Oogenesis and Oocyte maturation?

Question

Would this be the correct sequence of events?

progesterone activation of MPF, release from 1st arrest, GVBD, ovulation, release of 1st polar body, fertilization, cortical rotation, release from 2nd arrest, release of 2nd polar body, 1st cleavage division

Answer 1

Pretty much this is right. Let me break it down. Progesterone or other hormones activate ovulation in Xenopus adult females. Progesterone is the hormone released by follicule cells which is picked up by the oocyte. Gonadotropin is the activating hormone for follicule cells. When progesterone hits the oocyte, the signal transduction cascade commences, ending with activation of MPF. MPF does several things. First, it dissolves the GV (not sure how, probably phosphorylation of nuclear pore complex?), second, it phosphorylates lamins to produce chromosome condensation, and promotes activation of MPF through phosphroylation of cdc25 and Myt1 (which activates and represses them). So all these things are going on simultaneously. Meanwhile, the GV is actually progressing to the animal pole, to prepare for metaphase I, in which half of the chromosoems are lost in the first polar body. After activation of the APC, MPF is briefly inactivated by degradation of Cyclin B. Levels come up again very quickly and the cell procedes to the second part of meiosis, ending up with arrest at metaphase II. Outside the oocyte, the oocyte is being prepared for ovulation by addition of a proteous layer, and excretion through the cloacia (sp).

Now there is a mature oocyte in the media, ready for fertilization by any lucky sperm. Sperm entry promotes cortical rotation and release from metaphase II. The cell completes meiosis, and the zygote now merrily and furiously divides, approx every 45' for the first dozen or so cell cycles.

So basically you are right but some things are going on at the same time.