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Sally A McFadden, Natalia Bilton, Sheree Harrison, Marc HC Howlett; Crystalline lens thickness is modulated by spectacle lens defocus in chicks. Invest. Ophthalmol. Vis. Sci. 201657(12):.
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© 2017 Association for Research in Vision and Ophthalmology.
Eyes become myopic because they grow excessively. The signals underlying eye growth are studied using spectacle lens compensation (SLC), in which growth can be inhibited or enhanced after exposure to myopic or hyperopic defocus respectively. The underlying signals likely arise locally in the retina, and are rapidly translated to the choroid prior to subsequent eye growth in the posterior sclera. However, emmetropisation involves a balance between both posterior eye growth and the changing optics of the eye. Therefore, we looked for changes in the crystalline lens and the choroid during the early response to SLC in chicks.
63 chicks (White leghorn, Steggles, Amatil, NSW, Australian Poultry Ltd) were reared on a 12/12 h light/dark cycle, wearing a monocular spectacle lens of one of 8 different powers (-8D, -6D, -4D, -2D, +2D, +4D, +6D, +7.5D; n=6/group) or no lens (n=8) from 3 days of age for 24 h. The lens-wearing eye was repeatedly measured every 4 h over 24 h using a high frequency ultrasound (resolution 10 μm) in anaesthetised chicks.
Choroid thickness was bi-directionally modulated by the sign and the magnitude of imposed defocus (Fig 1A). The maximum change occurred in the middle of the day (Fig 2), and the gain was larger for hyperopic than myopic defocus. The thickness of the crystalline lens was also bi-directionally changed (Fig 1B), growing mostly during the day. These changes in the lens were not a passive consequence of eye size as they were in the opposite direction to that predicted by stretch, and after one day of lens-wear, there was no relationship between lens thickness and axial length (R2 = 0.003, p = 0.73). In contrast, choroid thickness was significantly correlated with crystalline lens thickness (R2 = 0.193; p = 0.004).
The depth of the crystalline lens is sensitive to early signals which predict the direction of eye growth and is thicker in eyes that subsequently develop myopia and thinner in eyes developing hyperopia. The source of the signal is unknown, but since SLC is unaffected after the elimination of lenticular accommodation in chicks, it may reflect a sensitivity of the crystalline lens or ciliary muscle to factors released from the retina in response to defocus exposure.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Fig 1. Mean choroid thickness (A) and crystalline lens thickness (B) in the lens-treated eye.
Fig 2. Choroidal rhythm after subtracting the increase in thickness over 24 h.
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