Development of a new surgical technique to infuse kynurenic acid to optic nerves in chickens for studying loss of myelination.

Prolonged infusion of a high dose of kynurenic acid (KYNA) reduces the myelin content in the rat spinal cord with preservation of the axonal integrity and without inducing an inflammatory response. We hypothesized that subdural infusion of a high concentration of KYNA can induce myelin loss in the optic nerves (ONs) of chickens. However, existing methods to deliver agents to the ON are inefficient, unlocalized and provide only acute exposure. Thus, we developed a surgical approach for sustained delivery of KYNA to the chicken ON. In brief, the novel surgical technique, which does not include excision of the extraocular muscles, involves incision of the skin and underlying fascial sheath to access the optic nerve within the muscle cone, implantation of a catheter in the dura of the optic nerve, the other end of which exits the orbit under the skin. The catheter runs under the skin near the lateral canthus, over the ears to the back of the neck, where a second incision is made to both implant the osmotic pump and to attach the catheter to the osmotic pump. India ink was used to confirm prolonged sustained administration to the optic nerves and across the chiasm. This surgical model was used to investigate KYNA's effect(s) on myelin loss in the ON. ONs of 7-day old chickens were infused with 50 mM KYNA or phosphate buffered saline (PBS) for seven days. Analysis of KYNA-infused contralateral ON g-ratios and protein levels indicated a reduction in myelin. These findings demonstrate the utility of our surgical approach for sustained delivery of KYNA into the ON and suggest a role for KYNA in modulating CNS myelination.

Morphological and functional changes in the rat retina associated with 2 months of intermittent moderate intraocular pressure elevation.

Morphological and functional changes in the rat retina and optic nerve head (ONH), associated with 8 weeks of intermittent moderately elevated intraocular pressure (IOP) were measured with a combined ultrahigh resolution optical coherence tomography (UHR-OCT) and electroretinography (ERG) system. The IOP of male Sprague-Dawley rats was raised in one eye to ~35 mmHg for 1 hour/day on 6 days each week using vascular loops. Single-flash ERG traces and volumetric UHR-OCT images of the ONH were acquired from both eyes before, during and after IOP elevations at weeks 1, 5 and 9 of the study. The UHR-OCT images showed depression of the posterior eye around the ONH during the IOP elevations, the magnitude of which increased significantly from week 1 to week 9 (p = 0.01). The ERG a-wave and b-wave amplitudes increased temporarily during IOP elevations and returned to normal ~30 minutes after loop removal. Recurrent intermittent IOP spikes caused > 30% decrease in the ERG a-wave and b-wave amplitudes measured during the IOP elevations over the course of 2 months. This study suggests that recurrent, relatively short-duration IOP spikes for extended period of time are associated with peri-ONH tissue hypercompliance and reduced retinal functional response to visual stimulation during acute IOP elevation.

Isoflurane and ketamine:xylazine differentially affect intraocular pressure-associated scotopic threshold responses in Sprague-Dawley rats.

PURPOSE: Amplitudes of electroretinograms (ERG) are enhanced during acute, moderate elevation of intraocular pressure (IOP) in rats anaesthetised with isoflurane. As anaesthetics alone are known to affect ERG amplitudes, the present study compares the effects of inhalant isoflurane and injected ketamine:xylazine on the scotopic threshold response (STR) in rats with moderate IOP elevation. METHODS: Isoflurane-anaesthetised (n = 9) and ketamine:xylazine-anaesthetised (n = 6) rats underwent acute unilateral IOP elevation using a vascular loop anterior to the equator of the right eye. STRs to a luminance series (subthreshold to -3.04 log scotopic cd s/m2) were recorded from each eye of Sprague-Dawley rats before, during, and after IOP elevation. RESULTS: Positive STR (pSTR) amplitudes for all conditions were significantly smaller (p = 0.0001) for isoflurane- than for ketamine:xylazine-anaesthetised rats. In addition, ketamine:xylazine was associated with a progressive increase in pSTR amplitudes over time (p = 0.0028). IOP elevation was associated with an increase in pSTR amplitude (both anaesthetics p < 0.0001). The absolute interocular differences in IOP-associated enhancement of pSTR amplitudes for ketamine:xylazine and isoflurane were similar (66.3 ± 35.5 vs. 54.2 ± 24.1 µV, respectively). However, the fold increase in amplitude during IOP elevation was significantly higher in the isoflurane- than in the ketamine:xylazine-anaesthetised rats (16.8 ± 29.7x vs. 2.1 ± 2.7x, respectively, p = 0.0004). CONCLUSIONS: The anaesthetics differentially affect the STRs in the rat model with markedly reduced amplitudes with isoflurane compared to ketamine:xylazine. However, the IOP-associated enhancement is of similar absolute magnitude for the two anaesthetics, suggesting that IOP stress and anaesthetic effects operate on separate retinal mechanisms.

Pigmented and albino rats differ in their responses to moderate, acute and reversible intraocular pressure elevation.

PURPOSE: To compare the electrophysiological and morphological responses to acute, moderately elevated intraocular pressure (IOP) in Sprague-Dawley (SD), Long-Evans (LE) and Brown Norway (BN) rat eyes. METHODS: Eleven-week-old SD (n = 5), LE (n = 5) and BN (n = 5) rats were used. Scotopic threshold responses (STRs), Maxwellian flash electroretinograms (ERGs) or ultrahigh-resolution optical coherence tomography (UHR-OCT) images of the rat retinas were collected from both eyes before, during and after IOP elevation of one eye. IOP was raised to ~35 mmHg for 1 h using a vascular loop, while the other eye served as a control. STRs, ERGs and UHR-OCT images were acquired on 3 days separated by 1 day of no experimental manipulation. RESULTS: There were no significant differences between species in baseline electroretinography. However, during IOP elevation, peak positive STR amplitudes in LE (mean ± standard deviation 259 ± 124 µV) and BN (228 ± 96 µV) rats were about fourfold higher than those in SD rats (56 ± 46 µV) rats (p = 0.0002 for both). Similarly, during elevated IOP, ERG b-wave amplitudes were twofold higher in LE and BN rats compared to those of SD rats (947 ± 129 µV and 892 ± 184 µV, vs 427 ± 138 µV; p = 0.0002 for both). UHR-OCT images showed backward bowing in all groups during IOP elevation, with a return to typical form about 30 min after IOP elevation. CONCLUSION: Differences in the loop-induced responses between the strains are likely due to different inherent retinal morphology and physiology.

The pattern ERG in chicks - Stimulus dependence and optic nerve section.

The chick is widely used in studies of eye growth regulation and myopia. The aim of this study was to explore the utility of pattern (p)ERG as a tool to assess retinal function in such studies. Effects of optical defocus and diffusing blur, manipulations used to alter eye growth experimentally, were evaluated. PERGs were recorded from White-Leghorn chickens, using a checkerboard pattern, including 8 spatial frequencies (0.05-2.2c/d SF), 13 contrast levels (1-100%), and 8 temporal reversal frequencies (0.5-20Hz). The acute effects of defocus and diffusing blur were examined. Flash- and pERGs were also recorded from chicks that underwent monocular optic nerve section (ONS), to explore the contribution of retinal ganglion cells (RGCs). Measurements were made up to 6weeks post-ONS, complemented with SD-OCT imaging. In normal chicks, the response to 1Hz, 100% contrast stimuli showed positive- and negative-going waveforms at 43ms (P1) and 75ms (N95), respectively, with 0.06-0.1c/d SF eliciting the largest P1 amplitudes of 21.9±2.5µV. Contrast levels above 5% yielded measurable P1 responses. Responses were transient and monophasic for 0.5-5Hzreversal rates, with higher temporal frequencies yielding steady state responses. Defocus and diffusing blur decreased pERG amplitude across all SFs. pERG responses remained normal after ONS, despite the loss of RGCs. In conclusion, chicks show robust pERG responses, which are attenuated by defocus and diffusing blur. The pERG response is not affected by ONS, suggesting that RGCs do not contribute to the chick pERG.

Short-Term Moderately Elevated Intraocular Pressure Is Associated With Elevated Scotopic Electroretinogram Responses.

PURPOSE: Moderately elevated intraocular pressure (IOP) is a risk factor for open-angle glaucoma. Some patients suffer glaucoma despite clinically measured normal IOPs. Fluctuations in IOP may have a significant role since IOPs are higher during sleep and inversion activities. Controlled transient elevations of IOPs in rats over time lead to optic nerve structural changes that are similar to the early changes observed in constant chronic models of glaucoma. Because early intervention decreases glaucoma progression, this study was done to determine if early physiological changes to the retina could be detected with noninvasive electrophysiological and optical imaging tests during moderately elevated IOP. METHODS: Intraocular pressures were raised to moderately high levels (35 mm Hg) in one eye of Sprague-Dawley rats while the other (control) eye was untreated. One group of rats underwent scotopic threshold response (STR) and electroretinogram (ERG) testing, while another 3 groups underwent optical coherence tomography (OCT) imaging, Western blot, or histologic evaluation. RESULTS: The amplitudes of the STR and ERG responses in eyes with moderately elevated IOPs were enhanced compared to the values before IOP elevation, and compared to untreated contralateral eyes. Structural changes to the optic nerve also occurred during IOP elevation. CONCLUSIONS: Although ischemic IOP elevations are well-known to globally reduce components of the scotopic ERG, acute elevation in rats to levels often observed in untreated glaucoma patients caused an increase in these parameters. Further exploration of these phenomena may be helpful in better understanding the mechanisms mediating early retinal changes during fluctuating or chronically elevated IOP.

The effects of actomyosin disruptors on the mechanical integrity of the avian crystalline lens.

PURPOSE: Actin and myosin within the crystalline lens maintain the structural integrity of lens fiber cells and form a hexagonal lattice cradling the posterior surface of the lens. The actomyosin network was pharmacologically disrupted to examine the effects on lenticular biomechanics and optical quality. METHODS: One lens of 7-day-old White Leghorn chickens was treated with 10 µM of a disruptor and the other with 0.01% dimethyl sulfoxide (vehicle). Actin, myosin, and myosin light chain kinase (MLCK) disruptors were used. The stiffness and the optical quality of the control and treated lenses were measured. Western blotting and confocal imaging were used to confirm that treatment led to a disruption of the actomyosin network. The times for the lenses to recover stiffness to match the control values were also measured. RESULTS: Disruptor-treated lenses were significantly less stiff than their controls (p≤0.0274 for all disruptors). The disruptors led to changes in the relative protein amounts as well as the distributions of proteins within the lattice. However, the disruptors did not affect the clarity of the lenses (p≥0.4696 for all disruptors), nor did they affect spherical aberration (p = 0.02245). The effects of all three disruptors were reversible, with lenses recovering from treatment with actin, myosin, and MLCK disruptors after 4 h, 1 h, and 8 min, respectively. CONCLUSIONS: Cytoskeletal protein disruptors led to a decreased stiffness of the lens, and the effects were reversible. Optical quality was mostly unaffected, but the long-term consequences remain unclear. Our results raise the possibility that the mechanical properties of the avian lens may be actively regulated in vivo via adjustments to the actomyosin lattice.

Pharmacologically stimulated pupil and accommodative changes in Guinea pigs.

PURPOSE: The guinea pig is being used increasingly as a model of human myopia. As accommodation may influence the effects of manipulations used in experimental myopia models, understanding the accommodative ability of guinea pigs is important. Here, nonselective muscarinic agonists were used as pharmacological tools to study guinea pig accommodation. METHODS: Measurements were made on 15 pigmented guinea pigs. For in vivo testing, animals were anesthetized and, following baseline measurements, 2% pilocarpine was applied topically. Measurements included A-scan ultrasonography, optical coherence tomography (OCT) imaging, corneal topography, and refraction. In vitro lens scanning experiments were performed using anterior segment preparations, with measurements before and during exposure to carbachol. Anterior segment structures were examined histologically and immunohistochemistry was done to characterize the muscarinic receptor subtypes present. RESULTS: In vivo, pilocarpine induced a myopic shift in refractive error coupled to a small, but consistent decrease in anterior chamber depth (ACD), a smaller and more variable increase in lens thickness, and a decrease in pupil size. Lens thickness increases were short-lived (10 minutes), while ACD and pupil size decreased over 20 minutes. Corneal curvature was not significantly affected. Carbachol tested on anterior segment preparations in vitro was without effect on lens back vertex distance, but did stimulate pupil constriction. Immunohistochemistry indicated the presence of muscarinic receptor subtypes 1 to 5 in the iris and ciliary body. CONCLUSIONS: The observed pilocarpine-induced changes in ACD, lens thickness, and refraction are consistent with active accommodation in the guinea pig, through cholinergic muscarinic stimulation.

Inner retinal cell development is impaired in Smoky Joe chickens.

Many different components of the retina can be affected by inherited degenerative diseases causing blindness. Currently, 5 different mutant strains of chicken have already been studied as potential models for inherited retinal degeneration; however, the potential for the blind strain of White Leghorns, called Smoky Joe (SJ), as a model remains unknown. Ocular symptoms observed within homozygous SJ birds show the birds have varying levels of blindness at hatch and by 8 wk posthatch are completely blind, but details about the development of the blindness are unclear (Salter et al., 1997). The objective of this study was to characterize the retinal development of blind and sighted SJ chicks during embryogenesis, and to monitor the numbers of the retinal cells with cell-type-specific markers. Blind SJ chicks showed less retinal cells throughout embryogenesis compared with sighted SJ chicks (P < 0.0001). Based on the histological analysis, it was determined that amacrine cells within the inner nuclear layer were the most affected cell type, showing lower numbers in the blind SJ compared with the sighted; amacrine cell development was also delayed in the blind birds, beginning 2 d later than in sighted SJ birds. Photoreceptors were also scarcely detected within the blind SJ and potentially may be an additional target of developmental impairment. Further analysis on posthatch SJ will aid in determining degenerative characteristics of a fully developed retina and its cells.

Inhibition of defocus-induced myopia in chickens.

PURPOSE: To determine the effect of wearing a lens with a unique peripheral optical design on the development and progression of defocus-induced myopia in newly hatched chickens. METHODS: Eighty-five newly hatched chickens underwent bilateral retinoscopy and A-scan ultrasound to determine their refractive error and axial length. They were randomly divided into Control and two Test groups, in which each chicken was fitted with a goggle-lens over the right eye, with the left eye remaining untreated. The Control group wore a lens of power -10.00 diopters (D) of standard spherical optical design. The two Test lenses both had a central optical power -10.00 D, but used different peripheral myopia progression control (MPC) designs. For all groups, retinoscopy was repeated on days 3, 7, 10, and 14; ultrasound was repeated on day 14. RESULTS: On day 0 there was no statistical difference in refractive error (mean +6.92 D) or axial length (mean 8.06 mm) between Test and Control groups or treated and untreated eyes (all P 0.05). At day 14, 37 (43.5%) of 85 chickens had not experienced goggle detachment and were included in the final analyses. in this cohort there was a significant refractive difference between the treated eyes of the control group (n = 17) and those of test 1 (n = 14) and Test 2 (N = 6) groups (both P <0.01): Control -4.65 ± 2.11 D, Test 1 +4.57 ± 3.11 D, Test 2 +1.08 ± 1.24 D (mean ± SEM). There was also a significant axial length difference (both P < 0.01): Control 10.55 ± 0.36 mm, Test 1 9.99 ± 0.14 mm, Test 2 10.17 ± 0.18 mm. CONCLUSIONS: Use of these unique MPC lens designs over 14 days caused a significant reduction in the development of defocus-induced myopia in chickens; the degree of reduction appeared to be design specific.

Stimulus-specific pupil dynamics measured in birds (Gallus gallus domesticus) in vivo with ultrahigh resolution optical coherence tomography.

PURPOSE: To demonstrate the ability of high speed, ultrahigh-resolution optical coherence tomography (UHR-OCT) to measure and characterize in vivo visual stimulus-specific pupil dynamics in birds. METHODS: Ten two-week old White Leghorn (Gallus gallus domesticus) chickens were imaged in this study. The chickens were dark-adapted for 1 hour and anesthetized with 2% isoflurane prior to the imaging procedure. Blue, green, and red single flash visual stimuli of 7 ms duration were used to evoke pupillary responses. UHR-OCT cross-sectional images of the pupil were acquired prior, during, and for several seconds after the visual stimuli onset. Images were processed with a novel custom automatic algorithm, designed to determine the pupil diameter changes over time. RESULTS: Results from this study show that the pupillary constriction begins with the onset of the visual stimuli; however, maximum pupil constriction occurs ∼150 ms later. No statistically significant variation in the timing of the maximum pupillary constriction was observed for stimuli of different colors. However, significant variation was observed in the maximum pupil constriction amplitudes, between red-green and red-blue stimuli, but not between blue-green stimuli. Furthermore, the magnitude of the maximum pupil constriction decreased monotonically with time under isoflurane anesthesia. CONCLUSIONS: We demonstrated, for the first time, measurements of visually evoked pupillary dynamics in animals using high speed UHR-OCT. The results suggest dependence of the pupillary dynamics on the color of the visual stimulus, and adverse effects of isoflurane anesthesia on the visually evoked pupillary responses in chickens.

In vivo assessment of thickness and reflectivity in a rat outer retinal degeneration model with ultrahigh resolution optical coherence tomography.

PURPOSE: To provide in vivo quantitative assessment of sodium iodate-induced retinal damage in a rat model of outer retinal degeneration using ultrahigh resolution optical coherence tomography (UHR-OCT). METHODS: Outer retinal degeneration was induced in four female Long Evans rats via tail vein injection of sodium iodate (40 mg/kg). Changes in the thickness and optical reflectivity of individual retinal layers were extracted using a semi-automatic segmentation algorithm and were assessed in vivo at 6 hours, days 1, 3, and 7, and up to 3 months post injection with UHR-OCT. Hematoxylin and eosin (H&E) histology was used to confirm the morphologic changes observed in the UHR-OCT images. RESULTS: UHR-OCT tomograms showed progressive structural damage in the rat retina over time, such as swelling, thinning, complete disintegration of individual retinal layers, and clustering of highly reflective cellular debris. Photoreceptor swelling was observed 6 hours after injection of sodium iodate, followed by progressive structural decomposition of the outer retina. At 3 months post injection, the outer retina was completely disintegrated, and the inner nuclear layer (INL) was in direct contact with the choroid. Changes in the reflectivity of individual retinal layers were observed over time and correlated well with the morphologic changes. CONCLUSIONS: UHR-OCT permits in vivo, noninvasive, longitudinal, quantitative assessment of the progressive changes in retinal morphology and optical reflectivity in a sodium iodate rodent model of outer retinal degeneration.

Correlation of visually evoked intrinsic optical signals and electroretinograms recorded from chicken retina with a combined functional optical coherence tomography and electroretinography system.

Visually evoked fast intrinsic optical signals (IOSs) were recorded for the first time in vivo from all layers of healthy chicken retina by using a combined functional optical coherence tomography (fOCT) and electroretinography (ERG) system. The fast IOSs were observed to develop within ∼5 ms from the on-set of the visual stimulus, whereas slow IOSs were measured up to 1 s later. The visually evoked IOSs and ERG traces were recorded simultaneously, and a clear correlation was observed between them. The ability to measure visually evoked fast IOSs non-invasively and in vivo from individual retinal layers could significantly improve the understanding of the complex communication between different retinal cell types in healthy and diseased retinas.

Automatic system for 3D reconstruction of the chick eye based on digital photographs.

The geometry of anatomical specimens is very complex and accurate 3D reconstruction is important for morphological studies, finite element analysis (FEA) and rapid prototyping. Although magnetic resonance imaging, computed tomography and laser scanners can be used for reconstructing biological structures, the cost of the equipment is fairly high and specialised technicians are required to operate the equipment, making such approaches limiting in terms of accessibility. In this paper, a novel automatic system for 3D surface reconstruction of the chick eye from digital photographs of a serially sectioned specimen is presented as a potential cost-effective and practical alternative. The system is designed to allow for automatic detection of the external surface of the chick eye. Automatic alignment of the photographs is performed using a combination of coloured markers and an algorithm based on complex phase order likelihood that is robust to noise and illumination variations. Automatic segmentation of the external boundaries of the eye from the aligned photographs is performed using a novel level-set segmentation approach based on a complex phase order energy functional. The extracted boundaries are sampled to construct a 3D point cloud, and a combination of Delaunay triangulation and subdivision surfaces is employed to construct the final triangular mesh. Experimental results using digital photographs of the chick eye show that the proposed system is capable of producing accurate 3D reconstructions of the external surface of the eye. The 3D model geometry is similar to a real chick eye and could be used for morphological studies and FEA.

In vivo imaging of intrinsic optical signals in chicken retina with functional optical coherence tomography.

Visually evoked intrinsic optical signals (IOSs) were measured in vivo for the first time to our knowledge from all retina layers of the chicken retina with a combined functional optical coherence tomography and electroretinography (ERG) system. IOS traces were recorded from a small volume in the retina with 3.5 µm axial resolution and 7 ms time resolution. Comparison of the IOS and ERG traces shows a correlation between the positive and negative IOS measured from different retinal layers and the timing of the a and b waves in the ERG recording.

Effects of a myosin light chain kinase inhibitor on the optics and accommodation of the avian crystalline lens.

PURPOSE: While many studies investigate the cytoskeletal properties of the lens with respect to cataract development, examinations of how these molecular structures interact are few. Myosin light chain kinase (MLCK), actin, and myosin are present on the crystalline lenses of chickens. The purpose of this experiment was to determine whether contractile proteins found on the lens play a role in the optical functions of the lens at rest, and during accommodation. METHODS: Eyes of 6-day old white Leghorn chicks (Gallus gallus domesticus) were enucleated, with the ciliary nerve intact. One eye was treated with the MLCK inhibitor 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-7) and the other eye with vehicle only. Three concentrations of ML-7 were used: 1 µM, 10 µM, and 100 µM. The back vertex focal lengths (BVFLs) were measured before, during, and after accommodation using an optical laser scanning monitor (Scantox™). To further confirm ML-7 activity, western blotting was performed to detect whether MLCK was inhibited. RESULTS: Western blots confirmed that MLCK was inhibited at all three ML-7 concentrations. Ten µM ML-7 treatments led to longer BVFLs at rest (p=0.0338), while 100 µM treatments led to opposite changes, resulting in shorter BVFLs (p=0.0220). While 1 µM treatments did not lead to significant optical changes (p=0.4416), BVFLs were similar in pattern to those of the 10 µM group. ML-7 had no effects on accommodative amplitudes (p=0.7848). CONCLUSIONS: Inhibition of MLCK by ML-7 led to differential changes in BVFLs that presumably affected lenticular integrity. No apparent effect on accommodative amplitudes was observed.

In vivo volumetric imaging of chicken retina with ultrahigh-resolution spectral domain optical coherence tomography.

The chicken retina is an established animal model for myopia and light-associated growth studies. It has a unique morphology: it is afoveate and avascular; oxygen and nutrition to the inner retina is delivered by a vascular tissue (pecten) that protrudes into the vitreous. Here we present, to the best of our knowledge, the first in vivo, volumetric high-resolution images of the chicken retina. Images were acquired with an ultrahigh-resolution optical coherence tomography (UHROCT) system with 3.5 µm axial resolution in the retina, at the rate of 47,000 A-scans/s. Spatial variations in the thickness of the nerve fiber and ganglion cell layers were mapped by segmenting and measuring the layer thickness with a semi-automatic segmentation algorithm. Volumetric visualization of the morphology and morphometric analysis of the chicken retina could aid significantly studies with chicken retinal models of ophthalmic diseases.

The role of the iris in chick accommodation.

PURPOSE: Peripheral defocus, higher-order aberrations, and accommodation interact with pupil size to influence retinal image quality and possibly eye growth. Iridectomy (ID) provides a fixed, enlarged pupil. Results from in vitro studies suggest that ID may reduce or eliminate accommodation in the chicken. This paper further investigates the effects of ID on chicken accommodation, eye growth, and refractive development. METHODS: Refraction, biometry, and corneal curvature were measured, before, and after topical instillation of nicotine in 43 White-Leghorn chickens that had undergone monocular ID. Intraocular pressure (IOP) was measured, and eyes were imaged with anterior segment optical coherence tomography (OCT) during accommodation. In vitro preparations were used to examine accommodation responses in a lens-scanning instrument. RESULTS: Iridectomy induced small but significant decreases in anterior and vitreous chamber depths and an increase in lens thickness (LT). IOP was similar in iridectomized and control eyes from 1 week on. In vivo, nicotine induced similar accommodative changes in iridectomized and control eyes. OCT images revealed a forward displacement of the iris during accommodation in control eyes. Iridectomized and control eyes showed similar increases in LT. In vitro, iridectomized eyes showed minimal nicotine-induced accommodation. CONCLUSIONS: Refraction and eye growth were minimally affected by ID in chickens, implying that emmetropization was unaffected and supporting the use of ID as a tool in emmetropization and myopia studies. The greatly attenuated accommodative responses in vitro for iridectomized eyes suggest a role of biomechanical factors in the chick. IOP was unaffected by the surgery, implying that the iris musculature is not essential for maintaining aqueous outflow pathways.

Density of ocular components of the bovine eye.

PURPOSE: Density is essential for acoustic characterization of tissues and provides a basic input for ultrasound backscatter and absorption models. Despite the existence of extensive compilations of acoustic properties, neither unified data on ocular density nor comparisons of the densities between all ocular components can be found. This study was undertaken to determine the mass density of all the ocular components of the bovine eye. METHODS: Liquid components were measured through mass/volume ratio, whereas solid tissues were measured with two different densitometry techniques based on Archimedes Principle. The first method determines the density by measuring dry and wet weight of the tissues. The second method consists of immersing the tissues in sucrose solutions of varying densities and observing their buoyancy. RESULTS: Although the mean densities for all tissues were found to be within 0.02 g/cm by both methods, only the sucrose solution method offered a consistent relative order for all measured ocular components, as well as a considerably smaller standard deviation (a maximum standard deviation of 0.004 g/cm for cornea). The lens was found to be the densest component, followed by the sclera, cornea, choroid, retina, aqueous, and vitreous humors. CONCLUSIONS: The consistent results of the sucrose solution tests suggest that the ocular mass density is a physical property that is more dependent on the compositional and structural characteristics of the tissue and than on population variability.

Colchicine attenuates compensation to negative but not to positive lenses in young chicks.

Optic nerve-sectioned (ONS) chick eyes are capable of emmetropisation, but these eyes also exhibit increased hyperopia without any visual manipulations, which suggests altered eye growth regulation. These altered growth changes may be related to the loss of retinal ganglion cells that follows nerve lesioning. Colchicine, which also destroys retinal ganglion cells in chicks, was used to further examine the effects of retinal ganglion cell loss on emmetropisation. Growth responses of +10D and -10D lens-wearing colchicine-injected eyes were compared to those of +10D and -10D lens-wearing saline-injected eyes, respectively. Changes after removal of lenses were also analysed. Prior to lens-wear, colchicine-injected eyes exhibited longer optical axial lengths (OL; distance from cornea to retina; p=0.0185) but no differences in refractive error (RE; p=0.6588). Although myopic shifts were not significant for -10D lens-wearing colchicine-injected eyes (p=0.5913), but were for the saline-injected eyes (p=0.0034), these changes were not different (p=0.1646). However, -10D lens-induced OL changes in colchicine-injected eyes showed insignificant (p=0.2214) and reduced (p=0.0102) changes compared to those of saline-injected eyes. +10D lens-treated colchicine-injected eyes showed significant hyperopic shifts (p<0.0001) and significant reductions in OL (p<0.0001) that were similar to those of saline-injected eyes (p=0.7990 and p=0.1495, respectively). Growth responses in eyes recovering from -10D lenses were minimal, with REs unaffected (p=0.3325), but OL reductions affected (p=0.0199) by colchicine. Colchicine-injected eyes recovering from +10D lenses showed significant myopic shifts (p=0.0003) and OL elongations (p<0.0001) that were similar to those of saline-injected eyes (p=0.3999 and p=0.4731, respectively). The results showing that colchicine suppresses the ability to respond to negative lenses but leaves compensation to positive lenses relatively unchanged, are opposite to those of optic nerve sectioned eyes. We speculate that the differences are probably related to the way retinal cells are lost.