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Debora L. Nickla, Christine F. Wildsoet, David Troilo; Endogenous Rhythms in Axial Length and Choroidal Thickness in Chicks: Implications for Ocular Growth Regulation. Invest. Ophthalmol. Vis. Sci. 2001;42(3):584-588.
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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 72μ
m/24 hours; P < 0.01), and the choroid becomes
significantly thinner (159 versus 210 μm; P <
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“
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