Ovulation

 

Ovulation occurs at midcycle when steroid hormone levels are highest. It is triggered off by LH surge which ends in ovulation after 36 hours. It is the process by which the eggs which are ripe and in the appropriate stage of meiotic stage of arrest after completion of first meiotic division are released into the peritoneal cavity from where they are siphoned into the oviduct for eventual fertilization in this tube. Several complex mechanisms are involved in the process of ovulation. Firstly it is believed the trigger is LH surge. Although there has been a gradual increase in blood circulating FSH during the proliferative phase, this is caused by low level of estrogen from the previous cycle or indeed the menses phase. The increase in estrogen during the late follicular phase increases the sensitivity of the pituitary gland to gonadotropin release factor produced from the hypothalamus (FSHRH or LHRH) which control the releases of LH and FSH. Shortly before ovulation therefore, and because of the feedback effects of estrogen on the pituitary gland and increase sensitivity brought about by the increasing estrogen, the level of FSH and LH rise sharply in blood due to increase release from pituitary stores. The egg is then released at about  36 hours after the beginning of LH surge. Experimentally or therapeutically, if estrogen is administered during the midfollicular phase, LH and FSH are suppressed initially, and this suppression is then followed by an acute surge 12-24 hours later. The egg is released as secondary oocyte which has completed its first meiotic division and is released into the preampulla of the oviduct. It is believed that the fimbriae aid ovum pick up by contraction, although the exact way in which it performs this function is not fully understood since fimbriectomy does not necessary block ovum pickup. It is also believed that some chemical agents in the preampulla, probably a glycoprotein, aid the molecular maturation of the ovum as it slides along the preampulla to the ampulla, even as the second meiotic division is under arrest. The hypothesised chemical component may also aid zona pellucida ripening or maturation since it has been shown that there is a need for the zona to become mature before it will be able to bind to spermatozoon through specific zona pellucida proteins called the ZP.

 

                                           

 

 

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