells. TRb was expressed in photoreceptor cells for several days and then was reduced in expression level, as the expression of Crx, a later photoreceptor gene, became more evident. Dio3 was expressed throughout the cell cycle in progenitor cells. TRa was in most, if not all, retinal cells. Dio2 appeared transiently in a ventral (high) to dorsal gradient, likely in a subset of photoreceptor cells.

CONCLUSION

Multiple TH components were expressed in dynamic patterns in cycling progenitor cells and photoreceptors cells across the developing chick retina. These dynamic patterns suggest that TH is playing several roles in retinal development, both within the cycling progenitor cells and possibly with respect to the timing of differentiation of photoreceptor cells.

Implications of Research on Assays to Characterize Thyroid Toxicants

Many aspects of thyroid endocrinology are very well conserved across vertebrate taxa. These aspects include thyroid hormone chemistry, the mechanism of its synthesis, and the proteins involved in these processes. In addition, the system by which the hormone is delived from the thyroid gland to target cells, including transport and regulation within the hypothalamic-pituitary-thyroid (HPT) axis, and the proteins that regulate the different components of this delivery system appear to be highly conserved across the vertebrates. Finally, the receptors that mediate thyroid hormone action and the roles thyroid hormone plays are very similar among the vertebrates. Thus, the goal of this chapter is to provide a brief synopsis of the literature supporting existing screening and testing strategies in different vertebrate taxa, and to provide insight into the strengths, weaknesses, and likely changes over time. It was determined during this review that, because of the complexity of the thyroid system, it is unlikely that current in vitro assays for thyroid toxicity will be able to sufficiently replace in vivo assays for thyroid toxicants. However, the in vitro assays serve an important purpose in providing mode of action information and could provide potential screening tools, and should continue to be developed for use. Moreover, because in vivo assays are added on to preexisting reproductive or developmental screens and tests, there are no additional animals required for the in vivo assays. Specific in vitro assays were identified for development, including the thyroid receptor binding and activation assays, and in vitro assays to evaluate thyroid hormone action. Some in vivo endpoints suggested for further research included neuronal differentiation and migration, measures of histogenesis, and measures for thyroid gland thyroid hormone content, which may be more sensitive indicators of TSH stimulation. The most commonly used endpoints currently used to monitor thyroid function are thyroid hormone levels (T3 and T4), TSH, thyroid gland weight, and thyroid histology. Thyroid endocrinology is rapidly advancing and new discoveries will certainly warrant incorporation into future assays. The development of additional endpoints that measure thyroid hormone's actions peripheral to the HPT axis and the development of new reagents for nonmammalian vertebrate species will significantly improve the ability of today's assays to detect chemicals that disrupt the thyroid system in multiple vertebrate species. It is our hope that this series of thyroid articles will provide regulators and research scientists the information needed for each individual to identify the assays and endpoints most suited for their specific purposes.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Neuromodulation and developmental plasticity in the locomotor system of anuran amphibians during metamorphosis

Metamorphosis in frogs has long fascinated laymen and scientists alike. This remarkable developmental transformation involves the simultaneous remodelling of almost every organ in the body, including the gut, associated with a switch in diet from filter feeder to predator, and the visual system, from laterally-directed monocular to forward-directed binocular vision. In the context of locomotion there is the complete loss of the tail, the main structure involved in generating thrust during swimming in larvae, and the gain of the limbs which produce rhythmic extension-flexion kicks during swimming and jumping. Here we review recent evidence from experiments utilizing novel in vitro isolated preparations of the Xenopus laevis spinal cord and brainstem which remain viable for several days and can generate motor rhythms similar to those that would normally drive locomotion in vivo. The results indicate that the developing limb circuitry is born from within the existing axial-based network, which acts like a functional scaffold. Initially the limb activity shares the same left-right alternation coordination and relatively high frequency as the tail swimming network. Only later, once the limbs are fully functional, does the limb network break free to produce left-right synchrony of limb motoneuron bursting and with a different, slower cadence than the tail-based system. During the initial formation of the limb networks nitric oxide-producing neurons appear in the spinal cord, but occupy regions other than those in which the new limb circuitry is developing. Now exogenous nitric oxide facilitates locomotor activity, in contrast to its inhibitory effects on swimming at earlier larval stages of development.

Regulation of photoreceptor gene expression by Crx-associated transcription factor network

Rod and cone photoreceptors in the mammalian retina are special types of neurons that are responsible for phototransduction, the first step of vision. Development and maintenance of photoreceptors require precisely regulated gene expression. This regulation is mediated by a network of photoreceptor transcription factors centered on Crx, an Otx-like homeodomain transcription factor. The cell type (subtype) specificity of this network is governed by factors that are preferentially expressed by rods or cones or both, including the rod-determining factors neural retina leucine zipper protein (Nrl) and the orphan nuclear receptor Nr2e3; and cone-determining factors, mostly nuclear receptor family members. The best-documented of these include thyroid hormone receptor beta2 (Tr beta2), retinoid related orphan receptor Ror beta, and retinoid X receptor Rxr gamma. The appropriate function of this network also depends on general transcription factors and cofactors that are ubiquitously expressed, such as the Sp zinc finger transcription factors and STAGA co-activator complexes. These cell type-specific and general transcription regulators form complex interactomes; mutations that interfere with any of the interactions can cause photoreceptor development defects or degeneration. In this manuscript, we review recent progress on the roles of various photoreceptor transcription factors and interactions in photoreceptor subtype development. We also provide evidence of auto-, para-, and feedback regulation among these factors at the transcriptional level. These protein-protein and protein-promoter interactions provide precision and specificity in controlling photoreceptor subtype-specific gene expression, development, and survival. Understanding these interactions may provide insights to more effective therapeutic interventions for photoreceptor diseases.

Development and neuromodulation of spinal locomotor networks in the metamorphosing frog

Metamorphosis in the anuran frog, Xenopus laevis, involves profound structural and functional transformations in most of the organism's physiological systems as it encounters a complete alteration in body plan, habitat, mode of respiration and diet. The metamorphic process also involves a transition in locomotory strategy from axial-based undulatory swimming using alternating contractions of left and right trunk muscles, to bilaterally-synchronous kicking of the newly developed hindlimbs in the young adult. At critical stages during this behavioural switch, functional larval and adult locomotor systems co-exist in the same animal, implying a progressive and dynamic reconfiguration of underlying spinal circuitry and neuronal properties as limbs are added and the tail regresses. To elucidate the neurobiological basis of this developmental process, we use electrophysiological, pharmacological and neuroanatomical approaches to study isolated in vitro brain stem/spinal cord preparations at different metamorphic stages. Our data show that the emergence of secondary limb motor circuitry, as it supersedes the primary larval network, spans a developmental period when limb circuitry is present but not functional, functional but co-opted into the axial network, functionally separable from the axial network, and ultimately alone after axial circuitry disappears with tail resorption. Furthermore, recent experiments on spontaneously active in vitro preparations from intermediate metamorphic stage animals have revealed that the biogenic amines serotonin (5-HT) and noradrenaline (NA) exert short-term adaptive control over circuit activity and inter-network coordination: whereas bath-applied 5-HT couples axial and appendicular rhythms into a single unified pattern, NA has an opposite decoupling effect. Moreover, the progressive and region-specific appearance of spinal cord neurons that contain another neuromodulator, nitric oxide (NO), suggests it plays a role in the maturation of limb locomotor circuitry. In summary, during Xenopus metamorphosis the network responsible for limb movements is progressively segregated from an axial precursor, and supra- and intra-spinal modulatory inputs are likely to play crucial roles in both its functional flexibility and maturation.

Transient expression of thyroid hormone nuclear receptor TRβ2 sets S opsin patterning during cone photoreceptor genesis

Cone photoreceptors in the murine retina are patterned by dorsal repression and ventral activation of S opsin. TR beta 2, the nuclear thyroid hormone receptor beta isoform 2, regulates dorsal repression. To determine the molecular mechanism by which TR beta 2 acts, we compared the spatiotemporal expression of TR beta 2 and S opsin from embryonic day (E) 13 through adulthood in C57BL/6 retinae. TR beta 2 and S opsin are expressed in cone photoreceptors only. Both are transcribed by E13, and their levels increase with cone genesis. TR beta 2 is expressed uniformly, but transiently, across the retina. mRNA levels are maximal by E17 at completion of cone genesis and again minimal before P5. S opsin is also transcribed by E13, but only in ventral cones. Repression in dorsal cones is established by E17, consistent with the occurrence of patterning during cone cell genesis. The uniform expression of TR beta 2 suggests that repression of S opsin requires other dorsal-specific factors in addition to TR beta 2. The mechanism by which TR beta 2 functions was probed in transgenic animals with TR beta 2 ablated, TR beta 2 that is DNA binding defective, and TR beta 2 that is ligand binding defective. These studies show that TR beta 2 is necessary for dorsal repression, but not ventral activation of S opsin. TR beta 2 must bind DNA and the ligand T3 (thyroid hormone) to repress S opsin. Once repression is established, T3 no longer regulates dorsal S opsin repression in adult animals. The transient, embryonic action of TR beta 2 is consistent with a role (direct and/or indirect) in chromatin remodeling that leads to permanent gene silencing in terminally differentiated, dorsal cone photoreceptors.

Photoreceptor differentiation during retinal development, growth, and regeneration in a metamorphic vertebrate

To test the hypothesis that growth and regeneration of the adult retina involves a mechanistic recapitulation of retinal development, the patterns of photoreceptor differentiation were investigated in the developing retina, as well as growing and regenerating adult retina, of a metamorphic vertebrate, the winter flounder. Only one opsin, of type RH2 (a "green" cone opsin), was expressed in premetamorphic (developing) retina, and a corresponding middle-wavelength visual pigment was observed. In premetamorphic retinas there was no evidence for any other cone opsins or pigments, rods, rod opsin expression, or rod visual pigment. In contrast, a rod opsin (RH1) and three cone opsins (SWS2, RH2, and LWS) were expressed in postmetamorphic (adult) retina, and these opsins were consistent with the observed repertoire of visual pigments. During postmetamorphic retinal growth and regeneration, cones were always produced before rods, but the different cone types were apparently produced simultaneously, suggesting that cone differentiation mechanisms might change after metamorphosis. The results support the hypothesis that photoreceptor differentiation during growth and regeneration of the adult retina involves a recapitulation of mechanisms that control the sequence of photoreceptor production during retinal development.

Thyroid hormone and retinal development: an emerging field

Thyroid hormone appears to play a critical, yet not fully understood, role in the development of the neuroretina. This review focuses on recent experiments in the rodent, chicken, and amphibian, with an emphasis on how the hormone and its receptor isoforms influence retinal cell proliferation and cell fate decisions. The initial results are fueling the next generation of experiments in the retina, which promise to provide insights into the mechanisms of thyroid hormone action in a wide variety of developing neural tissue.

Control of retinal growth and axon divergence at the chiasm: lessons from Xenopus

Metamorphosis in frogs is a critical developmental process through which a tadpole changes into an adult froglet. Metamorphic changes include external morphological transformations as well as important changes in the wiring of sensory organs and central nervous system. This review aims to provide an overview on the events that occur in the visual system of metamorphosing amphibians and to discuss recent studies that provide new insight into the molecular mechanisms that control changes in the retinal growth pattern as well as the formation of new axonal pathways in the central nervous system. BioEssays 23:319-326, 2001.

Localization of choline acetyltransferase in the developing and adult turtle retinas

Acetylcholine has important epigenetic roles in the developing retina. In this study, cells that expressed choline acetyltransferase (ChAT), the enzyme that synthesizes acetylcholine, were investigated in embryonic, postnatal, and adult turtle retinas by using immunofluorescence histochemistry. ChAT was present at stage 15 (S15) in cells near the vitreal surface. With the formation of the inner plexiform layer (IPL) at S18, ChAT-immunoreactive (-IR) cells were located in the inner nuclear layer (INL) and the ganglion cell layer (GCL). In the INL, presumed starburst amacrine cells were homogenous in appearance and formed a single row next to the IPL: This pattern was conserved until adulthood. In the GCL, however, there were multiple rows of ChAT-IR cells early in development, and this high density of labeled cells continued during the embryonic stages, until around birth. The high density of ChAT-IR cells in the GCL was due in part to a population of cells that expressed ChAT transiently. In postnatal stages and adult retinas, the presumed starburst amacrine ChAT-IR cells formed two mirror-like rows of homogenous cells on both borders of the IPL. Two cholinergic dendritic strata that were continuous with these cells were observed as early as S18, and their depths in the IPL were relatively stable throughout development. A third population of ChAT-IR cells was observed toward the middle of the INL around S25 and persisted into adulthood. Finally, cells in the outer nuclear layer (ONL) were ChAT-IR during the embryonic stages, were less immunoreactive during the postnatal stages, and were not immunoreactive in the adult retinas.

Ephrin-B regulates the Ipsilateral routing of retinal axons at the optic chiasm

In Xenopus tadpoles, all retinal ganglion cells (RGCs) send axons contralaterally across the optic chiasm. At metamorphosis, a subpopulation of EphB-expressing RGCs in the ventrotemporal retina begin to project ipsilaterally. However, when these metamorphic RGCs are grafted into embryos, they project contralaterally, suggesting that the embryonic chiasm lacks signals that guide axons ipsilaterally. Ephrin-B is expressed discretely at the chiasm of metamorphic but not premetamorphic Xenopus. When expressed prematurely in the embryonic chiasm, ephrin-B causes precocious ipsilateral projections from the EphB-expressing RGCs. Ephrin-B is also found in the chiasm of mammals, which have ipsilateral projections, but not in the chiasm of fish and birds, which do not. These results suggest that ephrin-B/EphB interactions play a key role in the sorting of axons at the vertebrate chiasm.

Asymmetric growth and development of the Xenopus laevis retina during metamorphosis is controlled by type III deiodinase

During the metamorphosis of the Xenopus laevis retina, thyroid hormone (TH) preferentially induces ventral ciliary marginal zone (CMZ) cells to both increase their proliferation and give rise to ipsilaterally projecting ganglion cells. Here we show that dorsal CMZ cells express type III deiodinase (D3), an enzyme that inactivates TH. The dorsal CMZ cells can be induced to proliferate if deiodinase activity is inhibited. D3 or dominant-negative thyroid hormone receptor transgenes inhibit both TH-induced proliferation of the ventral CMZ cells and the formation of the ipsilateral projection. Thus, the localized expression of D3 in the dorsal CMZ cells accounts for the asymmetric growth of the frog retina.

Amphibian metamorphosis as a model for studying the developmental actions of thyroid hormone

The thyroid hormones L-thyroxine and triiodo-L-thyronine have profound effects on postembryonic development of most vertebrates. Analysis of their action in mammals is vitiated by the exposure of the developing foetus to a number of maternal factors which do not allow one to specifically define the role of thyroid hormone (TH) or that of other hormones and factors that modulate its action. Amphibian metamorphosis is obligatorily dependent on TH which can initiate all the diverse physiological manifestations of this postembryonic developmental process (morphogenesis, cell death, re-structuring, etc.) in free-living embryos and larvae of most anurans. This article will first describe the salient features of metamorphosis and its control by TH and other hormones. Emphasis will be laid on the key role played by TH receptor (TR), in particular the phenomenon of TR gene autoinduction, in initiating the developmental action of TH. Finally, it will be argued that the findings on the control of amphibian metamorphosis enhance our understanding of the regulation of postembryonic development by TH in other vertebrate species.

Amphibian metamorphosis as a model for studying the developmental actions of thyroid hormone

The thyroid hormones L-thyroxine and triiodo-L-thyronine have profound effects on postembryonic development of most vertebrates. Analysis of their action in mammals is vitiated by the exposure of the developing foetus to a number of maternal factors which do not allow one to specifically define the role of thyroid hormone (TH) or that of other hormones and factors that modulate its action. Amphibian metamorphosis is obligatorily dependent on TH which can initiate all the diverse physiological manifestations of this postembryonic developmental process (morphogenesis, cell death, re-structuring, etc.) in free-living embryos and larvae of most anurans. This article will first describe the salient features of metamorphosis and its control by TH and other hormones. Emphasis will be laid on the key role played by TH receptor (TR), in particular the phenomenon of TR gene autoinduction, in initiating the developmental action of TH. Finally, it will be argued that the findings on the control of amphibian metamorphosis enhance our understanding of the regulation of postembryonic development by TH in other vertebrate species.

The molecular basis of thyroid hormone-dependent central nervous system remodeling during amphibian metamorphosis

Tadpole metamorphosis involves a coordinated series of changes in virtually every tissue of the body. This developmental process is induced by the single morphogen, thyroid hormone (TH). The amphibian central nervous system (CNS) is a primary target for TH, and it undergoes dramatic morphological and cytoarchitectural changes in response to the hormone. TH acts by regulating gene expression and its actions in metamorphosis are thought to result from its ability to induce tissue-specific genetic programs. Receptors for TH are ligand-dependent transcription factors whose mRNA expression is upregulated by TH during metamorphosis (receptor autoinduction). Studies on the tadpole CNS have identified four general classes of early TH response genes. These genes code for: (1) transcription factors, that are likely to be required for the expression of downstream genes (i.e. secondary response genes), (2) cellular enzymes, which carry out hormone conversions, energy transformations and may possibly mediate extranuclear effects of TH on neural cells, (3) cytoskeletal elements required for axonal development, and (4) secreted signaling molecules that control the production of TH. Recent studies suggest a critical, evolutionarily conserved role for the TH-induced transcription factor genes in controling neural cell proliferation and differentiation.

The type 2 and type 3 iodothyronine deiodinases play important roles in coordinating development in Rana catesbeiana tadpoles

In developing Rana catesbeiana tadpoles, the timing of the thyroid hormone (TH)-dependent metamorphic responses varies markedly among tissues. Yet at any one time these tissues are exposed to the same plasma concentration of TH, suggesting that TH action is regulated in part at the level of the peripheral tissues. A major factor in TH action is the intracellular level of the active TH, T3. This level is dependent not only on the plasma concentration of TH (mostly T4) but also on the intracellular activities of the type 2 5'-deiodinase (D2) and the type 3 5-deiodinase (D3), which are responsible, respectively, for generating and degrading T3. (D1 is not present in this species.) To determine whether differential expression of D2 and D3 among tissues could be a significant factor in the coordination of metamorphic events, the ontogenic profiles of the two enzyme activities and corresponding messenger RNA levels in most tissues of R. catesbeiana tadpoles have been documented. The profiles of D2 expression in tail, hindlimb, forelimb, intestine, skin, and eye differed markedly at both activity and messenger RNA levels, but it was notable that expression was invariably highest in a given tissue at the time of its major metamorphic change. D2 expression was very low in brain and heart and did not vary during development. D2 was not expressed in liver, kidney, or red blood cells. With the exception of red blood cells, D3 expression was detected in all tissues studied. Furthermore, it was evident that in tissues that expressed both deiodinase genes, the two expression profiles were comparable, indicating a potential for tight control of intracellular T3 levels. Direct evidence of the importance of the intracellular conversion of T4 to T3 for TH-dependent metamorphic events was obtained in tadpoles in which endogenous TH synthesis was blocked with methimazole, and the activities of D2 and D3 were inhibited by iopanoic acid. This treatment inhibited metamorphosis. The inhibition could be overcome by the concomitant administration of replacement levels of T3, but not T4. These results strongly support the view that coordinated development in amphibia depends in part on the tissue-specific expression patterns of the D2 and D3 genes, which ensure that the requisite level of intracellular T3 is attained in a given tissue, regardless of the current level of circulating TH, at the appropriate stage of metamorphosis.

Thyroid hormone-dependent gene expression program for Xenopus neural development

Although thyroid hormone (TH) plays a significant role in vertebrate neural development, the molecular basis of TH action on the brain is poorly understood. Using polymerase chain reaction-based subtractive hybridization we isolated 34 cDNAs for TH-regulated genes in the diencephalon of Xenopus tadpoles. Northern blots verified that the mRNAs are regulated by TH and are expressed during metamorphosis. Kinetic analyses showed that most of the genes are up-regulated by TH within 4-8 h and 13 are regulated by TH only in the brain. All cDNA fragments were sequenced and the identities of seven were determined through homology with known genes; an additional five TH-regulated genes were identified by hybridization with known cDNA clones. These include five transcription factors (including two members of the steroid receptor superfamily), a TH-converting deiodinase, two metabolic enzymes, a protein disulfide isomerase-like protein that may bind TH, a neural-specific cytoskeletal protein, and two hypophysiotropic neuropeptides. This is the first successful attempt to isolate a large number of TH-target genes in the developing vertebrate brain. The gene identities allow predictions about the gene regulatory networks underlying TH action on the brain, and the cloned cDNAs provide tools for understanding the basic molecular mechanisms underlying neural cell differentiation.

The developmental trajectory of ultraviolet photosensitivity in rainbow trout is altered by thyroxine

Small (< 30 g) juvenile rainbow trout (Oncorhynchus mykiss) possess retinal photoreceptor mechanisms sensitive to ultraviolet (UV), short (S), middle (M), and long (L) wavelengths. During normal development, UV photosensitivity is lost progressively until, by approx. 60 g, individuals are no longer sensitive in the UV. This shift in spectral sensitivity is associated with the disappearance of small accessory corner cones (ACCs) from the retinal photoreceptor cell mosaic: the UV cone mechanism is lost. Exposing small (< 16 g) rainbow trout to the thyroid hormone thyroxine (T4) for a period of 6 weeks induced a precocial loss of the UV cone mechanism that was indistinguishable from the events that occur during normal development. Six weeks after termination of hormone treatment, the same individuals that had lost their UV photosensitivity after exposure to T4 once again possessed a peak in spectral sensitivity at 360 nm. ACCs had reappeared in the retinae of these fish. After 6 weeks of exposure to thyroxine, large (> 90 g) juvenile rainbow trout, which had lost their UV photoreceptor mechanism during normal development, were once again UV photosensitive and ACCs were found in their retinae. These results imply that the UV photoreceptor mechanism, although lost at one point during development, can reappear at another time during the life history of the same individual. Thyroid hormones appear to be involved in both the loss and reappearance of UV photosensitivity.

Postembryonic development of gamma-aminobutyric acid-like immunoreactivity in the brain of the sphinx moth Manduca sexta

We have investigated the distribution of immunocytochemical staining for the neurotransmitter gamma-aminobutyric acid (GABA) in the brain of the sphinx moth Manduca sexta during larval, pupal, and adult development. In the larval brain, about 300 neurons are GABA-immunoreactive. All neuropil areas except the mushroom bodies and central complex show intense immunostaining. Only minor changes in the pattern of immunoreactivity occur during larval development. During metamorphosis, changes in immunostaining occur in two phases. Beginning in wandering fifth-instar larvae (stage W2), immunoreactivity appears in numerous neurons of the central body and optic lobe and becomes more intense during early pupal stages. At the same time, GABA-like immunoreactivity disappears in most neuropil areas of the brain and becomes faint in many immunoreactive somata. Neurons with arborizations in the ventrolateral protocerebrum, however, continue to exhibit intense immunostaining during this period, and strongly immunolabeled fibers connect these areas with the ventral nerve cord. The second phase of transformation begins around pupal stage P5/P6, when faint immunostaining appears in many previously nonimmunoreactive somata and most neuropil areas of the brain. In subsequent stages (P8-P10), this immunoreactivity disappears again in most somata, but in certain cell groups, it becomes more intense and gradually develops to the adult pattern. Most larval GABA-immunoreactive neurons appear to survive through metamorphosis into the adult. Neurons in the midbrain that acquire GABA-like immunoreactivity during metamorphosis usually lie adjacent to larval immunostained neurons, suggesting common lineages. The onsets of the two developmental phases of GABA-like immunoreactivity correlate with sharp rises in hemolymph titers of ecdysteroid hormones, suggesting a role for ecdysteroids in the regulation of GABA synthesis. We hypothesize that the disappearance of GABA in many areas of the brain starting 2 days prior to pupation dramatically alters its functional circuitry and thus may account for profound changes in the behavior of the animal.

Changing synaptic specificities in the nervous system of Caenorhabditis elegans: differentiation of the DD motoneurons

During postembryonic development, the DD motoneurons in the nematode Caenorhabditis elegans completely reorganize their pattern of synapses. Ablation of a pair of embryonic precursors results in the absence of this entire class of motoneurons. In their absence animals exhibit two developmentally distinct locomotory defects. The transition period from one defect to the other is correlated with the synaptic reorganization of the DD mns. Mutations in a gene (unc-123) have been isolated that exhibit locomotory defects similar to those of the ablated adult animals. Genetic and cellular analyses of one of these alleles suggest that the unc-123 gene product may be involved in the reestablishment of functional synapses in these neurons.

Gene expression during metamorphosis: an ideal model for post-embryonic development

The precocious induction in vivo and in culture of insect and amphibian metamorphosis by exogenous ecdysteroids and thyroid hormones, and its retardation or inhibition by juvenile hormone and prolactin, respectively, has allowed the analysis of such diverse processes of post-embryonic development as morphogenesis, tissue remodelling, functional reorganization, and programmed cell death. Metamorphosis in vertebrates also shares many similarities with mammalian development in the late foetal and perinatal period. This review describes the regulation of expression of some of the 'adult' gene products during metamorphosis in invertebrates and vertebrates. Recent studies on metamorphosis have revealed the important role played by auto-induction of hormone receptor genes, based on which a model will be presented to explain the activation of 'downstream' genes which give rise to the adult phenotype. It will also be argued that metamorphosis is an ideal model for analyzing some of the major mechanisms governing post-embryonic development.

Thyroxine induces a precocial loss of ultraviolet photosensitivity in rainbow trout (Oncorhynchus mykiss, Teleostei)

Small (< 30 g) juvenile rainbow trout (Oncorhynchus mykiss) possess retinal photoreceptor mechanisms sensitive to ultraviolet (UV), short (S), middle (M) and long (L) wavelengths. During normal development, the sensitivity peak of the UV cone mechanism (360 nm) shifts towards the S-wavelengths (to an intermediate lambda max of 390 nm) until, at approx. 60 g, individuals are no longer sensitive in the UV (only a S-wavelength peak at 430 nm remains). This shift in spectral sensitivity is associated with the loss of small accessory corner cones from the retinal photoreceptor cell mosaic. Treating small (< 30 g) rainbow trout with thyroid hormone induced a precocial loss of UV photosensitivity and an associated change in the retinal photoreceptor cell mosaic, identical to the events that occur during normal development.

Ipsilateral retinal projections into the tectum during regeneration of the optic nerve in the cichlid fish Haplochromis burtoni: a Dil study in fixed tissue

Retinal projections were experimentally manipulated in a bony fish to reveal conditions under which considerably enlarged ipsilateral projections developed and persisted. Three experimental groups were studied: animals after unilateral enucleation, after unilateral nerve crush, and after enucleation and crush of the remaining optic nerve. At 29 days after unilateral enucleation alone, no enhanced ipsilateral projection had developed. After nerve crush, however, large numbers of retinal fibers regenerated into the ipsilateral tectum. Retrogradely filled, ipsilaterally projecting ganglion cells were distributed throughout the entire retina. After 15 days regenerating retinal fibers covered the entire ipsilateral tectum. At later stages the ipsilateral projection showed progressive reduction in coverage of the tectum. Combining enucleation with nerve crush led to an ipsilateral projection that covered the tectum at 28 days and later. In this experimental situation the development of an ipsilateral projection appears to be a two-step process: (1) Fibers are rerouted to the ipsilateral side at the diencephalon, and (2) ipsilateral fibers persist in the tectum only in the absence of a contralateral projection while they appear to be eliminated in the other cases.

The development of ipsilateral retinal projections into the tectum in the cichlid fish Haplochromis burtoni: a Dil study in fixed tissue

The normal development of the retinal projection was studied in a bony fish with Dil. Between 5.5 and 10 days postfertilization the contralateral retinal projection grows from the rostral pole of the tectum across its center. A maximum of 15 retinal fibers reaches the ipsilateral tectum. In 33-day-old juvenile animals, less than 15 ipsilateral fibers terminate in the entire tectum. Ipsilaterally projecting ganglion cells (maximal number = 20 cells) are scattered throughout the entire retina, and the location of ganglion cells in the retina and axonal terminations in the tectum display a large interindividual variability. This suggests that the small adult contingent of ipsilateral fibers in this bony fish develops without an initial exuberant ipsilateral retinal projection that is later pruned back.

The eye in the brain: retinoic acid effects morphogenesis of the eye and pathway selection of axons but not the differentiation of the retina in Xenopus laevis

We have analyzed the effects of all-trans retinoic acid (RA) on the morphogenesis, differentiation and projection of the eye of Xenopus. RA was applied in concentrations of 10(-5), 5 x 10(-6) and 10(-6) M at stages 9-17. Animals were reared until stages 40-48. RA applied before stage 11 1/2, abated completely formation of an eye or a retina, at later stages it led to the formation of microphthalmic eyes. Even in the absence of an eye parts of the forebrain had characteristics of the retina, but rods and cones reached then into the lumen of the third ventricle. The projection of eyes of RA-treated animals was revealed with rhodamine dextran amine. Ganglion cell axons projected bilaterally to the tectum, to the hindbrain, the contralateral retina and, occasionally, to the olfactory bulb. RA affects both morphogenesis of the eye and pathway selectivity of ganglion cell axons but not differentiation of the neural retina.

The evolution of metamorphosis in amphibians

A survey is provided of the external transformations that coincide with metamorphosis or a water-to-land transition, and of transformations during water-to-land transition in the retinal projection, the brain stem, the lateral-line system, and the inner ear of amphibians. Among the three orders of amphibians, the frogs are characterized by more pronounced transformations during the water-to-land transition than are the other two orders. Some of the progressive and regressive changes in the sensory and nervous system are presented and a scenario is suggested for the evolution of these transformations among amphibians. Suggestions that metamorphosis in frogs can recapitulate the water-to-land transition of ancestral amniotic vertebrates are refuted.

Neurometamorphosis

This article introduces this special issue of the Journal of Neurobiology by reviewing several basic issues in metamorphosis as they specifically relate to the nervous system. It promotes the idea that metamorphic changes in the nervous system (neurometamorphosis) represent adaptive restructurings rather than recapitulations of evolutionary transitions. It introduces, but leaves unresolved, the question of whether neurometamorphosis is achieved primarily as a delayed phase of embryonic neurogenesis or as a special neurogenic period. It points out that respecification of old neurons and the addition of new neurons are the main contributory pathway of neural restructuring at metamorphosis, that respecification can be dramatic and seems to be preferred over the elimination and replacement of particular neurons. It also highlights the question of how much the central rewiring during metamorphosis is driven by trophic interactions with the changing body of the metamorphic animal and to what extent neurometamorphosis is driven by the direct action of metamorphic hormones on the neural elements themselves. Finally, this article introduces the question of the cellular and molecular pathways of neurometamorphosis, from the role of the nervous system in triggering the event to the receptor mediated changes in gene expression. Further details on all of these issues are to be found in the articles that make up the rest of this special issue.