Patterns of cellular proliferation and migration in the developing tectum mesencephali of the frog Rana temporaria and the salamander Pleurodeles waltl

Genome size drives morphological evolution in organ-specific ways

Morphogenesis is an emergent property of biochemical and cellular interactions during development. Genome size and the correlated trait of cell size can influence these interactions through effects on developmental rate and tissue geometry, ultimately driving the evolution of morphology. We tested whether variation in genome and body size is related to morphological variation in the heart and liver using nine species of the salamander genus Plethodon (genome sizes 29-67 gigabases). Our results show that overall organ size is a function of body size, whereas tissue structure changes dramatically with evolutionary increases in genome size. In the heart, increased genome size is correlated with a reduction of myocardia in the ventricle, yielding proportionally less force-producing mass and greater intertrabecular space. In the liver, increased genome size is correlated with fewer and larger vascular structures, positioning hepatocytes farther from the circulatory vessels that transport key metabolites. Although these structural changes should have obvious impacts on organ function, their effects on organismal performance and fitness may be negligible because low metabolic rates in salamanders relax selective pressure on function of key metabolic organs. Overall, this study suggests large genome and cell size influence the developmental systems involved in heart and liver morphogenesis.

Spatio-temporal neural stem cell behavior leads to both perfect and imperfect structural brain regeneration in adult newts

Adult newts can regenerate large parts of their brain from adult neural stem cells (NSCs), but how adult NSCs reorganize brain structures during regeneration remains unclear. In development, elaborate brain structures are produced under broadly coordinated regulations of embryonic NSCs in the neural tube, whereas brain regeneration entails exquisite control of the re-establishment of certain brain parts, suggesting that a yet-unknown mechanism directs NSCs upon partial brain excision. Here we report that upon excision of a quarter of the adult newt (Pleurodeles waltl) mesencephalon, active participation of local NSCs around specific brain subregions' boundaries leads to some imperfect and some perfect brain regeneration along an individual's rostrocaudal axis. Regeneration phenotypes depend on how wound closing occurs using local NSCs, and perfect regeneration replicates development-like processes, but takes more than 1 year. Our findings indicate that newt brain regeneration is supported by modularity of boundary-domain NSCs with self-organizing ability in neighboring fields.This article has an associated First Person interview with the first author of the paper.

Morphogenetic and Histogenetic Roles of the Temporal-Spatial Organization of Cell Proliferation in the Vertebrate Corticogenesis as Revealed by Inter-specific Analyses of the Optic Tectum Cortex Development

The central nervous system areas displaying the highest structural and functional complexity correspond to the so called cortices, i.e., concentric alternating neuronal and fibrous layers. Corticogenesis, i.e., the development of the cortical organization, depends on the temporal-spatial organization of several developmental events: (a) the duration of the proliferative phase of the neuroepithelium, (b) the relative duration of symmetric (expansive) versus asymmetric (neuronogenic) sub phases, (c) the spatial organization of each kind of cell division, (e) the time of determination and cell cycle exit and (f) the time of onset of the post-mitotic neuronal migration and (g) the time of onset of the neuronal structural and functional differentiation. The first five events depend on molecular mechanisms that perform a fine tuning of the proliferative activity. Changes in any of them significantly influence the cortical size or volume (tangential expansion and radial thickness), morphology, architecture and also impact on neuritogenesis and synaptogenesis affecting the cortical wiring. This paper integrates information, obtained in several species, on the developmental roles of cell proliferation in the development of the optic tectum (OT) cortex, a multilayered associative area of the dorsal (alar) midbrain. The present review (1) compiles relevant information on the temporal and spatial organization of cell proliferation in different species (fish, amphibians, birds, and mammals), (2) revises the main molecular events involved in the isthmic organizer (IsO) determination and localization, (3) describes how the patterning installed by IsO is translated into spatially organized neural stem cell proliferation (i.e., by means of growth factors, receptors, transcription factors, signaling pathways, etc.) and (4) describes the morpho- and histogenetic effect of a spatially organized cell proliferation in the above mentioned species. A brief section on the OT evolution is also included. This section considers how the differential operation of cell proliferation could explain differences among species.

Triploidy alters brain morphology in pre-smolt Atlantic salmon Salmo salar: possible implications for behaviour

Total brain mass and the volumes of five specific brain regions in diploid and triploid Atlantic salmon Salmo salar pre-smolts were measured using digital images. There were no significant differences (P > 0·05) in total brain mass when corrected for fork length, or the volumes of the optic tecta or hypothalamus when corrected for brain mass, between diploids and triploids. There was a significant effect (P < 0·01) of ploidy on the volume of the olfactory bulb, with it being 9·0% larger in diploids compared with triploids. The cerebellum and telencephalon, however, were significantly larger, 17 and 8% respectively, in triploids compared with diploids. Sex had no significant effect (P > 0·05) on total brain mass or the volumes of any measured brain region. As the olfactory bulbs, cerebellum and telencephalon are implicated in a number of functions, including foraging ability, aggression and spatial cognition, these results may explain some of the behavioural differences previously reported between diploids and triploids.

In vivo time-lapse imaging of cell proliferation and differentiation in the optic tectum of Xenopus laevis tadpoles

We analyzed the function of neural progenitors in the developing central nervous system of Xenopus laevis tadpoles by using in vivo time-lapse confocal microscopy to collect images through the tectum at intervals of 2-24 hours over 3 days. Neural progenitor cells were labeled with fluorescent protein reporters based on expression of endogenous Sox2 transcription factor. With this construct, we identified Sox2-expressing cells as radial glia and as a component of the progenitor pool of cells in the developing tectum that gives rise to neurons and other radial glia. Lineage analysis of individual radial glia and their progeny demonstrated that less than 10% of radial glia undergo symmetric divisions resulting in two radial glia, whereas the majority of radial glia divide asymmetrically to generate neurons and radial glia. Time-lapse imaging revealed the direct differentiation of radial glia into neurons. Although radial glia may guide axons as they navigate to the superficial tectum, we find no evidence that radial glia function as a scaffold for neuronal migration at early stages of tectal development. Over 3 days, the number of labeled cells increased 20%, as the fraction of radial glia dropped and the proportion of neuronal progeny increased to approximately 60% of the labeled cells. Tadpoles provided with short-term visual enhancement generated significantly more neurons, with a corresponding decrease in cell proliferation. Together these results demonstrate that radial glial cells are neural progenitors in the developing optic tectum and reveal that visual experience increases the proportion of neurons generated in an intact animal.

Regional distribution and migration of proliferating cell populations in the adult brain of Hyla cinerea (Anura, Amphibia)

We examined the distribution of adult cell proliferation throughout the brain of an anuran amphibian using 5-bromo-2'-deoxyuridine (BrdU). BrdU, a thymidine analog, is a commonly used cellular marker that is incorporated into actively dividing progenitor cells. Adult green treefrogs, Hyla cinerea, received injections of BrdU and were sacrificed 2 h, 2 days, 2 weeks, or 30 days later. Immunohistochemistry revealed BrdU-immunopositive (BrdU+) cells to be distributed in ventricular zones throughout the brain. The heaviest concentrations of cells were located in the telencephalon, primarily in the ventrolateral region of the lateral ventricles, and the ventricles of olfactory bulbs. Numerous BrdU+ cells were located around the preoptic and hypothalamic recesses and few around the third ventricle in the diencephalon. Proceeding caudally towards the midbrain, there was a marked decrease in BrdU labeling and few BrdU+ cells were found in the hindbrain. Consistent with previous studies in ectothermic vertebrates, BrdU+ cells were found predominantly in the ventricular zone (VZ) and immediately adjacent to the VZ; at later time points (i.e., 30 days), the cells appeared to have migrated into parenchymal regions. The extent of cellular proliferation in anurans is similar to that of fishes and reptiles and thus is more widespread compared to mammals.

BrdU-, neuroD (nrd)- and Hu-studies reveal unusual non-ventricular neurogenesis in the postembryonic zebrafish forebrain

In the postembryonic zebrafish forebrain, subpial locations of neurogenesis do exist in the early cerebellar external granular layer, and--unusually among vertebrates--in the primordial pretectal (M1) and preglomerular (M2) Anlagen as shown here with 5-bromo-2'-deoxyuridine (BrdU)/Hu-immunocytochemistry and in situ hybridization of neuroD. An intermediate BrdU incubation time of 12-16 h reveals in addition to proliferative ventricularly located cells those in M1 and M2. This BrdU saturation-labeling shows--in conjunction with a Hu-assay demonstrating earliest neuronal differentiation--that proliferating cells in M1 and M2 represent neuronal progenitors. This is demonstrated by single BrdU-labeled and double BrdU-/Hu-labeled cells in these aggregates. Further, expression of NeuroD--a marker for freshly determined neuronal cells--confirms this unusual subpial postembryonic forebrain neurogenesis.

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.

Morphology, axonal projection pattern, and response types of tectal neurons in plethodontid salamanders. II: intracellular recording and labeling experiments

In the plethodontid salamanders Plethodon jordani and P. glutinosus, the morphology and axonal projections of 140 tectal neurons and their responses to electrical optic nerve stimulation were determined by intracellular recording and biocytin labeling. Six types of neurons are distinguished morphologically. TO1 neurons have wide dendritic trees that arborize mainly in tectal layers 1 and 3; they project bilaterally to the tegmentum and contralaterally to the medulla oblongata. TO2 neurons have very wide dendritic trees that arborize mainly in layers 2 and 3; axons project bilaterally or unilaterally to the pretectum and thalamus and ipsilaterally to the medulla oblongata. TO3 neurons have very wide and flat dendritic trees confined to layers 3-5; some have the same axonal projection as TO2 neurons, whereas others have descending axons that reach only the level of the cerebellum. TO4 neurons have narrower dendritic trees that arborize in layers 2 and 3; they project to the ipsilateral pretectum, thalamus, and medulla oblongata. TO5 neurons have dendritic trees that arborize in layers 1 and 2 or 1-3 and project bilaterally or unilaterally to the pretectum and thalamus. TO-IN are interneurons, with a number of subtypes with respect to variations in dendritic arborization pattern. TO1-TO5 neurons generally have short latencies of 2-16 ms (average = 8.4 ms) at electrical optic nerve stimulation; first responses are always excitatory, often followed by inhibition. They are likely to be mono- or oligosynaptically driven by retinal afferents. TO-IN interneurons have long latencies of 20-80 ms (average = 38.6 ms) and appear to receive no direct retinal input. With their specific dendritic arborization, consequent dominant retinal input, specific axonal projections, the different types of tectal projection neurons constitute separate ascending and descending visual pathways. Hypotheses are presented regarding the nature of the information processed by these pathways.

Morphology, axonal projection pattern, and response types of tectal neurons in plethodontid salamanders. I: tracer study of projection neurons and their pathways

In three salamander species (Hydromantes italicus, H. genei, Plethodon jordani), the tectobulbospinal and tectothalamic pathways and their cells of origin were studied by means of anterograde and retrograde biocytin and tetramethylrhodamine tracing. In plethodontid salamanders, five types of tectal projection neurons were identified. TO1 neurons have widefield dendritic trees that arborize in the layers of retinal afferents and form a neuropil in the superficial layer; axons constitute the crossed tectospinal tract. Dendrites of TO2 cells have the largest dendritic trees that arborize in the intermediate and deep layers of retinal afferents; axons constitute a lateral uncrossed tectospinal tract. TO3 cells have widefield dendritic trees that arborize in the deep layer of retinal afferents and in the layer of tectal efferents; axons constitute a superficial uncrossed tectospinal tract. TO4 cells have slender primary dendrites and small-field dendritic trees that arborize in the intermediate layers of retinal afferents; axons constitute another lateral uncrossed tectospinal tract. TO2, TO3, and TO4 cells also have ascending axons that run to the ventral and dorsal thalamus. TO5 cells have slender primary dendrites and small-field dendritic trees that extend into the superficial layers of retinal afferents; their fine axons constitute the bulk of the pathways ascending to the ipsilateral and contralateral thalamus. These morphological types of projection neurons and their ascending and descending axonal pathways closely resemble those found in frogs, reptiles, and birds. Their role in visual and visuomotor functions is discussed.

Development of the tectum in Gymnophiones, with comparison to other amphibians

Tectal development in a number of caecilian (Gymnophiona: Amphibia) species was examined and compared with that in frogs and salamanders. The caecilian optic tectum develops along the same rostrocaudal and lateromedial gradients as those of frogs and salamanders. However, differences exist in the time course of development. Our data suggest that, as in salamanders, simplification of morphological complexity in caecilians is due to a retardation or loss of late developmental stages. Differences in the time course of development (heterochrony) among different caecilian species are correlated with phylogenetic history as well as with variation in life histories. The most pronounced differences in development occur between the directly developing Hypogeophis rostratus and all other species examined. In this species, the increase in the degree of morphological complexity is greatly accelerated. J. Morphol. 236:233-246, 1998. © 1998 Wiley-Liss, Inc.

Motor nuclei of nerves innervating the tongue and hypoglossal musculature in a caecilian (amphibia:gymnophiona), as revealed by HRP transport

The organization of the motor nuclei of the glossopharyngeal, vagal, occipital, first spinal and second spinal nerves of Typhlonectes natans (Amphibia: Gymnophiona: Caeciliaidae: Typhlonectinae) was studied by using horseradish peroxidase reaction staining. Each nucleus has discrete patterns of cytoarchitecture and of topography. Nuclei are elongate and some overlap anteroposteriorly. The brainstem is elongate, with no distinct demarcation of brainstem from spinal cord. The occipital nerve emerges through a separate foramen from that for the vagus and glossopharyngeal nerves in the species studied, is distinct from both, and its nucleus is more similar to spinal nuclei in cytoarchitecture. The occipital nerve fuses with spinal nerves 1 and 2 to contribute to the hypoglossal trunk. A spinal accessory nerve is absent.

Differentiation processes in the amphibian brain with special emphasis on heterochronies

Amphibians and caecilians exhibit a great variety of adult morphologies, life histories, and developmental strategies (biphasic development, direct development, viviparity, and neoteny). While early brain development and the differentiation of neural tissues in the three amphibian orders follow a basic pattern, differences exist in the onset and offset as well as the rate of growth and differentiation processes. These differences are described within a phylogenetic framework, and special emphasis is laid on the relationship between altered ontogenies and phylogenetic diversity. We concentrate on ontogenetic differentiation processes in the motor, olfactory, and visual system. We discuss the morphological consequences of secondary simplification of the brain in the context of paedomorphosis, which has happened several times independently among amphibians and consists in the abbreviation or truncation of late developmental processes. We deal with the cellular and molecular basis of brain development and the consequences for the adult nervous system in representative species of the three amphibian orders. Our analysis reveals that differences in brain morphology are largely due to heterochrony (i.e., the desynchronization of ontogenetic processes), a phenomenon that in turn is related to changes in genome sizes and life histories.

Immunohistological localization of tenascin-C in the developing and regenerating retinotectal system of two amphibian species

The expression pattern of the extracellular matrix molecule tenascin-C was investigated in the retinotectal system of the frog Discoglossus pictus and the salamander Pleurodeles waltl during development and optic nerve regeneration in the adult. In both species, the retina was devoid of tenascin-C immunoreactivity at all ages studied. During development, tenascin-C was distributed in a gradient in the optic nerve, with the highest immunoreactivity in the eye near part of the optic nerve. The myelin-associated glycoprotein was distributed in a gradient with opposite polarity. In Discoglossus, but not Pleurodeles, tenascin-C was detected in the anterior chiasm. In the tectum of both species, tenascin-C was observed in deep cellular and fiber layers but not in the layers receiving optic fibers or proliferative zones. The distribution patterns of tenascin-C were the same during development and in the adult, except for a disappearance of the molecule from the intraocular part of the optic nerve. After lesioning the optic nerve of adult animals, tenascin-C was strongly reexpressed in the intraocular part of the optic nerve but was only weakly upregulated in the distal optic nerve stump. In contrast, a chondroitin sulfate epitope was strongly upregulated in the distal optic nerve stump. These observations suggest that during development, tenascin-C serves as an attenuating barrier for myelinating cells in the optic nerve and contributes to the guidance of growing retinal ganglion cell axons. Due to its sustained expression in the adult, tenascin-C may have similar functions during regeneration of the lesioned adult retinotectal system.

Comparison of melatonin-binding sites in the brain of two amphibians: an autoradiographic study

Neuroanatomic comparison of the binding capability of 2-[125I] iodomelatonin in the crested newt Triturus carnifex Laur. and the green frog Rana esculenta, using quantitative autoradiographic techniques, revealed a heterogeneous distribution pattern. The highest and relatively high binding activities were shown to occur in the optic tracts and in the suprachiasmatic area of the hypothalamus and the optic tectum, respectively, of both species. Low or no 2-[125I] iodomelatonin binding values were obtained in the preoptic nucleus, the tuberal hypothalamus, the medulla oblongata, the septum and the dorsal pallium. A differential binding pattern was observed in the amygdaloid nucleus pars lateralis, the striatum and the hindbrain of these amphibians. Indeed, notably high binding levels were shown to occur in the former two brain areas of the crested newt, whereas high levels were displayed in the latter brain region of the green frog. On the basis of elevated quantities of melatonin receptors in mesencephalic, hypothalamic and telencephalic sites, it seems plausible to ascribe some important sensory functions to this receptor system in both species. The remarkably different binding activities in the brain of the two amphibians could be correlated with the simpler cytoarchitectonic brain structure of urodeles and with species-specific variations.

Multiple sources of the pituitary pars intermedia innervation in amphibians: a DiI retrograde tract-tracing study

Afferent projections to the pituitary pars intermedia were studied using the DiI tract-tracing technique in two amphibian species, the urodelan Triturus carnifex, and the anuran Rana esculenta. After DiI crystal application into the pituitary intermediate lobe, in both species cells were retrogradely labeled in the preoptic nucleus, in the supra- and retro-chiasmatic hypothalamus and in the brainstem (especially in the area indicated as locus coeruleus). The findings are discussed in relation to data on the neurochemical nature of the innervation of the pars intermedia in amphibians.

Distribution of NCAM-180 and polysialic acid in the developing tectum mesencephali of the frog Discoglossus pictus and the salamander Pleurodeles waltl

The 180 kDa component of the neural cell adhesion molecule (NCAM-180), total NCAM (NCAM-total) and the polysialic acid modification of NCAM (PSA) show similar temporal and spatial regulation in the developing tecta of Pleurodeles waltl (salamander) and Discoglossus pictus (frog). Whereas NCAM-total is found throughout the tectal tissue on neurons and glia, NCAM-180 is only found on non-proliferating neurons and in fiber layers. PSA is expressed by a subset of NCAM-180-positive cells. Western blots show that there is little polysialylated NCAM-140 in the developing amphibian tectum. Regions unstained for PSA and NCAM-180 correspond precisely to the growth zones of the tectum. NCAM-180 and PSA are not present in tecta of early larvae. Staining intensity is strongest at midlarval stages for both antigens. At metamorphosis, PSA is strongly downregulated, whereas NCAM-180 is downregulated in juvenile animals. Both antigens are still present in fiber layers of adult animals. In dissociated tissue culture of the frog tectum, NCAM-180 is not present on astrocytes, but on neuronal cells. Expression is enhanced at cell contact sites, suggesting that NCAM-180 is involved in cell contact stabilization. This study shows that general features of temporal and spatial regulation of NCAM isoforms and PSA are highly conserved in frog and salamander tecta, despite large differences in the rate of cell migration and the degree of lamination in these homologous brain regions.