Fig. 2
From: The G2-to-M transition from a phosphatase perspective: a new vision of the meiotic division
Resumption of Xenopus oocyte meiosis. G2-arrested oocytes contain pre-MPF, i.e. inactive Cdk1-Cyclin B complexes in which Cdk1 is inhibited by T14 and Y15 phosphorylation. Progesterone initiates a signalling pathway that leads to T14-Y15 dephosphorylation of Cdk1. MPF promotes the breakdown of the nuclear envelope (GVBD for germinal vesicle breakdown, identified by a white spot at the pole of the brown hemisphere) and formation of the metaphase I spindle. After extrusion of the first polar body, the metaphase II spindle is organized and the oocyte arrests at this stage, until fertilization. Top: two pictures of Xenopus oocytes, G2 arrest (left) and GVBD (right)