General Introduction to Animal Models of Human Conditions

Animal models serve to imitate (patho) physiological states and/or phenotypical characteristics known to occur in target species (usually man but sometimes other species as well). The use of animal models has had and may continue to have a tremendous impact on medical progress. Laboratory animals are now used in the study of basic (patho)physiological mechanisms, in the development, production, and evaluation of diagnostic and therapeutic agents and/or procedures, as well as in safety studies to assess carcinogenic, teratogenic, or reproductive toxicity of investigational chemical and biological agents, and in education and training. The quality or utility of a model often depends on its validity, which is highest in the so-called homologous models where the phenotypical presentation displayed as well as the cause of the condition in the animal are identical to those of the human condition. Isomorphic models display similar symptoms, but the condition is not provoked by the same events as the human condition. Partial models do not attempt to model the entire condition, but focus only on limited aspects. Models can be further classified into spontaneous, induced, negative, and “orphan” models. Uncritical extrapolation of animal findings to the human condition may lead to unreliable or even dangerous conclusions. Extrapolation tends to be most reliable when a plurispecies approach is taken, and when differences in metabolic patterns and speed, as well as several other potentially confounding variables are taken into account. Animal models have been crucial to neurological and psychiatric research, even though the search for valid models has been difficult in these fields because of the differences in brain structure and function between humans and other species.