Regeneration in general
Regeneration can be accomplished in three different manners:
1. dedifferentiation of adult structures to form a mass of undifferentiated cells, the regeneration blastema, that then can be used to form new tissue and new structures.
2. morphallaxis: regeneration by repatterning of existing tissues without much new growth. For instance the Hydra.
3. Compensatory regeneration. New cells are formed by they produce cells that are similar to themselves.
As usual nature doesn't confine itself to one solution but works with what it has. The first post of this thread regarding lens regeneration works through the first mechanism of dedifferentiation and growth of new tissue from the mass of undifferentiated cells. It is perhaps one of the more spectacular examples of regeneration.
The regeneration of the salamander limb
You can amputate the limb of a salamander and it will regrow completely without any defects; proper structures, proper size. If you amputate the limb at the wrist all structures are formed from the wrist onwards and not more. Apparently the salamander 'knows' where the amputation is taking place. This is rather fascinating since during the development the limb grows through rather specific mechanisms under the influence of specific regulatory genes. This system isn't really in place anymore at an adult stage.
The first thing that happens after amputation is a sort of woundhealing. The wound epidermis forms across the wound. From this epithelium another crucial structure is formed, the apical ectodermal ridge. Then the cells underneath the epidermis undergo dedifferentiation. All cells, bone, cartilage, neurons etc lose their fate,and begin to grow into a lump of dedifferentiated cells that all look rather similar. This mass of cells is called the regeneration blastema.
The cells proliferate, and more importantly they need to be patterned. A lump of cells does not make a good hand. You need a wrist, fingers etc all consisting of different kinds of tissue, bone, cartilage, fat, connective tissue, nerves etc.
In limb development during embryonic development there is a typical pattern of HOX genes that specify what's going to be what. In the adult limb this pattern is gone, but during regeneration some of this HOX gene patterning is miraculously re-established.
A similar system of patterning, growth and organizers of these processes, such as the apical ectodermal ridge, are re-established in the regenerating limb which get 'blank' tissue from the dedifferentiated lump of cells produced originally in the regeneration blastema. The progeny of what once was a bone cell can actually end up as a muscle cell. A remarkable feat.
The lens is part of the eye that, along with the cornea, helps to refract light to focus on the retina (see the picture below). It is therefore rather an important structure. Without it we are not exactly blind, but all we see is a blur.
Once damaged it is gone. That is in most vertebrate species. However, not in all species! The newt is a salamander which can regenerate not only its eye lens, but also limbs and its spinal cord.
It isn't really a recent discovery that the newt can regenerate its lens. There are three publications of different researchers ranging from the year 1880-1895 where this phenomenon was first described. That's more than 100 years ago. These researchers removed the lens and noticed it was completely regenerated.
The lens is regenerated by a process called transdifferentiation. A group of cells that is already committed to a specific cell fate or specialization returns to a blank state and then differentiates into a new cell fate. This is a very special event. It doesn't occur very often in complex organisms.
Some fish, avian and mammalian species also show lens regeneration, but the mechanism does not happen via transdifferentiation, but through another more complex mechanism.
In the figure below you see a summary of the events that occur during lens generation. After the lens is removed the iris pigment epithelial cells or PECs for short (that's just how they are called) on the dorsal side start de-differentiating at the tip of the iris (1). Nothing much happens at the ventral side.
These dedifferentiated cells will now re-enter the cell cycle which simply means that they start proliferating once again. After a few cell divisions they will form a lens vesicle (2). At this point already cells at the inside start thickening (in orange) and the synthesis of crystallins starts. Crystallins are structural molecules typical of the lens. This process continues, the vesicle grows larger, and the inside is filled up with cells producing more crystallins (3). And finally the lens is almost fully formed (4).
In the newt the entire process takes about 25 days.
Genetic Control of Newt lens regeneration
I don't really want to go to deeply into this subject here because I'm afraid it will pretty soon get too specialized to be of general interest. I will restrict myself here to discuss the role of Pax-6 in more general terms.
Pax-6 is a very interesting gene. It is a master regulator of eye development. Forced expression of this gene in the wrong place can lead to ectopic eyes. It is also very highly conserved between species. Mouse Pax-6 can trigger eye development in the fruitfly. Without Pax-6 there is no eye development.
It is also expressed in the dorsal and ventral iris. And as we might remember the tip of the dorsal iris is the place where lens regeneration starts in the newt with the de-differentiation of PEC cells. This pax-6 expression domain at this location also coincides with cellular proliferation during lens generation.
You cannot knock out pax-6 in the newt in a transgenic newt line like you might do in the mouse, because there is no transgenic technology available for the newt. However you can knock out a gene via other means. In one paper the researchers used morpholinos to knock down Pax-6 (ref: Rio-Tsonis 2006). Without Pax-6 expression cellular proliferation was diminished. Also crystalline production was reduced leading to a serious reduction in lens fibers.
It seems therefore that Pax-6 is not only needed to initiate eye development, also in eye regeneration it has a crucial role.
Regeneration in Folklore and popular culture
Trolls have the ability to regenerate. Trolls cannot regenerate tissue damaged by fire or acid.
Rio-Tsonis et al (2006) The role of Pax-6 in lens regeneration. PNAS, 103: p14848-14853
Rio-Tsonis and Tsonis (2003) Eye regeneration at the molecular Age. Developmental Dynamics 226: p211-224
Scott Gilbert, Developmental Biology.