Overview of the Hormones of the Female Reproductive Cycle: E, PG, GnRH, FSH, LH

The human female reproductive system is guided by an incredible coordination of hormones. This is the first of a series of posts reviewing the hormones involved in the ovarian and uterine cycles.  It is important to realize 2 things: 1) that the ovarian and uterine cycles operate on the same hormone signals: estrogen, progesterone, GnRH, LH, FSH.  The production of these hormones are intertwined. 2) These hormones are free flowing in the blood, so, if there is an increase GnRH, there will be an increase in FSH and LH systemically, which means there will be an increase in estrogen and progesterone systemically, which means that both the ovaries and the uterus will be fluctuating/reacting at the same time.

http://ib.berkeley.edu/courses/ib140/ovarian%20cycle,%20uterine%20menstrual%20cycle,%20menstruation_files/image016.jpg

Gonad/o/tropin Releasing Hormone: GnRH is a hormone released by the hypothalamus, that stimulates the anterior pituitary to release LH and FSH, which are hormones that act upon the sex organs, also known as "gonads" (hence the name GONAD-o-TROPIN.)  In most cases, the presence of both estrogen and progesterone in the bloodstream at the same time will inhibit the hypothalamus from releasing GnRH.  This is a type of negative feedback.  (However, during a part of the uterine and ovarian cycle, estrogen actually becomes a positive feedback loop--no one really knows how this happens, but I will attempt to explain how to identify when this happens in a later post.)

Follicle Stimulating Hormone: As the name implies, this hormone stimulates the follicles.  Follicles are a generic term for any type of little sac, or cavity.  In the case of ovarian follicles, we're talking about the little sack that surrounds a single egg before it is released from the ovary into the fallopian tube.  All of a woman's oocyte first starts off as primordial follicles-- primary oocytes surrounded by a single layer of squamous granulosa cells.  FSH is the hormone that helps each primary oocyte begin developing into a mature follicle with a gigantic fluid filled antrum.  The antrum builds up so much pressure it explodes, jettisoning the oocyte out of the ovary during ovulation.

https://classconnection.s3.amazonaws.com/427/flashcards/1500427/jpg/picture21335761453346.jpg

Luteinizing Hormone: This hormone causes the follicles to complete maturation and undergo ovulation in the "LH surge."  Right as the oocyte has exploded out of the ovary and into the waiting arms of the fimbriae, LH causes the leftover follicle bits (mostly granulosa cells) to crumple up into the corpus luteum (the fact that luteinizing hormone has the same root word as corpus luteum, should not escape your notice).  These granulosa cells change into luteal cells, and supports the corpus luteum in its task to secrete large amounts of progesterone and some estrogen.

Estrogen: Estrogen is secreted by granulosa cells.  The primary reason for granulosa cells secrete estrogen is for a positive feedback loop with LH to induce a ovulation.  The secondary reason for estrogen secretion is to increase the number of progesterone receptors in the uterine lining.  This is necessary to increase progesterone sensitivity in the uterine lining so that the uterine lining can proliferate, becoming thicker with more blood vessels to create nice, nutrient rich lining for the potential embryo.

Progesterone: The pregnancy hormone.  Progesterone is secreted by the corpus luteum, and later by the chorionic villi, and then, when the placenta finally becomes functioning, it is secreted by the placenta.  In order for a pregnancy to remain viable, there has to be a consistent, elevated level of progesterone.  Progesterone does many things, which I will cover in a different post, but the most important one is, it relaxes the muscles of the body, causing a decrease chance of uterine contraction (which would lead to spontaneous abortion).

https://courses.stu.qmul.ac.uk/smd/kb/microanatomy/humandev/placenta/index.htm

Human chorionic gonadotropin:  The pregnancy test hormone.  Human chorionic gonadotropin is only released by the syncytiotrophoblast of the blastocyst (see the picture above).  More specifically it is secreted by the most superficial epithelial layer of the embryonic placental villi, so there has to be an embryo mature enough to be embedding into the uterus for this hormone to appear.   (You should remember that embedding of the embryo occurs about 8 days after fertilization, so hCG would appear around day 22 of a standard menstrual cycle).  HCG is like the little brother of luteinizing hormone.  You know how sometimes there are two siblings who play the same sport but the little one not only plays at the same level as the older, he also seems to be able to land all the tricks the older one can't?  If LH were the older brother, hCG would be the little brother who can do the same thing as LH, except better, because it is secreted even in the presence of elevated estrogen (remember elevated levels of estrogen causes a negative feedback repression of GnRH, which would cut off the supply of LH from the pituitary).  Sometimes LH is referred to as the pituitary analogue.  As the analogue, hCG continues to support the corpus luteum after LH has faded from the blood stream, ensuring that there is a steady release of progesterone and estrogen even after the high concentrations of progesterone and estrogen have caused the pituitary to stop releasing LH.  Additionally, FSH is also suppressed, which is good, because the woman's body doesn't need to be wasting energy maturing extra oocytes when there's already a pregnancy underway.


References:

Perry, Shannon E., Marilyn J. Hockenberry, Deitra Leonard Lowdermilk, and David Wilson.  Maternal Child Nursing Care. St. Louis: Elsevier, 2014. 5th Ed.
Seeley, Rod, Cinnamon VanPutte, Jennifer Regan, Andrew Russo.  Seeley's Anatomy and Physiology. Boston: McGraw Hill, 2011. 9th Ed.
http://en.wikipedia.org/wiki/Syncytiotrophoblast