Tritium–labeled cholesterol was injected into two pregnant baboons, in the third trimester of gestation and 5 days later laparotomy was performed and 14C–acetate was injected into the umbilical vein and the circulation was intact for 110 and 119 min. In the first experiment the SA of the maternal 3H–cholesterol (free and esterified) reached a constant level by the 5th day at which time there was complete equilibrium between the maternal and fetal free, but not the esterified, cholesterol. The study of the SA of 14C– and 3H–cholesterol of various tissues indicated that the placenta actively incorporated the maternal cholesterol but did not show any de novo synthesis of cholesterol. The fetal liver was a major site of synthesis of cholesterol whereas the synthesis and incorporation of cholesterol in the brain was small. None of the several neutral steroids searched for could be isolated from the liver in the second experiment and from the adrer nals in both studies, indicating that the conversion of acetate and cholesterol to neutral steroids may be extremely small and undetectable by the experimental design employed for the study of these tissues.
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