In the ovary, oocytes are held at metaphase II of meiosis until fertilization. How is this
block released?
O Progesterone activates C-Mos, which phosphorylates maturation promotion factor
(MPF).
O A Ca2+ influx induces the activity of calmodulin, which induces Camodulin
dependent kinase (CAM PKII) and calpain and they degrade cystostatic factor (CSF).
o Calmodulin dependent kinase II (CamkII) and calpain are activated by progesterone
and they degrade maturation promotion factor (MPF).
o Progesterone activates c-Mos, which phosphorylates cytostatic factor (CSF)
releasing the block.

Progesterone is a steroid hormone that is well known to be involved in embryogenesis, which is secreted by the corpus luteum. During meiosis I, steroid production is limited to progesterone. Moreover, the oocyte maturation factor Mos (cMOS) is kinase expressed in germ-line cells when it is activated by progesterone, where this factor controls oocyte maturation and the arrest at the metaphase of meiosis II by modulating the activity of the Maturation-promoting factor (MPF). This protein (MPF) is a cyclin-dependent kinase complex that promotes mitotic entry via phosphorylation of different proteins required during mitosis.