[Oocyte maturation in Xenopus laevis. The entry of progesterone is necessary for maturation].

In vitro progesterone-induced maturation of Xenope ovocyte was studied as a function of progesterone uptake and extracellular progesterone concentration. Maturation depends on the initial total amount of progesterone in incubation medium and does not depend on the extracellular hormonal concentration. This result strongly argues for an intracellular receptor site for progesterone.