Pathology of human-mouse somatic cell hybrid tumors.

Somatic cell hybrids between mouse peritoneal macrophages and simian virus 40-transformed human cells injected sc into nude mice resulted in a formation of tumors with two distinct growth rates and growth patterns: 1) rapid growing, irregularly circumscribed, with necrotic foci and prominent vascular channels lined by tumor cells and 2) slow growing, small, well-circumscribed, and nonvascular. Individual tumor cells were ultrastructurally classified as undifferentiated mesenchymal cells indistinguishable within these two tumor types. Characteristics of each tumor type were retained during subsequent passages in nude mice.