309 The Mouse Model of Myopia

more brightness. Even owls do not have such a low f/number (1.1329), and it is clear that the mouse has probably one of the brightest retinal images among vertebrates. The thickness of the retina, relative to axial length (which grows from 0.178 mm at 22 days of age by 0.6 m per day), should give rise to a large difference in the position of the photoreceptor layer and of the light reflecting layer(s) in retinoscopy — resulting in large amounts of apparently measured hyperopia (the “small eye artifact”).26 From the schematic eye, the small eye artifact was calculated to more than 30 D.15 Since neither infrared photorefraction nor streak retinoscopy showed such large amounts of hyperopia, mice must either be myopic (which is not in agreement with behavioral data, see below), or the light reflecting layer(s) is (are) not at the vitreo-retinal interface, but rather deeper in the retina.

Techniques Currently Available for Myopia Studies in the Mouse, Both for Its Induction and Measurement

The large depth of field and relatively low visual acuity of the mouse suggest that the retinal image degradation must become quite severe before it is detectable for the retina, and before it can potentially trigger changes in eye growth.

Devices to induce refractive errors

The first attempts were made by gluing hemispherical frosted plastic goggles over the eye of C57BL/6 mice, using instant glue (Fig. 3, from Schaeffel et al.).22 The problem was that the mice tended to remove these goggles. A plastic collar was necessary to prevent the mice from reaching the goggles. Even then, it was demanding to keep the eye covers in place for two weeks.22 Barathi et al.21 managed to keep velcro rings, glued to the fur in Balb/cJ albino mice, in place from P10 to P56. These rings even carried hard contact lenses. Also, Tkatchenko and Tkatchenko30 managed to keep them in place without a collar. Because of the problems associated with gluing the devices to the fur around the eye, Faulkner et al.31 developed a head-mounted pedestal in which a holder was implanted in the skull and the diffusers were carried by a wire. These devices were well tolerated by the mice, but they were applied rather late at P28. They were kept in place until P84.25

Continuous light rearing, which causes severe flattening of the cornea and large hyperopic refractive errors in chickens32 was also attempted but