Abstract
Eyes of young chickens show diurnal oscillations in axial length and choroidal thickness that are out of phase. In eyes responding to myopic defocus induced by prior form deprivation, the two rhythms shift into phase. In order to elucidate the possible role for these rhythms in ocular growth regulation, they were measured under visual conditions that altered ocular growth rate. (1) Form deprivation to myopic defocus. Eyes of chicks were monocularly deprived for 5 days. Diffusers were removed. (2) Myopic defocus to hyperopic defocus. Eyes wore positive lenses for 6 days; lenses were removed. (3) Hyperopic to myopic defocus. Eyes wore negative lenses for 5 days; lenses were removed. Eyes were measured using A-scan ultrasonography at 6-h intervals for 24 h over various cycles. The rhythms shift into phase in eyes slowing their growth in response to myopic defocus in all three conditions. This shift precedes by 1 day the decrease in growth in both lens conditions, and is concomitant with it in recovering eyes. There is a positive correlation between the phase difference and growth rate. In conclusion, there is a consistent association between growth rate and phase relationships of the rhythms in axial elongation and choroidal thickness.
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Notes
The technique of subtracting the linear regression from the data has been shown to be an effective one in that for normal growing eyes, the residuals are very well fit to a sine wave (see Nickla et al. 1998). However, in some of these data the mean start and end values resulting from this analysis differ somewhat (Figs. 2, 5, 7). The phase differences that would result from adding a slope that would result in these differences onto a sine wave are 1.1, 1.5 and 1.3 h, respectively. These small errors are unlikely to bias the statistics.
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Acknowledgements
The author thanks Grace Lytle and Catherine Johnson for assistance with some of the ultrasound measurements. Thanks also to Heidi Denman and Kristen Totonelly for care of the chickens. This work was supported by NIH-EY-13636. All experiments conformed to the ARVO Resolution on the Use of Animals in Research.
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Nickla, D.L. The phase relationships between the diurnal rhythms in axial length and choroidal thickness and the association with ocular growth rate in chicks. J Comp Physiol A 192, 399–407 (2006). https://doi.org/10.1007/s00359-005-0077-2
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DOI: https://doi.org/10.1007/s00359-005-0077-2