Evidence for growth heterogeneity among foci with different phenotypes in the population of altered hepatocyte foci induced by a single neonatal treatment with carcinogen.

Relationships between phenotypic and growth characteristics of carcinogen-induced altered hepatocyte foci were investigated. Male and female rats were given a single i.p. injection of carcinogen (diethylnitrosamine or benzo[a]pyrene) within 1 day after birth and were exposed to dietary promoter (phenobarbital) beginning at weaning. Groups of these rats were then killed at intervals, and their livers were examined for foci exhibiting various phenotypic markers through the use of serial frozen sectioning techniques, histochemical staining and computer-assisted image analysis. These procedures permitted the identification and sizing of foci with different specific phenotypes (identities of focus markers) within each phenotypic complexity level (number of markers per focus). The data suggest that foci growth rates differ with respect to specific focus phenotypes within complexity levels. This observation complements previous demonstrations of a direct relationship between foci growth rates and levels of phenotypic complexity and indicates that the observed diversity of focus phenotypes reflects true biological diversity within the focus population. Given the prior evidence for (i) the stability of focus phenotypes; (ii) the rapid emergence of phenotypically dissimilar foci following a single carcinogen treatment; and (iii) the production of foci by single initiation events, we suggest that each proliferatively and phenotypically distinct member of the focus population reflects the occurrence of a lesion at a unique genetic locus during initiation.