The development of transgenic mice and of "knock out" mice has resulted in a revolution in our thinking. The mouse has emerged as the major research model for biology. The mouse has been referred to as the E. coli of modern biology and the surrogate for human biology. The mouse is inexpensive to maintain, is easy to manipulate and its genome is syntetic with the human. The mouse has become the mammal of choice for the analysis of interaction of specific genes with the whole animal. To understand the pathology of genetic manipulation in the transgenic or knock out mouse we need to understand the normal variations that occur in the animal and the target organ. Manipulation can result in unexpected alterations in the growth and development that defy traditional pathologic interpretation. Variations in normal can readily be misinterpreted as phenotypic changes. Non-neoplastic inflammatory infiltrates have been interpreted as malignancies. Animals with altered immune systems are particularly susceptible to opportunistic infections that can be misinterpreted as a "phenotype" by investigators eager to publish their "results".
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