Visible-Light Optical Coherence Tomography Fibergraphy for Quantitative Imaging of Retinal Ganglion Cell Axon Bundles

Visible-Light Optical Coherence Tomography Fibergraphy of the Tree Shrew Retinal Ganglion Cell Axon Bundles

We seek to develop techniques for high-resolution imaging of the tree shrew retina for visualizing and parameterizing retinal ganglion cell (RGC) axon bundles in vivo. We applied visible-light optical coherence tomography fibergraphy (vis-OCTF) and temporal speckle averaging (TSA) to visualize individual RGC axon bundles in the tree shrew retina. For the first time, we quantified individual RGC bundle width, height, and cross-sectional area and applied vis-OCT angiography (vis-OCTA) to visualize the retinal microvasculature in tree shrews. Throughout the retina, as the distance from the optic nerve head (ONH) increased from 0.5 mm to 2.5 mm, bundle width increased by 30%, height decreased by 67%, and cross-sectional area decreased by 36%. We also showed that axon bundles become vertically elongated as they converge toward the ONH. Ex vivo confocal microscopy of retinal flat-mounts immunostained with Tuj1 confirmed our in vivo vis-OCTF findings.

Study of tree shrew biology and models: A booming and prosperous field for biomedical research

The tree shrew ( Tupaia belangeri) has long been proposed as a suitable alternative to non-human primates (NHPs) in biomedical and laboratory research due to its close evolutionary relationship with primates. In recent years, significant advances have facilitated tree shrew studies, including the determination of the tree shrew genome, genetic manipulation using spermatogonial stem cells, viral vector-mediated gene delivery, and mapping of the tree shrew brain atlas. However, the limited availability of tree shrews globally remains a substantial challenge in the field. Additionally, determining the key questions best answered using tree shrews constitutes another difficulty. Tree shrew models have historically been used to study hepatitis B virus (HBV) and hepatitis C virus (HCV) infection, myopia, and psychosocial stress-induced depression, with more recent studies focusing on developing animal models for infectious and neurodegenerative diseases. Despite these efforts, the impact of tree shrew models has not yet matched that of rodent or NHP models in biomedical research. This review summarizes the prominent advancements in tree shrew research and reflects on the key biological questions addressed using this model. We emphasize that intensive dedication and robust international collaboration are essential for achieving breakthroughs in tree shrew studies. The use of tree shrews as a unique resource is expected to gain considerable attention with the application of advanced techniques and the development of viable animal models, meeting the increasing demands of life science and biomedical research.

Longitudinal imaging of vitreal hyperreflective foci in mice with acute optic nerve damage using visible-light optical coherence tomography

Hyperreflective foci (HRFs) appear in optical coherence tomography (OCT) images of the retina and vitreous of patients with various ocular diseases. HRFs are hypothesized to be immune cells that appear in response to ischemia or tissue damage. To accurately identify HRFs and establish their clinical significance, it is necessary to replicate the detection of similar patterns in vivo in a small animal model. We combined visible-light OCT with temporal speckle averaging (TSA) to visualize and track vitreal HRFs (VHRFs) densities for three days after an optic nerve crush (ONC) injury. Resulting vis-OCT images revealed that VHRF density significantly increased approximately 10-fold at 12 h after ONC and returned to baseline three days after ONC. Additional immunohistochemistry results confirmed these VHRFs as inflammatory cells induced from optic nerve damage.

Comparative In Vivo Imaging of Retinal Structures in Tree Shrews, Humans, and Mice

Rodent models, such as mice and rats, are commonly used to examine retinal ganglion cell damage in eye diseases. However, as nocturnal animals, rodent retinal structures differ from primates, imposing significant limitations in studying retinal pathology. Tree shrews (Tupaia belangeri) are small, diurnal paraprimates that exhibit superior visual acuity and color vision compared with mice. Like humans, tree shrews have a dense retinal nerve fiber layer (RNFL) and a thick ganglion cell layer (GCL), making them a valuable model for investigating optic neuropathies. In this study, we applied high-resolution visible-light optical coherence tomography to characterize the tree shrew retinal structure in vivo and compare it with that of humans and mice. We quantitatively characterize the tree shrew's retinal layer structure in vivo, specifically examining the sublayer structures within the inner plexiform layer (IPL) for the first time. Next, we conducted a comparative analysis of retinal layer structures among tree shrews, mice, and humans. We then validated our in vivo findings in the tree shrew inner retina using ex vivo confocal microscopy. The in vivo and ex vivo analyses of the shrew retina build the foundation for future work to accurately track and quantify the retinal structural changes in the IPL, GCL, and RNFL during the development and progression of human optic diseases.

Multiscale imaging of corneal endothelium damage and Rho-kinase inhibitor application in mouse models of acute ocular hypertension

We developed a multiscale optical imaging workflow, integrating and correlating visible-light optical coherence tomography, confocal laser scanning microscopy, and single-molecule localization microscopy to investigate mouse cornea damage from the in-vivo tissue level to the nanoscopic single-molecule level. We used electron microscopy to validate the imaged nanoscopic structures. We imaged wild-type mice and mice with acute ocular hypertension and examined the effects of Rho-kinase inhibitor application. We defined four types of intercellular tight junction structures as healthy, compact, partially-distorted, and fully-distorted types by labeling the zonula occludens-1 protein in the corneal endothelial cell layer. We correlated the statistics of the four types of tight junction structures with cornea thickness and intraocular pressure. We found that the population of fully-distorted tight junctions correlated well with the level of corneal edema, and applying Rho-kinase inhibitor reduced the population of fully-distorted tight junctions under acute ocular hypertension. Together, these data point to the utility of multiscale optical imaging in revealing fundamental biology relevant to disease and therapeutics.

Evaluating the performance of OCT in assessing static and potential dynamic properties of the retinal ganglion cells and nerve fiber bundles in the living mouse eye

Glaucoma is a group of eye diseases characterized by the thinning of the retinal nerve fiber layer (RNFL), which is primarily caused by the progressive death of retinal ganglion cells (RGCs). Precise monitoring of these changes at a cellular resolution in living eyes is significant for glaucoma research. In this study, we aimed to assess the effectiveness of temporal speckle averaging optical coherence tomography (TSA-OCT) and dynamic OCT (dOCT) in examining the static and potential dynamic properties of RGCs and RNFL in living mouse eyes. We evaluated parameters such as RNFL thickness and possible dynamics, as well as compared the ganglion cell layer (GCL) soma density obtained from in vivo OCT, fluorescence scanning laser ophthalmoscopy (SLO), and ex vivo histology.

Establishing Functional Retina in a Dish: Progress and Promises of Induced Pluripotent Stem Cell-Based Retinal Neuron Differentiation

The human eye plays a critical role in vision perception, but various retinal degenerative diseases such as retinitis pigmentosa (RP), glaucoma, and age-related macular degeneration (AMD) can lead to vision loss or blindness. Although progress has been made in understanding retinal development and in clinical research, current treatments remain inadequate for curing or reversing these degenerative conditions. Animal models have limited relevance to humans, and obtaining human eye tissue samples is challenging due to ethical and legal considerations. Consequently, researchers have turned to stem cell-based approaches, specifically induced pluripotent stem cells (iPSCs), to generate distinct retinal cell populations and develop cell replacement therapies. iPSCs offer a novel platform for studying the key stages of human retinogenesis and disease-specific mechanisms. Stem cell technology has facilitated the production of diverse retinal cell types, including retinal ganglion cells (RGCs) and photoreceptors, and the development of retinal organoids has emerged as a valuable in vitro tool for investigating retinal neuron differentiation and modeling retinal diseases. This review focuses on the protocols, culture conditions, and techniques employed in differentiating retinal neurons from iPSCs. Furthermore, it emphasizes the significance of molecular and functional validation of the differentiated cells.

Multiscale imaging of corneal endothelium damage and effects of Rho Kinase inhibitor application in mouse models of acute ocular hypertension

We developed a multiscale optical imaging workflow, integrating and correlating visible-light optical coherence tomography, confocal laser scanning microscopy, and single-molecule localization microscopy to investigate the mouse cornea damages from the in-vivo tissue level to the nanoscopic single-molecule level. We used electron microscopy to validate the imaged nanoscopic structures. We imaged wild-type mice and mice with acute ocular hypertension and examined the effects of Rho Kinase inhibitor application. We defined four types of intercellular tight junction structures as healthy, compact, partially-distorted, and fully-distorted types by labeling the Zonula occludens-1 protein in the corneal endothelial cell layer. We correlated the statistics of the four types of tight junction structures with cornea thickness and intraocular pressure. We found that the population of fully-distorted tight junctions correlated well with the level of cornea edema, and applying Rho Kinase inhibitor reduced the population of fully-distorted tight junctions under acute ocular hypertension.

Visible-Light Optical Coherence Tomography Fibergraphy of the Tree Shrew Retinal Ganglion Cell Axon Bundles

We seek to develop techniques for high-resolution imaging of the tree shrew retina for visualizing and parameterizing retinal ganglion cell (RGC) axon bundles in vivo. We applied visible-light optical coherence tomography fibergraphy (vis-OCTF) and temporal speckle averaging (TSA) to visualize individual RGC axon bundles in the tree shrew retina. For the first time, we quantified individual RGC bundle width, height, and cross-sectional area and applied vis-OCT angiography (vis-OCTA) to visualize the retinal microvasculature in tree shrews. Throughout the retina, as the distance from the optic nerve head (ONH) increased from 0.5 mm to 2.5 mm, bundle width increased by 30%, height decreased by 67%, and cross-sectional area decreased by 36%. We also showed that axon bundles become vertically elongated as they converge toward the ONH. Ex vivo confocal microscopy of retinal flat-mounts immunostained with Tuj1 confirmed our in vivo vis-OCTF findings.

Longitudinal Analysis of Retinal Ganglion Cell Damage at Individual Axon Bundle Level in Mice Using Visible-Light Optical Coherence Tomography Fibergraphy

Purpose

We developed a new analytic tool based on visible-light optical coherence tomography fibergraphy (vis-OCTF) to longitudinally track individual axon bundle transformation as a new in vivo biomarker for retinal ganglion cell (RGC) damage.

Methods

After acute optic nerve crush injury (ONC) in mice, we analyzed four parameters: lateral bundle width, axial bundle height, cross-sectional area, and the shape of individual bundles. We next correlated the morphological changes in RGC axon bundles with RGC soma loss.

Results

We showed that axon bundles became wider and taller at three days post ONC (pONC), which correlated with about 15% RGC soma loss. At six days pONC, axon bundles showed a significant reduction in lateral width and cross-sectional area, followed by a reduction in bundle height at nine days pONC. Bundle shrinking at nine days pONC correlated with about 68% RGC soma loss. Both experimental and simulated results suggested that the cross-sectional area of individual RGC axon bundles is more sensitive than bundle width and height to indicate RGC soma loss.

Conclusions

This study is the first to track and quantify individual RGC axon bundles in vivo after ONC injury.

Translational Relevance

Recognizing RGC loss at its earliest stage is crucial for disease diagnosis and treatment. However, current clinical methods to detect the functional and structural changes in the inner retina are not sensitive enough to directly assess RGC health. In this study, we developed vis-OCTF-based parameters to track RGC damage, making possible to establishing a quantifiable biomarker for glaucoma.

Longitudinal Observation of Retinal Response to Optic Nerve Transection in Rats Using Visible Light Optical Coherence Tomography

Purpose

To characterize rat retinal responses after optic nerve transection (ONT) by visible-light optical coherence tomography (vis-OCT).

Methods

Unilateral ONT was performed in Brown Norway rats (n = 8). In vivo, vis-OCT retinal imaging was performed on the experimental eyes before ONT (baseline), and two days, one week, two weeks, and four weeks (endpoint) after ONT, as well as on fellow eyes at the endpoint. The system was operated at a 70 kHz A-line sampling rate with both raster scans (512 × 2 × 512 A-lines), and circular scans (2048 × 100 A-lines) acquired around the optic disc. Retinal layers were segmented to calculate layer thicknesses and project en face images for visualization and quantifications. Vessel densities and oxygen saturation were used to evaluate the morphologic and functional impact on the retinal vasculature.

Results

After ONT, retinal nerve fiber bundles demonstrated significant degeneration, starting at two weeks, with a reduction of thicknesses quantified on the nerve fiber layer, ganglion cell complex, and total retina. Along with that, the activation of macrophage-like cells in the vitreoretinal interface was also observed. Vessel densities for all three retinal plexuses were unaffected over the period of observation. However, oxygen saturation in retinal arteries and veins was significantly reduced at four weeks after ONT.

Conclusions

Vis-OCT can provide high-definition, in vivo characterization of retinal responses to ONT in rats. Despite a significant reduction in retinal layer thickness, this was not accompanied by alterations in vascular density. Despite this, oximetry indicates reduced retinal oxygen saturation, suggesting that altered vascular physiology is not reflected in the anatomic appearance of retinal blood vessel density alone.

Comparative Analysis of Retinal Organotypic Cultures and In Vivo Axotomized Retinas

Retinal organotypic cultures (ROCs) are used as an in vivo surrogate to study retinal ganglion cell (RGC) loss and neuroprotection. In vivo, the gold standard to study RGC degeneration and neuroprotection is optic nerve lesion. We propose here to compare the course of RGC death and glial activation between both models. The left optic nerve of C57BL/6 male mice was crushed, and retinas analyzed from 1 to 9 days after the injury. ROCs were analyzed at the same time points. As a control, intact retinas were used. Retinas were studied anatomically to assess RGC survival, microglial, and macroglial activation. Macroglial and microglial cells showed different morphological activation between models and were activated earlier in ROCs. Furthermore, microglial cell density in the ganglion cell layer was always lower in ROCs than in vivo. RGC loss after axotomy and in vitro followed the same trend up to 5 days. Thereafter, there was an abrupt decrease in viable RGCs in ROCs. However, RGC somas were still immuno-identified by several molecular markers. ROCs are useful for proof-of-concept studies on neuroprotection, but long-term experiments should be carried out in vivo. Importantly, the differential glial activation observed between models and the concomitant death of photoreceptors that occurs in vitro may alter the efficacy of RGC neuroprotective therapies when tested in in vivo models of optic nerve injury.

Pan-retinal ganglion cell markers in mice, rats, and rhesus macaques

Univocal identification of retinal ganglion cells (RGCs) is an essential prerequisite for studying their degeneration and neuroprotection. Before the advent of phenotypic markers, RGCs were normally identified using retrograde tracing of retinorecipient areas. This is an invasive technique, and its use is precluded in higher mammals such as monkeys. In the past decade, several RGC markers have been described. Here, we reviewed and analyzed the specificity of nine markers used to identify all or most RGCs, i.e., pan-RGC markers, in rats, mice, and macaques. The best markers in the three species in terms of specificity, proportion of RGCs labeled, and indicators of viability were BRN3A, expressed by vision-forming RGCs, and RBPMS, expressed by vision- and non-vision-forming RGCs. NEUN, often used to identify RGCs, was expressed by non-RGCs in the ganglion cell layer, and therefore was not RGC-specific. γ-SYN, TUJ1, and NF-L labeled the RGC axons, which impaired the detection of their somas in the central retina but would be good for studying RGC morphology. In rats, TUJ1 and NF-L were also expressed by non-RGCs. BM88, ERRβ, and PGP9.5 are rarely used as markers, but they identified most RGCs in the rats and macaques and ERRβ in mice. However, PGP9.5 was also expressed by non-RGCs in rats and macaques and BM88 and ERRβ were not suitable markers of viability.

The Development and Clinical Application of Innovative Optical Ophthalmic Imaging Techniques

The field of ophthalmic imaging has grown substantially over the last years. Massive improvements in image processing and computer hardware have allowed the emergence of multiple imaging techniques of the eye that can transform patient care. The purpose of this review is to describe the most recent advances in eye imaging and explain how new technologies and imaging methods can be utilized in a clinical setting. The introduction of optical coherence tomography (OCT) was a revolution in eye imaging and has since become the standard of care for a plethora of conditions. Its most recent iterations, OCT angiography, and visible light OCT, as well as imaging modalities, such as fluorescent lifetime imaging ophthalmoscopy, would allow a more thorough evaluation of patients and provide additional information on disease processes. Toward that goal, the application of adaptive optics (AO) and full-field scanning to a variety of eye imaging techniques has further allowed the histologic study of single cells in the retina and anterior segment. Toward the goal of remote eye care and more accessible eye imaging, methods such as handheld OCT devices and imaging through smartphones, have emerged. Finally, incorporating artificial intelligence (AI) in eye images has the potential to become a new milestone for eye imaging while also contributing in social aspects of eye care.

In vivo imaging of the inner retinal layer structure in mice after eye-opening using visible-light optical coherence tomography

The growth of the mouse eye and retina after birth is a dynamic, highly regulated process. In this study, we applied visible-light optical coherence tomography (vis-OCT), a non-invasive imaging technique, to examine developing retinal layer structures after eye-opening. We introduced a resampled circumpapillary B-scan averaging technique to improve the inter-layer contrast, enabling retinal layer thickness measurements as early as postnatal day 13 (P13) - right after eye-opening. We confirmed vis-OCT measurements using ex vivo confocal microscopy of retinal sections at different ages. Our results demonstrate that vis-OCT can visualize the developmental murine retinal layer structure in vivo, which offers us new opportunities to better characterize the pathological alterations in mouse models of developmental eye diseases.