Induction of oocytic maturation and differentiation: mode of progesterone action

Full-grown amphibian oocytes, arrested in prophase I or meiosis, respond in vitro to progesterone and certain other steroids. They undergo an apparently normal sequence of nuclear maturation and cytoplasmic differentiation, which are necessary precedents for fertilization and embryogenesis. Individual oocytes or populations of these cells thus provide a model system for investigations concerning the nature and mechanism of hormone-cellular interactions. In this system, previous studies have shown that certain aspects of hormonal (progesterone, DOCA) induction of morphologic and biochemical differentiation in the nucleus can be induced in part, as a result of the formation of secondary cytoplasmic factors, some of which do not require the presence of the nucleus for their formation. In addition to initiating nuclear events, progesterone or DOCA alters the functional activity of the plasma membrane and establishes the conditions necessary for fertilization and activation. Uptake of radioactive vitellogenin, a yolk protein precursor, was inhibited by progesterone and DOCA. Maximum inhibition was dependent on induction of nuclear breakdown, the dose of steroid used, and was correlated with morphologic alterations at the oocytic surface. Estrone neither stimulated nor inhibited vitellogenin incorporation and had no effect on nuclear breakdown. Oocytic capacity to exhibit activation responses (vitelline membrane elevation) was dependent on oocytic exposure to progesterone or DOCA and development subsequent to the initiation of nuclear breakdown. Onset of the activation response after steroid treatment varied with the type of activation stimulus utilized (pricking or divalent ionophore A-23187). The results suggest that hormones cause ionic alterations in oocytes and that ions are directly involved in the activation response. To study steroid interaction with the cell surface, a method was developed for culturing oocytes that permits localized application of steroids to portions of the oocytic or follicular surface. The results obtained suggest that oocytes exposed to steroid over part of their surface do not respond, with regard to nuclear breakdown, in the same manner as do oocytes exposed over their entire surface to similar concentrations of steroid. Studies of isotopic distribution within the oocyte after local application of steroid indicate that hormone does not readily diffuse through the oocyte. Evidence for the role of cytoplasmic factors in the mediation of nuclear and cytoplasmic events is discussed.