Animal Models of Aging (Worms to Birds)

Article excerpt

Researchers are rapidly closing in on identification of fundamental aging processes.

Why study aging (or many medical problem) in animals, when it's people we really want to know about? This is a question that medical researchers hear often, and we have plenty of answers. First, studying animals is much more convenient. That is, it is cheaper in terms of time and dollars. Consider that an investigation of aging usually requires monitoring a group of individuals (the larger the group, the better) from birth until they are all dead. Such a study of humans might take too years, of mice or rats about three years, of fruit flies perhaps three months, and of tiny roundworms about a month. So we could perform almost 2,000 roundworm studies in the time it would take for one human study Since scientific knowledge moves forward by following up and refining successive experiments, progress would be glacially slow if we studied aging only directly in humans.

Aging rate is also difficult to assess in individuals, so it is generally measured by analyzing patterns of age-at-death in populations. Large populations allow much more sensitive measurements of this sort. An average-sized research laboratory can manage populations of perhaps several hundred mice or rats, up to a million fruit flies, and millions upon millions of roundworms. Short lives and the possibility of tracking death rates in larger populations, then, are two equally important virtues of aging research using animal models.

Experiments are the foundation of science. As the physicist Richard Feynman (1995) put it, "the test of all knowledge is experiment." the use of animals allows us to explore and examine processes at a fine level of detail that for practical and ethical reasons is beyond anything we could ever do in people. So, we can produce hundreds of genetically identical individuals and use them to transplant organs from the elderly into youngsters and vice versa. We can subject these individuals to extreme environmental conditions or unproven drugs that might affect aging, as is now routine, insert genes of one species into another, turn specific genes on or off in specific tissues or at specific times of life, or even dramatically overproduce specific gene products.

Additionally, experiments become more powerful with increased control of the environment in which the experimental subjects live, because factors extraneous to the experiment are Less likely to confound the results. Using animal models, we can dictate exactly what they eat and how much, standardize the physical conditions twenty-four hours a day, seven days a week for life, and even control the social interactions in which they can engage.

But for all the obvious advantages of using animals for aging research, we need to remember that they are different from people in as yet undefined ways, so we must give serious thought to how relevant research findings from other species will be for understanding aging in humans. Because almost all animal species age, there might well be fundamental aging processes that are conserved among all animals, but we do not know that this is the case for sure. A general principle of biology is that species more closely related to one another will share more traits. So we can be pretty sure that other primates will exhibit aging processes similar to those of humans and even more certain that our closest relatives, chimpanzees and bonobos, will. We can be less certain about rodents and even less certain about invertebrates such as roundworms and fruit flies. As I describe the various model species currently used, I will note how they might conceivably differ from humans.

WORMS

The worm used as a model in aging research is not the familiar earthworm, but a tiny roundworm, scientific name Caenorhabditis elegans, about half the size of a grain of rice. In the wild it lives in soil. For aging research, its specific advantages are that it has a short life (about two weeks at normal room temperature), develops rapidly (egg to adult in about two days), and usually reproduces by self fertilization, making its 300-350 offspring genetically identical to one another and to their parent. …