StoryImage( ‘/Images/Story//Auto-img-118894149117241.jpg’, ‘Photo courtesy of http://www.ohiohistorycentral.org‘, ‘Grab a fence lizard by the tail, and he will quickly amputate it and grow a new one.‘);
It is a basic fact of life that as animals grow older their body parts wear out or are apt to be damaged by traumatic events. It is also a fact that the lower we go down the ladder of life, from the most complex to the most primitive of animals, the ability to regenerate all or part of the body increases. This is especially true of animals without backbones, or the invertebrates. When the most complex animal of all, the human, is considered, the power of regeneration is almost non-existent.
This phenomenon is frequently demonstrated in the freshman biology laboratory by having the student take a tiny freshwater flatworm and cut it into several pieces. Each piece of the worm is placed in a watch glass, and in a few weeks each segment has regenerated itself into a complete worm, much to the amazement of the student.
Among the most advanced of invertebrate animals is the sea star, or starfish, This animal loves to prey on clams, much to the dismay of the men who dredge clams from the bottom of the sea for a living. For years, it was the usual practice of the clam dredger to cut into pieces any starfish brought up in their dredges and unceremoniously dump the parts back into the sea. The clammers, not knowing of the regenerative powers of the starfish, were increasing their problem by creating several new predators from one.
As we advance up the ladder of life to the vertebrates, or animals with backbones, we find the powers of regeneration are greatly curtailed. There is no vertebrate who can create a complete copy of itself from a piece of its anatomy. Many vertebrates, however, can regenerate portions of their makeup. We all have seen the numerous sharp teeth found in a sharks mouth from the movie Jaws or in an aquarium. If the shark breaks off some of these teeth while tussling with a prey animal, there is no need for him to worry that he will be incapacitated. A new tooth will grow back any number of times where the old one was lost, so the shark has an unlimited number of sets of teeth. Unfortunately for humans, and fortunate for dentists, we have only two sets of teeth; milk or baby teeth and our permanent set.
If the hollow fangs of a rattlesnake or other pit viper are lost, new ones will be quickly regenerated and take their place. An animal with an unlimited number of sets of teeth is said to be polyphyodont, whereas humans with only two sets are diphyodont.
I recall with amusement an incident that occurred to me as a child. I wanted to catch a common fence lizard and maintain it in a terrarium as a pet. These lizards were fairly common near my home in north Texas, but they were extremely hard to catch. After many frustrating attempts to capture one, I finally managed to grab one by the tail. Almost immediately, I was holding only the tail in my hand while the rest of the critter scampered off to safety.
Lizards have a protective adaptation that enables them to escape many predators, such as young boys. There is a ring of constrictor muscles around the base of the tail with which they can quickly amputate the appendage, if the need be. In a few months, however, a new tail is regenerated, complete with muscles, blood vessels and nerves.
We have all heard of cloning of higher animals, which is the production of a new individual from a single non-reproductive cell of a donor. This was first done in England with frogs in the late 1970s by Gurdon working at Oxford. Since that time, many other higher animals, including sheep, goats, cats and mice, have been successfully cloned. Cloning is based on the fact that every single cell of an organism contains the complete genetic complement necessary for the formation of a new individual.
Though there have been claims a human has been cloned, there is no proof this has occurred. I believe one of the next breakthroughs in reproductive biology will be the cloning of individual organs or specialized cells, such as those that produce hormones. Then, there would be no need for transplants from donors, and the problem of rejection would be solved. The new replacement organ for the individual will have been cloned from a cell taken from his own body.
Many say such a scenario is impossible. But remember, in science nothing is considered to be impossible.
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.from the Sept. 5 – Sept. 11, 2007, issue