Early events in the embryogenesis of the vertebrate visual system: cellular determination and pathfinding

Vision in laboratory rodents-Tools to measure it and implications for behavioral research

Mice and rats are nocturnal mammals and their vision is specialized for detection of motion and contrast in dim light conditions. These species possess a large proportion of UV-sensitive cones in their retinas and the majority of their optic nerve axons target superior colliculus rather than visual cortex. Therefore, it was a widely held belief that laboratory rodents hardly utilize vision during day-time behavior. This dogma is being questioned as accumulating evidence suggests that laboratory rodents are able to perform complex visual functions, such as perceiving subjective contours, and that declined vision may affect their performance in many behavioral tasks. For instance, genetic engineering may have unexpected consequences on vision as mouse models of Alzheimer's and Huntington's diseases have declined visual function. Rodent vision can be tested in numerous ways using operant training or reflex-based behavioral tasks, or alternatively using electrophysiological recordings. In this article, we will first provide a summary of visual system and explain its characteristics unique to rodents. Then, we present well-established techniques to test rodent vision, with an emphasis on pattern vision: visual water test, optomotor reflex test, pattern electroretinography and pattern visual evoked potentials. Finally, we highlight the importance of visual phenotyping in rodents. As the number of genetically engineered rodent models and volume of behavioral testing increase simultaneously, the possibility of visual dysfunctions needs to be addressed. Neglect in this matter potentially leads to crude biases in the field of neuroscience and beyond.

Xenopus sonic hedgehog guides retinal axons along the optic tract

The role of classic morphogens such as Sonic hedgehog (Shh) as axon guidance cues has been reported in a variety of vertebrate organisms (Charron and Tessier-Lavigne [2005] Development 132:2251-2262). In this work, we provide the first evidence that Xenopus sonic hedgehog (Xshh) signaling is involved in guiding retinal ganglion cell (RGC) axons along the optic tract. Xshh is expressed in the brain during retinal axon extension, adjacent to these axons in the ventral diencephalon. Retinal axons themselves express Patched 1 and Smoothened co-receptors during RGC axon growth. Blocking Shh signaling causes abnormal ventral pathfinding, and targeting errors at the optic tectum. Misexpression of exogenous N-Shh peptide in vivo also causes pathfinding errors. Retinal axons grown in culture respond to N-Shh in a dose-dependent manner, either by decreasing extension at lower concentrations, or retracting axons in the presence of higher doses. These data suggest that Shh signaling is required for normal RGC axon pathfinding and tectal targeting in the developing visual system of Xenopus. We propose that Shh serves as a ventral optic tract repellent that helps to define the caudal boundary for retinal axons in the diencephalon, and that this signaling is also required for initial target recognition at the optic tectum.

Periventricular notch activation and asymmetric Ngn2 and Tbr2 expression in pair-generated neocortical daughter cells

To understand the cellular and molecular mechanisms regulating cytogenesis within the neocortical ventricular zone, we examined at high resolution the spatiotemporal expression patterns of Ngn2 and Tbr2. Individually DiI-labeled daughter cells were tracked from their birth in slice cultures and immunostained for Ngn2 and Tbr2. Both proteins were initially absent from daughter cells during the first 2 h. Ngn2 expression was then initiated asymmetrically in one of the daughter cells, with a bias towards the apical cell, followed by a similar pattern of expression for Tbr2, which we found to be a direct target of Ngn2. How this asymmetric Ngn2 expression is achieved is unclear, but gamma-secretase inhibition at the birth of daughter cells resulted in premature Ngn2 expression, suggesting that Notch signaling in nascent daughter cells suppresses a Ngn2-Tbr2 cascade. Many of the nascent cells exhibited pin-like morphology with a short ventricular process, suggesting periventricular presentation of Notch ligands to these cells.

Expression patterns of chick Musashi-1 in the developing nervous system

Vertebrate homologues of musashi have recently been referred to as neural stem cell markers because of their expression patterns and RNA-binding interactions. In the context of the notch signaling pathway, Musashi-1 (Msi-1) is a regulator of neural cell generation, cooperating with notch to maintain mitosis. In an effort to identify definitive stem cell markers of the neural retina, a portion of the Msi-1 cDNA was cloned, and the expression of Msi-1 in the chick eye was analyzed. Using an Msi-1-specific antibody and RNA probe, we show that expression of Msi-1 in the early neural tube is consistent with neural stem identity. In the neural retina, expression starts shortly before embryonic day 3 (E3) and continues up to and including E18. A BrdU incorporation assay shows Msi-1 to be found in both proliferating and differentiating cells of E5 neural retina. At E8 (when proliferation is complete in the fundus of the retina) and E18 (mature retina) Msi-1 expression was found in the ciliary marginal zone (CMZ) as well as in a subpopulation of differentiated cells, including photoreceptors and ganglion cells.

A bHLH transcriptional network regulating the specification of retinal ganglion cells

In the developing retina, the production of ganglion cells is dependent on the proneural proteins NGN2 and ATH5, whose activities define stages along the pathway converting progenitors into newborn neurons. Crossregulatory interactions between NGN2, ATH5 and HES1 maintain the uncommitted status of ATH5-expressing cells during progenitor patterning, and later on regulate the transition from competence to cell fate commitment. Prior to exiting the cell cycle, a subset of progenitors is selected from the pool of ATH5-expressing cells to go through a crucial step in the acquisition of a definitive retinal ganglion cell fate. The selected cells are those in which the upregulation of NGN2, the downregulation of HES1 and the autostimulation of ATH5 are coordinated with the progression of progenitors through the last cell cycle. This coordinated pattern initiates the transcription of ganglion cell-specific traits and determines the size of the ganglion cell population.

Development of the visual system of the chick. II. Mechanisms of axonal guidance

The quest to understand axonal guidance mechanisms requires exact and multidisciplinary analyses of axon navigation. This review is the second part of an attempt to synthesise experimental data with theoretical models of the development of the topographic connection of the chick retina with the tectum. The first part included classic ideas from developmental biology and recent achievements on the molecular level in understanding cytodifferentiation and histogenesis [J. Mey, S. Thanos, Development of the visual system of the chick. (I) Cell differentiation and histogenesis, Brain Res. Rev. 32 (2000) 343-379]. The present part deals with the question of how millions of fibres exit from the eye, traverse over several millimetres and spread over the optic tectum to assemble a topographic map, whose precision accounts for the sensory performance of the visual system. The following topics gained special attention in this review. (i) A remarkable conceptual continuity between classic embryology and recent molecular biology has revealed that positional cellular specification precedes and determines the formation of the retinotectal map. (ii) Graded expression of asymmetric genes, transcriptional factors and receptors for signal transduction during early development seem to play a crucial role in determining the spatial identity of neurons within surface areas of retina and optic tectum. (iii) The chemoaffinity hypothesis constitutes the conceptual framework for development of the retinotopic organisation of the primary visual pathway. Studies of repulsive factors in vitro developed the original hypothesis from a theoretical postulate of chemoattraction to an empirically supported concept based on chemorepulsion. (iv) The independent but synchronous development of retina and optic tectum in topo-chronologically corresponding patterns ensures that ingrowing retinal axons encounter receptive target tissue at appropriate locations, and at the time when connections are due to be formed. (v) The growth cones of the retino-fugal axons seem to be guided both by local cues on glial endfeet and within the extracellular matrix. On the molecular level, the ephrins and their receptors have emerged as the most likely candidates for the material substrate of a topographic projection along the anterior-posterior axis of the optic tectum. Yet, since a number of alternative molecules have been proposed for the same function, it remains the challenge for the near future to define the proportional contribution of each one of the individual mechanisms proposed by matching theoretical predictions with the experimental evidence.

Overexpression of FGF-2 alters cell fate specification in the developing retina of Xenopus laevis

The developing vertebrate retina produces appropriate ratios of seven phenotypically and functionally distinct cell types. Retinal progenitors remain multipotent up until the last cell division, favoring the idea that extrinsic cues direct cell fate. We demonstrated previously that fibroblast growth factor (FGF) receptors are necessary for transduction of signals in the developing Xenopus retina that bias cell fate decisions (S. McFarlane et al., 1998, Development 125, 3967-3975). However, the precise identity of the signal remains unknown. To test whether an FGF signal is sufficient to influence cell fate choices in the developing retina, FGF-2 was overexpressed in Xenopus retinal precursors by injecting, at the embryonic 16-cell stage, a cDNA plasmid encoding FGF-2 into cells fated to form the retina. We found that FGF-2 overexpression in retinal precursors altered the relative numbers of transgene-expressing retinal ganglion cells (RGC) and Müller glia; RGCs were increased by 35% and Müller glia decreased by 50%. In contrast, the proportion of retinal precursors that became photoreceptors was unchanged. Within the photoreceptor population, however, we found a twofold increase in rod photoreceptors at the expense of cone photoreceptors. These data are consistent with an endogenous FGF signal influencing cell fate decisions in the developing vertebrate retina.

The course of later generated axons in the developing optic nerve of the chick embryo. A morphometric electron microscopic study

The topographic position of growth cones (GCs) shows the course of ingrowing axons within the optic nerve and allows to draw conclusions with respect to the fiber order in this pathway. Therefore, the topographic distribution and frequency of GCs as well as the proximal and distal axon shaft segments were studied within cross-sections of the distal, middle, and prechiasmatic part of the nerve of 3-8-day-old embryos using electron microscopy. The ingrowth of GCs was not confined to a particular region. Initially, GCs were found near the ventral periphery. With increasing age, simultaneous ingrowth occurred within an area that expanded dorsally. In parallel, GCs also occurred in dorsal regions and eventually in the dorsal periphery. GCs intermingled everywhere with more mature axon profiles. However, youngest profiles predominated ventrally, oldest dorsally. Hence, maturity increased from ventral to dorsal. This indicated that the time of arrival of axons and the topographic position in the cross-section correlated significantly. It is concluded that axons are chronotopically organized, but in a probabilistic sense. The predominant ingrowth of axons in the ventral part may be associated largely with the first wave of neurogenesis of retinal ganglion cells. The ingrowth in dorsal regions of the cross section may be related to later generated axons that enter the nerve following older axons of the same retinal sector as well as axons of neighboring ganglion cells which continue to leave the mitotic cycle while the front of neurogenesis has spread into the periphery.

The embryogenesis of rod photoreceptors in the teleost fish retina, Haplochromis burtoni

Development of the retina, like that of other tissues, occurs via an orderly sequence of cell division and differentiation, producing the functional retina. In teleost fish, however, cell division and differentiation in the retina continue throughout the life of the animal in two distinct ways. Stem cells in a circumferential germinal zone at the periphery of the retina give rise to all retinal cell types and progenitor cells located throughout the retina in the outer nuclear layer (ONL) produce new rod photoreceptors. These processes in adult retina recapitulate in space the embryonic events responsible for forming the retina. Analysis of these events in an African cichlid fish, Haplochromis burtoni, confirmed that cone photoreceptors differentiate first, followed by rod photoreceptors. Correspondingly, at the margin of the eye, cone photoreceptors differentiate nearer to the margin than do rods. Control of photoreceptor production is not understood. Here we present the time of appearance and distribution pattern of GABA and vimentin which are candidates for the control of retinal cell division and differentiation. Antibody staining reveals that both GABA and vimentin exhibit unique patterns of expression during embryonic retinal development. Vimentin immunoreactivity is evident throughout the retina in a spoke-like pattern between developmental Days 4 and 7, as both cone and rod photoreceptors are being formed. GABA is expressed in horizontal cells between Days 5 and 7, corresponding to the onset of rod differentiation in time and in position within the retina. Moreover, the wave of GABAergic staining in the horizontal cells parallels the wave of rod differentiation across the embryonic retina of H. burtoni. Thus, GABA may play a role in the development of rod photoreceptors.

[Tissue expression of a gene potentially implicated in some diseases with retinal and renal involvement]

We have produced a monoclonal antibody (HA34) that specifically reveals the pigmented epithelium in the eye and the proximal convoluted tubules of the kidney, whatever the developmental stage. The results obtained with the kidney of other mammals suggest that the antigen is human specific. Its molecular weight is approximately 200 kDa. The epitope recognized by HA34 is always present on cell lines grown in vitro. This allowed us to use somatic cell interspecific hybrids to localize the gene implicated in the cytogenetic band 11q13, between microsatellites D11S1777 (AFMa046wa9) and D11S913 (AFM164zf12) in a 9 cM space. This region is involved in forms of retinitis pigmentosa, some of which can also include kidney abnormalities. We propose that this gene is possibly implicated in some of these diseases.

The human fetal retinal nerve fiber layer and optic nerve head: a DiI and DiA tracing study

The organization of the primate nerve fiber layer and optic nerve head with respect to the positioning of central and peripheral axons remains controversial. Data were obtained from 32 human fetal retinae aged between 15 and 21 weeks of gestation. Crystals of the carbocyanine dyes, DiI or DiA, and fluorescence microscopy were used to identify axonal populations from peripheral retinal ganglion cells. Peripheral ganglion cell axons were scattered throughout the vitreal-scleral depth of the nerve fiber layer. Such a scattered distribution was maintained as the fibers passed through the optic nerve head and along the optic nerve. There was a rough topographic representation within the optic nerve head according to retinal quadrant such that both peripheral and central fibers were mixed within a wedge extending from the periphery to the center of the nerve. There was no indication that the fibers were reorganized in any way as they passed through the optic disc and into the nerve. The present results suggest that any degree of order present within the fiber layer and optic nerve is not an active process but a passive consequence of combining the fascicles of the retinal nerve fiber layer. Optic axons are not instructed to establish a retinotopic order and the effect of guidance cues in reordering fibers, particularly evident prechiasmatically and postchiasmatically, does not appear to be present within the nerve fiber layer or optic nerve head in humans.

Xrx1, a novel Xenopus homeobox gene expressed during eye and pineal gland development

We have isolated a novel Xenopus homeobox gene, Xrx1, belonging to the paired-like class of homeobox genes. Xrx1 is expressed in the anterior neural plate, and subsequently in the neural structures of the developing eye (neural retina and pigmented epithelium), and in other forebrain structures deriving from the anterior neural plate: in the pineal gland, throughout its development, in the diencephalon floor and in the hypophysis. Its rostral limit of expression corresponds to the chiasmatic ridge, which some authors consider as the anteriormost limit of the neural tube: thus, Xrx1 may represent one of the most anteriorly expressed homeobox genes reported to date. Moreover, its expression in organs implicated in the establishment of circadian rhythms, may suggest for Xrx1 a role in the genetic control of this function. Finally, analysis of Xrx1 expression in embryos subjected to various treatments, or microinjected with different dorsalizing agents (noggin, Xwnt-8), suggests that vertical inductive signals leading to head morphogenesis are required to activate Xrx1.

The ganglionic eminence may be an intermediate target for corticofugal and thalamocortical axons

In the nervous system of many species, growing axons associate transiently with cellular groupings along their path. Whether this mechanism applies to the development of corticothalamic and thalamocortical projections is unknown. Using carbocyanine dyes, we studied the early growth of both corticofugal and thalamocortical fibers in hamster embryos. At embryonic day 11.5 (E11.5), corticofugal fibers invade the lateral ganglionic eminence (LGE), and thalamocortical fibers invade the medial ganglionic eminence (MGE). At this age, both sets of fibers are not yet in contact with each other. At the same time, neurons in each subdivision of the GE grow toward the cortex and thalamus. During the next 24 hr, corticofugal and thalamocortical fibers remain within the confines of the GE, where they course at different radial levels and bear large and complex growth cones. In the LGE, corticofugal fibers are often found in close association with cells that are likely to be neuronal. Starting on E13.5, both early projections from the GE decrease, and corticothalamic and thalamocortical fibers invade their definitive target regions. To test whether the GE specifically orients the growth and trajectories of cortical fibers even in the absence of the reciprocal thalamic projection, we cocultured explants of cortex and GE from either hamster or mouse embryos. These experiments showed that the GE, but not other tested brain regions, is able specifically to orient the growth of cortical axons. We therefore suggest that the GE may be an intermediate target in the pathfinding of axons between the cortex and the thalamus.

Engrailed, retinotectal targeting, and axonal patterning in the midbrain during Xenopus development: an antisense study

Axonal tracts in the vertebrate brain seem to respect domains of homeobox gene expression. To test the role of engrailed in tract formation in the midbrain, we inhibited its expression using antisense (AS) oligonucleotides. Phosphorothioate-modified AS oligos caused navigational errors in both the optic projection (OP) and the intertectal commissure (ITC). These oligos, however, also inhibited bFGF binding to the brain. To determine whether these tract phenotypes were due to inhibition of bFGF function or engrailed expression, we used partially phosphorothioate-modified (pp) oligos, which inhibit engrailed expression but do not affect bFGF binding. These ppAS oligos caused the ITC phenotype but had no effect on the OP. Thus, interference with bFGF function correlates with the OP phenotype, while the ITC phenotype is directly related to engrailed expression.

Competition and position-dependent targeting in the development of the Drosophila R7 visual projections

The R7 photoreceptor neuron projections form a retinotopic map in the medulla of the Drosophila optic lobe. The more inner photoreceptors mutation, an allele of gap1, results in the differentiation of excess R7s in the eye, whose axons invade the brain and establish functional connections. We have used this hyperinnervation phenotype to explore the roles of photoreceptor-target regulation, competitive interactions, and chemoaffinity in map formation. We show that the extra axons are supported in a wild-type brain, with all R7s from a single ommatidium sharing a single termination site, and thus there is no evidence that the target regulates the size of the presynaptic population. In mosaic eyes, in which ommatidia containing extra R7s are surrounded by ommatidia lacking all R7 cells, R7 axons still target to appropriate retinotopic locations in a largely empty R7 terminal field. Axons at the edges of the projection, however, send collaterals into vacant areas of the field, suggesting they are normally restrained to share single termination sites by competitive interactions. In contrast, no sprouts are seen when the vacant sites are juxtaposed with singly innervated sites. In the third instar, R7 and R8 axons transiently display halos of filopodia that overlap adjacent terminals and provide a means to assess occupancy at adjacent sites. Finally, in sine oculis larvae in which only a small number of ommatidia develop, the R7/R8 axons target to predicted dorsoventral portions of the medulla despite the absence of their neighbors, suggesting that position in the eye field determines their connectivity in the brain. We suggest that the mechanisms used to set up this insect map are formally similar to strategies used by vertebrates. The availability of a genetic model for these events should facilitate studies aimed at understanding the molecular bases of retinotopic map development.

Expression of horizontal cell phenotypes in monolayer cultures from immature rabbit retina

Using the sandwich culture technique introduced by Brewer and Cotman we have studied the in vitro differentiation of A- and B-type horizontal cells which represent two well characterized cell types of the rabbit retina. Neurons from immature (postnatal day 3) rabbit retinae were dissociated and grown on inverted coverslips for up to 5 weeks in a chemically defined medium. On the basis of morphological criteria and the staining pattern for several immunocytochemical and autoradiographic horizontal cell markers we have examined to what extent expression of a distinct mature neuronal phenotype can take place under the artificial conditions of monolayer cultures. After 14 days in vitro neurons could be identified which had acquired elaborate morphological features closely resembling those of A- and B-type horizontal cells, respectively. Axonless A-like cells had 2-4 stout primary dendrites. In agreement with in situ observations these cells showed immunoreactivity for neurofilament proteins (68 kDa, 200 kDa), calbindin-28 kDa and less strongly for vimentin. B-like neurons reached varying states of development. Ideally, they had dendritic trees with 6-8 primary processes extending radially from the soma and a single axon-like process which branched extensively to form a profuse neuritic arbor strikingly similar to axon terminal systems of B-type cells in the intact retina. B-like cells also stained for vimentin, calbindin-28 kDa and unexpectedly also for neurofilament proteins. Interestingly, however, neurofilaments became redistributed during in vitro development eventually resulting in their restricted localization in the 'axon terminal system'. This apparently reflects a developmental process which has escaped detection in situ so far. Both cell types were intensely labelled with antibodies to gamma-aminobutyric acid (GABA), the presumed horizontal cell transmitter, but high affinity uptake of this transmitter was practically undetectable by [3H]-GABA autoradiography. This was in agreement with observations in intact retinae. These results support the notion that once a neuron has reached a certain developmental state further differentiation and maintenance of its particular morphological and functional properties are primarily governed by intrinsic factors, but do not exclude that extrinsic signals have important modulatory functions.

A discrete group of melanin containing cells are coincident with a major reorganization of retinal ganglion cell axons in the optic nerve of Xenopus

Further investigations into the abrupt reorganization of axons in the proximal optic nerve of the frog Xenopus are presented. This reorganization reverses the radial, age-related organization of the projection. At the site of axon divergence a discrete group of melanin-producing astrocytes occurs, which are coincident with the reorganization. These cells are identified as astrocytes by their ultrastructure, and by staining in vitro with an antibody to glial fibrillary acidic protein. In an attempt to see if this reorganization occurs in the absence of pigmented astrocytes, the periodic albino mutant of Xenopus was examined. In this albino a similar reorganization of the retinal axons occurs in the optic nerve, but there is also a population of pigment-containing astrocytes. Unlike the pigment of retinal pigment epithelium, which is lost during metamorphosis in these albinos, the pigmented astrocytes of the optic nerve retain their pigment throughout life. A second approach, used to provide support for the involvement of these pigmented astrocytes in the reorganization, was to study the development of the projection. In early development there is a period prior to gliogenesis in the optic nerve (Stage 47/48) when no pigmented cells are present. At these early stages of development there is no evidence for a reorganization of the retinal axons in the optic nerve. The possible role of these melanin-containing astrocytes in axon guidance is discussed.

Early opsin expression in Xenopus embryos precedes photoreceptor differentiation

The visual pigment which serves as the first step in the phototransduction cycle in vertebrate rod cells consists of a retinal chromophore which is linked to the transmembrane protein, opsin. Opsin genes have been isolated from a number of different organisms and studies have shown opsin to be developmentally regulated with both mRNA and protein expression associated with the morphological differentiation of photoreceptor cells. Due to its potential utility as a marker for rod photoreceptor determination in studies of retinal tissue interactions, and because no amphibian opsin genes have as yet been cloned, we isolated cDNA clones of the Xenopus laevis opsin gene. Sequence analysis shows that within the coding region Xenopus opsin shares a high degree of identity with other rod opsin genes, except at the C-terminal where it more closely resembles the mammalian color opsins. A developmental analysis, on the other hand, reveals that Xenopus opsin transcripts are detectable in a retina-specific fashion early in retinal development. Using in situ hybridization we find that Xenopus opsin mRNA is initially restricted to a few isolated cells in the presumptive photoreceptor layer which express the gene at relatively high levels. This suggests that rod photoreceptor determination occurs in single cells, and that the mechanisms controlling opsin expression in Xenopus are initiated well before any evidence of morphological differentiation.

Myelination of the optic radiation in Leber congenital amaurosis

We have studied the myelination of the visual pathway by magnetic resonance imaging in seven children (aged 5 months to 16 years) with Leber congenital amaurosis. The corpus geniculatum laterale and the retrogeniculate optic radiation had a normal appearance on MRI in all patients. Therefore we conclude that normal myelination of the optic radiation, as it can be grossly assessed by MRI, can take place even with absent or greatly reduced visual sensory input.

The glutamate-receptor linked cGMP cascade of retinal on-bipolar cells is pertussis and cholera toxin-sensitive

Whole-cell patch-clamp recordings were obtained from light-responsive on-bipolar cells in retinal slices of the dogfish. Inclusion of the A-subunit of pertussis toxin in the patch-pipette solution resulted in an increase in inward current and membrane conductance, and a block of light-evoked currents of on-bipolar cells. The opposite effect was obtained with the A-subunit of cholera toxin, which blocked light responses, and induced an outward current and a decrease in membrane conductance. These actions were NAD+ dependent. The results show that the G-protein(s) linking glutamate receptors to a cGMP cascade in on-bipolar cells possess sites which are ADP-ribosylated by pertussis and cholera toxins, with no homology to the adenylate cyclase system but possibly with a homology to transducin. Furthermore, inclusion of H-7, a kinase inhibitor in the patch-pipette solution, or of a non-hydrolysable ATP analogue (AMP-PNP) had no effect on light responses, membrane conductance or dark current of on-bipolar cells, suggesting that the components of this cGMP cascade are unlikely to be regulated by protein kinases.

Patterning of retinotectal connections in the vertebrate visual system

Recent work on the retinotectal projection clearly establishes the roles of neuronal activity and position-based cues in the patterning of nerve connections. In some species, the high degree of spatial order has been shown to emerge from a continued process of terminal growth and refinement. The future challenge is now to determine how multiple cues work together to guide the sculpting of the final pattern.

Fiber outgrowth from fetal vasopressin neurons of the suprachiasmatic nucleus, bed nucleus of the stria terminalis, and medial amygdaloid nucleus transplanted into adult Brattleboro rats

Outgrowth of fibers from different types of vasopressin (AVP) neurons was compared in the brains of AVP-deficient Brattleboro rats. Fetal grafts of the suprachiasmatic nucleus (SCN), the bed nucleus of the stria terminalis (BST), and the medial amygdaloid nucleus (MA) were implanted into the lateral ventricle. AVP-immunoreactive fibers from all grafts entered the host tissue in the lateral septum. SCN fibers were confined to the lateral margin of the septum. In contrast, MA and BST fibers formed equally dense networks spanning the width of the lateral septum. The data suggest that these transplanted neurons show specific outgrowth, and that the phylogenetically related BST and MA neurons follow similar cues to reach their targets.

The oblique muscle organizer in Hirudo medicinalis, an identified embryonic cell projecting multiple parallel growth cones in an orderly array

The oblique muscle layer in the leech body wall is built upon the processes of a unique identified embryonic cell, the Comb- or C-cell. Each C-cell is composed of a spindle-shaped soma that projects approximately 70 parallel processes through the developing body wall at an angle oblique to the long axis. The morphogenesis of this cell and the navigation of its growth cones were examined by intracellular dye filling and antibody staining. At the earliest stages described each C-cell had about six processes, with those near the center of the cell oriented obliquely. As processes were added at the axial ends of the soma they often projected along previously developed longitudinal or circular muscle founder cells and then secondarily aligned themselves parallel to the older processes from the same C-cell. All growth cones initially extended to a particular location in the body wall, where they ceased growing until all 70 processes had been added (over the course of about 5 days). As adjacent segmental homologs met, their growth cones intermingled, eventually sorting out to align parallel. When one of these cells was ablated early--but not later--in development, the remaining adjacent segmental homologs expanded into the vacant territory, consistent with a hypothesis of mutual avoidance between segmental homologs. Most processes that expanded into the experimentally induced vacancy remained correctly oriented and parallel; the few exceptions projected instead along the mirror-image trajectory. Thus, expression of specific avoidance between adjacent C-cell processes is developmentally regulated and functions as a guidance mechanism in vivo, in that it serves to restrict possible trajectories. After aligning its growth cones, each cell stopped adding processes and the processes rapidly extended in concert along relatively precise trajectories. Processes of contralateral homologs cross to form the orthogonal grid used as a scaffold by myocytes to form the oblique muscles. The advancing fronts of growth cones reached the dorsal midline at about the same time as body closure occurs (at about Embryonic Day 20) at which time the C-cells became granular, lost processes, and presumably died. This sequence of developmental events is consistent with temporal and spatial regulation of different morphogenetic strategies, including--but not limited to--specific avoidance, and further suggests testable hypotheses of mechanisms of growth cone navigation in the intact embryo.

Initial stages of retinofugal axon development in the hamster: evidence for two distinct modes of growth

In order to characterize differences in growth patterns of axons as they elongate toward their targets and during the initial stages of terminal arbor formation within the targets, we examined the primary visual system of fetal and newborn hamsters using three morphological methods: the Cajal-deCastro reduced silver method, the rapid Golgi technique, and anterograde transport of HRP. Axons emerge from the retina between the 10th and 11th embryonic days (E10-E11). The front of retinal axons crosses the chiasm, extends over the primitive dorsal nucleus of the lateral geniculate body (LGBd) by E13, and advances to the back of the superior colliculus (SC) by E13.5-E14. The rate of axon growth during this advance is nearly 2 mm/day. Collateral sprouts appear on axons around E15.5. In the LGBd and SC, these sprouts arise from multiple sites along the parent axons. Only one or a few of the sprouts continue to grow and branch, while others are eliminated. The net rate of axon collateral advance in this second phase is an order of magnitude slower than during the stage of axon elongation. Thus, formation of CNS projections may involve two qualitatively distinct modes of axon growth. The arborization mode contrasts with the elongation mode by the presence of branching, a lack of fasciculation and a slower average rate of extension. The stereotypic direct advance of axons during elongation also differs from the remodelling which occurs during arborization. The delay between axon arrival at targets and onset of arborization could be a reflection of axons "waiting" for a maturational change to occur in the retina or in targets. Arborization in the LGBd and SC is initiated around the same time, implicating the former possibility. However, a slower differentiation of retinal arbors in the SC, in addition to morphological differences of arbors in the two structures, suggests that alterations in substrate factors also play a critical role in triggering the early stages of arbor formation.

How do retinal axons find their targets in the developing brain?

Tissue culture studies show that cell survival and process outgrowth from retinal ganglion cells depend on the molecular composition of the substrates over which the neurites grow, and on diffusible factors present in the medium. Recent work has begun to show that at least some of these components might be interactive. Since the conditions in a culture dish, as well as the patterns of antigen expression on cells in vitro, can differ considerably from those encountered in vivo, it is important to design experiments in vivo that examine how growing neurites relate to their natural microenvironment. By the use of transplantation techniques, it has been possible to provide evidence for a comparable duality of substrate-dependent and target-derived controls of optic axon growth, which might provide insight into the normal developmental process.

Relationship of retinotopic ordering of axons in the optic pathway to the formation of visual maps in central targets

We examined in rats the relationship between the ordering of retinal axons in the optic pathway and the formation of a retinotopically organized projection to their primary target, the contralateral superior colliculus (SC). We have previously found that axons labeled by focal injections of 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) made in temporal or nasal retina of perinatal rats commonly mistarget along the medial-lateral and rostral-caudal axes of the SC. By postnatal day (P) 11-12, the retinocollicular projection attains an adult-like topography. Incorrectly targeted axons or axon segments are removed; axons that persist terminate in the topographically appropriate part of the SC (Simon and O'Leary: Dev Biol 137:125, 1990). In the present study, we made similar DiI injections, covering less than 2% of the retinal area, in peripheral temporal, nasal, superior, or inferior retina, in rats of two age groups, embryonic day (E) 21 to P (postnatal day) 2 and P11-P17. Whole mounts of retina, optic nerve and tract, and SC, and cross sections of the optic nerve, were examined. In E21-P2 rats, retinal axons labeled from each retinal site are diffusely distributed in the SC, and poorly ordered in the optic pathway. In retina, labeled axons travel in fascicles directly from the injection site to the optic disc, but neighbor relationships begin to degrade as fascicles split and mix. Retinotopic order is virtually lost in the optic nerve; axons labeled from each injection site disperse throughout its cross-sectional area, but the labeled axons tend to be concentrated toward a specific half of the nerve depending upon their retinal origin. This slight tendency toward retinotopic order increases in the optic tract, but axons are still poorly ordered as they leave the tract and enter the SC. Targeting errors along the medial-lateral axis of the SC, but apparently not along its rostral-caudal axis, are related to the positioning of axons across the width of the optic tract. In P11-P17 rats, axons labeled from each injection site arborize only in a small, topographically correct part of the SC. However, the distributions of labeled retinal axons observed in whole mounts of the retina and optic pathway have a degree of disorder similar to those in E21-P2 rats. Further, the scatter of labeled axons in optic nerve cross sections is comparable in both age groups. Therefore, the emergence of topographic order in the retinocollicular projection is not accompanied by an emergence of a retinotopic ordering of axons in the optic nerve.(ABSTRACT TRUNCATED AT 400 WORDS)

Topologically restricted appearance in the developing chick retinotectal system of Bravo, a neural surface protein: experimental modulation by environmental cues

A novel neural surface protein, Bravo, shows a pattern of topological restriction in the embryonic chick retinotectal system. Bravo is present on the developing optic fibers in the retina; however, retinal axons in the tectum do not display Bravo. The appearance of Bravo in vitro is modulated by environmental cues. Axons growing out from retinal explants on retinal basal lamina, their natural substrate, express Bravo, whereas such axons growing on collagen do not. Retinal explants provide a valuable system to characterize the mechanism of Bravo restriction, as well as the cellular signals controlling it. Bravo was identified with monoclonal antibodies from a collection generated against exposed molecules isolated by using a selective cell surface biotinylation procedure. The NH2-terminal sequence of Bravo shows similarity with L1, a neural surface molecule which is a member of the immunoglobulin superfamily. This possible relationship to L1, together with its restricted appearance, suggests an involvement of Bravo in axonal growth and guidance.