Two forms of tumors in nude mice generated by a neoplastic rat mammary stem cell line

The rat mammary (Rama) epithelial stem cell line Rama 25, isolated from a dimethylbenz[a]anthracene-induced tumor, differentiates in culture into elongated cells. The uncloned elongated cells or "floaters" and one cloned elongated cell line, Rama 29, have been identified previously as containing some cells with myoepithelial characteristics. Both Rama 29 and the floaters contained a spectrum of different morphological forms, the multipolar and bipolar representatives of which were isolated by cloning cell lines. After six passages in culture, the ratio of bipolar to multipolar forms, proliferation rates, and final saturation densities usually increased, and at still higher passages, morphologically transformed foci of criss-cross cells were observed in Rama 29 cultures. From these, two morphologically transformed cell lines were isolated, Rama 29:T and Rama 521. Rama 25 and a similar subclone Rama 259, Rama 29:T, and Rama 521 readily formed tumors when injected into nude mice, whereas both the cloned (Rama 29, Rama 502, and Rama 503) and uncloned elongated cell lines (floaters) obtained directly from Rama 25 failed to do so within 12 weeks. Rama 25 and Rama 259 formed tumors which contained glandular-like areas, sheets of cuboidal cells, and spindle-cell areas, whereas Rama 29:T and Rama 521 formed tumors which contained only spindle cells. Immunocytochemistry showed that glandular areas stained with antibodies to rat milk fat globule membranes; that spindle-cell areas stained with antibodies to laminin, type IV collagen, and myosin; and that Rama 25- and Rama 259-induced tumors contained spindle-cell areas which also stained with antibodies to keratins. Ultrastructural analysis confirmed the presence of glandular-like structures and showed that spindle-cell regions contained mesenchymal-like cells with varying myoepithelial characteristics. Thus, Rama 25 and Rama 259 cells yielded some glandular-like regions and some myoepithelial cells in their tumors, while Rama 29:T and Rama 521 cells yielded some myoepithelial-like cells in their tumors without any glandular elements.