Induced refractive anomalies affect chick orbital bone structure

Experiments have shown that it is possible to induce ametropias (myopia and hyperopia) in the eyes of young animals by distorting early visual experience through the use of negative and positive defocussing lenses mounted over the eye. Defocus lenses (+15 and -15 diopters) were mounted unilaterally over one eye of day old broiler chicks using a contact lens-goggle and velcro combination. Refractive states and ocular dimensions were measured by retinoscopy and ultrasound during the experiment. On the seventh day the birds were killed after which the eyes were removed, weighed and measured with calipers. The remaining heads were cleaned of all soft tissue to leave only the bones of the skull. Axial and equatorial orbital dimensions were then measured with vernier calipers. The frontal bone was prepared for histological analysis and sections were used to determine the relative proportions of formed bone to primitive mesenchymal cells. Prior to treatment there were no differences in refractive states or dimensions of the two eyes. After one week of defocus, the treated eyes were longer or shorter as well as more myopic or hyperopic than the contralateral eye by amounts close to the powers of the defocussing lenses (-12.3 and +11.8 diopters). Orbital sizes varied substantially. Orbital axes of myopic eyes were significantly (P < or = 0.05) longer (on average 0.77 +/- 0.23 mm) than the contralateral control orbits. The orbital axes associated with the hyperopic eyes were significantly (P < or = 0.05) shorter (on average 0.69 +/- 0.18 mm) than the contralateral control orbits. Similarly, significant differences (P < or = 0.05) were recorded for a variety of equatorial measures (naso-temporal, superior inferior, oblique (nasal-superior, temporal-superior). Histological analysis reveals that the frontal bone of the myopic chick is in a more mature state of development compared to the frontal bone of the hyperopic chick. The eyes and orbits of chicks with induced ametropias that were allowed to the recover were not significantly different from the control eyes and orbits. This study clearly shows that, in chicks, ocular refractive development is associated with orbital development and that experiments related to growth factors and retinal processing of defocus information should also consider growth and development of tissue beyond the ocular globe.