Synthesis of thromboplastin (factor III) in mouse placental cells in vitro

Mouse placental cells have been isolated and grown in cultures. These cells produce a procoagulant which is identical to thromboplastin (factor III) by two criteria. The procoagulant activity increases with time, culminating on the 5th day of culture. The increase is inhibited by cycloheximide, alpha-amanitin, and actinomycin D, showing that de novo synthesis of protein and RNA is necessary. About 90-95% of the total cellular thromboplastin activity in whole cells is available for inactivation by particle-bound trypsin and thus present on the cell surface. Endotoxin and phytohaemagglutinin did not further increase the procoagulant activity.