Endogenous rhythms in axial length and choroidal thickness in chicks: Implications for ocular growth regulation

Purpose. To determine whether the diurnal rhythms in axial length and choroidal thickness in the chick eye are endogenous circadian rhythms. Methods. Six chickens, 14 days of age, were put into darkness for 4 days. Beginning on the 3rd day, ocular dimensions were measured using high-frequency A-scan ultrasonography, in darkness, at 6-hour intervals over 48 hours. Five age-matched chickens reared in a normal light/dark (L/D) cycle and measured at 6-hour intervals for 5 days were controls. Results. The rhythms in axial length and choroidal thickness persist in constant darkness. The phases of these rhythms are approximately in antiphase to one another, similar to those of eyes in a L/D cycle: however, the peak of the rhythm in axial length occurs slightly earlier relative to that of eyes in L/D (12 PM versus 3 PM; P < 0.05, one-tailed t-test). By the 3rd day in darkness, the rate of growth is significantly higher than that in L/D (117 versus 72 μm/24 hours; P < 0.01), and the choroid becomes significantly thinner (159 versus 210 μm; P < 0.0001). Conclusions. The rhythms in axial length and choroid thickness are circadian rhythms, driven by an endogenous oscillator. The phase of the rhythm in axial length in constant darkness is slightly phase-advanced relative to eyes in L/D and thus is similar to eyes that are deprived of form vision. These findings suggest that in the absence of visual input, the eyes revert to a "default" growth state and that the similarities between the effects of constant darkness and of form deprivation suggest that deprivation may represent a type of "constant" condition.