Adaptation of the central retina for high acuity vision: cones, the fovea and the avascular zone

Patients and animal models of CNGβ1-deficient retinitis pigmentosa support gene augmentation approach

Retinitis pigmentosa (RP) is a major cause of blindness that affects 1.5 million people worldwide. Mutations in cyclic nucleotide-gated channel β 1 (CNGB1) cause approximately 4% of autosomal recessive RP. Gene augmentation therapy shows promise for treating inherited retinal degenerations; however, relevant animal models and biomarkers of progression in patients with RP are needed to assess therapeutic outcomes. Here, we evaluated RP patients with CNGB1 mutations for potential biomarkers of progression and compared human phenotypes with those of mouse and dog models of the disease. Additionally, we used gene augmentation therapy in a CNGβ1-deficient dog model to evaluate potential translation to patients. CNGB1-deficient RP patients and mouse and dog models had a similar phenotype characterized by early loss of rod function and slow rod photoreceptor loss with a secondary decline in cone function. Advanced imaging showed promise for evaluating RP progression in human patients, and gene augmentation using adeno-associated virus vectors robustly sustained the rescue of rod function and preserved retinal structure in the dog model. Together, our results reveal an early loss of rod function in CNGB1-deficient patients and a wide window for therapeutic intervention. Moreover, the identification of potential biomarkers of outcome measures, availability of relevant animal models, and robust functional rescue from gene augmentation therapy support future work to move CNGB1-RP therapies toward clinical trials.

Mechanisms of macular edema: Beyond the surface

Macular edema consists of intra- or subretinal fluid accumulation in the macular region. It occurs during the course of numerous retinal disorders and can cause severe impairment of central vision. Major causes of macular edema include diabetes, branch and central retinal vein occlusion, choroidal neovascularization, posterior uveitis, postoperative inflammation and central serous chorioretinopathy. The healthy retina is maintained in a relatively dehydrated, transparent state compatible with optimal light transmission by multiple active and passive systems. Fluid accumulation results from an imbalance between processes governing fluid entry and exit, and is driven by Starling equation when inner or outer blood-retinal barriers are disrupted. The multiple and intricate mechanisms involved in retinal hydro-ionic homeostasis, their molecular and cellular basis, and how their deregulation lead to retinal edema, are addressed in this review. Analyzing the distribution of junction proteins and water channels in the human macula, several hypotheses are raised to explain why edema forms specifically in the macular region. "Pure" clinical phenotypes of macular edema, that result presumably from a single causative mechanism, are detailed. Finally, diabetic macular edema is investigated, as a complex multifactorial pathogenic example. This comprehensive review on the current understanding of macular edema and its mechanisms opens perspectives to identify new preventive and therapeutic strategies for this sight-threatening condition.

Foveal Development in Infants Treated with Bevacizumab or Laser Photocoagulation for Retinopathy of Prematurity

PURPOSE

To characterize and quantify early foveal development in preterm infants and to compare this development between eyes treated with intravitreal bevacizumab or laser photocoagulation (LPC) and untreated eyes.

DESIGN

Observational case series.

PARTICIPANTS

One hundred thirty-one preterm infants undergoing retinopathy of prematurity (ROP) screenings.

METHODS

Handheld OCT imaging was performed longitudinally on all patients. Thickness measurements of the inner and outer retinal layers were obtained at the foveal center and the nasal and temporal foveal rims. Comparisons between treated and untreated eyes were adjusted for age and other confounding variables.

MAIN OUTCOME MEASURES

Weekly change in inner and outer retinal thickness and presence of inner retinal layers, ellipsoid zone (EZ), and cystoid macular changes (CMCs).

RESULTS

Outer retinal thickness at the foveal center increased by 3.1 μm/week in untreated eyes and 7.2 μm/week in bevacizumab-treated eyes (P = 0.038). Eyes treated with LPC had a lower probability of having all inner retinal layers present at the foveal center (odds ratio, 0.04; P = 0.001) and a lower probability of having the EZ present at the foveal center (odds ratio, 0.07; P = 0.024) compared with untreated eyes. Cystoid macular changes were found in 53% of patients and 22% of imaging sessions. The age-adjusted incidence of CMCs was not correlated with bevacizumab or LPC treatment.

CONCLUSIONS

Intravitreal bevacizumab therapy for ROP is associated with more rapid outer retinal thickening at the foveal center, whereas LPC is associated with earlier extrusion of the inner retinal layers and delayed development of the EZ at the foveal center. Long-term follow-up is needed to determine the visual significance of these findings.

Quantitative Assessment of Microstructural Changes of the Retina in Infants With Congenital Zika Syndrome

Importance

A better pathophysiologic understanding of the neurodevelopmental abnormalities observed in neonates exposed in utero to Zika virus (ZIKV) is needed to develop treatments. The retina as an extension of the diencephalon accessible to in vivo microcopy with spectral-domain optical coherence tomography (SD-OCT) can provide an insight into the pathophysiology of congenital Zika syndrome (CZS).

Objective

To quantify the microstructural changes of the retina in CZS and compare these changes with those of cobalamin C (cblC) deficiency, a disease with potential retinal maldevelopment.

Design, Setting, and Participants

This case series included 8 infants with CZS and 8 individuals with cblC deficiency. All patients underwent ophthalmologic evaluation at 2 university teaching hospitals and SD-OCT imaging in at least 1 eye. Patients with cblC deficiency were homozygous or compound heterozygotes for mutations in the methylmalonic aciduria and homocystinuria type C (MMACHC) gene. Data were collected from January 1 to March 17, 2016, for patients with CZS and from May 4, 2015, to April 23, 2016, for patients with cblC deficiency.

Main Outcomes and Measures

The SD-OCT cross-sections were segmented using automatic segmentation algorithms embedded in the SD-OCT systems. Each retinal layer thickness was measured at critical eccentricities using the position of the signal peaks and troughs on longitudinal reflectivity profiles.

Results

Eight infants with CZS (5 girls and 3 boys; age range, 3-5 months) and 8 patients with cblC deficiency (3 girls and 5 boys; age range, 4 months to 15 years) were included in the analysis. All 8 patients with CZS had foveal abnormalities in the analyzed eyes (8 eyes), including discontinuities of the ellipsoid zone, thinning of the central retina with increased backscatter, and severe structural disorganization, with 3 eyes showing macular pseudocolobomas. Pericentral retina with normal lamination showed a thinned (<30% of normal thickness) ganglion cell layer (GCL) that colocalized in 7 of 8 eyes with a normal photoreceptor layer. The inner nuclear layer was normal or had borderline thinning. The central retinal degeneration was similar to that of cblC deficiency.

Conclusions and Relevance

Congenital Zika syndrome showed a central retinal degeneration with severe GCL loss, borderline inner nuclear layer thinning, and less prominent photoreceptor loss. The findings provide the first, to date, in vivo evidence in humans for possible retinal maldevelopment with a predilection for retinal GCL loss in CZS, consistent with a murine model of the disease and suggestive of in utero depletion of this neuronal population as a consequence of Zika virus infection.

Duplication events downstream of IRX1 cause North Carolina macular dystrophy at the MCDR3 locus

Autosomal dominant North Carolina macular dystrophy (NCMD) is believed to represent a failure of macular development. The disorder has been linked to two loci, MCDR1 (chromosome 6q16) and MCDR3 (chromosome 5p15-p13). Recently, non-coding variants upstream of PRDM13 (MCDR1) and a duplication including IRX1 (MCDR3) have been identified. However, the underlying disease-causing mechanism remains uncertain. Through a combination of sequencing studies on eighteen NCMD families, we report two novel overlapping duplications at the MCDR3 locus, in a gene desert downstream of IRX1 and upstream of ADAMTS16. One duplication of 43 kb was identified in nine families (with evidence for a shared ancestral haplotype), and another one of 45 kb was found in a single family. Three families carry the previously reported V2 variant (MCDR1), while five remain unsolved. The MCDR3 locus is thus refined to a shared region of 39 kb that contains DNAse hypersensitive sites active at a restricted time window during retinal development. Publicly available data confirmed expression of IRX1 and ADAMTS16 in human fetal retina, with IRX1 preferentially expressed in fetal macula. These findings represent a major advance in our understanding of the molecular genetics of NCMD and provide insights into the genetic pathways involved in human macular development.

Fgf8 Expression and Degradation of Retinoic Acid Are Required for Patterning a High-Acuity Area in the Retina

Species that are highly reliant on their visual system have a specialized retinal area subserving high-acuity vision, e.g., the fovea in humans. Although of critical importance for our daily activities, little is known about the mechanisms driving the development of retinal high-acuity areas (HAAs). Using the chick as a model, we found a precise and dynamic expression pattern of fibroblast growth factor 8 (Fgf8) in the HAA anlage, which was regulated by enzymes that degrade retinoic acid (RA). Transient manipulation of RA signaling, or reduction of Fgf8 expression, disrupted several features of HAA patterning, including photoreceptor distribution, ganglion cell density, and organization of interneurons. Notably, patterned expression of RA signaling components was also found in humans, suggesting that RA also plays a role in setting up the human fovea.

CrxRdy Cat: A Large Animal Model for CRX-Associated Leber Congenital Amaurosis

Purpose

Mutations in the retinal transcription factor cone-rod homeobox (CRX) gene result in severe dominant retinopathies. A large animal model, the Rdy cat, carrying a spontaneous frameshift mutation in Crx, was reported previously. The present study aimed to further understand pathogenesis in this model by thoroughly characterizing the Rdy retina.

Methods

Structural and functional changes were found in a comparison between the retinas of Crx^Rdy/+ kittens and those of wild-type littermates and were determined at various ages by fundus examination, electroretinography (ERG), optical coherence tomography, and histologic analyses. RNA and protein expression changes of Crx and key target genes were analyzed using quantitative reverse-transcribed PCR, Western blot analysis, and immunohistochemistry. Transcription activity of the mutant Crx was measured by a dual-luciferase transactivation assay.

Results

Crx^Rdy/+ kittens had no recordable cone ERGs. Rod responses were delayed in development and markedly reduced at young ages and lost by 20 weeks. Photoreceptor outer segment development was incomplete and was followed by progressive outer retinal thinning starting in the cone-rich area centralis. Expression of cone and rod Crx target genes was significantly down-regulated. The mutant Crx allele was overexpressed, leading to high levels of the mutant protein lacking transactivation activity.

Conclusions

The Crx^Rdy mutation exerts a dominant negative effect on wild-type Crx by overexpressing mutant protein. These findings, consistent with those of studies in a mouse model, support a conserved pathogenic mechanism for CRX frameshift mutations. The similarities between the feline eye and the human eye with the presence of a central region of high cone density makes the Crx^Rdy/+ cat a valuable model for preclinical testing of therapies for dominant CRX diseases.

Development of cone photoreceptors and their synapses in the human and monkey fovea

During retinal development, ribbon synapse assembly in the photoreceptors is a crucial step involving numerous molecules. While the developmental sequence of plexiform layers in human retina has been characterized, the molecular steps of synaptogenesis remain largely unknown. In the present study, we focused on the central rod-free region of primate retina, the fovea, to specifically investigate the development of cone photoreceptor ribbon synapses. Immunocytochemistry and electron microscopy were utilized to track the expression of photoreceptor transduction proteins and ribbon and synaptic markers in fetal human and Macaca retina. Although the inner plexiform layer appears earlier than the outer plexiform layer, synaptic proteins, and ribbons are first reliably recognized in cone pedicles. Markers first appear at fetal week 9. Both short (S) and medium/long (M/L) wavelength-selective cones express synaptic markers in the same temporal sequence; this is independent of opsin expression which takes place in S cones a month before M/L cones. The majority of ribbon markers, presynaptic vesicular release and postsynaptic neurotransduction-related machinery is present in both plexiform layers by fetal week 13. By contrast, two crucial components for cone to bipolar cell glutamatergic transmission, the metabotropic glutamate receptor 6 and voltage-dependent calcium channel α1.4, are not detected until fetal week 22 when bipolar cell invagination is present in the cone pedicle. These results suggest an intrinsically programmed but nonsynchronous expression of molecules in cone synaptic development. Moreover, functional ribbon synapses and active neurotransmission at foveal cone pedicles are possibly present as early as mid-gestation in human retina.

Microstructural visual system changes in AQP4-antibody-seropositive NMOSD

Objective:

To trace microstructural changes in patients with aquaporin-4 antibody (AQP4-ab)-seropositive neuromyelitis optica spectrum disorders (NMOSDs) by investigating the afferent visual system in patients without clinically overt visual symptoms or visual pathway lesions.

Methods:

Of 51 screened patients with NMOSD from a longitudinal observational cohort study, we compared 6 AQP4-ab–seropositive NMOSD patients with longitudinally extensive transverse myelitis (LETM) but no history of optic neuritis (ON) or other bout (NMOSD-LETM) to 19 AQP4-ab–seropositive NMOSD patients with previous ON (NMOSD-ON) and 26 healthy controls (HCs). Foveal thickness (FT), peripapillary retinal nerve fiber layer (pRNFL) thickness, and ganglion cell and inner plexiform layer (GCIPL) thickness were measured with optical coherence tomography (OCT). Microstructural changes in the optic radiation (OR) were investigated using diffusion tensor imaging (DTI). Visual function was determined by high-contrast visual acuity (VA). OCT results were confirmed in a second independent cohort.

Results:

FT was reduced in both patients with NMOSD-LETM (p = 3.52e⁻¹⁴) and NMOSD-ON (p = 1.24e⁻¹⁶) in comparison with HC. Probabilistic tractography showed fractional anisotropy reduction in the OR in patients with NMOSD-LETM (p = 0.046) and NMOSD-ON (p = 1.50e⁻⁵) compared with HC. Only patients with NMOSD-ON but not NMOSD-LETM showed neuroaxonal damage in the form of pRNFL and GCIPL thinning. VA was normal in patients with NMOSD-LETM and was not associated with OCT or DTI parameters.

Conclusions:

Patients with AQP4-ab–seropositive NMOSD without a history of ON have microstructural changes in the afferent visual system. The localization of retinal changes around the Müller-cell rich fovea supports a retinal astrocytopathy.

The neural retina in retinopathy of prematurity

Retinopathy of prematurity (ROP) is a neurovascular disease that affects prematurely born infants and is known to have significant long term effects on vision. We conducted the studies described herein not only to learn more about vision but also about the pathogenesis of ROP. The coincidence of ROP onset and rapid developmental elongation of the rod photoreceptor outer segments motivated us to consider the role of the rods in this disease. We used noninvasive electroretinographic (ERG), psychophysical, and retinal imaging procedures to study the function and structure of the neurosensory retina. Rod photoreceptor and post-receptor responses are significantly altered years after the preterm days during which ROP is an active disease. The alterations include persistent rod dysfunction, and evidence of compensatory remodeling of the post-receptor retina is found in ERG responses to full-field stimuli and in psychophysical thresholds that probe small retinal regions. In the central retina, both Mild and Severe ROP delay maturation of parafoveal scotopic thresholds and are associated with attenuation of cone mediated multifocal ERG responses, significant thickening of post-receptor retinal laminae, and dysmorphic cone photoreceptors. These results have implications for vision and control of eye growth and refractive development and suggest future research directions. These results also lead to a proposal for noninvasive management using light that may add to the currently invasive therapeutic armamentarium against ROP.

Evaluating outer segment length as a surrogate measure of peak foveal cone density

Adaptive optics (AO) imaging tools enable direct visualization of the cone photoreceptor mosaic, which facilitates quantitative measurements such as cone density. However, in many individuals, low image quality or excessive eye movements precludes making such measures. As foveal cone specialization is associated with both increased density and outer segment (OS) elongation, we sought to examine whether OS length could be used as a surrogate measure of foveal cone density. The retinas of 43 subjects (23 normal and 20 albinism; aged 6-67years) were examined. Peak foveal cone density was measured using confocal adaptive optics scanning light ophthalmoscopy (AOSLO), and OS length was measured using optical coherence tomography (OCT) and longitudinal reflectivity profile-based approach. Peak cone density ranged from 29,200 to 214,000cones/mm² (111,700±46,300cones/mm²); OS length ranged from 26.3 to 54.5μm (40.5±7.7μm). Density was significantly correlated with OS length in albinism (p<0.0001), but not normals (p=0.99). A cubic model of density as a function of OS length was created based on histology and optimized to fit the albinism data. The model includes triangular cone packing, a cylindrical OS with a fixed volume of 136.6μm³, and a ratio of OS to inner segment width that increased linearly with increasing OS length (R²=0.72). Normal subjects showed no apparent relationship between cone density and OS length. In the absence of adequate AOSLO imagery, OS length may be used to estimate cone density in patients with albinism. Whether this relationship exists in other patient populations with foveal hypoplasia (e.g., premature birth, aniridia, isolated foveal hypoplasia) remains to be seen.

Optical coherence tomography angiography of foveal hypoplasia

AIMS

To discuss foveal development in the context of detailed retinal vasculature imaging in foveal hypoplasia using optical coherence tomography angiography.

METHODS

In this case series, the optical coherence tomography angiography results of four patients with idiopathic foveal hypoplasia and two patients with foveal hypoplasia secondary to oculocutaneous albinism are presented.

RESULTS

Cases with intact visual acuity demonstrated lower grades of foveal hypoplasia on optical coherence tomography, while those with poor vision demonstrated high grades of foveal hypoplasia. The superficial retinal capillary plexus was intact in the foveal area in all cases, with no demonstrable foveal avascular zone. The deep retinal capillary plexus was absent to variable degrees in most cases, but was most persistent in those cases with reduced vision.

CONCLUSIONS

The superficial retinal capillary plexus is present in cases with foveal hypoplasia, while the deep retinal capillary plexus is absent to varying degrees. Our findings support the hypothesis that an intact foveal avascular zone of the deep capillary plexus allows for outer retinal photoreceptor specialisation to occur unimpeded, resulting in preserved visual acuity, while this process may be inhibited by an absent deep capillary foveal avascular zone with resultant poor vision.

In vivo genome editing as a potential treatment strategy for inherited retinal dystrophies

In vivo genome editing represents an emerging field in the treatment of monogenic disorders, as it may constitute a solution to the current hurdles in classic gene addition therapy, which are the low levels and limited duration of transgene expression. Following the introduction of a double strand break (DSB) at the mutational site by highly specific endonucleases, such as TALENs (transcription activator like effector nucleases) or RNA based nucleases (clustered regulatory interspaced short palindromic repeats - CRISPR-Cas), the cell's own DNA repair machinery restores integrity to the DNA strand and corrects the mutant sequence, thus allowing the cell to produce protein levels as needed. The DNA repair happens either through the error prone non-homologous end-joining (NHEJ) pathway or with high fidelity through homology directed repair (HDR) in the presence of a DNA donor template. A third pathway called microhomology mediated endjoining (MMEJ) has been recently discovered. In this review, the authors focus on the different DNA repair mechanisms, the current state of the art tools for genome editing and the particularities of the retina and photoreceptors with regard to in vivo therapeutic approaches. Finally, current attempts in the field of retinal in vivo genome editing are discussed and future directions of research identified.

Biophysical Properties of Optogenetic Tools and Their Application for Vision Restoration Approaches

Optogenetics is the use of genetically encoded light-activated proteins to manipulate cells in a minimally invasive way using light. The most prominent example is channelrhodopsin-2 (ChR2), which allows the activation of electrically excitable cells via light-dependent depolarization. The combination of ChR2 with hyperpolarizing-light-driven ion pumps such as the Cl(-) pump halorhodopsin (NpHR) enables multimodal remote control of neuronal cells in culture, tissue, and living animals. Very soon, it became obvious that this method offers a chance of gene therapy for many diseases affecting vision. Here, we will give a brief introduction to retinal function and retinal diseases; optogenetic vision restoration strategies will be highlighted. We will discuss the functional and structural properties of rhodopsin-based optogenetic tools and analyze the potential for the application of vision restoration.

Error compensation in random vector double step saccades with and without global adaptation

In saccade sequences without visual feedback endpoint errors pose a problem for subsequent saccades. Accurate error compensation has previously been demonstrated in double step saccades (DSS) and is thought to rely on a copy of the saccade motor vector. However, these studies typically use fixed target vectors on each trial, calling into question the generalizability of the findings due to the high stimulus predictability. We present a random walk DSS paradigm (random target vector amplitudes and directions) to provide a more complete, realistic and generalizable description of error compensation in saccade sequences. We regressed the vector between the endpoint of the second saccade and the endpoint of a hypothetical second saccade that does not take first saccade error into account on the ideal compensation vector. This provides a direct and complete estimation of error compensation in DSS. We observed error compensation with varying stimulus displays that was comparable to previous findings. We also employed this paradigm to extend experiments that showed accurate compensation for systematic undershoots after specific-vector saccade adaptation. Utilizing the random walk paradigm for saccade adaptation by Rolfs et al. (2010) together with our random walk DSS paradigm we now also demonstrate transfer of adaptation from reactive to memory guided saccades for global saccade adaptation. We developed a new, generalizable DSS paradigm with unpredictable stimuli and successfully employed it to verify, replicate and extend previous findings, demonstrating that endpoint errors are compensated for saccades in all directions and variable amplitudes.

Multivendor Spectral-Domain Optical Coherence Tomography Dataset, Observer Annotation Performance Evaluation, and Standardized Evaluation Framework for Intraretinal Cystoid Fluid Segmentation

Development of image analysis and machine learning methods for segmentation of clinically significant pathology in retinal spectral-domain optical coherence tomography (SD-OCT), used in disease detection and prediction, is limited due to the availability of expertly annotated reference data. Retinal segmentation methods use datasets that either are not publicly available, come from only one device, or use different evaluation methodologies making them difficult to compare. Thus we present and evaluate a multiple expert annotated reference dataset for the problem of intraretinal cystoid fluid (IRF) segmentation, a key indicator in exudative macular disease. In addition, a standardized framework for segmentation accuracy evaluation, applicable to other pathological structures, is presented. Integral to this work is the dataset used which must be fit for purpose for IRF segmentation algorithm training and testing. We describe here a multivendor dataset comprised of 30 scans. Each OCT scan for system training has been annotated by multiple graders using a proprietary system. Evaluation of the intergrader annotations shows a good correlation, thus making the reproducibly annotated scans suitable for the training and validation of image processing and machine learning based segmentation methods. The dataset will be made publicly available in the form of a segmentation Grand Challenge.

The organization of the cone photoreceptor mosaic measured in the living human retina

The cone photoreceptors represent the initial fundamental sampling step in the acquisition of visual information. While recent advances in adaptive optics have provided increasingly precise estimates of the packing density and spacing of the cone photoreceptors in the living human retina, little is known about the local cone geometric arrangement beyond a tendency towards hexagonal packing. We analyzed the cone mosaic in data from 10 normal subjects. A technique was applied to calculate the local average cone mosaic structure which allowed us to determine the hexagonality, spacing and orientation of local regions. Using cone spacing estimates, we find the expected decrease in cone density with retinal eccentricity and higher densities along the horizontal as opposed to the vertical meridians. Orientation analysis reveals an asymmetry in the local cone spacing of the hexagonal packing, with cones having a larger local spacing along the horizontal direction. This horizontal/vertical asymmetry is altered at eccentricities larger than 2 degrees in the superior meridian and 2.5 degrees in the inferior meridian. Analysis of hexagon orientations in the central 1.4° of the retina shows a tendency for orientation to be locally coherent, with orientation patches consisting of between 35 and 240 cones.

Assessing the spatial relationship between fixation and foveal specializations

Increased cone photoreceptor density, an avascular zone (FAZ), and the displacement of inner retinal neurons to form a pit are distinct features of the human fovea. As the fovea provides the majority of our vision, appreciating how these anatomical specializations are related is important for understanding foveal development, normal visual function, and retinal disease. Here we evaluated the relationship between these specializations and their location relative to the preferred retinal locus of fixation (PRL). We measured foveal pit volume, FAZ area, peak cone density, and location of the PRL in 22 subjects with normal vision using optical coherence tomography and adaptive optics scanning light ophthalmoscopy. Foveal pit volume was positively correlated with FAZ area; however, peak cone density was not correlated with pit volume. In addition, there was no systematic offset of the location of any of these specializations relative to PRL, and there was no correlation between the magnitude of the offset from PRL and the corresponding foveal specialization measurements (pit volume, FAZ area, peak cone density). The standard deviation of our PRL measurements was consistent with previous measurements of fixational stability. These data provide insight into the sequence of events during foveal development and may have implications for visual function and retinal disease.

Outer retinal deformity detected by optical coherence tomography in eyes with foveal hypoplasia

PURPOSE

To investigate the relationship between vision and foveal maturity, especially in foveal hypoplasia exhibiting severe structural immaturity.

METHODS

This retrospective observational case series included 42 eyes of 23 patients (mean age, 7.0 ± 5.0 years; 9 patients with foveal hypoplasia as an isolated entity and 14 patients with aniridia). A complete ophthalmic examination included measurement of best-corrected visual acuity (BCVA) and spectral-domain optical coherence tomography (SD-OCT). The sensory retina, ganglion cell complex (GCC), and outer retinal layers, including Henle's fiber layer (HFL), were measured and analyzed.

RESULTS

Using SD-OCT images, eyes were classified as having a differentiated (6 eyes), diffuse (19 eyes), or no HFL (17 eyes), based on the appearance of the HFL around the foveal region. The logMAR BCVA was significantly worse (p < 0.0001) in eyes with diffuse HFL and those with no HFL than in those with differentiated HFL. Outer retinal thickness (outer plexiform layer + HFL + outer nuclear layer) was less (p = 0.0051) in eyes with no HFL than in those with differentiated HFL. The logMAR BCVA, GCC thickness, and outer retinal thickness in eyes with foveal hypoplasia with aniridia were significantly worse (p = 0.0083), thicker (p = 0.0039), and thinner (p = 0.0001), respectively, than in eyes with foveal hypoplasia as an isolated entity.

CONCLUSIONS

In eyes with foveal hypoplasia with severe structural immaturity, diffuse HFL or no HFL was associated with worse vision. There was greater foveal immaturity in eyes with foveal hypoplasia with aniridia compared with foveal hypoplasia as an isolated entity.

Analysis of foveal characteristics and their asymmetries in the normal population

The advance of optical coherence tomography (OCT) enables a detailed examination of the human retina in-vivo for clinical routine and experimental eye research. Only few investigations to date captured human foveal morphology in a large subject group on the basis of a detailed analysis employing mathematical models. However, even for important foveal characteristics unified terminology and clear definitions were not implemented so far. This might be a reason, why to this day the human fovea is considered to be a mostly symmetric and round structure. Therefore, the most important finding of this work is the detailed analysis of the asymmetric structure of the human fovea. We employed five clinically highly relevant foveal characteristics, which are derived from a previously published fovea model. For each, an accurate mathematical description is given. The presented properties include (1) mean retinal thickness inside a defined radius, (2) foveal bowl area, (3) a new, exact definition of foveal radius, (4) maximum foveal slope, and (5) the maximum height of the foveal rim. Furthermore, minimum retinal thickness was derived and analyzed. 220 strictly controlled healthy Caucasian subjects of European decent with an even distribution of age and gender were imaged with an Heidelberg Spectralis OCT. Detailed analysis demonstrated the following general results: (1) significant gender difference regarding the central foveal subfield thickness (CFST) but no significant differences for the minimum central retinal thickness, (2) a strong correlation between right and left eye of the same subject, and, as essential finding, (3) strong structural differences of the fovea form in the different anatomical directions (nasal, temporal, inferior and superior). In the analysis of the foveal asymmetry, it will be demonstrated that the foveal radius is larger in nasal and temporal direction compared to inferior and superior position. Furthermore, it will be shown that the circular fovea rather has an elliptic form with the larger axis along the nasal to temporal direction. Interestingly, the foveal slope shows a divergent behavior as the temporal direction has the smallest slope angle and both, inferior and superior angles are clearly larger than the others. The findings in this work can be used for an exact quantification of changes in early stages of various retinal diseases and as a marker for initial diagnosis.

Subclinical primary retinal pathology in neuromyelitis optica spectrum disorder

Foveal thickness may be a more sensitive indicator of primary retinal pathology than retinal nerve fiber layer thickness since the fovea contains no or sparse retinal nerve fiber layer, which coalesces into axons of the optic nerve. To our knowledge, few quantitative in vivo studies have investigated foveal thickness. By using optical coherence tomography, we measured foveal thickness to evaluate intrinsic retinal pathology. Seventy-two neuromyelitis optica spectrum disorder patients (99 eyes with optic neuritis and 45 eyes without optic neuritis) and 34 age-matched controls were included. Foveal thinning was observed both in eyes with non-optic neuritis (185.1 µm, p < 0.001) and optic neuritis (185.0 µm, p < 0.001) relative to controls (205.0 µm). Compared to controls, eyes with non-optic neuritis did not have peripapillary retinal nerve fiber layer thinning, but showed foveal thinning (p < 0.001). In neuromyelitis optica spectrum disorder, foveal thickness correlated with 2.5 % low contrast visual acuity, while retinal nerve fiber layer thickness correlated with high or low contrast visual acuity, extended disability status scale, and disease duration. In this study, we observed foveal thinning irrespective of optic neuritis; thus, we believe that subclinical primary retinal pathology, prior to retinal nerve fiber layer thinning, may exist in neuromyelitis optica spectrum disorder.

FOVEA: a new program to standardize the measurement of foveal pit morphology

The fovea is one of the most studied retinal specializations in vertebrates, which consists of an invagination of the retinal tissue with high packing of cone photoreceptors, leading to high visual resolution. Between species, foveae differ morphologically in the depth and width of the foveal pit and the steepness of the foveal walls, which could influence visual perception. However, there is no standardized methodology to measure the contour of the foveal pit across species. We present here FOVEA, a program for the quantification of foveal parameters (width, depth, slope of foveal pit) using images from histological cross-sections or optical coherence tomography (OCT). FOVEA is based on a new algorithm to detect the inner retina contour based on the color variation of the image. We evaluated FOVEA by comparing the fovea morphology of two Passerine birds based on histological cross-sections and its performance with data from previously published OCT images. FOVEA detected differences between species and its output was not significantly different from previous estimates using OCT software. FOVEA can be used for comparative studies to better understand the evolution of the fovea morphology in vertebrates as well as for diagnostic purposes in veterinary pathology. FOVEA is freely available for academic use and can be downloaded at: http://estebanfj.bio.purdue.edu/fovea.

Control of Retinal Ganglion Cell Positioning and Neurite Growth: Combining 3D Printing with Radial Electrospun Scaffolds

Retinal ganglion cells (RGCs) are responsible for the transfer of signals from the retina to the brain. As part of the central nervous system, RGCs are unable to regenerate following injury, and implanted cells have limited capacity to orient and integrate in vivo. During development, secreted guidance molecules along with signals from extracellular matrix and the vasculature guide cell positioning, for example, around the fovea, and axon outgrowth; however, these changes are temporally regulated and are not the same in the adult. Here, we combine electrospun cell transplantation scaffolds capable of RGC neurite guidance with thermal inkjet 3D cell printing techniques capable of precise positioning of RGCs on the scaffold surface. Optimal printing parameters are developed for viability, electrophysiological function and, neurite pathfinding. Different media, commonly used to promote RGC survival and growth, were tested under varying conditions. When printed in growth media containing both brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF), RGCs maintained survival and normal electrophysiological function, and displayed radial axon outgrowth when printed onto electrospun scaffolds. These results demonstrate that 3D printing technology may be combined with complex electrospun surfaces in the design of future retinal models or therapies.

Multi-nucleate retinal pigment epithelium cells of the human macula exhibit a characteristic and highly specific distribution

BACKGROUND

The human retinal pigment epithelium (RPE) is reportedly 3% bi-nucleated. The importance to human vision of multi-nucleated (MN)-RPE cells could be clarified with more data about their distribution in central retina.

METHODS

Nineteen human RPE-flatmounts (9 ≤ 51 years, 10 > 80 years) were imaged at 12 locations: 3 eccentricities (fovea, perifovea, near periphery) in 4 quadrants (superior, inferior, temporal, nasal). Image stacks of lipofuscin-attributable autofluorescence and phalloidin labeled F-actin cytoskeleton were obtained using a confocal fluorescence microscope. Nuclei were devoid of autofluorescence and were marked using morphometric software. Cell areas were approximated by Voronoi regions. Mean number of nuclei per cell among eccentricity/quadrant groups and by age were compared using Poisson and binominal regression models.

RESULTS

A total of 11,403 RPE cells at 200 locations were analyzed: 94.66% mono-, 5.31% bi-, 0.02% tri-nucleate, and 0.01% with 5 nuclei. Age had no effect on number of nuclei. There were significant regional differences: highest frequencies of MN-cells were found at the perifovea (9.9%) and near periphery (6.8%). The fovea lacked MN-cells almost entirely. The nasal quadrant had significantly more MN-cells compared to other quadrants, at all eccentricities.

CONCLUSION

This study demonstrates MN-RPE cells in human macula. MN-cells may arise due to endoreplication, cell fusion, or incomplete cell division. The topography of MN-RPE cells follows the topography of photoreceptors; with near-absence at the fovea (cones only) and high frequency at perifovea (highest rod density). This distribution might reflect specific requirements of retinal metabolism or other mechanisms addressable in further studies.

Eye Morphology and Retinal Topography in Hummingbirds (Trochilidae: Aves)

Hummingbirds are a group of small, highly specialized birds that display a range of adaptations to their nectarivorous lifestyle. Vision plays a key role in hummingbird feeding and hovering behaviours, yet very little is known about the visual systems of these birds. In this study, we measured eye morphology in 5 hummingbird species. For 2 of these species, we used stereology and retinal whole mounts to study the topographic distribution of neurons in the ganglion cell layer. Eye morphology (expressed as the ratio of corneal diameter to eye transverse diameter) was similar among all 5 species and was within the range previously documented for diurnal birds. Retinal topography was similar in Amazilia tzacatl and Calypte anna. Both species had 2 specialized retinal regions of high neuron density: a central region located slightly dorso-nasal to the superior pole of the pecten, where densities reached ∼45,000 cells·mm-2, and a temporal area with lower densities (38,000-39,000 cells·mm-2). A weak visual streak bridged the two high-density areas. A retina from Phaethornis superciliosus also had a central high-density area with a similar peak neuron density. Estimates of spatial resolving power for all 3 species were similar, at approximately 5-6 cycles·degree-1. Retinal cross sections confirmed that the central high-density region in C. anna contains a fovea, but not the temporal area. We found no evidence of a second, less well-developed fovea located close to the temporal retina margin. The central and temporal areas of high neuron density allow for increased spatial resolution in the lateral and frontal visual fields, respectively. Increased resolution in the frontal field in particular may be important for mediating feeding behaviors such as aerial docking with flowers and catching small insects.

Spectral-Domain Optical Coherence Tomography in Wagner Syndrome: Characterization of Vitreoretinal Interface and Foveal Changes

PURPOSE

To evaluate the spectrum of morphologic abnormalities in patients with Wagner syndrome by spectral-domain optical coherence tomography (SD OCT).

DESIGN

Retrospective comparative case study.

METHODS

Institutional study of patients entered into the French Vitreoretinopathy Study Group database. Twelve eyes of 9 patients from 3 unrelated families with genetically confirmed Wagner syndrome and 28 eyes from 15 age- and sex-matched healthy family controls were scanned by SD OCT. Morphology and layer thickness of the total retina, inner retinal layers, outer retinal layers, and photoreceptor layer at different degrees of eccentricity from the fovea were compared between the 2 groups.

RESULTS

A thick multilayered membrane adherent to the perifovea but completely detached from the fovea, thus forming a bridge over the foveal pit, was observed in 84% of eyes from patients with Wagner syndrome. At the equatorial area, SD OCT imaging allowed visualization of the architecture of an avascular vitreous veil with localized retinal traction. Most retinal layers were significantly thinner in patients with Wagner syndrome compared to the control group, except at the foveal center where abnormal persistence of 1 or more inner retinal layers could be observed.

CONCLUSION

SD OCT provides better structural insight into the range of retinal defects at the vitreoretinal interface and fovea, which is not only useful for improving diagnosis and management, but also for understanding the pathogenesis of Wagner syndrome.

The temporal profile of retinal cell genesis in the marmoset monkey

The New World marmoset monkey (Callithrix jacchus) has a relatively short gestational period compared with other primates but possesses a retina at a similar stage of maturation by birth. Previous studies have highlighted that the complex fovea of the marmoset undergoes a more rapid postnatal development in comparison with the Macaca monkey, reaching a mature stage earlier than these species. In this current study, we examined the prenatal proliferation profile of cells in the entire retina employing the thymidine analogs and also determined their phenotype by double-label immunocytochemistry using type-specific markers. Akin to other primate species, we demonstrate a centroperipheral gradient in the emergence of both neurons and Müller glia with cones, ganglion cells, and horizontal cells generated first in the fovea at fetal day (Fd)70-74 and with the last generated at the retinal edge at Fd115. Rods, bipolar cells, amacrine cells, displaced amacrine cells, and Müller glia were generated between Fd76 and Fd135 along the same gradient. Similar to foveal development, marmoset neuronal generation was rapid, only taking 51% of gestation whereas in Macaca this takes 81%.

Advances in understanding the molecular basis of the first steps in color vision

Serving as one of our primary environmental inputs, vision is the most sophisticated sensory system in humans. Here, we present recent findings derived from energetics, genetics and physiology that provide a more advanced understanding of color perception in mammals. Energetics of cis-trans isomerization of 11-cis-retinal accounts for color perception in the narrow region of the electromagnetic spectrum and how human eyes can absorb light in the near infrared (IR) range. Structural homology models of visual pigments reveal complex interactions of the protein moieties with the light sensitive chromophore 11-cis-retinal and that certain color blinding mutations impair secondary structural elements of these G protein-coupled receptors (GPCRs). Finally, we identify unsolved critical aspects of color tuning that require future investigation.

The marmoset monkey as a model for visual neuroscience

The common marmoset (Callithrix jacchus) has been valuable as a primate model in biomedical research. Interest in this species has grown recently, in part due to the successful demonstration of transgenic marmosets. Here we examine the prospects of the marmoset model for visual neuroscience research, adopting a comparative framework to place the marmoset within a broader evolutionary context. The marmoset's small brain bears most of the organizational features of other primates, and its smooth surface offers practical advantages over the macaque for areal mapping, laminar electrode penetration, and two-photon and optical imaging. Behaviorally, marmosets are more limited at performing regimented psychophysical tasks, but do readily accept the head restraint that is necessary for accurate eye tracking and neurophysiology, and can perform simple discriminations. Their natural gaze behavior closely resembles that of other primates, with a tendency to focus on objects of social interest including faces. Their immaturity at birth and routine twinning also makes them ideal for the study of postnatal visual development. These experimental factors, together with the theoretical advantages inherent in comparing anatomy, physiology, and behavior across related species, make the marmoset an excellent model for visual neuroscience.

Enhanced photovoltaics inspired by the fovea centralis

The fovea centralis is a closely-packed vertical array of inverted-cone photoreceptor cells located in the retina that is responsible for high acuity binocular vision. The cones are operational in well-lit environments and are responsible for trapping the impinging illumination. We present the vertical light-funnel silicon array as a light-trapping technique for photovoltaic applications that is bio-inspired by the properties of the fovea centralis. We use opto-electronic simulations to evaluate the performance of light-funnel solar cell arrays. Light-funnel arrays present ~65% absorption enhancement compared to a silicon film of identical thickness and exhibit power conversion efficiencies that are 60% higher than those of optimized nanowire arrays of the same thickness although nanowire arrays consist of more than 2.3 times the amount of silicon. We demonstrate the superior absorption of the light-funnel arrays as compared with recent advancements in the field. Fabrication of silicon light-funnel arrays using low-cost processing techniques is demonstrated.

A novel in vivo model of focal light emitting diode-induced cone-photoreceptor phototoxicity: neuroprotection afforded by brimonidine, BDNF, PEDF or bFGF

We have investigated the effects of light-emitting diode (LED)-induced phototoxicity (LIP) on cone-photoreceptors and their protection with brimonidine (BMD), brain-derived neurotrophic factor (BDNF), pigment epithelium-derived factor (PEDF), ciliary neurotrophic factor (CNTF) or basic fibroblast growth factor (bFGF). In anesthetized, dark adapted, adult albino rats a blue (400 nm) LED was placed perpendicular to the cornea (10 sec, 200 lux) and the effects were investigated using Spectral Domain Optical Coherence Tomography (SD-OCT) and/or analysing the retina in oriented cross-sections or wholemounts immune-labelled for L- and S-opsin and counterstained with the nuclear stain DAPI. The effects of topical BMD (1%) or, intravitreally injected BDNF (5 µg), PEDF (2 µg), CNTF (0.4 µg) or bFGF (1 µg) after LIP were examined on wholemounts at 7 days. SD-OCT showed damage in a circular region of the superotemporal retina, whose diameter varied from 1,842.4±84.5 µm (at 24 hours) to 1,407.7±52.8 µm (at 7 days). This region had a progressive thickness diminution from 183.4±5 µm (at 12 h) to 114.6±6 µm (at 7 d). Oriented cross-sections showed within the light-damaged region of the retina massive loss of rods and cone-photoreceptors. Wholemounts documented a circular region containing lower numbers of L- and S-cones. Within a circular area (1 mm or 1.3 mm radius, respectively) in the left and in its corresponding region of the contralateral-fellow-retina, total L- or S-cones were 7,118±842 or 661±125 for the LED exposed retinas (n = 7) and 14,040±1,860 or 2,255±193 for the fellow retinas (n = 7), respectively. BMD, BDNF, PEDF and bFGF but not CNTF showed significant neuroprotective effects on L- or S-cones. We conclude that LIP results in rod and cone-photoreceptor loss, and is a reliable, quantifiable model to study cone-photoreceptor degeneration. Intravitreal BDNF, PEDF or bFGF, or topical BMD afford significant cone neuroprotection in this model.

Niche convergence suggests functionality of the nocturnal fovea

The fovea is a declivity of the retinal surface associated with maximum visual acuity. Foveae are widespread across vertebrates, but among mammals they are restricted to haplorhine primates (tarsiers, monkeys, apes, and humans), which are primarily diurnal. Thus primates have long contributed to the view that foveae are functional adaptations to diurnality. The foveae of tarsiers, which are nocturnal, are widely interpreted as vestigial traits and therefore evidence of a diurnal ancestry. This enduring premise is central to adaptive hypotheses on the origins of anthropoid primates; however, the question of whether tarsier foveae are functionless anachronisms or nocturnal adaptations remains open. To explore this question, we compared the diets of tarsiers (Tarsius) and scops owls (Otus), taxa united by numerous anatomical homoplasies, including foveate vision. A functional interpretation of these homoplasies predicts dietary convergence. We tested this prediction by analyzing stable isotope ratios that integrate dietary information. In Borneo and the Philippines, the stable carbon isotope compositions of Tarsius and Otus were indistinguishable, whereas the stable nitrogen isotope composition of Otus was marginally higher than that of Tarsius. Our results indicate that species in both genera consumed mainly ground-dwelling prey. Taken together, our findings support a functional interpretation of the many homoplasies shared by tarsiers and scops owls, including a retinal fovea. We suggest that the fovea might function similarly in tarsiers and scops owls by calibrating the auditory localization pathway. The integration of auditory localization and visual fixation during prey detection and acquisition might be critical at low light levels.

Relationship between foveal cone specialization and pit morphology in albinism

PURPOSE

Albinism is associated with disrupted foveal development, though intersubject variability is becoming appreciated. We sought to quantify this variability, and examine the relationship between foveal cone specialization and pit morphology in patients with a clinical diagnosis of albinism.

METHODS

We recruited 32 subjects with a clinical diagnosis of albinism. DNA was obtained from 25 subjects, and known albinism genes were analyzed for mutations. Relative inner and outer segment (IS and OS) lengthening (fovea-to-perifovea ratio) was determined from manually segmented spectral domain-optical coherence tomography (SD-OCT) B-scans. Foveal pit morphology was quantified for eight subjects from macular SD-OCT volumes. Ten subjects underwent imaging with adaptive optics scanning light ophthalmoscopy (AOSLO), and cone density was measured.

RESULTS

We found mutations in 22 of 25 subjects, including five novel mutations. All subjects lacked complete excavation of inner retinal layers at the fovea, though four subjects had foveal pits with normal diameter and/or volume. Peak cone density and OS lengthening were variable and overlapped with that observed in normal controls. A fifth hyper-reflective band was observed in the outer retina on SD-OCT in the majority of the subjects with albinism.

CONCLUSIONS

Foveal cone specialization and pit morphology vary greatly in albinism. Normal cone packing was observed in the absence of a foveal pit, suggesting a pit is not required for packing to occur. The degree to which retinal anatomy correlates with genotype or visual function remains unclear, and future examination of larger patient groups will provide important insight on this issue.

Cholesterol in the retina: the best is yet to come

Historically understudied, cholesterol in the retina is receiving more attention now because of genetic studies showing that several cholesterol-related genes are risk factors for age-related macular degeneration (AMD) and because of eye pathology studies showing high cholesterol content of drusen, aging Bruch's membrane, and newly found subretinal lesions. The challenge before us is determining how the cholesterol-AMD link is realized. Meeting this challenge will require an excellent understanding these genes' roles in retinal physiology and how chorioretinal cholesterol is maintained. In the first half of this review, we will succinctly summarize physico-chemical properties of cholesterol, its distribution in the human body, general principles of maintenance and metabolism, and differences in cholesterol handling in human and mouse that impact on experimental approaches. This information will provide a backdrop to the second part of the review focusing on unique aspects of chorioretinal cholesterol homeostasis, aging in Bruch's membrane, cholesterol in AMD lesions, a model for lesion biogenesis, a model for macular vulnerability based on vascular biology, and alignment of AMD-related genes and pathobiology using cholesterol and an atherosclerosis-like progression as unifying features. We conclude with recommendations for the most important research steps we can take towards delineating the cholesterol-AMD link.

Canine retina has a primate fovea-like bouquet of cone photoreceptors which is affected by inherited macular degenerations

Retinal areas of specialization confer vertebrates with the ability to scrutinize corresponding regions of their visual field with greater resolution. A highly specialized area found in haplorhine primates (including humans) is the fovea centralis which is defined by a high density of cone photoreceptors connected individually to interneurons, and retinal ganglion cells (RGCs) that are offset to form a pit lacking retinal capillaries and inner retinal neurons at its center. In dogs, a local increase in RGC density is found in a topographically comparable retinal area defined as the area centralis. While the canine retina is devoid of a foveal pit, no detailed examination of the photoreceptors within the area centralis has been reported. Using both in vivo and ex vivo imaging, we identified a retinal region with a primate fovea-like cone photoreceptor density but without the excavation of the inner retina. Similar anatomical structure observed in rare human subjects has been named fovea-plana. In addition, dogs with mutations in two different genes, that cause macular degeneration in humans, developed earliest disease at the newly-identified canine fovea-like area. Our results challenge the dogma that within the phylogenetic tree of mammals, haplorhine primates with a fovea are the sole lineage in which the retina has a central bouquet of cones. Furthermore, a predilection for naturally-occurring retinal degenerations to alter this cone-enriched area fills the void for a clinically-relevant animal model of human macular degenerations.

A new recessively inherited disorder composed of foveal hypoplasia, optic nerve decussation defects and anterior segment dysgenesis maps to chromosome 16q23.3-24.1

PURPOSE

We have previously described two families with unique phenotypes involving foveal hypoplasia. The first family (F1) presented with foveal hypoplasia and anterior segment dysgenesis, and the second family (F2) presented with foveal hypoplasia and chiasmal misrouting in the absence of albinism. A genome-wide linkage search in family F1 identified a 6.5 Mb locus for this disorder on chromosome 16q23.2-24.1. The aim of this study was to determine if both families have the same disorder and to see if family F2 is also linked to the 16q locus.

METHODS

Family members underwent routine clinical examination. Linkage was determined by genotyping microsatellite makers and calculating logarithm of the odds (LOD) scores. Locus refinement was undertaken with single nucleotide polymorphism (SNP) microarray analysis.

RESULTS

The identification of chiasmal misrouting in family F1 and anterior segment abnormalities in family F2 suggested that the families have the same clinical phenotype. This was confirmed when linkage analysis showed that family F2 also mapped to the 16q locus. The single nucleotide polymorphism microarray analysis excluded a shared founder haplotype between the families and refined the locus to 3.1 Mb.

CONCLUSIONS

We report a new recessively inherited syndrome consisting of foveal hypoplasia, optic nerve decussation defects and anterior segment dysgenesis, which we have abbreviated to FHONDA syndrome. The gene mutated in this disorder lies within a 3.1 Mb interval containing 33 genes on chromosome 16q23.3-24.1 (chr16:83639061 - 86716445, hg19).