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 choro idal thickness in the chick eye are endogenous circadian rhythms. METHODS. Six chickens, 14 days of age, were put into darkness for 4 days. B eginning on the 3rd day, ocular dimensions were measured using high-frequen cy 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 mea sured at 6-hour intervals for 5 days were controls. RESULTS. The rhythms in axial length and choroidal thickness persist in con stant 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 e yes in L/D (12 Phl versus 3 PM; P < 0.05, one-tailed t-test). By the 3rd da y in darkness, the rate of growth is significantly higher than that in L/D (117 versus 72 <mu>m/24 hours; P < 0.01), and the choroid becomes significa ntly thinner (159 versus 210 <mu>m; P < 0.0001). CONCLUSIONS. The rhythms in axial length and choroid thickness are circadia n rhythms, driven by an endogenous oscillator. The phase of the rhythm in a xial length in constant darkness is slightly phase-advanced relative to eye s in L/D and thus is similar to eyes that are deprived of form vision. Thes e 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 c onstant darkness and of form deprivation suggest that deprivation may repre sent a type of "constant" condition.