Cytogenesis in the monkey retina

Developmental plasticity of photoreceptors

During development, retinal ganglion cells undergo conspicuous structural remodeling as they gradually attain their mature morphology and connectivity. Alterations in their dendritic organization and in their axonal projections can also be achieved following early insult to their targets or their afferents. Other retinal cell types are thought not to display this same degree of developmental plasticity. The present review will consider the evidence, drawn largely from recent experimental studies in the carnivore retina, that photoreceptors also undergo structural remodeling, extending their terminals transiently into inner plexiform layer before retracting to the outer plexiform layer. The determinants of this transient targeting to the inner plexiform layer are considered, and the role of cholinergic amacrine cells is discussed. The factors triggering this retraction are also considered, including the concurrent maturational changes in outer segment formation and in the differentiation of the outer plexiform layer. These results provide new insight into the life history of the photoreceptor cell and its connectivity, and suggest a transient role for the photoreceptors in the circuitry of the inner retina during early development, prior to the onset of phototransduction.

Morphology and physiology of primate M- and P-cells

Catarrhines and platyrrhines, the so-called Old- and New-World anthropoids, have different cone photopigments. Postreceptoral mechanisms must have co-evolved with the receptors to provide trichromatic color vision, and so it is important to compare postreceptoral processes in these two primate groups, both from anatomical and physiological perspectives. The morphology of ganglion cells has been studied in the retina of catarrhines such as the diurnal and trichromatic Macaca, as well as platyrrhines such as the diurnal, di- or trichromatic Cebus, and the nocturnal, monochromatic Aotus. Diurnal platyrrhines, both di- and trichromats, have ganglion cell classes very similar to those found in catarrhines: M (parasol), P (midget), small-field bistratified, and several classes of wide-field ganglion cells. In the fovea of all diurnal anthropoids, P-cell dendritic trees contact single midget bipolars, which contact single cones. The Aotus retina has far fewer cones than diurnal species, but M- and P-cells are similar to those in diurnal primates although of larger size. As in diurnal anthropoids, in the Aotus, the majority of midget bipolar cells, found in the central 2 mm of eccentricity, receive input from a single cone and the sizes of their axon terminals match the sizes of P-cell dendritic fields in the same region. The visual responses of retinal ganglion cells of these species have been studied using single-unit electrophysiological recordings. Recordings from retinal ganglion cells in Cebus and Aotus showed that they have very similar properties as those in the macaque, except that P-cells of mono- and dichromatic animals lack cone opponency. Whatever the original role of the M- and P-cells was, they are likely to have evolved prior to the divergence of catarrhines and platyrrhines. M- and P-cell systems thus appear to be strongly conserved in the various primate species. The reasons for this may lie in the roles of these systems for both achromatic and chromatic vision.

Development of the outer retina in the mouse

Mice represent a valuable species for studies of development and disease. With the availability of transgenic models for retinal degeneration in this species, information regarding development and structure of mouse retina has become increasingly important. Of special interest is the differentiation and synaptogenesis of photoreceptors since these cells are predominantly involved in hereditary retinal degenerations. Thus, some of the keys to future clinical management of these retinal diseases may lie in understanding the molecular mechanisms of outer retinal development. In this study, we describe the expression of markers for photoreceptors (recoverin), horizontal cells (calbindin), bipolar cells (protein kinase C; PKC) and cytoskeletal elements pivotal to axonogenesis (beta-tubulin and actin) during perinatal development of mouse retina. Immunocytochemical localization of recoverin, calbindin, PKC and beta-tubulin was monitored in developing mouse retina (embryonic day (E) 18.5 to postnatal day (PN) 14), whereas f-actin was localized by Phalloidin binding. Recoverin immunoreactive cells, presumably the photoreceptors, were observed embryonically (E 18.5) and their number increased until PN 14. Neurite projections from the immunoreactive cells towards the outer plexiform layer (OPL) were noted at PN 0 and these processes reached the OPL at PN 7 coincident with histological evidence for the differentiation of the OPL. Outer segments, all the cell bodies in the ONL, as well as the OPL were immunoreactive to recoverin at PN 14. Calbindin immunoreactive horizontal cells were also present in E 18.5 retinas. These cells became progressively displaced proximally as the ONL developed. A calbindin immunoreactive plexus was seen in the OPL at PN 7. PKC immunoreactive bipolar cells developed postnatally, becoming distinguished at PN 7. Both beta-tubulin and actin immunoreactive cells were present in the IPL as early as E 18.5; however, appearance of processes labeled with these markers in the OPL was delayed until PN 7, concurrent with the first appearance of photoreceptor neurites, development of the horizontal cell plexus, and development of synaptophysin immunoreactivity at this location. These results provide a developmental timeframe for the expression of recoverin, calbindin, synaptophysin, beta-tubulin and actin. Our findings suggest that the time between PN 3 and PN 7 represents a critical period during which elements of the OPL are assembled.

Age-related changes in the dynamics of human albino visual pathways

A deficiency of melanin in the retinal pigment epithelium, which regulates the development of neural retina, leads to chiasmal misrouting such that the uncrossed pathway (to the ipsilateral hemisphere) is reduced relative to the crossed pathway (to the contralateral hemisphere). This study examines age-related changes in the flash and pattern appearance visual evoked potentials (VEP) of human albinos. Scalp recorded cortical VEPs to flash (FVEP) and pattern appearance stimulation were recorded in 58 albino (8 months to 60 years) and 34 normal subjects (4-55 years). VEPs were analysed by amplitude and latency. The contralateral hemisphere FVEP amplitude decreased with age in albino subjects, as in both hemispheres in normals. However, the ipsilateral hemisphere FVEP amplitude was significantly lower in young albino subjects, initially giving a marked interhemispheric asymmetry, but this normalized with age. Significant interhemispheric FVEP latency asymmetries were not observed. The contralateral pattern appearance VEP latency in albino subjects decreased with age, as in both hemispheres in normals; the ipsilateral latency increased significantly with age. Significant interhemispheric pattern appearance VEP amplitude asymmetries were not observed. These novel and unexpected observations indicate significant age-related changes in the retinocortical pathways of the human albino. These changes have implications for our understanding of development and plasticity of the central visual pathways.

Dlx1, Dlx2, Pax6, Brn3b, and Chx10 homeobox gene expression defines the retinal ganglion and inner nuclear layers of the developing and adult mouse retina

Distal-less homeobox genes are expressed in the developing forebrain. We assessed Dlx gene expression in the developing and adult mouse retina. Dlx1 and Dlx2 are detected in retinal neuroprogenitors by embryonic day (E) 12.5 (Eisenstat et al. [1999] J. Comp. Neurol. 217-237). At E13.5, the expression of four homeodomain proteins, DLX2, BRN3b, PAX6, and CHX10, define distinct yet overlapping domains in the retinal neuroepithelium. By postnatal day (P) 0, DLX2 is expressed in the neuroblastic layer and the ganglion cell layer (GCL) consisting of ganglion and displaced amacrine cells. DLX1 expression resembles DLX2 to P0 but decreases postnatally. In the adult, DLX2 is localized to ganglion, amacrine, and horizontal cells as determined by coexpression with retinal cell-specific markers. There is coincident expression of DLX2 with gamma-aminobutyric acid (GABA), glutamic acid decarboxylase (GAD)65, and GAD67 in the inner nuclear layer (INL) and GCL. In the adult, DLX2 is coexpressed with BRN3b in ganglion cells; PAX6 in amacrine, horizontal, and ganglion cells; and Chx10 in some bipolar cells. We predict that a combinatorial code of these homeobox genes and others specify retinal cell fate. Our results support a possible role for Dlx1 and Dlx2 in inner retinal development and in the terminal differentiation and/or maintenance of INL interneurons and ganglion cells in the adult. The correlation of DLX2 with GABA expression in the mouse retina closely mirrors the relationship of DLX2 to GABAergic neuronal differentiation in the embryonic forebrain, including neocortex, olfactory bulb and hippocampus, signifying a conservation of function of Dlx genes in the developing central nervous system.

Differential expression and cellular localization of doublecortin in the developing rat retina

Doublecortin is 40 kDa microtubule-associated phosphoprotein required for neuronal migration and differentiation in various regions of the developing central nervous system. We have investigated the expression and cellular localization of doublecortin in the developing rat retina using immunocytochemistry and Western blot analysis. The expression of doublecortin was high from embryonic day 18 (E18) until E20 and was low during the postnatal period. The doublecortin immunoreactivity first appeared in a few radially orientated cells in the mantle zone of the primitive retina at E15. From E16 onward, the immunoreactivity appeared in two different regions: the inner part of the retina and middle of the neuroblastic layer. In the inner part, the somata of cells in the ganglion cell layer, in the distal row of the neuroblastic layer and profiles in the inner plexiform layer showed doublecortin immunoreactivity up to postnatal day 1 (P1). Afterwards, the doublecortin immunoreactivity persisted in the inner plexiform layer until P15, although the intensity decreased gradually with the maturation of the retina. In the middle of the neuroblastic layer, doublecortin immunoreactivity appeared in the radially orientated cells. These cells transformed into horizontal cells. The doublecortin immunoreactivity persisted in these cells up to P21. Given these results, doublecortin may play an important role in the migration and differentiation of specific neuronal populations in developmental stages of the rat retina.

Spatiotemporal features of early neuronogenesis differ in wild-type and albino mouse retina

In albino mammals, lack of pigment in the retinal pigment epithelium is associated with retinal defects, including poor visual acuity from a photoreceptor deficit in the central retina and poor depth perception from a decrease in ipsilaterally projecting retinal fibers. Possible contributors to these abnormalities are reported delays in neuronogenesis (Ilia and Jeffery, 1996) and retinal maturation (Webster and Rowe, 1991). To further determine possible perturbations in neuronogenesis and/or differentiation, we used cell-specific markers and refined birth dating methods to examine these events during retinal ganglion cell (RGC) genesis in albino and pigmented mice from embryonic day 11 (E11) to E18. Our data indicate that relative to pigmented mice, more ganglion cells are born in the early stages of neuronogenesis in the albino retina, although the initiation of RGC genesis in the albino is unchanged. The cellular organization of the albino retina is perturbed as early as E12. In addition, cell cycle kinetics and output along the nasotemporal axis differ in retinas of albino and pigmented mice, both absolutely, with the temporal aspect of the retina expanded in albino, and relative to the position of the optic nerve head. Finally, blocking melanin synthesis in pigmented eyecups in culture leads to an increase in RGC differentiation, consistent with a role for melanin formation in regulating RGC neuronogenesis. These results point to spatiotemporal defects in neuronal production in the albino retina, which could perturb expression of genes that specify cell fate, number, and/or projection phenotype.

Neurogenesis in adult primate neocortex: an evaluation of the evidence

Reports of continuous genesis and turnover of neurons in the adult primate association neocortex--the site of the highest cognitive functions--have generated great excitement. Here, I review the available evidence, and question the scientific basis of this claim.

Cellular competence plays a role in photoreceptor differentiation in the developing Xenopus retina

Factors in the environment appear to be responsible for inducing many of the cell fates in the retina, including, for example, photoreceptors. Further, there is a conserved order of histogenesis in the vertebrate retina, suggesting that a temporal mechanism interacts in the control of cellular determination. The temporal mechanism involved could result from different inducing signals being released at different times. Alternatively, the inducing signals might be present at many stages, but an autonomous clock could regulate the competence of cells to respond to them. To differentiate between these mechanisms, cells from young embryonic retinas were dissociated and grown together with those from older embryos, and the timing of photoreceptor determination assayed. Young cells appeared uninfluenced by older cells, expressing photoreceptor markers on the same time schedule as when cultured alone. A similar result was obtained when the heterochronic mixing was done in vivo by grafting a small plug of optic vesicle from younger embryos into older hosts. Even the graft cells at the immediate margin of the transplant failed to express photoreceptor markers earlier than normal, despite their being in contact with older, strongly expressing host cells. We conclude that retinal progenitors intrinsically acquire the ability to respond to photoreceptor-inducing cues by a mechanism that runs on a cell autonomous schedule, and that the conserved order of histogenesis is based in part on this competence clock.

Architecture of the optic chiasm and the mechanisms that sculpt its development

At the optic chiasm the two optic nerves fuse, and fibers from each eye cross the midline or turn back and remain uncrossed. Having adopted their pathways the fibers separate to form the two optic tracts. Research into the architecture and development of the chiasm has become an area of increasing interest. Many of its mature features are complex and vary between different animal types. It is probable that numerous factors sculpt its development. The separate ganglion cell classes cross the midline at different locations along the length of the chiasm, reflecting their distinct periods of production as the chiasm develops in a caudo-rostral direction. In some mammals, uncrossed axons are mixed with crossed axons in each hemi-chiasm, whereas in others they remain segregated. These configurations are the product of different developmental mechanisms. The morphology of the chiasm changes significantly during development. Neurons, glia, and the signals they produce play a role in pathway selection. In some animals fiber-fiber interactions are also critical, but only where crossed and uncrossed pathways are mixed in each hemi-chiasm. The importance of the temporal dimension in chiasm development is emphasized by the fact that in some animals uncrossed ganglion cells are generated abnormally early in relation to their retinal location. Furthermore, in albinos, where many cells do not exit the cell cycle at normal times, there are systematic chiasmatic abnormalities in ganglion cell projections.

M and P retinal ganglion cells of the owl monkey: morphology, size and photoreceptor convergence

We have estimated photoreceptor convergence to M and P retinal ganglion cells of two closely related nocturnal (owl monkey, Aotus) and diurnal (capuchin monkey, Cebus) anthropoids. Rod convergence is higher in the owl monkey retina while cone convergence to both M and P cells are very similar in the retinas of the owl monkey and the capuchin monkey. These results indicate that during evolution, the owl monkey retina has undergone changes compatible with a more nocturnal lifestyle, but kept a cone to ganglion cell relation similar to that found in diurnal primates.

Harnessing the power of forward genetics--analysis of neuronal diversity and patterning in the zebrafish retina

The seven major cell classes of the vertebrate retina are organized with remarkable precision into distinct layers. The appearance of this architecture during embryogenesis raises two questions of general importance. How do individual cell classes acquire their specialized structures and functions if they all originate from a morphologically uniform cell population? What mechanisms are responsible for the formation of such a complex and exact pattern? Recent advances present an opportunity to apply the tools of forward genetic analysis to identify mutations that affect these mechanisms in zebrafish. Molecular characterization will follow, providing insight into the basis of neuronal patterning in the vertebrate CNS.

The mammalian photoreceptor mosaic-adaptive design

Unlike in birds and cold-blooded vertebrates' retinas, the photoreceptors of mammalian retinas were long supposed to be morphologically uniform and difficult to distinguish into subtypes. A number of new techniques have now begun to overcome the previous limitations. A hitherto unexpected variability of spectral and morphological subtypes and topographic patterns of distribution in the various retinas are being revealed. We begin to understand the design of the photoreceptor mosaics, the constraints of evolutionary history and the ecological specialization of these mosaics in all the mammalian subgroups. The review discusses current cytological identification of mammalian photoreceptor types and speculates on the likely "bottleneck-scenario" for the origin of the basic design of the mammalian retina. It then provides a brief synopsis of current data on the photoreceptors in the various mammalian orders and derives some trends for phenomena such as rod/cone dualism, spectral range, preservation or loss of double cones and oil droplets, photopigment co-expression and mono- and tri-chromacy. Finally, we attempt to demonstrate that, building on the limits of an ancient rod dominant (probably dichromatic) model, mammalian retinas have developed considerable radiation. Comparing the nonprimate models with the intensively studied primate model should provide us with a deeper understanding of the basic design of the mammalian retina.

Vascular endothelial cell growth factors promote the in vitro development of rat photoreceptor cells

We have identified and characterized a novel trophic effect of vascular endothelial cell growth factor (VEGF) on photoreceptor cells. Treatment of retinal cultures, derived from postnatal day 1 (P1) rats, with VEGF-2 resulted in a dose- and time-dependent increase in the level of rhodopsin protein, as determined by ELISA assay. After 7-9 d of treatment the VEGF-1 or VEGF-2, at a concentration of 10 ng/ml, induced a 200-300% increase in rhodopsin protein and a 220% increase in the number of rhodopsin-immunopositive cells. Treatment with VEGF-2 induced a 250% increase in the number of syntaxin-immunopositive cells and a 67% increase in high-affinity GABA uptake, both markers for amacrine cells. In contrast, there was no increase in the non-neuronal cell populations. VEGF-2 induced an approximately 300% increase in the number of bromodeoxyuridine-labeled (BrdU) retinal cells within 48 hr of treatment. After 3 d in culture both the basal and stimulated levels of BrdU incorporation were reduced, suggesting that the proliferative effect of VEGF was restricted developmentally. Furthermore, there was a developmentally dependent increase in the mitogenic response to VEGF-2, with retinal cultures derived from E15, E20, or P1 animals demonstrating a 50, 100, and 300% increase in thymidine incorporation, respectively. However, VEGF treatment resulted in an increase in the number of rhodopsin-immunopositive cells only when the cultures were derived from P1 animals. Therefore, retinal progenitor cells appear to be targets for VEGF, and thus VEGF may be involved in the regulation of the early developmental program of retinal neurogenesis.

Cell fate decisions in a human retinal precursor cell line: basic fibroblast growth factor- and transforming growth factor-alpha-mediated differentiation

The purpose of this study was to determine if immortalized human retinal precursor cells could serve as a model to investigate cues that modulate cell fate and differentiation. We investigated the effects of a variety of growth factors broadly but specifically tested the effects of basic fibroblast growth factor (bFGF) and transforming growth factor (TGF)a in retinal cell differentiation and commitment. To determine the role of exogenously added growth factors in a human retinal precursor cell line (KGLDMSM), established from a first-trimester retina, cells were adapted to grow in a defined medium and exposed to a variety of trophic factors (epidermal growth factor [EGF], neuron growth factor [NGF], TGFalpha, TGFbeta, acidic FGF, and bFGF). Dose-response curves were developed to arrive at optimal concentrations. The neurotrophic potential of growth factors was determined by 3H-thymidine incorporation and bromodeoxyuridine (BrdU) labeling. The identity of the emerging neuronal phenotypes were determined by phase-contrast microscopy, immunolabeling for the neuron-specific antigens neurofilament protein (NF) and neuron-specific enolases (NSE), and photoreceptor-specific antigens (Rho1D4, 7G6) using immunocytochemistry and Western blot analysis. To identify some of the early response genes (c-fos, c-myc) expressed in response to growth factors, Northern blot analysis was performed. Almost all of the factors tested increased the total number of cells with a neuronal phenotype. Potency of growth factors to generate neurons was TGFalpha > bFGF > EGF > NGF. Both TGFalpha and bFGF, alone or in combination, increased the total number of neurons. Most of the neurons generated were photoreceptors, as depicted by the polarized phenotype, expression of photoreceptor-specific antigens, and processes resembling rudimentary outer segments. The increase in photoreceptor-like neurons is possibly attributable to an increase in numbers rather than greater survival. Additionally, the majority of the photoreceptors generated labeled with BrdU and for photoreceptor-specific antigens, suggesting that an inductive effect of bFGF and TGFalpha could occur in the cell cycle or shortly thereafter. Both bFGF and TGFalpha induced the expression of the early response gene c-fos while not altering the expression of c-actin or c-myc. The emergence of a photoreceptor phenotype was confirmed by both immunocytochemistry and Western blot analysis. The immortalized retinal precursor cell line could prove valuable in determining the role of exogenously added growth factors in retinal development and differentiation. Both bFGF and TGFalpha enhance the photoreceptor phenotype in medium-density cultures under conditions of defined medium. The same was confirmed by phase-contrast microscopy, immunocytochemistry, and Western blot analysis. Furthermore, cell fate determination in cultured precursor cells could occur during the late part of the cell cycle or shortly after completion of cell division. The effects of TGFalpha and bFGF seem to be slightly additive. The cell line will be extremely valuable in studying mechanisms of cell commitment and generation of retinal cell types, which could be tested for their potential for transplantation.

Developmental changes in astrocyte density in the macaque perifoveal region

We studied astrocyte density both in the perifoveal region and in extrafoveal regions within the same distance of the optic disc (OD) over a time period from foveal pit formation (embryonic day E112) until 2 months after birth. The study was prompted by earlier observations that the adult macaque displays an almost astrocyte-free region around the fovea which, however, at birth is occupied by astrocytes. Thus, we wanted to determine if the perifoveal region is invaded by astrocytes during early development to the same degree as other regions in the central retina, and how the reduction in density can be explained. From the earliest age we studied (embryonic day 112), less astrocytes were found in the perifovea than in other regions equidistant from the OD. In addition, the number of astrocytes steadily declined both in the perifovea and outside until birth. During the first week after birth, there was a further dramatic decline in perifoveal astrocyte density. Double-labelling with glial fibrillary acidic protein (GFAP) immunocytochemistry and the TUNEL method showed that during the whole observation period astrocytes undergo DNA fragmentation and presumably die. However, the rate of TUNEL-positive astrocytes did not significantly differ between perifovea and other regions equidistant to the OD, and at no time did we find a significant peak of apoptosis rate. Thus, the reduction in perifoveal astrocyte density cannot be explained by missing invasion or by selectively elevated apoptosis rates in the foveal and perifoveal regions. Alternative hypotheses are discussed.

Retinal maturation is delayed by repeated, but not single, maternal injections of betamethasone in sheep

PURPOSE

The safety and efficacy of prescribing a single maternal course of corticosteroid during pregnancy has been documented in human trials. However, the current trend is to prescribe repeated courses of corticosteroid. We investigated an aspect of the safety of this practice in an animal model.

METHODS

Date-mated ewes received saline, single or four corticosteroid injections between days 104 and 124 of gestation (term = 150). Lambs were delivered on day 125 or 145 by caesarian section after spinal anaesthesia. Eye diameters were measured and semi-thin toluidine-blue-stained transverse sections of retinae were analysed using an Optimus Image Analysis program.

RESULTS

At 125 days, retinal measures in the ventral periphery and area centralis were significantly thinner than control (p = 0.0001). At 145 days, total eye size was significantly reduced compared with control (p = 0.03), and retinal measures in the ventral periphery (p = 0.0001), but not the area centralis (p = 0.19), remained significantly different from control.

CONCLUSION

Repeated maternal administration of corticosteroid may affect retinal maturation in the fetus.

IBMX, taurine and 9-cis retinoic acid all act to accelerate rhodopsin expression in postmitotic cells

Birth dating studies in the rodent retina have shown that rod photoreceptors are generated throughout most of retinal development, yet the majority do not begin to express rhodopsin until the first postnatal week. We show that treatment with 3-isobutyl-1-methylxanthine (IBMX) enhances rod development in reaggregate, explant, and monolayer cultures of embryonic and newborn rat neural retina and is more potent than another rod-promoting factor, taurine, but less potent than 9-cis retinoic acid (RA). The effect of IBMX on rod development is not associated with an increase in precursor cell proliferation, rod survival, or a reduction in the development of other retinal cell types. We provide evidence that IBMX, as well as the rod promoting molecules taurine and RA, all act on postmitotic rhodopsin(-)cells to accelerate their differentiation into rhodopsin(+)cells.

Astrocyte proliferation during development of the human retinal vasculature

Wholemounts of human fetal retinas were labeled with antibodies to Ki67 or proliferating cell nuclear antigen, to map the distribution of proliferating cells in the developing primary vasculature and neural retina. Double labeling was used to determine the relative proportions of endothelial cells (CD34), astrocytes (glial fibrillary acidic protein - GFAP) and microglia (major histocompatability complex class II) associated with the developing vessels. The differentiated region of neural retina (cold spot) was 3.5 mm(2)at 15 weeks gestation (WG), centred on the incipient fovea, and increased in size with age to 80.5 mm(2)by 23-24 WG. Ki67 immunoreactive cells were distributed throughout the developing vasculature at all ages. The mean density of dividing cells in the neural retina increased with gestational age from 146 mm(-2)at 15 WG, to 624 mm(-2)at 23-24 WG. By 20 WG proliferation in the vasculature overlapped the margins of the cold spot, which was almost completely vascularized by 23-24 WG, except for a narrow strip on the horizontal meridian, which included the incipient fovea. Counts of CD34/Ki67 immunoreactive cells indicated that 15-52% of proliferations in the developing vasculature at 18 WG are endothelial cells. In contrast, in the fellow retina 65-85% cells were Ki67/GFAP immunoreactive, indicating proliferation of astrocytes in situ. No dividing microglia were observed. The findings suggest that large numbers of proliferating astrocytes accompany the developing vessels as they migrate across the primate retina.

In vitro analysis of a mammalian retinal progenitor that gives rise to neurons and glia

In vivo lineage studies have shown that retinal cells arise from multipotential progenitors whose fates are regulated by cell-cell interactions. To understand the mechanism underlying their maintenance and differentiation, we have analyzed the differentiation potential of progenitors derived from embryonic rat retina in vitro. These progenitors proliferate and remain undifferentiated in vitro in the presence of epidermal growth factor (EGF) and display properties similar to stem cells. In addition to expressing nestin, the neuroectodermal stem cell marker, retinal progenitors are multipotential. Upon withdrawal of EGF and addition of serum, the progenitors downregulate the expression of nestin and express cell-type specific markers corresponding to neurons and glia. In addition to expressing cell-type specific markers, retinal progenitors and their progeny could be distinguished on the basis of their distinct voltage gated current profile. A proportion of progenitors is lineage restricted and the fate of these cells can be influenced by the microenvironment, suggesting that stage-specific interactions mediated by the local environment influence the progression of progenitors towards acquisition of differentiated phenotypes.

Rod photoreceptor maturation does not vary with retinal eccentricity in mammalian retina

PURPOSE

Test the hypothesis that the development of mammalian rod outer segments (ROS) varies with retinal eccentricity.

METHODS

During the period of photoreceptor cell development, ROS lengths, opsin mRNA and (rhod)opsin were measured in central and peripheral retina of cows and pigmented rats. Published ROS length and/or rhodopsin data from albino rats, cows and monkeys were re-analyzed. Logistic growth curves were fitted to the newly obtained and published data. Within a species, growth in central and peripheral regions was compared.

RESULTS

The logistic growth curves fit all the data well and provide an excellent view of the developmental increases in ROS length, opsin mRNA and (rhod)opsin in each retinal region. Within a species, the growth curves for ROS length, opsin mRNA and (rhod)opsin concentration are superimposable. The age at which ROS length reaches 50% of its adult value is invariant with eccentricity. An exception to this pattern is the simian parafoveal ROS, which appears to have a delayed course of development.

CONCLUSIONS

The hypothesis is disproved. Unlike rod photoreceptor cell genesis, ROS development is invariant with retinal eccentricity. Primate parafoveal ROS appear to have a different pattern of development.

Neurogenesis and commitment of corticospinal neurons in reeler

In the homozygous (but not the heterozygous) reeler mutant, disruption of neuron migration leads to a major perturbation of the cortical environment that in turn could modify (1) the specification of neuronal fate and (2) the proliferation dynamics of cortical precursors. To investigate these issues, tritiated thymidine injections during cortical neurogenesis were coupled with postnatal injections of a retrograde tracer in the spinal cord to accurately measure the neurogenesis of corticospinal neurons in the heterozygous and homozygous mutant. The homozygous reeler shows (1) strict conservation of area-specific timetables of corticospinal neuron generation; (2) neurons with the appropriate birthdates show an enhanced probability of projecting to the spinal cord; (3) during early stages of corticogenesis, there is a reduced rate of neuron production followed at later stages by an increased rate of neuron production; and (4) these changes in the rate of neuron production were shown to be at least partially attributable to changes in the proportions of differentiative divisions. Taken together, our results show that in the developing cortex, the neurogenesis and specification of a given neuronal phenotype are partially controlled by the postmigratory compartment. On the other hand, neither areal identity nor the chronology of production of layer-specific neuronal phenotype seems to depend on the integrity of the cellular environment.

A role for the fibroblast growth factor receptor in cell fate decisions in the developing vertebrate retina

The mature vertebrate retina contains seven major cell types that develop from an apparently homogenous population of precursor cells. Clonal analyses have suggested that environmental influences play a major role in specifying retinal cell identity. Fibroblast growth factor-2 is present in the developing retina and regulates the survival, proliferation and differentiation of developing retinal cells in culture. Here we have tested whether fibroblast growth factor receptor signaling biases retinal cell fate decisions in vivo. Fibroblast growth factor receptors were inhibited in retinal precursors in Xenopus embryos by expressing a dominant negative form of the receptor, XFD. Dorsal animal blastomeres that give rise to the retina were injected with cDNA expression constructs for XFD and a control non-functional mutant receptor, D48, and the cell fates of transgene-expressing cells in the mature retina determined. Fibroblast growth factor receptor blockade results in almost a 50% loss of photoreceptors and amacrine cells, and a concurrent 3.5-fold increase in Muller glia, suggesting a shift towards a Muller cell fate in the absence of a fibroblast growth factor receptor signal. Inhibition of non-fibroblast-growth-factor-mediated receptor signaling with a third mutant receptor, HAVO, alters cell fate in an opposite manner. These results suggest that it is the balance of fibroblast growth factor and non-fibroblast growth factor ligand signals that influences retinal cell genesis.

Neural mechanisms underlying stereoscopic vision

The progressive frontalization of both eyes in mammals causes overlap of the left and right visual fields, having as a consequence a region of binocular field with single vision and stereopsis. The horizontal separation of the eyes makes the retinal images of the objects lying in this binocular field have slight horizontal and vertical differences, termed disparities. Horizontal disparities are the main cue for stereopsis. In the past decades numerous physiological studies made on monkeys, which have in many aspects a similar visual system to humans, showed that a population of visual cells are capable of encoding the amplitude and sign of horizontal disparity. Such disparity detectors were found in cortical visual areas V1, V2, V3, V3A, VP, MT (V5) and MST of monkeys and in the superior colliculus of the cat and opossum. According to their disparity tuning function, these cells were first grouped into tuned excitatory, tuned inhibitory, near and far sub-groups. Subsequent studies added two more categories, tuned near and tuned far cells. Asymmetries between left and right receptive field position, on and off regions, and intra-receptive field wiring are believed to be the neural mechanisms of disparity detection. Because horizontal disparity alone is insufficient to compute reliable stereopsis, additional information about fixation distance and angle of gaze is required. Thus, while there is unequivocal evidence of cells capable of detecting horizontal disparities, it is not known how horizontal disparity is calibrated. Sensitivity to vertical disparity and information about the vergence angle or eye position may be the source of this additional information.

Xath5 participates in a network of bHLH genes in the developing Xenopus retina

We examined the function of basic-helix-loop-helix (bHLH) transcription factors during retinal neurogenesis. We identified Xath5, a Xenopus bHLH gene related to Drosophila atonal, which is expressed in the developing Xenopus retina. Targeted expression of Xath5 in retinal progenitor cells biased the differentiation of these cells toward a ganglion cell fate, suggesting that Xath5 can regulate the differentiation of retinal neurons. We examined the relationship between the three bHLH genes Xash3, NeuroD, and Xath5 during retinal neurogenesis and found that Xash3 is expressed in early retinoblasts, followed by coexpression of Xath5 and NeuroD in differentiating cells. We provide evidence that the expression of Xash3, NeuroD, and Xath5 is coupled and propose that these bHLH genes regulate successive stages of neuronal differentiation in the developing retina.

Regulation of neuroblast cell-cycle kinetics plays a crucial role in the generation of unique features of neocortical areas

Cortical neurons are generated in the germinal zones lining the ventricles before migrating predominantly radially. To investigate regional differences in the cell-cycle kinetics of neuroblasts, pulse [3H]-thymidine injections were made throughout corticogenesis, and labeled neuron counts were compared in areas 3, 6, 17, and 18a in the adult mouse. The relationship between height in the cortex and intensity of autoradiographic signal distinguishes first generation and subsequent generations of neurons. This provides the mitotic history of defined sets of neurons and is a powerful tool for analyzing areal differences in cell-cycle kinetics. The infragranular laminar labeling indices of different generations show significant differences in areas 3 and 6. The labeling index of first generation neurons shows that the rate of neuron production is higher in area 3 than in area 6. This increased generation rate in area 3 was accompanied by two major changes. First, computation of the labeling index of the subsequent generation neurons (which reflects percentages of precursors in S-phase at the moment of the pulse) indicates a shorter cell cycle in area 3. Second, the total population of labeled neurons contains a higher proportion of first generation neurons in area 3, implying a higher leaving fraction in this area. Computer simulations of these areal differences of cell-cycle kinetics generate neuron numbers that are in close agreement with published data. Altogether these findings reveal an early regionalization of the ventricular zone that serves to generate unique features of future cortical areas.

Muller-cell-derived leukaemia inhibitory factor arrests rod photoreceptor differentiation at a postmitotic pre-rod stage of development

In the present study, we examine rod photoreceptor development in dissociated-cell cultures of neonatal mouse retina. We show that, although very few rhodopsin+ rods develop in the presence of 10% foetal calf serum (FCS), large numbers develop in the absence of serum, but only if the cell density in the cultures is high. The rods all develop from nondividing rhodopsin- cells, and new rods continue to develop from rhodopsin- cells for at least 6–8 days, indicating that there can be a long delay between when a precursor cell withdraws from the cell cycle and when it becomes a rhodopsin+ rod. We show that FCS arrests rod development in these cultures at a postmitotic, rhodopsin-, pre-rod stage. We present evidence that FCS acts indirectly by stimulating the proliferation of Muller cells, which arrest rod differentiation by releasing leukaemia inhibitory factor (LIF). These findings identify an inhibitory cell-cell interaction, which may help to explain the long delay that can occur both in vitro and in vivo between cell-cycle withdrawal and rhodopsin expression during rod development.

Spatiotemporal coordination of rod and cone photoreceptor differentiation in goldfish retina

In this study, we have compared spatial and temporal aspects of development of new rods and cones in the adult goldfish by using a combination of bromodeoxyuridine immunocytochemistry and opsin in situ hybridization to determine the intervals between terminal mitosis (cell "birth") and expression of opsin mRNA for each photoreceptor cell type. The goldfish opsins include rod opsin and four different cone opsins: red, green, blue, and ultraviolet. In a cohort of photoreceptors born at the same time, rods expressed opsin mRNA within 3 days of cell birth, while expression of cone opsin mRNA required at least 7 days. This temporal discrepancy in differentiation, coupled with a discordance in the site of cell genesis of rods and cones, allowed opsin expression to commence in both cell types in approximately the same retinal location. Commitment to the generic cone phenotype occurred within approximately 6 days throughout the cone cohort, as indicated by expression of interphotoreceptor retinoid-binding protein (IRBP) mRNA, but expression of a specific spectral phenotype was delayed until rods differentiated nearby. Onset of expression of cone opsin mRNA followed a phenotype-specific sequence: red, then green, then blue, and finally ultraviolet; in situ hybridization with two opsin probes confirmed that individual photoreceptors expressed only one type of opsin as they differentiated. This stepwise process of cone differentiation is consistent with the hypothesis that cell-cell interactions among developing photoreceptors may coordinate selection of specific photoreceptor phenotypes.

Influences on the global structure of cortical maps

Cortical maps often contain global spatial structure: however, theoretical accounts for their development have generally concentrated on reproducing only local structure. We show that the elastic net model of cortical map formation can closely approximate the global structure of the ocular dominance column map observed in macaque primary visual cortex. A key component is the assumption of spatially non-uniform and anisotropic correlations in the retina. This work shows how genetic and epigenetic effects could combine to establish characteristic global structure in cortical maps.

Delta-1 is a regulator of neurogenesis in the vertebrate retina

In the retina, cell fate determination is thought to be regulated by a series of local cell-cell interactions. Evidence suggests that retinal precursors utilize Notch-mediated intercellular signaling to regulate their fates. However, the identity of the endogenous ligand and its role in the Notch-signaling pathway is not well understood. We have identified C-Delta-1 as the putative endogenous ligand for Notch, in the developing chick retina. C-Delta-1 is coexpressed spatially and temporally with C-Notch-1 and their expression is associated with the temporal aspects of cell birth in the developing retina. This suggests that Delta-Notch signaling is utilized to maintain progenitors in an uncommitted state and that a subtle fluctuation in this signaling helps to sort out competent cells during successive cell-fate determination. We have tested the latter possibility in the specification of the ganglion cells. In early stages of retinal development when ganglion cells are the predominant cells born, decreasing C-Delta-1 expression with antisense oligonucleotides increases the proportion of RA4 antigen-expressing ganglion cells which are recruited predominantly in the periphery. Conversely, use of exogenous Drosophila Delta leads to a decrease in the RA4 antigen-expressing ganglion cells. Our results suggest that C-Delta-1 activation of the Notch pathway regulates the specification of retinal neurons in general and of ganglion cells in particular.

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.

cDNA cloning and expression analysis of NeuroD mRNA in human retina

We have shown that bHLH proteins are involved in mammalian retinal development. Here we report the identification and analysis of the expression of a neurogenic differentiation gene, NeuroD, in human retina. In situ hybridization and immunocytochemical analyses of adult retina showed that NeuroD transcripts and NeuroD immunoreactivity are predominantly localized to the outer nuclear layer which contains the photoreceptors. Southern analysis of PCR-amplified cDNA revealed that NeuroD mRNA is also expressed in fetal human retina. Fetal monkey retina was used to analyse the spatial distribution of NeuroD in the developing retina. Both NeuroD transcripts and immunoreactivity are largely detected in the outer neuroblastic layer. Therefore, NeuroD may be involved in the differentiation as well as maintenance of the differentiated properties of photoreceptors.

Regulation of neuronal diversity in the Xenopus retina by Delta signalling

To generate the variety of mature neurons and glia found in the developing retina, the competence of pluripotent progenitor cells to respond to extracellular signals must be controlled. Delta, a ligand of the Notch receptor, is a candidate for regulating progenitor competence on the grounds that activation of the pathway involving Notch and Delta can inhibit cellular differentiation. Here we test this possibility in the developing Xenopus retina by misexpression of Delta messenger RNA. We find that Delta-misexpressing cells with wild-type neighbours adopt earlier fates, primarily becoming ganglion cells and cone photoreceptors. Progenitors transfected with Delta later in development also produce rod photoreceptors, but not the latest-generated cell types, demonstrating the importance of timing in Delta function. We conclude that Delta signalling in the vertebrate retina is a basic regulatory mechanism that can be used to generate neuronal diversity.

Glial, vascular, and neuronal cytogenesis in whole-mounted cat retina

A method was developed for detecting cytogenesis in retinal whole-mount preparations by bromodeoxyuridine (BrdU) immunohistochemistry. Because BrdU is a nonspecific marker that labels all cells in the S phase of the cell cycle, it is ideally combined with other cell-specific markers to study the cytogenesis of specific cell types. Double-label protocols to visualize mitotically active astrocytes and cells associated with the forming vasculature have been developed and applied to the retina. This approach revealed that, during normal development of the kitten retina, vascular mitogenesis occurs predominantly in the ganglion cell and nerve fiber layers, where the inner retinal plexus is formed by a process involving transformation of mesenchymal precursor cells and division of vascular endothelial cells. The peak density of vascular mitogenesis moved in a central-to-peripheral manner and was associated with the leading edge of the forming capillary plexus. A small number of dividing vascular endothelial cells was also associated with angiogenesis, the process responsible for the formation of the outer retinal plexus, vessels at the area centralis, and the radial peripapillary capillaries. Cytogenesis associated with astrocytes occurred in the ganglion cell and nerve fiber layers but was apparent predominantly at or close to the optic nerve head. Confirming earlier studies, neuronal mitogenesis was shown to occur predominantly at the ventricular zone, first at the area centralis and spreading peripherally with increasing maturity. A second region of neuronal cytogenesis, at the subventricular zone, was also apparent. Tissue hyperoxia decreased the rate of vasculogenic cell division but had no apparent effect on neurogenic or astrocytic cell division. Four distinct zones of cell generation were therefore identified within the retina, each associated with either glial, vascular, or neuronal cytogenesis. Thus, BrdU immunohistochemistry in whole-mounted retinal preparations offers a fast and reliable alternative to [3H]thymidine autoradiography for the study of the topography of cytogenesis during development.

Spatiotemporal pattern of ontogenetic expression of calbindin-28/kD in the retinorecipient layers of rat superior colliculus

Using an antibody against calbindin-28kD, we have studied the spatial pattern of expression of this protein in the superior colliculi (SC) of four strains of mature laboratory rats. In all four strains, calbindin-expressing cells (CECs) formed horizontally oriented tiers in the retinorecipient and intermediate gray layers but were diffusely distributed throughout the deep layers. Ontogenetically, calbindin-28kD was expressed for the first time in the retinorecipient layers at postconceptional day 20 (PCD 20), by cells located in the rostrolateral region where the first born retinal ganglion cells (RGCs) are represented. Although on the day of birth (PCD 22/23), the CECs were distributed more widely, they were still absent in the most medial part of the SC, that is, the region where the latest born RGCs are represented. The spatial distribution of CECs became adultlike only by PCD 29, that is, at the end of the period of the naturally occurring death of the RGCs. Monocular eye enucleations on PCD 23 prevented the expression of calbindin in the medial fifth of the retinorecipient layers of the contralateral SC, while the unilateral removal of the visual cortices had no discernable effect on the numbers and distribution of the CECs in either SC. Thus, the spatiotemporal pattern of ontogenetic expression of calbindin-28kD in the retinorecipient layers of SC reflects the spatiotemporal pattern of generation of the RGCs, and the retinal input appears to induce neuronal expression of calbindin-28kD in these layers.

Flk-1, a receptor for vascular endothelial growth factor (VEGF), is expressed by retinal progenitor cells

Throughout development of the vertebrate retina, progenitor cells are multipotential, producing a variety of distinctive cell types. Little is known of the molecular mechanisms directing the determination of cell fate. We have examined retinal progenitor cells for expression of receptor tyrosine kinases in an attempt to define receptors that could allow a progenitor to respond to its environment. We found that the receptor tyrosine kinase Flk-1, previously shown to be expressed in endothelial cells, is also expressed in neural progenitor cells of the mouse retina. Flk-1 RNA expression in the retinal progenitors commences with the onset of neuronal differentiation and persists throughout retinal neurogenesis. Flk-1 RNA and protein levels in the retina vary temporally during development, as shown by in situ hybridization and Western blot analysis. Patterns of beta-galactosidase expression in mice containing the lacZ gene in place of the Flk-1 gene are consistent with Flk-1 being expressed in retinal progenitors. In addition, we show that the ligand of Flk-1, vascular endothelial growth factor (VEGF), is expressed in the developing retina by differentiated cells and that a chimeric ligand of VEGF fused to alkaline phosphatase binds to proliferating retinal progenitors. Furthermore, the neural retina-derived Flk-1 protein kinase is activated by VEGF in vitro. Thus, the Flk-1 receptor protein kinase is expressed on the surface of neural progenitors in mouse retina and may play a critical role in neurogenesis as well as in vasculogenesis.