Transparency and vision
The lens of the eye is a remarkable organ. It is the only cellular tissue in the body, which is perfectly transparent. We take this for granted, but transparency in nature is very rare and incredibly difficult to achieve. To become invisible, the cells of the lens have to specialise more than any other cell type – and yet always run the risk of clouding over.
In contrast to other tissues, the lens has no blood supply and does not contain any connective or nervous tissues. It is composed of only one cell type. These cells have to be very precisely arranged. They form what could be called a „biological crystal“. This is, however, not yet sufficient for the lens to become transparent. The cells that make up the lens also have to be radically different from any other cell type found in our bodies.
Inside most cells there is a confusing mixture of organelles, the „organs“ of a cell: the nucleus with the DNA, the energy-producing mitochondria, the protein and lipid factories Golgi apparatus and endoplasmatic reticulum (ER) and many more. The problem with organelles is that they scatter and absorb light and thus obstruct transparency. Hence, lens cells have to eliminate all organelles to become transparent.
This may not seem very dramatic. However, when other cells encounter so much as a little damage to their DNA, they can embark on a process called apoptosis or programmed cell death. They commit suicide to save the rest of the organism from the spread of the damage, e.g. in the form of a cancer. Lens cells, however, destroy not only their entire DNA and the nucleus that contained it, but also every other organelle. This is literally the equivalent of reducing the cell to its skin and bones.
Despite this apparent damage, lens cells do not die. They miraculously survive their suicide attempt and remain functional for a lifetime. This potential life span of up to 100 years contrasts spectacularly with the vast majority of other cells that expire after only a few days, weeks or months at most. Although the lens cells survive, the organelle destruction has drastic implications. Lens cells cannot produce new molecules to renew or repair themselves anymore. This is a major problem, as the capacity to replace damaged parts is one of the main advantages of biological systems. The molecules that comprise cells typically have half-lives of a few minutes to several days. That means that within a few months, most of the molecules that make up our bodies will be replaced by new ones. This is no longer possible in the lens. The molecules in these cells must be eked out until the organism dies.
Once the lens cells have rid themselves of their organelles, their cytoplasm is the total opposite of the „organelle chaos“ seen in other cells: it is completely homogenous. It consists of an unusually thick solution of very special proteins, the so-called crystallins. The close packing of crystallins creates a short-range order that prevents fluctuations of refractive index and thus prevents light scattering. It is these tightly packed crystallin proteins that finally render the cells‘ cytoplasm totally transparent.
To find out more about how the lens becomes transparent, please see the following reference:
- Dahm R: Dying to see. Scientific American, October 2004, p. 82-89. [Feature article on the development and transparency of the eye lens.]
Ralf Dahm has been engaged in vision research since 1994, first as an MSc and PhD student at the University of Dundee (UK), where he studied the changes which the cells of the developing lens have to undergo to become transparent. As a postdoctoral researcher in the group of Nobel Laureate Christiane Nüsslein-Volhard at the Max Planck Institute for Developmental Biology (Germany) he stayed true to transparent things, working with the translucent embryos of the zebrafish as a model to understand how genes control the development of the eye. As a group leader at the Medical University of Vienna (Austria), he continued working on the genetics of eye development, but also expanded his research interests to include how nerve cells change when we form memories. [to “Molecules and Memory”]
Ralf Dahm has published numerous peer-reviewed research papers, review articles, book chapters and popular scientific articles on different aspects of the development and function of the eye, most notably on the development, structure and function of the lens.