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Ronald H. Kroger, Ola S. Gustafsson; Evolution of Lens Suspension and Accommodation. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1538.
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© ARVO (1962-2015); The Authors (2016-present)
Previously poorly investigated eyes of phylogenetically basal vertebrates were studied in order to gain an evolutionary perspective on lens suspension and accommodation. With this comparative approach we want to contribute valuable information on the development and ageing of the human eye.
We used light and electron microscopy to study structures holding the crystalline lens in the eye at its correct position. The gained knowledge was combined with published data on extensively studied groups, such as teleosts (modern ray-finned bony fishes) and tetrapods (four-footed animals including humans), in order to recognize evolutionary patterns.
Three general types of lens suspension were identified. In chondrichthyans (cartilaginous fishes such as sharks and rays), the lens is suspended by a dorsal and a ventral ligament, as well as a ventral papilla protruding towards and in some cases touching the lens. Both ligaments are wide and may in some species be joined to a washer-like structure encircling the lens. Accommodation is achieved by protraction of the lens. In actinopterygians (ray-finned fishes including teleosts), the lens is suspended by up to seven ligaments and the tendon of a muscle that retracts the lens for accommodation. Surprisingly, the sturgeons, usually classified as basal actinopterygians, have the chondrichthyan type of lens suspension. However, it is unknown if or how sturgeons accommodate. In lampreys (the most basal group of vertebrates) and sarcopterygians (lobe-finned fishes that are the closest extant aquatic relatives of the tetrapods), the lens is suspended by a membrane that extends from the equator of the lens to the ciliary region. No accommodative muscles were found in these animals. Their lenses appeared softer than the hard lenses of chondrichthyans and actinopterygians.
Lens suspension by a radially symmetric set of zonule fibers and accommodation by action of the ciliary muscle seems to have evolved directly from the ancestral state present in lampreys and sarcopterygians. The studies will be extended to the structural and biochemical differences between hard and soft crystalline lenses.
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