Photoperiod, early post-natal eye growth, and visual deprivation

(1) We studied the influence of photoperiod and unilateral lid suture on post-natal ocular growth in two types of White Leghorn chicks previously reported to respond differently to visual deprivation, Truslow and Cornell K chicks. We analyzed the chicks after 2 weeks of rearing, a time period commonly used in neuropharmacological studies of eye growth but much shorter than in most prior studies of photoperiod effects on the chick eye. (2) Altering the photoperiod length significantly influenced the refraction and growth of both open and sutured eyes even at this early time, with differences between the two types of chicks. (3) The most prominent effect on the open eyes was the development of hyperopia with rearing under constant light, a response especially prominent in the Cornell K chicks. In the open eyes under this condition, the anterior chamber shallowed and the vitreous chamber elongated in the axial dimension only, reciprocal changes that resulted in no net alteration of axial length at 2 weeks. A high variability in refraction of open eyes reared with constant illumination suggests the need for a dark period in the regulation of eye growth. (4) Compared to contralateral open eyes, the lid-sutured eyes of both types of chicks developed longer total axial lengths and enlarged vitreous chambers in both axial and equatorial dimensions under each photoperiod. The effects on anterior chamber depth and refraction were complex and differed between the two types of chicks. (5) The responses in open eyes support the notion that growth of the vitrious chamber of the chick eye is differentially regulated in the axial and equatorial dimensions, previously indicated by pharmacological studies. The responses in both open and sutured eyes indicate different control mechanisms for anterior chamber and vitreous cavity growth.