Human placental tissue is also used. Various other tissues are collected at autopsy for an array of research uses. In vitro approaches are appropriate for some research in biology. For instance, much of the study of intermediary metabolism uses synthesized biochemicals in a manner similar to that of any chemistry experiment. Studies of reaction rates and the role of catalysts are typical examples.
Mathematical models can supplement experimental work or occasionally replace it.
Such models can increase the effectiveness of experiments by defining variables and checking theories, thus making experiments on biological systems more effective and economical. The preceding discussion has focused primarily on means of reducing the number of animals used or replacing mammals with other organisms or with in vitro or mathematical models. The third alternative is to refine experimental techniques to lessen the discomfort of the animals involved. Such refinements of protocol constantly occur, as researchers expand the range and uses of anesthetics, improve or eliminate restraining devices, minimize invasive techniques, or use noninvasive methods to obtain the required results. Each success in this area minimizes discomfort to experimental animals.