No problem (if you are a newt)
When in 1987 I applied to obtain research funds to continue a project on spinal cord regeneration that I had started with Dr Alibardi from Italy, I was unsuccessful. However, the question of spinal cord regeneration is still a compelling one. Mammals and birds are poor regenerators and usually cannot replace lost or malfunctioning parts of the central nervous system, but in lower vertebrates and many invertebrates the situation is different. Starfish, as most people would know, have no problem re-growing severed arms, but they aren’t vertebrate animals. Among the vertebrates most lizards can at least replace a lost tail, but in newts (my favourite animals) and salamanders the ability to replace lost or injured body parts is even more remarkable and goes much further.
In these amphibians a surgically-removed eye is completely replaced and amputated legs totally re-form, recreating a copy of the original extremity with all its toes, blood supply, nerves, muscle and bone. In frogs and toads (which are also amphibians) none of this would work, except if one were to operate on their tadpoles. So what makes newts and salamanders so special? Even if a small part of their vertebral column with its spinal cord is removed from an adult newt (thus turned into a paraplegic animal), it would regain its full use of its legs after a couple of months. How do these animals do it? After all, they are vertebrates like us.
We simply do not have all the answers yet – and will probably never get them unless proper funding for this kind of research is available. What we do know from research conducted so far is that cells of the wound blastema undergo de-differentiation followed by re-differentiation and cell proliferation (cell number increases). In other words, some cells of the skin begin to behave like embryological cells, making use of the information that is coded in the DNA of the nucleus of every cell of the body. What “switches” them on and how the de-differentiated cell selects the appropriate DNA-sequence of the code to re-create muscle tissue, nerve fibres, bone and vascular structures remains largely unknown at this stage. To understand how this happens is doubly important if we consider the similarity between regenerative and tumourous growths, a similarity I had pointed out in a review published in 1987 with Prof Asashima and Dr Oinuma based on both involving cell de-differentiation and rapid cell proliferation.
That the system isn’t fool-proof and can be “tricked” becomes apparent when a newt’s leg is not removed, but simply rotated by 180° and then left attached to the body. This manipulation results in an additional rather than replacement limb. Closest to limb regeneration in mammals is the annual antler regrowth in deer males (and, I was told, apparently regrowth of digits and tail tips in newborn possums). Antlers unlike horns consist of bone and grow anew each year from the stump they have broken off the year before. Deer antlers, newt limbs and lizard tails may seem to have as little in common as the telephone, the radio and the toaster, but chances are that there is indeed a unifying basic mechanism for all large-scale regenerative phenomena in vertebrates. All we have to do is find it – and the one who does is almost certainly going to be rewarded with a Nobel prize.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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