Analysis of postnatal eye development in the mouse with high-resolution small animal magnetic resonance imaging

Fundus autofluorescence and photoreceptor cell rosettes in mouse models

PURPOSE

This study was conducted to study correlations among fundus autofluorescence (AF), RPE lipofuscin accumulation, and photoreceptor cell degeneration and to investigate the structural basis of fundus AF spots.

METHODS

Fundus AF images (55° lens; 488-nm excitation) and spectral-domain optical coherence tomography (SD-OCT) scans were acquired in pigmented Rdh8(-/-)/Abca4(-/-) mice (ages 1-9 months) with a confocal scanning laser ophthalmoscope (cSLO). For quantitative fundus AF (qAF), gray levels (GLs) were calibrated to an internal fluorescence reference. Retinal bisretinoids were measured by quantitative HPLC. Histometric analysis of outer nuclear layer (ONL) thicknesses was performed, and cryostat sections of retina were examined by fluorescence microscopy.

RESULTS

Quantified A2E and qAF intensities increased until age 4 months in the Rdh8(-/-)/Abca4(-/-) mice. The A2E levels declined after 4 months of age, but qAF intensity values continued to rise. The decline in A2E levels in the Rdh8(-/-)/Abca4(-/-) mice paralleled reduced photoreceptor cell viability as reflected in ONL thinning. Hyperautofluorescent puncta in fundus AF images corresponded to photoreceptor cell rosettes in SD-OCT images and histological sections stained with hematoxylin and eosin. The inner segment/outer segment-containing core of the rosette emitted an autofluorescence detected by fluorescence microscopy.

CONCLUSIONS

When neural retina is disordered, AF from photoreceptor cells can contribute to noninvasive fundus AF images. Hyperautofluorescent puncta in fundus AF images are attributable, in at least some cases, to photoreceptor cell rosettes.

Refractive index measurement of the mouse crystalline lens using optical coherence tomography

In recent years, there has been a growing interest for using mouse models in refractive development and myopia research. The crystalline lens is a critical optical component of the mouse eye that occupies greater than 50% of the ocular space, and significant increases in thickness with age. However, changes in refractive index of the mouse crystalline lens are less known. In this study, we examined the changes in thickness and refractive index of the mouse crystalline lens for two different strains, wild-type (WT) and a nyx mutant (nob) over the course of normal visual development or after form deprivation. Refractive index and lens thickness measurements were made on ex vivo lenses using spectral domain optical coherence tomography (SD-OCT). Comparison of refractive index measurements on 5 standard ball lenses using the SD-OCT and their known refractive indices (manufacturer provided) indicated good precision (intra-class correlation coefficient, 0.998 and Bland-Altman coefficient of repeatability, 0.116) of the SD-OCT to calculate mouse lens refractive index ex vivo. During normal visual development, lens thickness increased significantly with age for three different cohorts of mice, aged 4 (average thickness from both eyes; WT: 1.78 ± 0.03, nob: 1.79 ± 0.08 mm), 10 (WT: 2.02 ± 0.05, nob: 2.01 ± 0.04 mm) and 16 weeks (WT: 2.12 ± 0.06, nob: 2.09 ± 0.06 mm, p < 0.001). Lens thickness was not significantly different between the two strains at any age (p = 0.557). For mice with normal vision, refractive index for isolated crystalline lenses in nob mice was significantly greater than WT mice (mean for all ages; WT: 1.42 ± 0.01, nob: 1.44 ± 0.001, p < 0.001). After 4 weeks of form deprivation to the right eye using a skull-mounted goggling apparatus, a thinning of the crystalline lens was observed in both right and left eyes of goggled animals compared to their naïve controls (average from both the right and the left eye) for both strains (p = 0.052). In form deprived mice, lens refractive index was significantly different between the goggled animals and non-goggled naïve controls in nob mice, but not in WT mice (p = 0.009). Both eyes of goggled nob mice had significantly greater lens refractive index (goggled, 1.49 ± 0.01; opposite, 1.47 ± 0.03) compared to their naïve controls (1.45 ± 0.02, p < 0.05). The results presented here suggest that there are genetic differences in the crystalline lens refractive index of the mouse eye, and that the lens refractive index in mice significantly increase with form deprivation. Research applications requiring precise optical measurements of the mouse eye should take these lens refractive indices into account when interpreting SD-OCT data.

Growth of the eye lens: II. Allometric studies

PURPOSE

The purpose of this study was to examine the ontogeny and phylogeny of lens growth in a variety of species using allometry.

METHODS

Data on the accumulation of wet and/or dry lens weight as a function of bodyweight were obtained for 40 species and subjected to allometric analysis to examine ontogenic growth and compaction. Allometric analysis was also used to compare the maximum adult lens weights for 147 species with the maximum adult bodyweight and to compare lens volumes calculated from wet and dry weights with eye volumes calculated from axial length.

RESULTS

Linear allometric relationships were obtained for the comparison of ontogenic lens and bodyweight accumulation. The body mass exponent (BME) decreased with increasing animal size from around 1.0 in small rodents to 0.4 in large ungulates for both wet and dry weights. Compaction constants for the ontogenic growth ranged from 1.00 in birds and reptiles up to 1.30 in mammals. Allometric comparison of maximum lens wet and dry weights with maximum bodyweights also yielded linear plots with a BME of 0.504 for all warm blooded species except primates which had a BME of 0.25. When lens volumes were compared with eye volumes, all species yielded a scaling constant of 0.75 but the proportionality constants for primates and birds were lower.

CONCLUSIONS

Ontogenic lens growth is fastest, relative to body growth, in small animals and slowest in large animals. Fiber cell compaction takes place throughout life in most species, but not in birds and reptiles. Maximum adult lens size scales with eye size with the same exponent in all species, but birds and primates have smaller lenses relative to eye size than other species. Optical properties of the lens are generated through the combination of variations in the rate of growth, rate of compaction, shape and size.

Photopic visual input is necessary for emmetropization in mice

It was recently demonstrated that refractive errors in mice stabilize around emmetropic values during early postnatal development, and that they develop experimental myopia in response to both visual form deprivation and imposed optical defocus similar to other vertebrate species. Animal studies also suggest that photopic vision plays critical role in emmetropization in diurnal species; however, it is unknown whether refractive eye development is guided by photopic vision in the mouse, which is a nocturnal species. We used an infrared mouse photorefractor and a high-resolution MRI to clarify the role of photopic visual input in refractive eye development in the mouse. Refractive eye development and form-deprivation myopia in P21-P89 C57BL/6J mice were analyzed under 12:12 h light-dark cycle, constant light and constant darkness regimens. Animals in all experimental groups were myopic at P21 (-13.2 ± 1.6 D, light-dark cycle; -12.5 ± 0.9 D, constant light; -12.5 ± 2.0 D, constant dark). The mean refractive error in the light-dark-cycle-reared animals was -0.5 ± 1.3 D at P32 and, and did not change significantly until P40 (+0.3 ± 0.6 D, P40). Animals in this group became progressively hyperopic between P40 and P89 (+2.2 ± 0.6 D, P67; +3.7 ± 2.0 D, P89). The mean refractive error in the constant-light-reared mice was -1.0 ± 0.7 D at P32 and remained stable until P89 (+0.1 ± 0.6 D, P40; +0.3 ± 0.6 D, P67; 0.0 ± 0.4 D, P89). Dark-reared animals exhibited highly hyperopic refractive errors at P32 (+5.2 ± 1.8 D) and became progressively more hyperopic with age (+8.7 ± 1.9 D, P40; +11.2 ± 1.4 D, P67). MRI analysis revealed that emmetropization in the P40-P89 constant-light-reared animals was associated with larger eyes, a longer axial length and a larger vitreous chamber compared to the light-dark-cycle-reared mice. Constant-light-reared mice also developed 4 times higher degrees of form-deprivation myopia on average compared to light-dark-cycle-reared animals (-12.0 ± 1.4 D, constant light; -2.7 ± 0.7 D, light-dark cycle). Dark-rearing completely prevented the development of form-deprivation myopia (-0.3 ± 0.5 D). Thus, photopic vision plays important role in normal refractive eye development and ocular response to visual form deprivation in the mouse.

SLITRK6 mutations cause myopia and deafness in humans and mice

Myopia is by far the most common human eye disorder that is known to have a clear, albeit poorly defined, heritable component. In this study, we describe an autosomal-recessive syndrome characterized by high myopia and sensorineural deafness. Our molecular investigation in 3 families led to the identification of 3 homozygous nonsense mutations (p.R181X, p.S297X, and p.Q414X) in SLIT and NTRK-like family, member 6 (SLITRK6), a leucine-rich repeat domain transmembrane protein. All 3 mutant SLITRK6 proteins displayed defective cell surface localization. High-resolution MRI of WT and Slitrk6-deficient mouse eyes revealed axial length increase in the mutant (the endophenotype of myopia). Additionally, mutant mice exhibited auditory function deficits that mirrored the human phenotype. Histological investigation of WT and Slitrk6-deficient mouse retinas in postnatal development indicated a delay in synaptogenesis in Slitrk6-deficient animals. Taken together, our results showed that SLITRK6 plays a crucial role in the development of normal hearing as well as vision in humans and in mice and that its disruption leads to a syndrome characterized by severe myopia and deafness.

Investigating mechanisms of myopia in mice

Genetic and environmental factors have been shown to control visually-guided eye growth and influence myopia development. However, investigations into the intersection of these two factors in controlling refractive development have been limited by the lack of a genetically modifiable animal model. Technological advances have now made it possible to assess refractive state and ocular biometry in the small mouse eye and therefore to exploit the many genetic mouse mutants to investigate mechanisms of visually-guided eye growth. This review considers the benefits and challenges of studying refractive development in mice, compares the results of refractive error and ocular biometry from wild-type strains and genetic models in normal laboratory visual environments or with disrupted visual input, and discusses some of the remaining challenges in interpreting data from the mouse to validate and standardize methods between labs.

Variational analysis of the mouse and rat eye optical parameters

Rodent models are increasingly used to study refractive eye development and development of refractive errors; however, there is still some uncertainty regarding the accuracy of the optical models of the rat and mouse eye primarily due to high variability in reported ocular parameters. In this work, we have systematically evaluated the contribution of various ocular parameters, such as radii of curvature of ocular surfaces, thicknesses of ocular components, and refractive indices of ocular refractive media, using variational analysis and a computational model of the rodent eye. Variational analysis revealed that not all variation in ocular parameters has critical impact on the refractive status of the eye. Variation in the depth of the vitreous chamber, thickness of the lens, radius of the anterior surface of the cornea, radius of the anterior surface of the lens, as well as refractive indices for the lens and vitreous, appears to have the largest impact on the refractive error. The radii of the posterior surfaces of the cornea and lens have much smaller contributions to the refractive state. These data provide the framework for further refinement of the optical models of the rat and mouse eye and suggest that extra efforts should be directed towards increasing the linear resolution of the rodent eye biometry and obtaining more accurate data for the refractive indices of the lens and vitreous.

Review: magnetic resonance imaging techniques in ophthalmology

Imaging the eye with magnetic resonance imaging (MRI) has proved difficult due to the eye's propensity to move involuntarily over typical imaging timescales, obscuring the fine structure in the eye due to the resulting motion artifacts. However, advances in MRI technology help to mitigate such drawbacks, enabling the acquisition of high spatiotemporal resolution images with a variety of contrast mechanisms. This review aims to classify the MRI techniques used to date in clinical and preclinical ophthalmologic studies, describing the qualitative and quantitative information that may be extracted and how this may inform on ocular pathophysiology.

Assessment of axial length measurements in mouse eyes

PURPOSE

To compare measurements of murine ocular axial lengths (ALs) made with 780 nm partial coherence interferometry (PCI) and 1310 nm spectral domain-optical coherence tomography (SD-OCT).

METHODS

AL was measured at postnatal day (P) 58 in C57BL/6J mice. Repeated AL measurements were taken using a custom-made 780 nm PCI and a commercial 1310 nm SD-OCT. Intra- and interuser variability was assessed along the central optical axis and 2-degree off-axes angles with the SD-OCT. Data were collected and analyzed using Cronbach alpha (α), Bland-Altman coefficient of repeatability, agreement plots, and intraclass correlation coefficients (ICC).

RESULTS

AL measurements agreed well between the two instruments (3.262 ± 0.042 mm for PCI; 3.264 ± 0.047 mm for SD-OCT; n = 20 eyes). The ICC for PCI compared with SD-OCT was 0.92, confirming high agreement between the two instruments. Intrauser ICC for the PCI and SD-OCT were 0.814 and 0.995, respectively. Similarly, interuser ICC for PCI and SD-OCT were 0.970 and 0.943, respectively. Using SD-OCT, a 2-degree misalignment of the eye along the horizontal meridian produced mean differences in AL of -0.002 ± 0.017 mm relative to the centrally aligned images, whereas similar misalignment along the vertical meridian created 0.005 ± 0.018 mm differences in AL measurements.

CONCLUSIONS

AL measurements from the 780 nm PCI and 1310 nm SD-OCT correlate well. Multiple statistical indices indicate that both instruments have good precision and agreement for measuring murine ocular AL in vivo. Although the vertical meridian had the greater variability in AL in the small mouse eye; 2-degree off-axes differences were within the SD of centrally aligned AL.

Heritability of ocular component dimensions in mice phenotyped using depth-enhanced swept source optical coherence tomography

The range of genetic and genomic resources available makes the mouse a powerful model for the genetic dissection of complex traits. Because accurate, high-throughput phenotypic characterisation is crucial to the success of such endeavours, we recently developed an optical coherence tomography (OCT) system with extended depth range scanning capability for measuring ocular component dimensions in mice. In order to test whether the accuracy and reproducibility of our OCT system was sufficient for gene mapping studies, we carried out an experiment designed to estimate the heritability of mouse ocular component dimensions. High-resolution, two dimensional tomograms were obtained for both eyes of 11 pairs of 8 week-old outbred MF1 mice. Subsequently, images were obtained when their offspring were aged 8 weeks. Biometric data were extracted after image segmentation, reconstruction of the geometric shape of each surface, and calculation of intraocular distances. The repeatability of measurements was evaluated for 12 mice scanned on consecutive days. Heritability estimates were calculated using variance components analysis. Sets of tomograms took ∼2 s to acquire. Biometric data could be obtained for 98% of the 130 eyes scanned. The 95% limits of repeatability ranged from ±6 to ±16 μm for the axial ocular component dimensions. The heritability of the axial ocular components was 0.6-0.8, except for corneal thickness, which had a heritability not significantly different from zero. In conclusion, axial ocular component dimensions are highly heritable in mice, as they are in humans. OCT with extended depth range scanning can be used to rapidly phenotype individual mice with sufficient accuracy and precision to permit gene mapping studies.

Postnatal elongation of eye size in DBA/2J mice compared with C57BL/6J mice: in vivo analysis with whole-eye OCT

PURPOSE

To characterize postnatal changes in eye size in glaucomatous DBA/2J (D2) mice and in nonglaucomatous C57BL/6J mice (B6) in vivo by means of whole-eye optical coherence tomography (OCT).

METHODS

D2 (n = 32) and B6 (n = 36) mice were tested between 2 and 20 months of age in eight age bins. A custom time-domain OCT system with a center wavelength of 825 nm and an axial scan length of 7.1 mm produced axial A-scan interferograms at a rate of 20 A-lines/s with a resolution of 8 μm. Axial length (AL), corneal thickness (CT), anterior chamber depth (ACD), lens thickness (LT), vitreous chamber depth (VCD), and retinal thickness (RT) were measured in the optical axis and adjusted with corresponding refractive indices. Corneal curvature (CC) and IOP were also measured.

RESULTS

AL increased (P < 0.001) more in the D2 (21%) than in the B6 (9%) mice. There was an interaction effect (two-way ANOVA, P < 0.001) between age and strain for AL, CT, ACD, and VCD. In the D2 mice, the lens became dislocated posteriorly. Multiple regression analysis in the D2 mice revealed an independent effect of age and IOP (P ≤ 0.01) on axial length. CC steepened in the older D2 mice, whereas it flattened in the B6 mice.

CONCLUSIONS

In D2 mice, postnatal elongation of AL is larger than that in B6 mice and is associated with a greater increase in ACD and IOP, which seems to be a causal factor. The ease of use, short acquisition time, and noninvasiveness of whole-eye OCT make it suitable for routine use in longitudinal studies of mouse models.

Fibromodulin regulates collagen fibrillogenesis during peripheral corneal development

Fibromodulin regulates collagen fibrillogenesis, but its existence/role(s) in the cornea is controversial. We hypothesize that fibromodulin regulates fibrillogenesis during postnatal development of the anterior eye. Fibromodulin is weakly expressed in the limbus at post-natal day (P) 4, increases and extends into the central cornea at P14, becomes restricted to the limbus at P30, and decreases at P60. This differential spatial and temporal expression of fibromodulin is coordinated with emmetropization; the developmental increase in axial length and globe size. Genetic analysis demonstrated that fibromodulin regulates fibrillogenesis in a region-specific manner. At the limbus, fibromodulin is dominant in regulating fibril growth during postnatal development. In the posterior peripheral cornea, cooperative interactions of fibromodulin and lumican regulate fibrillogenesis. These data indicate that fibromodulin plays important roles in the regulation of region-specific fibrillogenesis required for the integration of the corneal and scleral matrices and sulcus development required for establishment of the visual axis.

Ketamine-xylazine anesthesia causes hyperopic refractive shift in mice

Mice have increasingly been used as a model for studies of myopia. The key to successful use of mice for myopia research is the ability to obtain accurate measurements of refractive status of their eyes. In order to obtain accurate measurements of refractive errors in mice, the refraction needs to be performed along the optical axis of the eye. This represents a particular challenge, because mice are very difficult to immobilize. Recently, ketamine-xylazine anesthesia has been used to immobilize mice before measuring refractive errors, in combination with tropicamide ophthalmic solution to induce mydriasis. Although these drugs have increasingly been used while refracting mice, their effects on the refractive state of the mouse eye have not yet been investigated. Therefore, we have analyzed the effects of tropicamide eye drops and ketamine-xylazine anesthesia on refraction in P40 C57BL/6J mice. We have also explored two alternative methods to immobilize mice, i.e. the use of a restraining platform and pentobarbital anesthesia. We found that tropicamide caused a very small, but statistically significant, hyperopic shift in refraction. Pentobarbital did not have any substantial effect on refractive status, whereas ketamine-xylazine caused a large and highly significant hyperopic shift in refraction. We also found that the use of a restraining platform represents good alternative for immobilization of mice prior to refraction. Thus, our data suggest that ketamine-xylazine anesthesia should be avoided in studies of refractive development in mice and underscore the importance of providing appropriate experimental conditions when measuring refractive errors in mice.

Highly reproducible swept-source, dispersion-encoded full-range biometry and imaging of the mouse eye

We report a high-speed, dispersion-encoded, full-range (DEFR) swept-source optical coherence tomography system for in vivo ocular imaging and biometry of small animals. The fast DEFR algorithm removes the depth ambiguity, gives access to objects located at the zero delay position, and doubles the sampling depth to 2x5.0 mm (at -101 to -71 dB sensitivity) in a single scan using 2048 samples/depth scan 0.43 nm line width of a light source operating at 1056 nm with 70 nm tuning range. The acquisition speed (frames of 512 depth scans in 18.3 ms) permits precise on-line monitoring during positioning and provides cross-sectional views of the mouse eye. Preliminary studies demonstrate high-throughput, reproducible assessment of multiple biometric features (e.g., day-to-day reproducibility of axial length measurement +/-5.3 microm) that is insensitive to eye motion sufficient for long-term monitoring.

Non-contact measurement of linear external dimensions of the mouse eye

Biometric analyses of quantitative traits in eyes of mice can reveal abnormalities related to refractive or ocular development. Due to the small size of the mouse eye, highly accurate and precise measurements are needed to detect meaningful differences. We sought a non-contact measuring technique to obtain highly accurate and precise linear dimensions of the mouse eye. Laser micrometry was validated with gauge block standards. Simple procedures to measure eye dimensions on three axes were devised. Mouse eyes from C57BL/6J and rd10 on a C57BL/6J background were dissected and extraocular muscle and fat removed. External eye dimensions of axial length (anterior-posterior (A-P) axis) and equatorial diameter (superior-inferior (S-I) and nasal-temporal (N-T) axes) were obtained with a laser micrometer. Several approaches to prevent or ameliorate evaporation due to room air were employed. The resolution of the laser micrometer was less than 0.77 microm, and it provided accurate and precise non-contact measurements of eye dimensions on three axes. External dimensions of the eye strongly correlated with eye weight. The N-T and S-I dimensions of the eye correlated with each other most closely from among the 28 pair-wise combinations of the several parameters that were collected. The equatorial axis measurements correlated well from the right and left eye of each mouse. The A-P measurements did not correlate or correlated poorly in each pair of eyes. The instrument is well suited for the measurement of enucleated eyes and other structures from most commonly used species in experimental vision research and ophthalmology.

Ex vivo magnetic resonance imaging of crystalline lens dimensions in chicken

PURPOSE

A reduction in the power of the crystalline lens during childhood is thought to be important in the emmetropization of the maturing eye. However, in humans and model organisms, little is known about the factors that determine the dimensions of the crystalline lens and in particular whether these different parameters (axial thickness, surface curvatures, equatorial diameter, and volume) are under a common source of control or regulated independently of other aspects of eye size and shape.

METHODS

Using chickens from a broiler-layer experimental cross as a model system, three-dimensional magnetic resonance imaging (MRI) scans were obtained at 115-microm isotropic resolution for one eye of 501 individuals aged 3-weeks old. After fixation with paraformaldehyde, the excised eyes were scanned overnight (16 h) in groups of 16 arranged in a 2x2x4 array. Lens dimensions were calculated from each image by fitting a three-dimensional mesh model to the lens, using the semi-automated analysis program mri3dX. The lens dimensions were compared to measures of eye and body size obtained in vivo using techniques that included keratometry and A-scan ultrasonography.

RESULTS

A striking finding was that axial lens thickness measured using ex vivo MRI was only weakly correlated with lens thickness measured in vivo by ultrasonography (r=0.19, p<0.001). In addition, the MRI lens thickness estimates had a lower mean value and much higher variance. Indeed, about one-third of crystalline lenses showed a kidney-shaped appearance instead of the typical biconvex shape. Since repeat MRI scans of the same eye showed a high degree of reproducibility for the scanning and mri3dX analysis steps (the correlation in repeat lens thickness measurements was r=0.95, p<0.001) and a recent report has shown that paraformaldehyde fixation induces a loss of water from the human crystalline lens, it is likely that the tissue fixation step caused a variable degree of shrinkage and a change in shape to the lenses examined here. Despite this serious source of imprecision, we found significant correlations between lens volume and eye/body size (p<0.001) and between lens equatorial diameter and eye/body size (p<0.001) in these chickens.

CONCLUSIONS

Our results suggest that certain aspects of lens size (specifically, lens volume and equatorial diameter) are controlled by factors that also regulate the size of the eye and body (presumably, predominantly genetic factors). However, since it has been shown previously that axial lens thickness is regulated almost independently of eye and body size, these results suggest that different systems might operate to control lens volume/diameter and lens thickness in normal chickens.

Mouse experimental myopia has features of primate myopia

PURPOSE

Several recent studies have suggested that experimental myopia can be induced in mice. However, it is not clear what role the photopic visual input plays in this process and whether mouse myopia is similar to human myopia. The purpose of this study was to carry out an in vivo high-resolution analysis of changes in ocular components and refractive state of the eye upon induction of experimental myopia in mice.

METHODS

A high-resolution small animal MRI system and a high-resolution automated eccentric infrared photorefractor were used to analyze changes of the refractive state and ocular components in C57BL/6J mice associated with experimental myopia induced by diffusers and -25 D lenses under photopic conditions.

RESULTS

The authors found that both diffusers and -25 D lenses induce myopia in C57BL/6J mice under photopic conditions (continuous light, 200 +/- 15 lux). The extent of myopic shift induced by -25 D lenses was greater than the shift induced by diffusers (-15.2 +/- 0.7 D, lenses; -12.0 +/- 1.4 D, diffusers). Myopia in mice is attributed to an increase in size of the postequatorial segment of the eye. Experimental myopia in mice can be induced only during the susceptible period in postnatal development, which ends around postnatal day 67.

CONCLUSIONS

Both diffusers and spectacle lenses induce myopia in mice under photopic conditions, during the susceptible period in postnatal development. Myopia in mice is associated with elongation of the vitreous chamber of the eye, as in humans and nonhuman primates.