Hybrid cells in solid tumors

Cells of murine lines Sarcoma‐180 (Foley) and L‐5178Y lymphoblast were noted to undergo spontaneous hybridization when cultured together in vitro. Since hybridization of cells in solid tumors has not been demonstrated using chromosome techniques, an in vivo study of hybridization between the 2 lines was undertaken. The cell lines were grown as separate and mixed tumors in the thigh of 131 C3H mice. Growth characteristics of the inoculated tumors were observed over a 21‐day period, and karyotype analyses were made on cells from separate and mixed tumors. The Sarcoma‐180 grew progressively as a solid tumor; L‐5178Y regressed after the eighth day following inoculation; and the mixed tumor followed an intermediate course. Hybrid cells were consistently noted in low frequency (3.1% of tumor cells) in the mixed tumors 21 days following inoculation. The demonstration of hybrid cells in vivo in solid tumors makes hybridization a mechanism whereby a tumor cell population might evolve from clones of cells within the tumor.