INDUCED REFRACTIVE ANOMALIES AFFECT CHICK ORBITAL BONE-STRUCTURE

Experiments have shown that it is possible to induce ametropias (myopi a and hyperopia) in the eyes of young animals by distorting early visu al experience through the use of negative and positive defocussing len ses mounted over the eye. Defocus lenses(+l5 and -15 diopters) were mo unted unilaterally over one eye of day old broiler chicks using a cont act lens-goggle and velcro combination. Refractive states and ocular d imensions were measured by retinoscopy and ultrasound during the exper iment, On the seventh day the birds were killed after which the eyes w ere removed, weighed and measured with calipers. The remaining heads w ere cleaned of all soft tissue to leave only the bones of the skull. A xial 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 bo ne to primitive mesenchymal cells. Prior to treatment there were no di fferences in refractive states or dimensions of the two eyes. After on e week of defocus, the treated eyes were longer or shorter as well as more myopic or hyperopic than the contralateral eye by amounts close t o the powers of the defocussing lenses (- 12 . 3 and + 11 . 8 diopters ). Orbital sizes varied substantially. Orbital axes of myopic eyes wer e significantly (P less than or equal to 0 . 05) longer (on average 0 . 77 + 0 . 23 mm) than the contralateral control orbits. The orbital a xes associated with the hyperopic eyes were significantly (P less than or equal to 0 . 05) shorter (on average 0 . 69 +/- 0 . 18 mm) than th e contralateral control orbits. Similarly, significant differences (P less than or equal to 0 . 05) were recorded for a variety of equatoria l measures (naso-temporal, superior inferior, oblique (nasal-superior, temporal-superior). Histological analysis reveals that the frontal bo ne of the myopic chick is in a more mature state of development compar ed to the frontal bone of the hyperopic chick. The eyes and orbits of chicks with induced ametropias that were allowed to recover were not s ignificantly different from the control eyes and orbits. This study cl early shows that, in chicks, ocular refractive development is associat ed with orbital development and that experiments related to growth fac tors and retinal processing of defocus information should also conside r growth and development of tissue beyond the ocular globe. (C) 1997 A cademic Press Limited.