The role of the lens in refractive development of the eye: animal models of ametropia.

Research with young mammals and chicks has shown that the visual environment can affect the refractive development of the eye by enhancing or slowing axial eye growth, but the effect on the refractive components of the eye, the lens and cornea, are less clear. A review of the literature indicates that the lens is minimally affected, if at all, and results vary depending on whether the lens is studied in an isolated state or with the accommodative apparatus intact. Research has shown that the development of myopia or hyperopia in young chicks alters lens focal length and magnitude of the accommodative response. However, the result may be indirect or passive due to the effect of the change in size and shape of the globe on the articulation between the ciliary body and lens. Recent research has also investigated the role of the lens in induced refractive error development in a fish, tilapia (Oreochromis niloticus). Translucent goggles were sutured over one eye for 4 weeks to induce form deprivation myopia while the untreated eye served as an untreated contralateral control. In addition to measuring refractive state and intraocular dimensions, a scanning laser system was used to determine the optical quality of excised lenses. All the deprived fish eyes developed significant amounts of myopia and the vitreous and anterior chambers of the treated eye were significantly longer axially than those of the untreated contralateral eyes. No significant change in optical quality was found between lenses of the myopic and non-myopic eyes and the fish recovered completely from the myopia five days after the goggle was removed. The results show that although fish, unlike higher vertebrates, are capable of lifelong growth, the visual environment is an important factor controlling ocular development in this group as well, and eye development is not strictly genetically determined. This review indicates that lens growth and optical development is independent from the refractive development of the whole eye.