Growth of rat mammary tumor line 64-24 in liposome-supplemented defined medium. II. Effect of liposome B and prolactin on colony forming efficiency.

During studies on serum-free clonal growth of rat mammary tumor line 64-24, we observed that liposome B causes a major increase in colony-forming efficiency. This phenomenon was studied with a short-term assay based on phase contrast microscopic observation of the effects of liposome B on recently plated, serum-depleted cells. In the serum-free medium, colony forming efficiency is not determined primarily by viability (measured by dye exclusion) or cell attachment. Instead, most of the effect is determined by the extent of degeneration that occurs during cell elongation and the first division. Factors that prevent degeneration, such as liposome B, restore colony forming efficiency. Prolactin alone stimulates cell elongation but does not prevent degeneration. However, the combination of prolactin and liposome B increases colony formation to levels observed in the presence of serum.