Direct visualization, by β-galactosidase histochemistry, of differentiated normal cells derived from malignant teratocarcinoma in allophenic mice

Abstract

Mice obtained from blastocysts injected with malignant teratocarcinoma stem cells may comprise tumor-derived cells in their tissues. Evidence for their presence has hitherto been indirect, i.e., through detection of tissue-specific products of the tumor genotype or of strain-specific enzyme variants in tissue homogenates from healthy mice. Direct visualization and identification of the tumor-derived cells would permit their normalcy and their state of differentiation to be assessed. For this purpose, a histochemical marker is required. The marker chosen was β-galactosidase (BGS), which allows high- vs low-activity cell strains to be distinguished in situ by their differences in staining intensity. BGS has previously been employed for such visualization only in brain [Dewey, M., Gervais, A., and Mintz, B. (1976). Develop. Biol. 50, 68–81] and has here been shown to be applicable to other tissues, including kidney, pancreas, and salivary gland. Two unexpected results concerning the marker itself were obtained and affected its application to histochemical comparisons: BGS activity in some tissues of some inbred strains was not concordant with that of brain, on which the existing genotypic classification is based; and some cell types within a tissue varied independently in BGS levels among strains (e.g., exocrine vs endocrine pancreas). BGS visualization clearly disclosed the presence of large numbers of fully differentiated normal cells of the teratocarcinoma strain in tissues, including the Purkinje layer of the cerebellum, the kidney tubules, and the exocrine pancreas of experimental animals. In one individual, the relevant brain region was almost entirely derived from the teratocarcinoma. Yet all tissues were indistinguishable in structure and differentiation from adult controls, and showed no malignant growth. The pattern of cell-strain distribution, which was fine-grained in the brain and patchy in the other tissues named, also resembled that of ordinary allophenic mice produced from blastomere aggregates of two strains. Thus, teratocarcinoma stem cells are here seen to undergo normal histogenesis after they are successfully incorporated into a developing host embryo.