Initial Causes of Ovulation and Menstration

[ihatebluescrubs]

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Had a couple of questions that gave me second thoughts so I just wanted to double check if I understood the main cause of ovulation and menstration.

For Ovulation:
-- before ovulation, estrogen exerts negative feedback over LH FSH secretion
-- but high increase in estrogen turns negative feedback to pos feedback --> LH rises --> ovulation

For Menstration (this one im not very sure on what specific hormone causes menstration):
-- drop in progestrone / estrogen (or both?) causes the endometrium to fall off?


Can someone confirm if this is right / detailed enough for the mcat?

Thanks

 

[Alan245]

I think so.


For menstruation at least, the endometrium sheds due to a lack of progesterone. Progesterone is secreted by the corpus luteum. If the egg is not fertilized, the corpus luteum dissolves and progesterone is no longer secreted. The fall of progesterone levels causes the uterus to contract and shed the endometrium.

 

[image187]

I think so.


For menstruation at least, the endometrium sheds due to a lack of progesterone. Progesterone is secreted by the corpus luteum. If the egg is not fertilized, the corpus luteum dissolves and progesterone is no longer secreted. The fall of progesterone levels causes the uterus to contract and shed the endometrium.

The menstrual cycle can be broken down into 2 phases: follicular (proliferative) phase, and luteal (secretory) phase. Duiring the follicular phase estrogen is the main hormone present and serves to cause proliferative changes to the endometrium, building it up. Ovulation occurs following the LH surge, and marks the beginning of the 2nd phase of the cycle. During this luteal phase, progesterone is the main hormone at play. It is secreted by the corpus luteum (which used to be a follicle containing an egg) and serves to cause secretory changes to the endometrium such as gland maturation and secretion, basically preparing the built up endometrium for the implantation and maintenance of the blastocyst and maintenance of a pregnancy. After a certain number of days, the corpus luteum does not dissolve. It atrophies, and becomes a different structure known as a corpus albicans which is still present on the ovary and can be visualized on the ovary with a microscope. Nonetheless, when the corpus luteum atrophies, it stops producing progesterone, and it is this sudden drop of the hormone that causes shedding of the endometrium and passage of the tissue (menses).

Just an aside, if a pregnancy were to implant in the uterus, the placenta would begin secreting a hormone called beta-HCG (human chorionic gonadotropin). This hormone had the same alpha subunit as other pituitary hormones but specifically serves as an analog to LH, which stimulates the corpus luteum to stay alive and keep producing progesterone in order to maintain the endometrium for the pregnancy. After some time, the placenta actually takes over and is able to produce its own progesterone. This hormone I mentioned (beta-HCG) is the basis for essentially every pregnancy test.