Emmetropization and schematic eye models in developing pigmented guinea pigs

Abstract

A model of the axial change in ocular parameters of the guinea pig eye from 2 to 825 days of age was developed and a corresponding paraxial schematic eye model applicable from 2 to 100 days of age was constructed. Axial distances increased logarithmically over time except for the lens in which growth was more complex. Over the first 30 days, ocular elongation was approximately linear: ocular length increased by 37 microm/day, the majority due lens expansion. The choroid and sclera thickened with age, while the retina thinned in proportion to the increased ocular size, and the model suggests that there is no small eye artefact for white light retinoscopy. Refractive error just after birth was +4.8D but halved within the first week. Emmetropization occurred within the first month of life similar to that in other species when aligned at the point of sexual maturity and scaled by the time taken to reach adulthood. The power of the eye was 227D at 2 days of age and reduced by 19.7D by 100 days due to a 22% decrease in the power of the cornea. The posterior nodal distance (PND) was 4.7 mm at 30 days of age, with a maximum rate of change of 13 microm/day during the first week. The ratio of PND to axial length declined until at least 100 days of age, well after emmetropia was reached. This suggests that the maintenance of emmetropia is not sustained through proportional axial growth, but involves some active mechanism beyond simple scaling. The model predicts that 1D of myopia requires an elongation of between 23 and 32 microm, depending upon age, suggesting that a resolution of at least 50 microm is required in methods used to determine the significance of ocular length changes in guinea pig models of refractive development. Retinal magnification averaged 80 microm/degree, and the maximum potential brightness of the retinal image was high, which together with a ratio of lens power to corneal power of 1.7-2.0 suggests that the guinea pig eye is adapted for nocturnal conditions.

Emmetropisation and the aetiology of refractive errors

“…69 It seems reasonable to assume that human infant emmetropisation reflects the optically guided growth mechanisms that have been identified in lens-rearing studies in animals. Both are most active early in life 74 and human infants display other features that are predicted by an optically guided process. 75,76 Partial hyperopic correction in infants does not seem to impair the end result of emmetropisation though it does appear to slow the process.…”

Three-dimensional OCT based guinea pig eye model: relating morphology and optics

“…Matlab (MathWorks, Natick, MA) was used to create a suitable input file into ZEMAX DDE toolbox. Refractive indices of 1.376, 1.3346 and 1.401 were used for the cornea, aqueous-vitreous humor and crystalline lens, respectively [11]. The refractive index of the crystalline lens was calculated from an ex vivo guinea pig crystalline lens using the method proposed by Uhlhorn et al [23].…”

“…We specifically explored the relationships between the axes of astigmatism and higher-order surface irregularities across the different surfaces of the ocular components, and used eye's specific anatomical data to build computer eye models for the 30-40 day old guinea pig. This age corresponds to the time at which emmetropization has been previously reported to be completed [11]. A comparison with refraction and aberrometry data obtained in the same eyes allowed identifying the morphological contributing factors to refractive error and optical aberrations in the guinea pig eye, as well as other parameters potentially needed in the eye model, such as the gradient index distribution in the crystalline lens.…”

“…We have previously reported optical aberration measurements (using Hartmann Shack wavefront sensors) in other animal models, such as chickens [5,6], marmosets [7], macaques [8] and mice [9]. However, despite the increasing use of the guinea pig model, and extensive research on the regulation of its eye growth (in terms of axial length and refractive error) [4,10,11], there is scarce information on its optical properties in vivo.…”

“…To date, all the data available in guinea pigs are limited to 1-dimentional axial distances in vivo, or 2-D geometric information from crosssectional profiles from frozen sections in excised eyes. State-of-the art models of the guinea pig eye are based on that information [11].…”