Variational analysis of the mouse and rat eye optical parameters

Declines in PDE4B activity promote myopia progression through downregulation of scleral collagen expression

Myopia is the most common cause of a visual refractive error worldwide. Cyclic adenosine monophosphate (cAMP)-linked signaling pathways contribute to the regulation of myopia development, and increases in cAMP accumulation promote myopia progression. To pinpoint the underlying mechanisms by which cAMP modulates myopia progression, we performed scleral transcriptome sequencing analysis in form-deprived mice, a well-established model of myopia development. Form deprivation significantly inhibited the expression levels of genes in the cAMP catabolic pathway. Quantitative real-time polymerase chain reaction analysis validated that the gene expression level of phosphodiesterase 4B (PDE4B), a cAMP hydrolase, was downregulated in form-deprived mouse eyes. Under visually unobstructed conditions, loss of PDE4B function in Pde4b-knockout mice increased the myopic shift in refraction, -3.661 ± 1.071 diopters, more than that in the Pde4b-wildtype littermates (P < 0.05). This suggests that downregulation and inhibition of PDE4B gives rise to myopia. In guinea pigs, subconjunctival injection of rolipram, a selective inhibitor of PDE4, led to myopia in normal eyes, and it also enhanced form-deprivation myopia (FDM). Subconjunctival injection of dibutyryl-cyclic adenosine monophosphate, a cAMP analog, induced only a myopic shift in the normal visually unobstructed eyes, but it did not enhance FDM. As myopia developed, axial elongation occurred during scleral remodeling that was correlated with changes in collagen fibril thickness and distribution. The median collagen fibril diameter in the FDM + rolipram group, 55.09 ± 1.83 nm, was thinner than in the FDM + vehicle group, 59.33 ± 2.06 nm (P = 0.011). Thus, inhibition of PDE4 activity with rolipram thinned the collagen fibril diameter relative to the vehicle treatment in form-deprived eyes. Rolipram also inhibited increases in collagen synthesis induced by TGF-β2 in cultured human scleral fibroblasts. The current results further support a role for PDE enzymes such as PDE4B in the regulation of normal refractive development and myopia because either loss or inhibition of PDE4B function increased myopia and FDM development through declines in the scleral collagen fibril diameter.

Three-dimensional data capture and analysis of intact eye lenses evidences emmetropia-associated changes in epithelial cell organization

Organ and tissue development are highly coordinated processes; lens growth and functional integration into the eye (emmetropia) is a robust example. An epithelial monolayer covers the anterior hemisphere of the lens, and its organization is the key to lens formation and its optical properties throughout all life stages. To better understand how the epithelium supports lens function, we have developed a novel whole tissue imaging system using conventional confocal light microscopy and a specialized analysis software to produce three-dimensional maps for the epithelium of intact mouse lenses. The open source software package geometrically determines the anterior pole position, the equatorial diameter, and three-dimensional coordinates for each detected cell in the epithelium. The user-friendly cell maps, which retain global lens geometry, allow us to document age-dependent changes in the C57/BL6J mouse lens cell distribution characteristics. We evidence changes in epithelial cell density and distribution in C57/BL6J mice during the establishment of emmetropia between postnatal weeks 4-6. These epithelial changes accompany a previously unknown spheroid to lentoid shape transition of the lens as detected by our analyses. When combined with key findings from previous mouse genetic and cell biological studies, we suggest a cytoskeleton-based mechanism likely underpins these observations.

Characterization of retinal biomechanical properties using Brillouin microscopy

SIGNIFICANCE

The retina is critical for vision, and several diseases may alter its biomechanical properties. However, assessing the biomechanical properties of the retina nondestructively is a challenge due to its fragile nature and location within the eye globe. Advancements in Brillouin spectroscopy have provided the means for nondestructive investigations of retina biomechanical properties.

AIM

We assessed the biomechanical properties of mouse retinas using Brillouin microscopy noninvasively and showed the potential of Brillouin microscopy to differentiate the type and layers of retinas based on stiffness.

APPROACH

We used Brillouin microscopy to quantify stiffness of fresh and paraformaldehyde (PFA)-fixed retinas. As further proof-of-concept, we demonstrated a change in the stiffness of a retina with N-methyl-D-aspartate (NMDA)-induced damage, compared to an undamaged sample.

RESULTS

We found that the retina layers with higher cell body density had higher Brillouin modulus compared to less cell-dense layers. We have also demonstrated that PFA-fixed retina samples were stiffer compared with fresh samples. Further, NMDA-induced neurotoxicity leads to retinal ganglion cell (RGC) death and reactive gliosis, increasing the stiffness of the RGC layer.

CONCLUSION

Brillouin microscopy can be used to characterize the stiffness distribution of the layers of the retina and can be used to differentiate tissue at different conditions based on biomechanical properties.

Safety Profile of Slit-Lamp-Delivered Retinal Laser Photobiomodulation

Purpose

Photobiomodulation (PBM) refers to therapeutic irradiation of tissue with low-energy, 630- to 1000-nm wavelength light. An increasing body of evidence supports a beneficial effect of PBM in retinal disorders. To date, most studies have utilized light-emitting diode irradiation sources. Slit-lamp-mounted retinal lasers produce a coherent beam that can be delivered with precisely defined dosages and predetermined target area; however, the use of retinal lasers raises safety concerns that warrant investigation prior to clinical application. In this study, we determined safe dosages of laser-delivered PBM to the retina.

Methods

A custom-designed, slit-lamp-delivered, 670-nm, red/near-infrared laser was used to administer a range of irradiances to healthy pigmented and non-pigmented rat retinas. The effects of PBM on various functional and structural parameters of the retina were evaluated utilizing a combination of electroretinography, Spectral Domain Optical Coherence (SD-OCT), fluorescein angiography, histology and immunohistochemistry.

Results

In non-pigmented rats, no adverse events were identified at any irradiances up to 500 mW/cm². In pigmented rats, no adverse events were identified at irradiances of 25 or 100 mW/cm²; however, approximately one-third of rats that received 500 mW/cm² displayed very localized photoreceptor damage in the peripapillary region, typically adjacent to the optic nerve head.

Conclusions

A safety threshold exists for laser-delivered PBM in pigmented retinas and was identified as 500 mW/cm² irradiance; therefore, caution should be exercised in the dosage of laser-delivered PBM administered to pigmented retinas.

Translational Relevance

This study provides important data necessary for clinical translation of laser-delivered PBM for retinal diseases.

Estimation of Refractive Index for Biological Tissue Using Micro-Optical Coherence Tomography

The refractive index of a biological tissue is required for investigating the tissue's optical properties. Efforts have been made to characterize the refractive indices of biological tissues at a single wavelength, but it is more convenient to know the Cauchy's coefficients, which provide refractive index over a wide range of wavelengths. We demonstrate a method to noninvasively provide the Cauchy's dispersion coefficients of biological tissues using a micro-optical coherence tomography. Using the short-frequency Fourier transforms, the relative optical thickness of the sample in the wavelength range of the broadband source was obtained from interferograms. The coefficients of the Cauchy's equation were estimated based on the wavelength-dependent sample thickness. We validated the proposed method using distilled water and fresh rat cornea ex vivo, and the experimental results were consistent with the reference data.

Two-photon phosphorescence lifetime microscopy of retinal capillary plexus oxygenation in mice

Impaired oxygen delivery and/or consumption in the retinal tissue underlies the pathophysiology of many retinal diseases. However, the essential tools for measuring oxygen concentration in retinal capillaries and studying oxygen transport to retinal tissue are still lacking. We show that two-photon phosphorescence lifetime microscopy can be used to map absolute partial pressures of oxygen (pO2) in the retinal capillary plexus. Measurements were performed at various retinal depths in anesthetized mice under systemic normoxic and hyperoxic conditions. We used a newly developed two-photon phosphorescent oxygen probe, based on a two-photon absorbing platinum tetraphthalimidoporphyrin, and commercially available optics without correction for optical aberrations of the eye. The transverse and axial distances within the tissue volume were calibrated using a model of the eye's optical system. We believe this is the first demonstration of in vivo depth-resolved imaging of pO2 in retinal capillaries. Application of this method has the potential to advance our understanding of oxygen delivery on the microvascular scale and help elucidate mechanisms underlying various retinal diseases.

Effect of a contact lens on mouse retinal in vivo imaging: Effective focal length changes and monochromatic aberrations

For in vivo mouse retinal imaging, especially with Adaptive Optics instruments, application of a contact lens is desirable, as it allows maintenance of cornea hydration and helps to prevent cataract formation during lengthy imaging sessions. However, since the refractive elements of the eye (cornea and lens) serve as the objective for most in vivo retinal imaging systems, the use of a contact lens, even with 0 Dpt. refractive power, can alter the system’s optical properties. In this investigation we examined the effective focal length change and the aberrations that arise from use of a contact lens. First, focal length changes were simulated with a Zemax mouse eye model. Then ocular aberrations with and without a 0 Dpt. contact lens were measured with a Shack-Hartmann wavefront sensor (SHWS) in a customized AO-SLO system. Total RMS wavefront errors were measured for two groups of mice (14-month, and 2.5-month-old), decomposed into 66 Zernike aberration terms, and compared. These data revealed that vertical coma and spherical aberrations were increased with use of a contact lens in our system. Based on the ocular wavefront data we evaluated the effect of the contact lens on the imaging system performance as a function of the pupil size. Both RMS error and Strehl ratios were quantified for the two groups of mice, with and without contact lenses, and for different input beam sizes. These results provide information for determining optimum pupil size for retinal imaging without adaptive optics, and raise critical issues for design of mouse optical imaging systems that incorporate contact lenses.

High-intensity corneal collagen crosslinking with riboflavin and UVA in rat cornea

Corneal collagen cross-linking (CXL) halts human corneal ectasias progression by increasing stromal mechanical stiffness. Although some reports describe that this procedure is effective in dealing with some infectious and immunologic corneal thinning diseases, there is a need for more animal models whose corneal thickness more closely resemble those occurring in these patients. To meet this need, we describe here high-intensity protocols that are safe and effective for obtaining CXL in rat corneas. Initially, a range of potentially effective UVA doses were evaluated based on their effectiveness in increasing tissue enzymatic resistance to dissolution. At UVA doses higher than a threshold level of 0.54 J/cm², resistance to enzymatic digestion increased relative to that in non-irradiated corneas. Based on the theoretical threshold CXL dose, a CXL regimen was established in which the UVA tissue irradiance was 9 mW/cm², which was delivered at doses of either 2.16, 2.7 or 3.24 J/cm². Their dose dependent effects were evaluated on ocular surface morphological integrity, keratocyte apoptotic frequency, tissue thickness and endothelial cell layer density. Doses of 2.16 and 2.7 J/cm² transiently decreased normal corneal transparency and increased thickness. These effects were fully reversed after 14 days. In contrast, 3.24 J/cm² had more irreversible side effects. Three days after treatment, apoptotic frequency in the CXL-2.16 group was lower than that at higher doses. Endothelial cell losses remained evident only in the CXL-3.24 group at 42 days posttreatment. Stromal fiber thickening was evident in all the CXL-treated groups. We determined both the threshold UVA dose using the high-intensity CXL procedure and identified an effective dose range that provides optimal CXL with minimal transient side effects in the rat cornea. These results may help to provide insight into how to improve the CXL outcome in patients afflicted with a severe corneal thinning disease.

Orbital venous pattern in relation to extraorbital venous drainage and superficial lymphatic vessels in rats

The purpose of this study was to demonstrate the normal and variant anatomy of extraorbital and intraorbital venous drainage together with retroorbital communication, and determine the lymphatic drainage from the superficial orbital region with a potential outlet of lymphatic vessel into the venous bloodstream. The study of the venous system was carried out on 32 Wistar rats by using corrosion casts methods and radiography, while the lymphatic system was studied in 12 Wistar rats following ink injection. Superficially, orbital veins are connected with extraorbital veins running through angular vein of the eye and the superficial temporal vein, and via the pterygoid plexus with the maxillary vein, which provide readily accessible communication routes in the spread of infection. The extent of intraorbital and periorbital venous drainage was ensured by the dorsal and ventral external ophthalmic vein through the infraorbital vein, which together formed the principal part of the ophthalmic plexus. Venous drainage of the eyeball was carried out mainly by the vortex veins, ciliary veins and internal ophthalmic vein. The highest variability, first presented by differences in structural arrangement and formation of anastomoses, was observed within the ventral external ophthalmic vein (22 cases) and the medial vortex vein (10 cases). Four vortex veins, one vein in each quadrant of the eye, were observed in rats. The vortex vein located on the ventral side of the eyeball was occasionally found as two veins (in four cases) in the present study. The lymphatic vessel from the lower eyelid entered into the mandibular lymph centre, and from the upper eyelid entered into the superficial cervical lymph centre, but both drained into the deep cranial cervical lymph node. The direct entry of lymph entering the veins without passing through lymph nodes was not observed.

Species Differences in the Geometry of the Anterior Segment Differentially Affect Anterior Chamber Cell Scoring Systems in Laboratory Animals

PURPOSE

To determine the impact of anterior segment geometry on ocular scoring systems quantifying anterior chamber (AC) cells in humans and 7 common laboratory species.

METHODS

Using normative anterior segment dimensions and novel geometric formulae, ocular section volumes measured by 3 scoring systems; Standardization of Uveitis Nomenclature (SUN), Ocular Services On Demand (OSOD), and OSOD-modified SUN were calculated for each species, respectively. Calculated volumes were applied to each system's AC cell scoring scheme to determine comparative cell density (cells/mm(3)). Cell density values for all laboratory species were normalized to human values and conversion factors derived to create modified scoring schemes, facilitating interspecies comparison with each system, respectively.

RESULTS

Differences in anterior segment geometry resulted in marked differences in optical section volume measured. Volumes were smaller in rodents than dogs and cats, but represented a comparatively larger percentage of AC volume. AC cell density (cells/mm(3)) varied between species. Using the SUN and OSOD-modified SUN systems, values in the pig, dog, and cat underestimated human values; values in rodents overestimated human values. Modified normalized scoring systems presented here account for species-related anterior segment geometry and facilitate both intra- and interspecies analysis, as well as translational comparison.

CONCLUSIONS

Employment of modified AC cell scoring systems that account for species-specific differences in anterior segment anatomy would harmonize findings across species and may be more predictive for determining ocular toxicological consequences in ocular drug and device development programs.