Dialyzed fetal bovine serum contains two distinct growth-controlling macromolecular fractions: one stimulates and the other inhibits proliferation of primary cultured differentiated fetal rat hepatocytes.

Both fractions are precipitated by ammonium sulfate (50% saturation, pH 7.4, 4°C). Serum fraction I (SFI, mol wt ≥ 120,000 daltons estimated by gel filtration with Bio-gel P200) appears to contain at least two factors which function, respectively, to initiate DNA synthesis (activity pH 4–10 stable) and to increase the rate at which initiated cells traverse the cell cycle (activity pH 4 and pH 10 labile). Intraperitoneal injections of SFI into adult rats have produced detectable stimulation of hepatic but not renal DNA synthesis. Serum fraction II (SFII, mol wt 40,000–80,000 daltons) suppresses in vitro incorporation of CH3-[3H]thymidine into DNA under conditions which diminish neither cell viability nor cell attachment.

Mixing experiments indicate that SFI and SFII mutually antagonize each other with respect to DNA synthesis and cell multiplication. Thus, both the relative and absolute serum levels of multiple factors control in vitro fetal hepatocyte proliferation.

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