Development of vertebrate eyes
The high transparency of the lens is due to the denucleation of lens cells during maturation. However, vertebrate crystalline lenses are gradient-index structures and grow throughout life by addition of cell layers at the lens surface, even in mammals. The permanent growth and unusual cell biology of the lens lead to interesting questions concerning the development and maintenance of its optical properties, which are extremely sensitive to the distribution of refractive index within the lens.
We have detected regulatory mechanisms that adjust refractive index in lens cells, even in denucleated ones that cannot synthesize proteins. Regulation takes place on various time scales, from fast and reversible changes between day and night to long-term adjustments during lens growth.
Understanding the developmental plasticity of the lens is important also for understanding the evolution of vertebrate vision and the fine-tuning of the visual system to the environment. In addition, many fish species change behavior, habitat, and/or preferred depth from larval stages to reproducing adults. Such transitions in lifestyle and optimization of the visual system at each stage put particular demands on the developing eye and lens. In some fish species, eye and lens volume increases by a factor of 10^6 from early larval stages to large adults, and vision is of crucial importance at all times during this tremendous growth.
The work performed so far is just the beginning of our quest to understand how sophisticated gradient-index lenses develop and are maintained throughout life. We expect most vertebrates to share the basic mechanisms, since vertebrate color vision and multifocal lenses evolved at least 500 million year ago and the fundamental demands have not changed since then.