Cytogenesis in the monkey retina

Visually guided gradation of prey capture movements in larval zebrafish

A mechanistic understanding of goal-directed behavior in vertebrates is hindered by the relative inaccessibility and size of their nervous systems. Here, we have studied the kinematics of prey capture behavior in a highly accessible vertebrate model organism, the transparent larval zebrafish (Danio rerio), to assess whether they use visual cues to systematically adjust their movements. We found that zebrafish larvae scale the speed and magnitude of turning movements according to the azimuth of one of their standard prey, paramecia. They also bias the direction of subsequent swimming movements based on prey azimuth and select forward or backward movements based on the prey's direction of travel. Once within striking distance, larvae generate either ram or suction capture behaviors depending on their distance from the prey. From our experimental estimations of ocular receptive fields, we ascertained that the ultimate decision to consume prey is likely a function of the progressive vergence of the eyes that places the target in a proximal binocular 'capture zone'. By repeating these experiments in the dark, we demonstrate that paramecia are only consumed if they contact the anterior extremities of larvae, which triggers ocular vergence and tail movements similar to close proximity captures in lit conditions. These observations confirm the importance of vision in the graded movements we observe leading up to capture of more distant prey in the light, and implicate somatosensation in captures in the absence of light. We discuss the implications of these findings for future work on the neural control of visually guided behavior in zebrafish.

Aged peripheral retinal lesions originating from the ciliary body sweep away the retinal pigmented epithelium

Aims

To investigate age-related lesions in the far-anterior retina that migrate from the ciliary body (CB) and how they affect the neural retina and retinal pigmented epithelium (RPE).

Methods

One eye from three healthy subjects aged 87, 92 and 93 years were used. Retinae were photographed, embedded in resin and then sectioned at 2 μm.

Results

Multiple elliptically shaped lesions were present in the CB. Larger lesions extended into the peripheral retina. Lesions resulted from deposits that had lenticular qualities. These develop centrally along Bruch's membrane sweeping away the RPE, such that piles of RPE cells were present around the deposits that resulted in retinal atrophy. The internal composition of the deposits revealed large numbers of spherical bodies, unlike those seen in drusen. RPE cells adjacent to these deposits and their underlying lesions became highly irregular, with melanin granules spacing themselves out within the cell and adopting similar orientations. This is a highly distinctive feature.

Conclusions

These far-anterior deposits were different in nature from drusen in terms of morphology, composition and origin. They swept away the RPE, exposing the Bruch's membrane and isolating the retina, leading to atrophy. They appeared to originate from the CB and progressed centrally. The deposits may have developed from the ciliary muscle, which would account for their elongated orientation. Their impact on melanin distribution in RPE cells was unexpected and unusual, implying that they release a signal that influences melanin organisation.

Retinal histogenesis and cell differentiation in an elasmobranch species, the small-spotted catshark Scyliorhinus canicula

Here we present a detailed study of the major events in the retinal histogenesis in a slow-developing elasmobranch species, the small-spotted catshark, during embryonic, postnatal and adult stages using classical histological and immunohistological methods, providing a complete neurochemical characterization of retinal cells. We found that the retina of the small-spotted catshark was fully differentiated prior to birth. The major developmental events in retinal cell differentiation occurred during the second third of the embryonic period. Maturational features described in the present study were first detected in the central retina and, as development progressed, they spread to the rest of the retina following a central-to-peripheral gradient. While the formation of both plexiform layers occurs simultaneously in the retina of the most common fish models, in the small-spotted catshark retina the emergence of the outer plexiform layer was delayed with respect to the inner plexiform layer. According to the expression of the markers used, retinal cell differentiation followed a vitreal-to-scleral gradient, with the exception of Müller cells that were the last cell type generated during retinogenesis. This vitreal-to-scleral progression of neural differentiation seems to be specific to slow-developing fish species.

Human retinal development in an in situ whole eye culture system

Phenotypic characterization of human retinogenesis may be facilitated by use of the tissue culture system paradigm. Traditionally, most culture protocols involve isolation of retinal tissue and/or cells, imposing various degrees of trauma, which in many cases leads to abnormal development. In this paper, we present a novel culture technique using whole embryonic eyes to investigate whether the retina in situ can develop normally in vitro. All procedures were carried out in accordance with the Declaration of Helsinki. Human embryos were obtained from elective abortions with the informed consent of the women seeking abortion. A total of 19 eyes were enucleated. The ages of the embryonic retinas were 6-7.5 weeks. Eyecups from 2 eyes were fixed immediately, to be used as controls. The remaining 17 eyes were placed on culture plates and divided into 3 groups kept for 7 (n = 4), 14 (n = 7) and 28 (n = 6) days in vitro (DIV). After fixation, the specimens were processed for hematoxylin and eosin staining, immunohistochemistry and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). Antibodies against recoverin (rods and cones), protein kinase C (PKC; rod bipolar cells) and vimentin (Müller cells) were used. TUNEL was used to detect apoptotic cells. In hematoxylin- and eosin-stained sections, the control retinas displayed a neuroblast cell layer (NBL) and an inner marginal zone. Specimens kept 7-14 DIV had a similar appearance, while 28-day specimens consisted of an NBL with almost no marginal zone. Thirteen of the 17 cultured retinas displayed completely normal lamination without rosettes or double folds. Pyknotic cells were found at the inner margin of the retinas, and the proportion of these cells increased with time in vitro. TUNEL staining revealed a few scattered cells in 7-DIV specimens, and the amount of stained cells in the inner part of the retinas progressively increased in 14- and 28-DIV specimens. Vimentin labeling showed cells arranged in a vertical pattern in all retinas. Labeling with recoverin revealed photoreceptors in 4 of the retinas kept for 14 DIV, and in all retinas kept for 28 DIV. After 28 DIV, 2 of the eyes labeled with PKC contained rod bipolar cells with minimal axons. Here we showed that human embryonic retinas can be kept in culture in situ within the eye for at least 4 weeks. Abnormal lamination is not as frequent as in isolated full-thickness retinas, indicating that physical and biochemical contact with surrounding tissues is vital for proper development. Several types of the retina-specific neuronal and glial cells were seen to differentiate according to the in vivo schedule. The results are important for future studies of retinal development, and the technique can also be used for testing the effects of various drugs on the immature retina.

Ascl1 expression defines a subpopulation of lineage-restricted progenitors in the mammalian retina

The mechanisms of cell fate diversification in the retina are not fully understood. The seven principal cell types of the neural retina derive from a population of multipotent progenitors during development. These progenitors give rise to multiple cell types concurrently, suggesting that progenitors are a heterogeneous population. It is thought that differences in progenitor gene expression are responsible for differences in progenitor competence (i.e. potential) and, subsequently, fate diversification. To elucidate further the mechanisms of fate diversification, we assayed the expression of three transcription factors made by retinal progenitors: Ascl1 (Mash1), Ngn2 (Neurog2) and Olig2. We observed that progenitors were heterogeneous, expressing every possible combination of these transcription factors. To determine whether this progenitor heterogeneity correlated with different cell fate outcomes, we conducted Ascl1- and Ngn2-inducible expression fate mapping using the CreER™/LoxP system. We found that these two factors gave rise to markedly different distributions of cells. The Ngn2 lineage comprised all cell types, but retinal ganglion cells (RGCs) were exceedingly rare in the Ascl1 lineage. We next determined whether Ascl1 prevented RGC development. Ascl1-null mice had normal numbers of RGCs and, interestingly, we observed that a subset of Ascl1+ cells could give rise to cells expressing Math5 (Atoh7), a transcription factor required for RGC competence. Our results link progenitor heterogeneity to different fate outcomes. We show that Ascl1 expression defines a competence-restricted progenitor lineage in the retina, providing a new mechanism to explain fate diversification.

Mouse retinal development: a dark horse model for systems biology research

The developing retina is an excellent model to study cellular fate determination and differentiation in the context of a complex tissue. Over the last decade, many basic principles and key genes that underlie these processes have been experimentally identified. In this review, we construct network models to summarize known gene interactions that underlie determination and fundamentally affect differentiation of each retinal cell type. These networks can act as a scaffold to assemble subsequent discoveries. In addition, these summary networks provide a rational segue to systems biology approaches necessary to understand the many events leading to appropriate cellular determination and differentiation in the developing retina and other complex tissues.

A single enhancer regulating the differential expression of duplicated red-sensitive opsin genes in zebrafish

A fundamental step in the evolution of the visual system is the gene duplication of visual opsins and differentiation between the duplicates in absorption spectra and expression pattern in the retina. However, our understanding of the mechanism of expression differentiation is far behind that of spectral tuning of opsins. Zebrafish (Danio rerio) have two red-sensitive cone opsin genes, LWS-1 and LWS-2. These genes are arrayed in a tail-to-head manner, in this order, and are both expressed in the long member of double cones (LDCs) in the retina. Expression of the longer-wave sensitive LWS-1 occurs later in development and is thus confined to the peripheral, especially ventral-nasal region of the adult retina, whereas expression of LWS-2 occurs earlier and is confined to the central region of the adult retina, shifted slightly to the dorsal-temporal region. In this study, we employed a transgenic reporter assay using fluorescent proteins and P1-artificial chromosome (PAC) clones encompassing the two genes and identified a 0.6-kb “LWS-activating region” (LAR) upstream of LWS-1, which regulates expression of both genes. Under the 2.6-kb flanking upstream region containing the LAR, the expression pattern of LWS-1 was recapitulated by the fluorescent reporter. On the other hand, when LAR was directly conjugated to the LWS-2 upstream region, the reporter was expressed in the LDCs but also across the entire outer nuclear layer. Deletion of LAR from the PAC clones drastically lowered the reporter expression of the two genes. These results suggest that LAR regulates both LWS-1 and LWS-2 by enhancing their expression and that interaction of LAR with the promoters is competitive between the two genes in a developmentally restricted manner. Sharing a regulatory region between duplicated genes could be a general way to facilitate the expression differentiation in duplicated visual opsins.

Among vertebrates, fish may have the most advanced color vision. They have greatly varied repertoires of color sensors called visual opsins, possibly reflecting evolutionary adaptation to their diverse photic environments in water, and are an excellent model to study the evolution of vertebrate color vision. This is achieved by multiplying opsin genes and differentiating their absorption light spectra and expression patterns. However, little is understood regarding how the opsin genes are regulated to achieve the differential expression pattern. In this study, we focused on the duplicated red-sensitive opsin genes of zebrafish to tackle this problem. We discovered an “enhancer” region near the two red opsin genes that plays a crucial role in their differential expression pattern. Our results suggest that the two red opsin genes interact with the enhancer competitively in a developmentally restricted manner. Sharing a regulatory region could be a general way to facilitate the expression differentiation in duplicated visual opsin genes.

Birthdays of retinal amacrine cell subtypes are systematically related to their molecular identity and soma position

The mammalian retina contains six major cell types, several of which are divided into multiple molecularly and morphologically distinct subtypes. To understand how subtype diversity arises during development, we focused on amacrine interneurons in the mouse retina; approximately 30 amacrine subtypes have been identified in mammals. We used antibody markers to identify the two main amacrine subsets--GABAergic and glycinergic--and further subdivided these groups into smaller subsets based on expression of neurotransmitter and transcription factor markers. We then used bromodeoxyuridine (BrdU) labeling to see whether amacrine subsets are born (become postmitotic) at different times, as is the case for lamina-specified subsets of cortical projection neurons. We found that GABAergic amacrines are generated on average 2-3 days before glycinergic amacrines. Moreover, subsets of GABAergic amacrines are born at distinct times. We also found a strong correlation between amacrine cell birthday and soma position in the mature retina, another point of similarity with cortical projection neurons. This relationship raised the possibility that amacrine subtype identity is determined by signals that uncommitted cells receive after they migrate to their destinations. However, cells labeled with BrdU in vivo, then dissociated and allowed to develop in vitro, acquired the amacrine subtype-specific markers appropriate for their birthdays, supporting the idea that they become specified near the time and place of their birth. Together, our results suggest that the birthdays of amacrine cells independently specify their destinations and subtype identities.

Retinal progenitor cells, differentiation, and barriers to cell cycle reentry

Neurogenesis in the retina occurs via the coordination of proliferation, cell cycle exit and differentiation of retinal progenitor cells. Until recently, it was widely assumed that once a retinal progenitor cell produced a postmitotic neuron, there was no possibility for cell-cycle re-entry. However, recent studies have shown that mature differentiated horizontal neurons with reduced Rb pathway function can re-enter the cell cycle and proliferate while maintaining their differentiated features. This chapter will explore the molecular and cellular mechanisms that help to keep differentiated retinal neurons and glia postmitotic. We propose that there are cell-type specific barriers to cell-cycle re-entry by differentiated neurons and these may include apoptosis, chromatin/epigenetics mechanisms, cellular morphology and/or metabolic demands that are distinct across cell populations. Our data suggest that differentiated neurons span a continuum of cellular properties related to their ability to re-enter the cell cycle and undergo cytokinesis while maintaining their differentiated features. A deeper understanding of these processes may allow us to begin to explain the cell type specificity of neuronal cell death and tumor susceptibility. For example, neurons that have more barriers to cell-cycle re-entry may be less likely to form tumors but more likely to undergo degeneration. Conversely, neurons that have fewer barriers to cell-cycle re-entry may be more likely to form tumors but less likely to undergo degeneration.

Defining retinal progenitor cell competence in Xenopus laevis by clonal analysis

Extrinsic cues and intrinsic competence act in concert for cell fate determination in the developing vertebrate retina. However, what controls competence and how precise is the control are largely unknown. We studied the regulation of competence by examining the order in which individual retinal progenitor cells (RPCs) generate daughters. Experiments were performed in Xenopus laevis, whose full complement of retinal cells is formed in 2 days. We lineage-labeled RPCs at the optic vesicle stage. Subsequently we administered a cell cycle marker, 5-bromodeoxyuridine (BrdU) at early, middle or late periods of retinogenesis. Under these conditions, and in this animal, BrdU is not cleared by the time of analysis, allowing cumulative labeling. All retinal cell types were generated throughout nearly the entire retinogenesis period. When we examined the order that individual RPCs generated daughters, we discovered a regular and consistent sequence according to phenotype: RGC, Ho, CPr, RPr, Am, BP, MG. The precision of the order between the clones supports a model in which RPCs proceed through stepwise changes in competence to make each cell type, and do so unidirectionally. Because every cell type can be generated simultaneously within the same retinal environment, the change in RPC competence is likely to be autonomous.

Horizontal cell progenitors arrest in G2-phase and undergo terminal mitosis on the vitreal side of the chick retina

We have addressed the question when horizontal cells in the chick retina are generated and undergo their terminal mitosis. Horizontal cell progenitors replicate their DNA early and migrate bi-directionally to the horizontal cell layer. It was hypothesized that the cells undergo mitosis directly after replication and migrate as post-mitotic transition cells before differentiating to horizontal cells. However, our results show that cells expressing markers for the axon-bearing and the axon-less subtypes of horizontal cells undergo terminal mitosis while residing on the vitreal side of the retina. By combining horizontal cell transcription factors Lim1, Isl1 and Prox1 labeling with phospho-histone H3, a marker for mitosis, we demonstrate that all or a clear majority of vitreal mitoses are undertaken by the horizontal cell committed progenitors. The pattern of cells that incorporated the thymidine analogue EdU implied that the progenitors replicated their genome while migrating towards the vitreal side. Upon arrival to the vitreal retina they become arrested for about two days prior to mitosis. Hence, cells expressing horizontal cell markers are arrested in G2-phase on the vitreal side of the retina. These results support the existence of committed progenitors that give rise to horizontal cells and that those cells become arrested in G2-phase before undergoing terminal mitosis on the vitreal side of the retina followed by migration to the horizontal cell layer. The results also indicate that the regulation of the transition from G2-phase to mitosis is important for the development of these committed progenitor cells.

Retinal neurogenesis and ontogenetic changes in the visual system of the brown banded bamboo shark, Chiloscyllium punctatum (Hemiscyllidae, Elasmobranchii)

The development of the eye of the oviparous brown banded bamboo shark, Chiloscyllium punctatum, was monitored from egg deposition through adulthood. The order and timing of retinal cell differentiation were assessed by light and transmission electron microscopy. As in other vertebrates, the ganglion cells are the first to differentiate, in this case by 81 days post-egg deposition (dpd). The order then deviates from what is typically quoted for vertebrates, with the Müller and amacrine cells differentiating morphologically around the same time, followed by the bipolar cells (101 dpd) and finally the horizontal cells and photoreceptors (124 dpd). The neural retina is fully differentiated and synaptic connections are formed approximately 1 month prior to hatching, which occurs at about 158 dpd. The mature retina is duplex, with a peak rod to cone ratio of approximately 12:1. The eye and lens of C. punctatum continue to grow throughout life and become less aspherical with growth; the equatorial (nasotemporal) lens diameter is 12% larger than the axial (anterior-posterior) lens diameter in embryos and 8% larger in adults. Access to developmental stages and the protracted gestational period of C. punctatum make it a highly valuable model for developmental studies of the visual system. This study also provides an evolutionary perspective on retinal neurogenesis in an elasmobranch.

Expression of synaptic and phototransduction markers during photoreceptor development in the marmoset monkey Callithrix jacchus

Marmoset photoreceptor development was studied to determine the expression sequence for synaptic, opsin, and phototransduction proteins. All markers appear first in cones within the incipient foveal center or in rods at the foveal edge. Recoverin appears in cones across 70% of the retina at fetal day (Fd) 88, indicating that it is expressed shortly after photoreceptors are generated. Synaptic markers synaptophysin, SV2, glutamate vesicular transporter 1, and CTBP2 label foveal cones at Fd 88 and cones at the retinal edge around birth. Cones and rods have distinctly different patterns of synaptic protein and opsin expression. Synaptic markers are expressed first in cones, with a considerable delay before they appear in rods at the same eccentricity. Cones express synaptic markers 2-3 weeks before they express opsin, but rods express opsin 2-4 weeks before rod synaptic marker labeling is detected. Medium/long-wavelength-selective (M&L) opsin appears in foveal cones and rod opsin in rods around the fovea at Fd 100. Very few cones expressing short-wavelength-selective (S) opsin are found in the Fd 105 fovea. Across peripheral retina, opsin appears first in rods, followed about 1 week later by M&L cone opsin. S cone opsin appears last, and all opsins reach the retinal edge by 1 week after birth. Cone transducin and rod arrestin are expressed concurrently with opsin, but cone arrestin appears slightly later. Marmoset photoreceptor development differs from that in Macaca and humans. It starts relatively late, at 56% gestation, compared with Macaca at 32% gestation. The marmoset opsin expression sequence is also different from that of either Macaca or human.

Gradual morphogenesis of retinal neurons in the peripheral retinal margin of adult monkeys and humans

The adult mammalian retina has for long been considered to lack a neurogenerative capacity. However, retinal stem/progenitor cells, which can originate retinal neurons in vitro, have been recently reported in the ciliary body of adult mammals. Here we explored the possibility of retinal neurogenesis occurring in vivo in adult monkeys and humans. We found the presence of cells expressing molecular markers of neural and retinal progenitors in the nonlaminated retinal margin and ciliary body pars plana of mature primates. By means of immunohistochemistry and electron microscopy we also observed photoreceptors and other retinal cell types in different stages of morphological differentiation along the peripheral retinal margin. These findings allow us to extend to primates the idea of neurogenesis aimed at retinal cell turnover throughout life.

Topography of the long- to middle-wavelength sensitive cone ratio in the human retina assessed with a wide-field color multifocal electroretinogram

The topographical distribution of relative sensitivity to red and green lights across the retina was assayed using a custom-made wide-field color multifocal electroretinogram apparatus. There were increases in the relative sensitivity to red compared to green light in the periphery that correlate with observed increases in the relative amount of long (L) compared to middle (M) wavelength sensitive opsin mRNA. These results provide electrophysiological evidence that there is a dramatic increase in the ratio of L to M cones in the far periphery of the human retina. The central to far peripheral homogeneity in cone proportions has implications for understanding the developmental mechanisms that determine the identity of a cone as L or M and for understanding the circuitry for color vision in the peripheral retina.

Sox9 is expressed in mouse multipotent retinal progenitor cells and functions in Müller glial cell development

It is widely accepted that the process of retinal cell fate determination is under tight transcriptional control mediated by a combinatorial code of transcription factors. However, the exact repertoire of factors necessary for the genesis of each retinal cell type remains to be fully defined. Here we show that the HMG-box transcription factor, Sox9, is expressed in multipotent mouse retinal progenitor cells throughout retinogenesis. We also find that Sox9 is downregulated in differentiating neuronal populations, yet expression in Müller glial cells persists into adulthood. Furthermore, by employing a conditional knockout approach, we show that Sox9 is essential for the differentiation and/or survival of postnatal Müller glial cells.

Rod photoreceptor differentiation in fetal and infant human retina

Human rods and cones are arranged in a precise spatial mosaic that is critical for optimal functioning of the visual system. However, the molecular processes that underpin specification of cell types within the mosaic are poorly understood. The progressive differentiation of human rods was tracked from fetal week (Fwk) 9 to postnatal (P) 8 months using immunocytochemical markers of key molecules that represent rod progression from post-mitotic precursors to outer segment-bearing functional photoreceptors. We find two phases associated with rod differentiation. The early phase begins in rods on the foveal edge at Fwk 10.5 when rods are first identified, and the rod-specific proteins NRL and NR2e3 are detected. By Fwk 11-12, these rods label for interphotoreceptor retinoid binding protein, recoverin, and aryl hydrocarbon receptor interacting protein-like 1. The second phase occurs over the next month with the appearance of rod opsin at Fwk 15, closely followed by the outer segment proteins rod GTP-gated sodium channel, rod arrestin, and peripherin. TULP is expressed relatively late at Fwk 18-20 in rods. Each phase proceeds across the retina in a central-peripheral order, such that rods in far peripheral retina are only entering the early phase at the same time that cells in central retina are entering their late phase. During the second half of gestation rods undergo an intracellular reorganization of these proteins, and cellular and OS elongation which continues into infancy. The progression of rod development shown here provides insight into the possible mechanisms underlying human retinal visual dysfunction when there are mutations affecting key rod-related molecules.

Number and topography of cones, rods and optic nerve axons in New and Old World primates

To better understand the evolution of spatial and color vision, the number and spatial distributions of cones, rods, and optic nerve axon numbers were assessed in seven New World primates (Cebus apella, Saimiri ustius, Saguinus midas niger, Alouatta caraya, Aotus azarae, Calllithrix jacchus, and Callicebus moloch). The spatial distribution and number of rods and cones was determined from counts of retinal whole mounts. Optic axon number was determined from optic nerve sections by electron microscopy. These data were amassed with existing data on retinal cell number and distribution in Old World primates, and the scaling of relative densities and numbers with respect to retinal area, eye and brain sizes, and foveal specializations were evaluated. Regular scaling of all cell types was observed, with the exceptionally large, rod-enriched retina of the nocturnal owl monkey Aotus azarae, and the unusually high cone density of the fovea of the trichromatic howler monkey Alouatta caraya presenting interesting variations on this basic plan. Over all species, the lawful scaling of rods, cones, and retinal ganglion cell number is hypothesized to result from a conserved sequence of cell generation that defends retinal acuity and sensitivity over a large range of eye sizes.

Expression of the homeodomain transcription factor Meis2 in the embryonic and postnatal retina

Members of the Meis subfamily of homeodomain-containing transcription factors play important roles during development and disease. Here we report that the Meis family protein Meis2 is expressed by a subpopulation of gamma-aminobutyric acid (GABA)ergic amacrine (AM) cells in the adult and embryonic retina of different vertebrate species. In mice, Meis2-expressing (Meis2+) AM cells are not cholinergic or dopaminergic, but some are immunoreactive for neuronal nitric oxide synthase (bNOS). About 50% of the mouse Meis2+ AM cell population expresses the calcium-binding protein calretinin, and some Meis2+ AM cells show characteristics of Type II CD-15+ cells. AM cell expression of Meis2 is lost in a conditional knockout mouse model for Pax6, indicating a dependency upon Pax6. Bromodeoxyuridine pulse labeling experiments and immunohistochemical staining for the neuronal marker NeuN in embryonic mouse retinae indicate that Meis2 is an early marker for newly postmitotic AM cells. In addition, taking advantage of the protracted retinal development in humans, we show that newly generated AM cells express Meis2 before adopting the GABAergic or glycinergic neurotransmitter phenotype. As development proceeds, some AM cells lose Meis2 expression concomitantly with the appearance of glycine, while other AM cells retain Meis2 expression after they express GABA. These data identify Meis2 as a suitable marker for the study of AM cell diversity and development in addition to providing evidence for the stepwise specification of the glycinergic and GABAergic neurotransmitter phenotypes during AM cell differentiation.

Dlx2 homeobox gene transcriptional regulation of Trkb neurotrophin receptor expression during mouse retinal development

Dlx homeobox genes are first expressed in embryonic retina at E11.5. The Dlx1/Dlx2 null retina has a reduced ganglion cell layer (GCL), with loss of late-born differentiated retinal ganglion cells (RGCs) due to increased apoptosis. TrkB signaling is proposed to regulate the dynamics of RGC apoptosis throughout development. DLX2 expression markedly precedes the onset of TrkB expression in the GCL; TrkB co-expression with Dlx2 and RGC markers is well-established by E13.5. In the Dlx1/Dlx2 null retina, TrkB expression is significantly reduced by E16.5. We demonstrated that DLX2 binds to a specific region of the TrkB promoter in retinal neuroepithelium during embryogenesis. In vitro confirmation and the functional consequences of DLX2 binding to this TrkB regulatory region support TrkB as a Dlx2 transcriptional target. Furthermore, ectopic Dlx2 expression in retinal explants activates TrkB expression and Dlx2 knockdown in primary retinal cultures results in reduced TrkB expression. RGC differentiation and survival require the coordinated expression of transcription factors. This study establishes a direct transcriptional relationship between a homeodomain protein involved in RGC differentiation and a neurotrophin receptor implicated in RGC survival. Signaling mediated by TrkB may contribute to survival of late-born RGCs whose terminal differentiation is regulated by Dlx gene function.

Expression patterns of calretinin, calbindin and parvalbumin and their colocalization in neurons during development of Macaca monkey retina

The developmental expression of calbindin (CalB), calretinin (CaR) and parvalbumin (PV) was followed in Macaca monkey retina using single and double immunolabeling to identify which proteins provide distinctive labels for specific cell types and to clarify the role of these proteins during development. Ganglion cells (GC) expressed PV at fetal day (Fd)55 and CaR and CalB by Fd85. CaR was downregulated after birth. Separate subsets of amacrine (AM) cells expressed CaR and CalB at Fd65-70. After Fd115, many CaR+ AM coexpressed CalB. After Fd120 a few AM expressed PV and these added CaR and CalB after birth. A subset of horizontal cells (HZ) expressed CaR and CalB at Fd70. Slightly later all HZ express PV and CaR while the early subset is CalB+/PV+/CaR+. CaR downregulates in all HZ after birth. The DB3 cone bipolar cells (BP) under the HZ label for CalB by Fd90-110 while a probable OFF BP cell body just above the AM layer becomes CaR+ near birth with labeling increasing after birth. All cones outside of the fovea label for CalB by Fd125. Foveal cones, rods, most BP and Müller glia do not label for these proteins at any age. The complex patterns of up- and down-regulation found in Macaca retina are similar to previous reports of expression in human retina, but in many instances are quite different than earlier reports of CaR, CalB and PV expression patterns in monkey central visual centers. This makes it highly likely that each protein plays a specific but undetermined role(s) in each visual center, and that its expression is controlled at a given stage of retinal development by multiple intrinsic and extrinsic factors.

Development of scotopic visual thresholds in retinopathy of prematurity

PURPOSE

To test the hypothesis that the late-maturing parafoveal rod photoreceptors are more vulnerable than peripheral rods to the effects of retinopathy of prematurity (ROP).

METHODS

Twenty-four infants with a history of preterm birth (gestational age at birth </=31 weeks) participated in a longitudinal study: 12 had mild ROP that resolved without treatment, and 12 had never had ROP. Thresholds for detecting stimuli (2 degrees diameter, 50 ms duration) presented 10 degrees (parafoveal) and 30 degrees (peripheral) from a central fixation target were estimated by using a preferential-looking

METHOD

At each visit, thresholds at both sites were obtained in random order. Thresholds of the preterm subjects were compared with those of previously reported term infants.

RESULTS

The course of threshold maturation in subjects with ROP was significantly prolonged (P </= 0.01) compared with those who had never had ROP and with term-born control subjects. On average, parafoveal thresholds in subjects with ROP reached the adult level at a median age of 12 (range, 6-18) months, and peripheral thresholds reached the adult level at 9 (range, 5-12) months. Median thresholds in subjects who had never had ROP reached adult levels at both sites by approximately 7 months.

CONCLUSIONS

The slower development of parafoveal compared with peripheral thresholds in subjects with a history of ROP is evidence that the late-maturing parafoveal rods are more affected by the ROP disease process.

Identification of a locus control region for quadruplicated green-sensitive opsin genes in zebrafish

Duplication of opsin genes has a crucial role in the evolution of visual system. Zebrafish have four green-sensitive (RH2) opsin genes (RH2-1, RH2-2, RH2-3, and RH2-4) arrayed in tandem. They are expressed in the short member of the double cones (SDC) but differ in expression areas in the retina and absorption spectra of their encoding photopigments. The shortest and the second shortest wavelength subtypes, RH2-1 and RH2-2, are expressed in the central-to-dorsal retina. The longer wavelength subtype, RH2-3, is expressed circumscribing the RH2-1/RH2-2 area, and the longest subtype, RH2-4, is expressed further circumscribing the RH2-3 area and mainly occupying the ventral retina. The present report shows that a 0.5-kb region located 15 kb upstream of the RH2 gene array is an essential regulator for their expression. When the 0.5-kb region was deleted from a P1-artificial chromosome (PAC) clone encompassing the four RH2 genes and when one of these genes was replaced with a reporter GFP gene, the GFP expression in SDCs was abolished in the zebrafish to which a series of the modified PAC clones were introduced. Transgenic studies also showed that the 0.5-kb region conferred the SDC-specific expression for promoters of a non-SDC (UV opsin) and a nonretinal (keratin 8) gene. Changing the location of the 0.5-kb region in the PAC clone conferred the highest expression for its proximal gene. The 0.5-kb region was thus designated as RH2-LCR analogous to the locus control region of the L-M opsin genes of primates.

Mechanisms controlling Pax6 isoform expression in the retina have been conserved between teleosts and mammals

The Pax6 gene plays several roles in retinal development, including control of cell proliferation, maintenance of the retinogenic potential of progenitor cells, and cell fate specification. Emerging evidence suggests that these different aspects of Pax6 gene function are mediated by different isoforms of the Pax6 protein; however, relatively little is known about the spatiotemporal expression of Pax6 isoforms in the vertebrate retina. Using bacterial artificial chromosome (BAC) technology, we modified a zebrafish Pax6a BAC such that we could distinguish paired-containing Pax6a transcripts from paired-less Pax6a transcripts. In the zebrafish, the spatial and temporal onset of expression of these transcripts suggests that the paired-less isoform is involved in the cell fate decision leading to the generation of amacrine cells; however, because of limitations associated with transient transgenic analysis, it was not feasible to establish whether this promoter was active in all amacrine cells or in a specific population of amacrine cells. By making mice transgenic for the zebrafish Pax6a BAC reporter transgene, we were able to show that paired-containing and paired-less Pax6a transcripts were differentially expressed in amacrine subpopulations. Our study also directly demonstrates the functional conservation of the regulatory mechanisms governing Pax6 transcription in teleosts and mammals.

An urn model of the development of L/M cone ratios in human and macaque retinas

A model of the development of L/M cone ratios in the Old World primate retina is presented. It is supposed that during gestation, the cone cycles randomly between states in which it transcribes either L or M opsin. The current state determines and increases the probability that it will transcribe the same opsin in future cycles. These assumptions are sufficient to formalize the process as a Markov chain that can be modeled as an urn containing two types of balls, L and M. Drawing one ball results in the increase of its species and the decrease of the other. Over the long run, the urn will become populated with a single type of ball. This state corresponds to the photoreceptor adopting a fixed identity for its lifetime. We investigate the effect of the number of states and the rule that regulates the advantage of transition toward one cone type or another on the relation between fetal and adult L/M cone ratios. In the range of 100 to 1000 states, small variations of the initial L/M ratio or the transition advantage can each generate large changes in the final L/M ratio, in qualitative accord with the variation seen in human adult retinas. The time course to attain stable L/M ratios also varies with these parameters. If it is believed that the cycling follows a circadian rhythm, then final L/M cone ratios would be expected to stabilize shortly after birth in the human being and the macaque.

Topography of long- and middle-wavelength sensitive cone opsin gene expression in human and Old World monkey retina

The topographical distributions of the relative ratio of long- (L) and middle- (M) wavelength sensitive cone opsin messenger RNA (mRNA) in human and baboon retinas were mapped using real-time polymerase chain reaction. The L:M mRNA ratio increased in a central-to-peripheral gradient in both species, being quite pronounced for humans.

Neurogenesis and cell death in the ganglion cell layer of vertebrate retina

The correct formation of all central nervous system tissues depends on the proper balance of neurogenesis and developmental cell death. A model system for studying these programs is the ganglion cell layer (GCL) of the vertebrate retina because of its simple and well-described structure and amenability to experimental manipulations. The GCL contains approximately equal numbers of ganglion cells and displaced amacrine cells. Ganglion cells are the first or among the first cells born in the retina in all the studied vertebrates. Neurogenesis and cell death have been studied extensively in the GCL of various amniotes (rodents, chicks, and monkeys) and anamniotes (fish and frogs), and the two processes highlight developmental differences between the groups. In amniotes, neurogenesis occurs during a defined period prior to birth/hatch or the opening of the eyes, whereas in anamniotes, neurogenesis extends past hatching into adulthood-sometimes for years. Roughly half of GCL neurons die during development in amniotes, whereas developmental cell death does not occur in the GCL neurons of anamniotes. This review discusses the spatial and temporal patterns of neurogenesis, cell death, and possible explanation of cell death in the GCL. It also examines markers widely used to distinguish between ganglion cells and displaced amacrine cells, and methods employed to birth date neurons.

ERG oscillatory potentials in infants

PURPOSE

Study the scotopic and photopic oscillatory potentials (OPs) of the electroretinogram (ERG) in 10-week old infants.

METHODS

Term-born 10-week old infants (n = 15) and adults (n = 12) were tested. Full-field ERGs were recorded under scotopic and photopic conditions. The records were filtered (75-300 Hz) to demonstrate the oscillatory wavelets. The amplitudes and implicit times of the infants' OPs were compared to those in adults and also to amplitudes of the saturated photoreceptor responses.

RESULTS

In infants, the mean OP amplitudes are similar in scotopic and photopic conditions and do not vary significantly with OP number. Infants' OPs are significantly smaller than in adults, with scotopic OPs averaging 19% of that in adults and photopic OPs averaging 47%, whereas the amplitudes of the saturated photoreceptor responses are 43% and 66% of those in adults. Mean interpeak intervals are similar in infants and adults, indicating oscillatory behavior at a frequency of 155 Hz in scotopic conditions and 135 Hz in photopic conditions.

CONCLUSIONS

In young infants, the OPs are relatively immature compared to the photoreceptor responses, with the immaturity of the scotopic responses being more marked than that of the photopic responses.

Spatial and temporal patterns of proliferation and differentiation in the developing turtle eye

Here we show for the first time different aspects of the pattern of neurogenesis in the developing turtle retina by using different morphological and molecular clues. We show the chronotopographical fashion of occurrence of three major aspects of retinal development: (1) morphogenesis of the optic primordia and emergence of the different retinal layers, (2) the temporal progression of neurogenesis by the cessation of proliferative activity, and (3) the apparition and cellular localization of different antigens and neuroactive substances. Retinal cells were generated in a conserved temporal order with ganglion cells born first, followed by amacrine, photoreceptor, horizontal and bipolar/Müller cells. While eventually expressed in many types of retinal neurons, Islet1 was permanently expressed in differentiating and mature ganglion cells. Calbindin-immunoreactive elements were found in the ganglion cell layer and the inner nuclear layer. Interestingly, at later stages the amount of expressing cells in these layers was reduced dramatically. On the contrary, the number of calbindin-immunoreactive photoreceptors increased as development proceeded. In addition, calretinin expressing cells were prominent in the horizontal cell bodies, and their processes extending into the outer plexiform layer were also strongly labeled. Finally, the synthesis of gamma-aminobutyric acid (GABA) was detected in developing and matured horizontal and amacrine cells. All these maturational features began in the dorso-central area, in a region slightly displaced towards the temporal retina.

Epidermal Growth Factor Is a Neuronal Differentiation Factor for Retinal Stem Cells In Vitro

Stem cells are a tool for in vitro elucidation of the putative role of factors on cell fate. Herein we analyze the role of epidermal growth factor (EGF) on progeny derived from retinal stem cells (RSCs). We isolated cells from neuroretinas of neonate mice. All the proliferating cells harbored the radial glia marker RC2, expressed transcription factors usually found in radial glia (Mash1, Pax6), and met the criteria of stem cells: high capacity of expansion, maintenance of an undifferentiated state, and multipotency demonstrated by clonal analysis. We analyzed the differentiation 7 days after transfer of the cells in different culture media. In absence of serum, EGF led to the expression of the neuronal marker beta-tubulin-III and acquisition of neuronal morphology in 15% of the cells. Analysis of cell proliferation by bromodeoxyuridine incorporation revealed that EGF mainly induced the formation of neurons without stimulating cell cycle progression. Moreover, a pulse of 2-hour EGF stimulation was sufficient to induce neuronal differentiation. Some neurons were committed to the retinal ganglion cell (RGC) phenotype, as revealed by the expression of retinal ganglion markers (Ath5, Brn3b, and melanopsin) and in a few cases to other retinal phenotypes (photoreceptors [PRs] and bipolar cells). We confirmed that the late RSCs were not restricted over time and that they conserved their multipotency by generating retinal phenotypes that usually appear at early (RGC) or late (PRs) developmental stages. Our results show that EGF is not only a factor controlling glial development, as previously shown, but also a potent differentiation factor for retinal neurons, at least in vitro.

Early and rapid targeting of eye-specific axonal projections to the dorsal lateral geniculate nucleus in the fetal macaque

The emergence of eye-specific axonal projections to the dorsal lateral geniculate nucleus (dLGN) is a well established model system for exploring the mechanisms underlying afferent targeting during development. Using modern tract tracing methods, we examined the development of this feature in the macaque, an Old World Primate with a visual system similar to that of humans. Cholera toxin beta fragment conjugated to Alexa 488 was injected into the vitreous of one eye, and CTbeta conjugated to Alexa 594 into the other eye of embryos at known gestational ages. On embryonic day 69 (E69), which is approximately 100 d before birth, inputs from the two eyes were extensively intermingled in the dLGN. However, even at this early age, portions of the dLGN were preferentially innervated by the right or left eye, and segregation is complete within the dorsalmost layers 5 and 6. By E78, eye-specific segregation is clearly established throughout the parvocellular division of the dLGN, and substantial ocular segregation is present in the magnocellular division. By E84, segregation of left and right eye axons is essentially complete, and the six eye-specific domains that characterize the mature macaque dLGN are clearly discernable. These findings reveal that targeting of eye-specific axonal projections in the macaque occurs much earlier and more rapidly than previously reported. This segregation process is completed before the reported onset of ganglion cell axon loss and retino-dLGN synapse elimination, suggesting that, in the primate, eye-specific targeting occurs independent of traditional forms of synaptic plasticity.

Optic chiasm formation in humans is independent of foveal development

A failure of human foveal development only occurs in two genetically determined conditions; aniridia (Pax6 mutation) and albinism (tyrosinase mutation). The chiasmatic pathways from this region are disrupted in albinism and central retinal blood vessel patterns are abnormal. It is assumed that these three abnormalities have a common mechanism. Here we investigate whether similar abnormalities are present in subjects with aniridia. Using fundus photographs it is shown that abnormal blood vessel patterns are present in aniridia, but these significantly differ from those in albinos. Using electrophysiological techniques, abnormal hemispheric projections through the chiasm are demonstrated in albinos, but aniridics do not differ from normal subjects. These results demonstrate that although mutations in Pax6 and tyrosinase both affect central retinal development, they have a fundamentally different impact on the formation of the retinal vasculature and the projections from this region. This strongly suggests that separate mechanisms regulate the development of the central retina and decussation patterns at the optic chiasm.

Cell birth and death in the mouse retinal ganglion cell layer

Here we describe quantitatively the birth and death of the two separate populations of neurons, ganglion cells and displaced amacrine cells, in the mouse retinal ganglion cell layer (GCL). The two cell types, which are roughly equally numerous, were distinguished pre- and postnatally by labeling the ganglion cells retrogradely with fluorescent dye. Embryos were labeled cumulatively with bromodeoxyuridine (BrdU) delivered by an osmotic minipump implanted in the mother; cell birth dates were established as having occurred before or after pump implantation. Early cohorts (GCL cells born before embryonic day [E] 11.8 and E12.8) were 98+/-1.1% and 99+/-0.2% ganglion cells (mean+/-SEM), respectively, and a late cohort (born after E15.8) was 97+/-1.2% displaced amacrines. Thus birth date was a strong predictor of a GCL cell's ultimate identity. Cell death in each cohort was estimated by counting cells at different time points (soon after the cohort was produced and later) and subtracting the later from the earlier number. This method avoids the problem of simultaneous birth and death that has plagued many of the earlier attempts to assess cell death. Negligible numbers died during the first week after a cell's birthday. The amount of cell death differed in the two cohorts; 48.5+/-15% and 29.0+/-12.4% in early and late, respectively, and most of it was postnatal. These findings disagree sharply with an earlier conclusion that ganglion cells die within 5 days of their birthdays or not at all.

Development of the primate area of high acuity, 3: Temporal relationships between pit formation, retinal elongation and cone packing

By establishing an avascular, highly elastic, region within the fetal area of high acuity (AHA), the developing primate eye has created a unique substrate on which the mechanical forces of intraocular pressure (IOP) and growth-induced retinal stretch (stretch) can act. We proposed (Springer & Hendrickson, 2004b) that these forces generate both the pit and high cone density found in the adult AHA. In this paper, we use quantitative measures to determine the temporal relationships between nasal and temporal retinal elongation, changes in pit depth, cone packing, and cone morphology overM. nemestrinaretinal development. Retinal length increased rapidly to about 105 days postconception (dpc; Phase 1) and then elongation virtually ceased (Phase 2) until just after birth (180 dpc). Retinal elongation due to stretch resumed during Phase 3 until approximately 315 dpc (4–5 months), after which time the retina appeared mature (Phase 4). The pit appeared during the quiescent Phase 2, suggesting that IOP acts, in conjunction with molecular changes in the inner retina, on the highly elastic, avascular, AHA to generate a deep, narrow pit and causes inner retinal cellular displacements. Subsequently (Phase 3), the pit widened, became 50% shallower and central inner retinal lamina thinned slightly due to a small amount of retinal stretch occurring in the AHA. Centripetal movement of cones was minimal until just after birth when the pit reached 88% of its maximal depth. Accelerated cone packing during Phase 3 was temporally correlated with increased stretch. A slight stretching of the central inner retina generates “lift” forces that cause the pit to become shallower and wider. In turn, these “lift” forces draw cones toward the center of the AHA (Springer, 1999). Localized changes in cone morphology associated with packing, included smaller cell body size, a change from a monolayer to a multilayered mound of cell bodies, elongation of inner segments and tilting of the apical portion toward the AHA. These changes began in cones overlying the edges of the pit, not its center. Henle cone axons formed initially in association with centrifugal displacement of the inner retina during pit formation, with an additional subsequent elongation due to cones moving centripetally. An integrated, two-factor model of AHA formation is presented. Initially, during the second half of gestation (Phase 2), IOP acts on the hyperelastic avascular zone of the AHA to generate a deep pit in the inner retina. In the first 4 months after birth (Phase 3), central retinal stretch generates tensile “lift” forces that remodel the pit and pack cones by drawing them toward the AHA center.

Regional abnormalities in retinal development are associated with local ocular hypopigmentation

The retinal pigment epithelium (RPE) plays a key role in regulating retinal development. The critical enzyme in pigment production is tyrosinase. Transgenic mice with a tyrosinase construct where the locus control region was deleted (YRT4) display a variegated phenotype of tyrosinase expression. Their central retina is largely pigment free, whereas more peripheral regions are heavily pigmented. We have used this model to ask whether the influence of pigmented RPE over the retina during development is fundamentally governed by local interactions or is global. Our data show that YRT4 eyes have intermediate melanin content and relatively low tyrosinase activity compared with wild-type and albino animals. Rod counts are comparable to those in pigmented mice in peripheral regions but similar to those in albinos centrally. Anterograde labelling of retinal pathways demonstrates the presence of relatively normal ipsilateral chiasmatic projection in YRT4 mice, comparable with that in pigmented animals and consistent with the peripheral pigmented origin of this pathway. Examination of cellular proliferation levels during retinal development reveals that YRT4 mice display an extended period of mitosis, similar to that found in albinos. Hence, our results show that the regulatory influence of the RPE over the developing retina depends on localized interactions between these tissues.

Immunocytochemical development of the guinea pig retina

The aim of the present study was to establish the neurochemical profile of amacrine and horizontal cells during ontogeny in the guinea pig, a precocial species where significant retinal development occurs prenatally as opposed to altricial species where development largely occurs postnatally. The expression of neurochemical markers of horizontal cells and specific amacrine cell populations was investigated from 20 days of gestation (dg, term approximately 67 dg) to adulthood. Amacrine cell populations were identified immunohistochemically using antibodies to gamma-amino-butyric acid, cholineacetyltransferase, calbindin, calretinin, neuronal nitric oxide synthetase and tyrosine hydroxylase; horizontal cells were labelled with calbindin. All markers were present at 30 dg and had attained their mature (adult) laminar distribution and expression by 60 dg. Horizontal cells appeared in their final location at 30 dg with amacrine cell populations appearing in their final locations by 45 dg. Thus, in the guinea pig retina, the amacrine and horizontal cell populations investigated in this study are fully mature prior to birth.

Development of the primate area of high acuity. 2. Quantitative morphological changes associated with retinal and pars plana growth

Mechanisms underlying the development of the primate area of high acuity (AHA) remain poorly understood. Finite-element models have identified retinal stretch and intraocular pressure (IOP) as possible mechanical forces that can form a pit (Springer & Hendrickson, 2004). A series of Macaca nemestrina monkey retinas between 68 days postconception (dpc) and adult were used to quantify growth and morphological changes. Retinal and pars plana length, optic disc diameter, disc-pit distance, and inner and outer retinal laminar thickness were measured over development to identify when and where IOP or stretch might operate. Horizontal optic disc diameter increased 500 mum between 115 dpc and 2 months after birth when it reached adult diameter. Disc growth mainly influences the immediate surrounding retina, presumably displacing retinal tissue centrifugally. Pars plana elongation also began at 115 dpc and continued steadily to 3-4 years postnatal, so its influence would be relatively constant over retinal development. Unexpectedly, horizontal retinal length showed nonlinear growth, divided into distinct phases. Retinal length increased rapidly until 115 dpc and then remained unchanged (quiescent phase) between 115-180 dpc. After birth, the retina grew rapidly for 3 months and then very slowly into adulthood. The onset of pit development overlapped the late fetal quiescent phase, suggesting that the major mechanical factor initiating pit formation is IOP, not retinal growth-induced stretch. Developmental changes in the thickness of retinal layers were different for inner and outer retina at many, but not all, of the ten eccentricities examined.

Retinal stretching limits peripheral visual acuity in myopia

Axial elongation of the myopic eye has the potential to stretch the retina, thereby reducing the sampling density of retinal neurons. Resolution acuity in the peripheral field of normal eyes is known to be sampling-limited, which suggests that retinal stretching in the myopic eye should have a direct effect on resolution acuity everywhere in the visual field except perhaps the fovea, which is usually optically limited. We tested this prediction that neural sampling density is reduced in myopic eyes by measuring resolution acuity for sinusoidal gratings in the fovea plus five peripheral locations in 60 myopic subjects exhibiting a wide range of refractive errors. Control experiments using a detection paradigm to provoke spatial aliasing verified that peripheral resolution was sampling limited. Retinal spatial frequencies of the grating stimulus were computed assuming Knapps' Law of visual optics, which ensures that retinal image size (in mm) is independent of refractive error when axial myopia is corrected by a spectacle lens located in the anterior focal plane of the eye. Results obtained at every retinal locus showed that resolution acuity declined linearly with magnitude of refractive error. Regression of the population data indicated that approximately 15 D of refractive error doubles the spacing between retinal neurons, thereby halving peripheral resolution acuity relative to the emmetropic eye. Several subjects also demonstrated sampling-limited performance in the fovea, which indicated that optical filtering by the eye's optical system failed to protect the fovea from aliasing artifacts of neural undersampling in these eyes. We conclude that stretching of the retina is a primary cause of reduced spatial resolution of the peripheral field, and occasionally of the fovea, in myopic eyes. Stretching appears to be locally uniform over the central +/-15 degrees of visual field but is globally non-uniform since the foveal region appears to stretch more than the globe itself.

Recruitment of the rod pathway by cones in the absence of rods

In mammalian retinas, rods and cones connect to distinct sets of bipolar cells. In the retina of Nrl (neural retina leucine zipper) knock-out mice, in which rods fail to form and all photoreceptors are cones, rod bipolar cells have normal morphology, pattern of staining, and lamination, but they form synaptic connections with cones. Hence, retinal interneurons are not entirely committed to the choice of their synaptic partners; also, their morphology and pattern of distribution of their processes in the synaptic layers of the retina are not instructed by the establishment of synaptic connections with cognate photoreceptor(s). These findings are of considerable relevance for our understanding of mechanisms responsible for cell recognition during development and in the context of using transplanted tissue to restore vision in retinal degeneration.

Fundus pigment distribution in rhesus monkeys

There is growing evidence for an interaction among fundus pigmentation, character, geography and the function and structure of the outer retina. We examined three inbred groups of rhesus macaques (132 eyes) including all ages. One was a smaller (18 eyes) group. Coat colors were variations of brown-tan however, the smaller group had lighter 'golden' coats and colors. Fundus images were classified for pigmentation and its geographic distribution. In golden-coated animals there was bias toward nasal fundus hypopigmentation with the optic disk as a watershed demarcation zone, which extended in the superior-inferior direction. Temporal fundus hypopigmentation did not occur in the absence of nasal hypopigmentation. More common, darker coated samples showed a characteristic diffuse fundus pigmentation. There was no evidence for albinism or large variations in macular pigmentation. Rhesus monkeys can exhibit geographically controlled genetic development of fundus pigmentation. Hypopigmentation provides for access to the choroidal infrastructure.

A gene regulatory hierarchy for retinal ganglion cell specification and differentiation

Retinal ganglion cells (RGCs) are the first cell type to be specified during vertebrate retinogenesis. Specification and differentiation of the RGC lineage are a stepwise process involving a hierarchical gene regulatory network. During the past decade, a framework of the network has emerged and key transcriptional regulators have been identified. Pax6, Notch, Ath5, and the Brn3 (Pou4f) factors act at different levels of the regulatory hierarchy. In this review, we summarize the current understanding of the functions of these and other transcriptional factors in the specification and differentiation of the RGC lineage. We emphasize the regulatory relationships among transcription factors at different steps of RGC development. We discuss critical issues that need to be addressed before a complete understanding of the gene regulatory network for RGC development can be achieved. Future directions in RGC development will inevitably rely on combined genetic and genomics approaches.

Topographic and Laminar Maturation of Striate Cortex in Early Postnatal Marmoset Monkeys, as Revealed by Neurofilament Immunohistochemistry

The maturation of pyramidal neurons in the primary visual cortex (V1) of marmoset monkeys was investigated using an antibody (SMI-32) to non-phosphorylated neurofilament protein (NNF). Analysis of animals aged between birth and postnatal day 91 (PD 91, which corresponds approximately to the peak of synaptogenesis in this species) revealed discrete changes in both the laminar and the areal distribution of NNF. At PD 0, the upper part of layer 6 contained darkly labelled neurons and associated neuropil, including axons. In this layer a centroperipheral gradient, with more labelled cells in the foveal representation, was apparent at PD 0. This topographic gradient gradually disappeared, and by PD 91 a similar density of labelled layer 6 cells was observed throughout V1. Labelled cells were not apparent in layer 3C until PD 7, and were not distributed according to a topographic gradient. Labelled cells were first observed in layer 3B(alpha) at PD 28, when they formed a centroperipheral gradient similar to that seen in layer 6. This gradient was still evident in an adult animal. These results demonstrate an inside-out profile of postnatal cortical development, with the topographic pattern of maturation of V1 mimicking the centroperipheral gradient of maturation in the retina.

Distribution of short-wavelength-sensitive cones in human fetal and postnatal retina: early development of spatial order and density profiles

We analysed spatial density and distribution of short-wavelength-sensitive photoreceptors (S-cones) in developing and adult human retinae using antibody against short-wavelength-sensitive opsin. Statistical tests indicate that before 20 weeks of gestation (WG) the S-cone mosaic is not distinguishable from a random distribution, but by 20 WG is significantly different from a random distribution in the perifoveal region, as reported previously for adult retina. Changes in spatial density during development are consistent with displacement of the photoreceptor population towards the incipient fovea so that prior to 20 WG, peak S-cone density is >1.7 mm from the fovea, but is within 800 microm of the fovea by 20 WG.

Timing and topography of cell genesis in the rat retina

To understand the mechanisms of cell fate determination in the vertebrate retina, the time course of the generation of the major cell types needs to be established. This will help define and interpret patterns of gene expression, waves of differentiation, timing and extent of competence, and many of the other developmental processes involved in fate acquisition. A thorough retinal cell "birthdating" study has not been performed for the laboratory rat, even though it is the species of choice for many contemporary developmental studies of the vertebrate retina. We investigated the timing and spatial pattern of cell genesis using 3H-thymidine (3H-TdR). A single injection of 3H-TdR was administered to pregnant rats or rat pups between embryonic day (E) 8 and postnatal day (P) 13. The offspring of prenatally injected rats were delivered and all animals survived to maturity. Labeled cells were visualized by autoradiography of retinal sections. Rat retinal cell genesis commenced around E10, 50% of cells were born by approximately P1, and retinogenesis was complete near P12. The first postmitotic cells were found in the retinal ganglion cell layer and were 9-15 microm in diameter. This range includes small to medium diameter retinal ganglion cells and large displaced amacrine cells. The sequence of cell genesis was established by determining the age at which 5, 50, and 95% of the total population of cells of each phenotype became postmitotic. With few exceptions, the cell types reached these developmental landmarks in the following order: retinal ganglion cells, horizontal cells, cones, amacrine cells, rods, bipolar cells, and Müller glia. For each type, the first cells generated were located in the central retina and the last cells in the peripheral retina. Within the sequence of cell genesis, two or three phases could be detected based on differences in timing, kinetics, and topographic gradients of cell production. Our results show that retinal cells in the rat are generated in a sequence similar to that of the primate retina, in which retinogenesis spans more than 100 days. To the extent that sequences reflect underlying mechanisms of cell fate determination, they appear to be conserved.

Development of the primate area of high acuity. 1. Use of finite element analysis models to identify mechanical variables affecting pit formation

Most primate retinas have an area dedicated for high visual acuity called the fovea centralis. Little is known about specific mechanisms that drive development of this complex central retinal specialization. The primate area of high acuity (AHA) is characterized by the presence of a pit that displaces the inner retinal layers. Virtual engineering models were analyzed with finite element analysis (FEA) to identify mechanical mechanisms potentially critical for pit formation. Our hypothesis is that the pit emerges within the AHA because it contains an avascular zone (AZ). The absence of blood vessels makes the tissue within the AZ more elastic and malleable than the surrounding vascularized retina. Models evaluated the contribution to pit formation of varying elasticity ratios between the AZ and surrounding retina, AZ shape, and width. The separate and interactive effects of two mechanical variables, intraocular pressure (IOP) and ocular growth-induced retinal stretch, on pit formation were also evaluated. Either stretch or IOP alone produced a pit when applied to a FEA model having a highly elastic AZ surrounded by a less elastic region. Pit depth and width increased when the elasticity ratio increased, but a pit could not be generated in models lacking differential elasticity. IOP alone produced a deeper pit than did stretch alone and the deepest pit resulted from the combined effects of IOP and stretch. These models predict that the pit in the AHA is formed because an absence of vasculature makes the inner retinal tissue of the AZ very deformable. Once a differential elasticity gradient is established, pit formation can be driven by either IOP or ocular growth-induced retinal stretch.

Cumulative labeling of embryonic mouse neural retina with bromodeoxyuridine supplied by an osmotic minipump

I describe a method for providing cumulative label of bromodeoxyuridine (BrdU) to mouse embryos. Commercially available osmotic pumps, which release their contents at a steady rate, were implanted subcutaneously in the interscapular space of pregnant mice on embryonic day (E) 9.5-12.5. Survival times varied from 4h to 37 days. Tissues (embryonic and neonatal eyes and maternal intestine) were immunochemically labeled for BrdU and examined histologically. The first detectably labeled cells appeared 4-7h post-implantation (hpi) and all cycling cells were labeled for at least 7 days post-implantation (dpi). Retinal development appeared normal. Terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) stained retinas that had been exposed to BrdU showed no more apoptotic cells than those unexposed. I conclude that the maternally implanted osmotic pump successfully provides cumulative BrdU labeling in the mouse embryo.