The oocytes of several organisms—most frequently those of the African clawed toadXenopus laevis—have been used for many years as an excellent system to study regulation of transcription, translation, protein modification processes, secretion, and protein compartmentalization, as well as the expression of heterologous-membrane receptors and their association to specific signaling cascades. Full-grown oocytes are large cells (over 1.2 mm in diameter) that are arrested in late-G2 phase of the first meiosis (Meiosis I), and must progress after physiologrcal stimulus by progesterone to the second meiotrc metaphase (Meiosis II) before fertilization takes place. This process, called oocyte maturation or germinal vesicle breakdown (GVBD), can be easily visualized by the appearance of a small white spot in the animal pole, a consequence of the dissociation of the nuclear envelope. After GVBD is completed, if the oocytes have been fertilized, DNA synthesis takes places with the consequent initiation of the Meiosis II.
