Female Reproductive System #1

Which one of the following is associated with the shedding of endometrium during menstruation?


A. Formation of the corpus luteum
B. Rise in estrogen and progesterone
C. Rise in hCG
D. Low LH level

ANSWER: 
D. Low LH level 


SHORT-AND-SWEET:


Menstruation is usually considered to be the beginning of the menstrual cycle, but we suggest you consider it the last phase, which happens if there is no conception.  
No conception means no placenta, which subsequently means no hCG (which eliminates C).  The formation of corpus luteum is associated with the luteal phase (as the name suggests), which occurs after the ovulation and before menstruation (which eliminates A).  Estrogen and progesterone are after ovulation produced by the corpus luteum, so when the corpus luteum dies towards the end of the cycle, levels of estrogen and progesterone drop, which leads to the endometrial shedding (this eliminates B).  
We are left with answer choice D, which is the correct answer.  Remember that at the time of menstruation levels of all hormones are low.


THE WHOLE STORY:  


Female reproductive system is one of the most dreaded topics in the MCAT Biological Sciences section, which is exactly why we included it in our blog.  While others are wasting their time in needless memorization, we will help you understand the female reproductive system.  And there will be no diagrams -- we will think of it as a story.

We will start off with the big picture:  The purpose of the female reproductive system is to enable reproduction!!!  In order for this to happen, what does this system need to provide?

1. A mature egg (oocyte)

2. A way to deliver the egg to a location where sperm can fertilize it (ovulation)

3. A nice home for an embryo (in the uterus)

Think of the uterus, ovaries, and the anterior pituitary as the key players in the female reproductive system, and in particular, think of the anterior pituitary as the brains of the operation.  What do we need to memorize about them?  (We will ask you to memorize only two things in this topics, and this is one of them.) 
   -Non-gravid uterus does not produce hormones.  
   -Ovaries produce estrogen and progesterone.  
   -The anterior pituitary produces FSH (follicle-stimulating hormone) and LH (luteinizing hormone).  


Now that we know the primary purpose of the female reproductive system, the key players, and what they produce, we can start understanding the logic behind the menstrual cycle. 


Let's start by thinking about what happens after the menstruation is over.  The uterus lining is completely bare.  The ovaries, with their numerous immature follicles, are resting, taking a nice little nap.  The anterior pituitary, the brain of the operation, is resting as well.  For now.  


The brain knows that the purpose of this system is to reproduce, and BAM! -- the brain wakes up!  It starts producing FSH to wake up the ovarian follicles, stimulating them to mature.  As the ovarian follicles mature, they produce more and more estrogen.  Estrogen in turn acts on the uterus, causing the proliferation of the endometrial lining.  

Because of the endometrial proliferation and follicular maturation, which occur during this phase, this phase is called the proliferative or follicular phase.  

The product of this phase: 1. MATURE EGG. 

At the same time estrogen, which is produced by the ovaries, exerts negative feedback on the anterior pituitary, inhibiting the secretion of FSH and LH (mechanism seen in the other endocrine systems).  However, when estrogen reaches critically high level, instead of inhibiting LH production, it causes transient LH surge (positive feedback), which subsequently causes follicle rupture, or ovulation.  The LH surge tires the anterior pituitary, which now wants to rest, leading to low LH and FSH levels.

The product of this phase:  2. MATURE EGG DELIVERED TO THE FALLOPIAN TUBE, WHERE IT CAN BE FERTILIZED BY SPERM.

Now that the oocyte is delivered to the fallopian tube, waiting to be fertilized by a sperm, all eyes turn to the uterine endometrium, which now needs to make sure to provide a suitable home for a possible embryo.  How does this happen?


This happens in coordination with the ovaries.  The cells that were surrounding the oocyte in the follicle remain in the form of corpus luteum, and they continue to produce estrogen and progesterone (as they did during the proliferative (follicular) phase).  It is the progesterone (and to a lesser degree, estrogen) secreted by corpus luteum that acts on the endometrium, causing it to develop a thick juicy lining, called the secretory lining.  The secretory lining makes for a perfect embryo home.  

Because of the secretory endometrium and the formation of corpus luteum, this phase is called secretory or luteal phase.  

The product of this phase:  3. NICE HOME FOR A POSSIBLE EMBRYO.

But what happens if there is no fertilization?  
Here is that second thing to memorize: Corpus luteum has an internal "timer", and without hormonal stimulation it will die on its own after 14 days.  As it dies, estrogen and progesterone levels drop, and without these hormones the thick endometrial lining sheds, causing menstrual bleeding.


This brings us back to our initial question about what happens during menstruation.  Like we just said, the menstruation is what occurs when corpus luteum dies, which eliminates A.  As the corpus luteum dies, estrogen and progesterone will decrease, which eliminates B.  hCG (human chorionic gonadotropin) is a hormone produced by the placenta, which forms at the start of pregnancy.  There is no menstruation during pregnancy, which eliminates C. 


Answer D is the correct answer.  Like we said, during menstruation and immediately afterwards, the female reproductive system is "resting".  This includes the pituitary, which secretes minimal amounts of LH and FSH.  It is when this brain of the operation wakes up that the cycle starts anew.     


BIG PICTURE:  

The purpose of the female reproductive system is to enable reproduction!!!  In order for this to happen, the system needs to provide:

1. A mature egg (oocyte) - oocyte maturation is stimulated by FSH (follicle-stimulating hormone).

2. A way to deliver the egg to a location where sperm can fertilize it (think ovulation) - triggered by LH surge.

3. A nice home for an embryo (in the uterus) - stimulated initially by estrogen produced by the ovarian follicle, and then by progesterone produced by corpus luterum, the ovarian follicle remnant.

~The MCAT POD Team~