A three-step scheme for gray crescent formation in the rotated axolotl oocyte

It has been shown that various inhibitors of protein synthesis can elicit the precocious appearance of a gray crescent (GC) in in vitro maturing, nonactivated Ambystoma mexicanum oocytes. However, evidence has now been obtained that these treatments fail to induce GC formation when the oocytes are enucleated before initiation of maturation. The ability to form a GC is reestablished in enucleated oocytes by the injection of nucleoplasm from a normal oocyte, either before or after the injection of the inhibitor. In the latter case, the GC appears very rapidly, even though protein synthesis is at about 1/10th that of the control enucleated oocyte, after treatment with diphtheria toxin (final concentration 10(-8) M) as an inhibitor. One or several nuclear factors, in conjunction with inhibition of protein synthesis, are therefore essential for early symmetrization. The corrective nuclear factor is already present in the germinal vesicle of young oocytes, at the very beginning of vitellogenesis. It is not species specific, since enucleated axolotl oocytes can be symmetrized with Pleurodeles or even Xenopus oocyte nucleoplasm. Moreover, it has been shown that the nuclear-cytoplasmic interaction is possible only when cytoplasmic maturation has been proceeding for at least 10 hr after exposure to progesterone (at 18 degrees C). A three-step process as a prerequisite of GC formation in the oocyte is proposed: Cytoplasmic maturation must proceed till a reactive state is attained, allowing interactions with nuclear factors; Nuclear factor(s) interact(s) with matured cytoplasm; Inhibition of protein synthesis triggers GC formation. Sequence of steps 2 and 3 can be experimentally inverted but must always be preceded by step 1. Since a sharp reduction in amino acid incorporation has also been found in normally fertilized eggs just prior to GC formation, it is suggested that the scheme described above could be also applicable to normal symmetrization in this model system.