Lens of the Eye : Overview
Lens of the eye is formed from –
Where Do Lenses Come From? a) Surface ectoderm
d) None of the above
The vertebrate eye develops from a diverse collection of embryonic sources through a complex set of inductive events . Whereas the neural retina is derived from the diencephalon and is a part of the brain, the lens comes from surface ectoderm and the iris and ciliary body arise primarily from the neural crest. Mapping the genes known to play a role in mouse eye development, for example, shows that some of these genes are present on every chromosome . The apparent patchwork assembly of the eye makes it all the more surprising that common developmental programs seem to produce comparable outcomes across a broad phylogenetic divide .
Could we use the composition of lenses to gain insight into eye evolution?
Vertebrate lenses are formed from modified epithelial cells that contain high concentrations of soluble proteins known as crystallins because they are packed in a highly organized fashion. It is the change in relative concentration of these proteins from the periphery to the center of vertebrate lenses that produces the refractive index gradient necessary for a lens to be useful to the animal. In fact, the identity of the proteins seems not to be important since the crystallin proteins are not more transparent than others. Instead, the distribution of protein concentration as a function of radius is the key to a successful lens. Thus, the challenge in understanding lens evolution lies in discovering how the distribution of proteins within a lens is established and maintained.
Of the eleven lens crystallins now known, only three, alpha-, beta- and gamma-crystallins, are common to all vertebrates. In fact, until recently, all crystallins were thought to be unique to lens tissue and to have evolved for this special function. However, despite their apparently specialist role, most of the crystallins are neither structural proteins nor lens specific. There are two major groups of lens crystallins, those present in all vertebrates and those specific to a particular taxon. For example, in crocodiles and some bird species, the glycolytic enzyme lactate dehydrogenase B is a major protein in the lens. Indeed, 4 of the 8 taxon-specific crystallins are identical to metabolic enzymes and products of the same genes, suggesting these products share a gene.
Why might enzymes be recruited to make vertebrate lenses?
Perhaps the robust regulation of enzyme production is advantageous for producing sufficient protein for a lens, but there is not much beyond speculation to support this notion. There may be some deeper reason, however, because this molecular opportunism seemed such a good idea, that certain invertebrates, e.g. mollusks, independently evolved the same strategy . Squids have lenses whose protein content is nearly entirely the enzyme glutathione S-transferase. The common strategy of constructing lenses from different proteins seems to be a convergent evolutionary solution. This convergence of molecular strategy suggests that enzymes as lenses may have a functional meaning, or that it is easy to get lens cells to make a lot of enzyme, or there may be other as yet not understood reasons.
Eyes: Convergence or Homology?
Have the structural similarities among eyes resulted from evolutionary convergence due to similar selective pressures (analogous) or from descent from a common ancestor (homologous)? This distinction is particularly hard to draw when comparing eyes because the physical laws governing light greatly restrict the construction of eyes. Similar eye structures may have arisen in unrelated animals simply because of constraints imposed by light.
The most commonly cited example of evolutionary convergence are the eyes of squids and fish. Both of these are 'camera-type' eyes, in which an image is formed on the photosensitive retinal layer at the back. Moreover, both have evolved a spherical lens with an exquisitely constructed gradient of refractive index that allows good focus despite their spherical shape. In addition, both types of eyes use the same light-sensitive molecule, opsin, to convert photons into neural energy. However, the fish retina is inverted, meaning the light-sensing cells are at the very back of the eye (inverse) while those in squid are at the front of the retina (everse). Moreover, the parts of the eyes of fish and squid arise from very different embryological sources during development, suggesting different origins for these eye types.
Paired eyes in the three major phyla, vertebrates, arthropods and mollusks (fig. 4), have long been considered to be classic examples of evolutionary convergence. At the macroscopic level, this must be true since they arise from different tissues and have evolved radically different solutions to the common problem of collecting and focusing light. However, as discussed above, opsin has a significant DNA sequence homology across all phyla. Remarkably, recent work by Gehring and Ikeo has shown that features of ocular development in different phyla can be coordinated by a homologous 'master' gene, Pax-6. That a single gene could trigger construction of an animal's eye in diverse species led to their proposal that eyes are monophyletic, i.e. evolved only once. This is an interesting hypothesis that goes against all the previous suggestions of multiple (i.e. polyphyletic) origins for eyes. There are several reasons why this hypothesis seems difficult to support. It is well known that Pax-6 organizes other structures besides eyes and is even necessary for the onset of various actions outside the nervous system. Also, other genes can cause development of eyes [reviewed in 10]. Whether eyes are monophyletic or not, the work of Gehring and his colleagues has stimulated a great deal of new work on eye evolution, which is a good thing in itself.
Clearly, eyes have common molecular constituents whether they be opsins, Pax-6, or others. Yet, homology at the molecular level of organization does not predict homology at the organ or organismic level. Molecules are not eyes.
Eyes exist in a variety of shapes, sizes, optical designs and locations on the body, but they all provide similar information about wavelength and intensity of light to their owners. Different tissues have been recruited to build lenses and retinas across the phyla. In contrast, all eyes share the same mechanism of absorbing photons, i.e. the opsin-chromophore combination has been conserved across phylogeny. Despite new findings yielded by powerful molecular techniques, all evidence still suggests that eyes have a polyphyletic origin, with the caveat that they contain homologous molecules responsible for many structural, functional and even developmental features (fig. 5). Given a growing list of homologous gene sequences amongst molecules in the eye across vast phylogenetic distances, the challenge is now to discover what makes the eyes of Drosophila, squid and mouse so different. Since strictly homologous developmental processes must produce homologous structures, key elements responsible for the development of nonhomologous eyes remain missing. Understanding what makes eyes different may be a bigger challenge than finding what they have in common.
Development of the Eye in Vertebrates