ABSTRACT
Hayflick has proposed that the finite proliferative capacity is an expression, at the cellular level, of aging in vivo, and he has termed the phenomenon in vitro cellular senescence. In the more than 25 years since Hayflick’s original observations, the finite proliferative capacity of normal diploid cells (both fibroblastic and nonfibroblastic) of various species has been studied extensively. Clonal analysis has shown that the larger, nonproliferative cells (types III and IV) are derived from the smaller, highly proliferative cells (types I and II) in a sequence of morphological differentiation that parallels the loss of proliferative capacity of the mass culture. Laboratory stocks of Syrian hamsters are derived from three animals obtained from their natural habitat in Syria in 1930. Since the introduction of this species as a laboratory animal in biomedical research, it has become an established model system in a variety of research disciplines, notably toxicology and carcinogenesis.
