Replication of mouse mammary tumor cells in monolayer cultures stimulated with embryo extract

A particulate fraction derived from homogenized chicken embryos was added to primary monolayer cultures of mouse mammary tumor cells. All culture media contained fetal calf serum (10%). After 4 days, the extract had stimulated a twofold increase (relative to controls lacking the extract) in (1) total DNA per culture; (2) the rate of incorporation of labeled DNA precursors; (3) epithelial cell number, and (4) total protein. This integrated growth activity is discussed in terms of the nature and exogenous regulation of neoplastic growth.