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It is recognized that organismal death of complex metazoan animals cannot usually be ascribed to the large-scale, simultaneous death or degeneration of their constituent cells. Although age-associated degenerative changes can be identified by both morphological and functional criteria, most cells, tissues, and organs of recently deceased animals are alive and in satisfactory condition at the time of natural death. 1-3 One of the fundamental problems in gerontology is therefore to determine whether such somatic cells have a potentially limited or unlimited lifetime if maintained under conditions which are optimal for growth, survival, and function. Attempts to propagate vertebrate cells serially in vitro have yielded conflicting results, although recent work suggests that most diploid cell types proliferate in culture for a finite and relatively small number of cell generations.4' It is difficult to extend this conclusion to include cells inl vivo because of the necessarily artificial conditions of culture. The corresponding in vivo experiments serial transfer of normal tissue between isogenic animals appear simple but have not yielded definitive answers. When normal tissues are transplanted, it is often impossible to distinguish clearly between host and transplant. Even with skin,

The in vivo life span of normal and preneoplastic mouse mammary glands: a serial transplantation study.