The Female Menstrual cycle is a part of the Endocrine System because it is controlled by the same Hypothalamus and Pituitary. Let’s dive into an endocrine analogy.
The endocrine system is like a factory that is owned by the Hypothalamus, and Pituitary. The factory produces a variety of hormones, and hence it has multiple assembly lines with different skilled workers. In this analogy the “corporate office” so to speak, where the CEO and Vice President sit, is in the brain which is quite far away from the workers in the reproductive system. Because of this distance, extra communication must take place to ensure that all employees are in sync. The communication and commands happen through hormones via roads or blood vessels that connect the organs.
The assembly line begins with the CEO giving the command to the Vice President saying “whatever hormone that should be made for this gender should be released”. This command is called “Gonado –trophin (gender specific hormone) Releasing Hormone” or “GnRH”. The Vice President, Ms. Pituitary, takes the command from the Madame Hypothalamus and has choice between two commands to send to the Reproductive System. Her choices are Follicle Stimulating Hormone and Luteinizing hormone. Since we are starting our story with day one, and we mentioned earlier that the first phase in the ovary is the maturation of the follicle, we obviously know that the Ms. Pituitary has to pick the FSH command! But how does she know which phase the ovary is in unless the ovary tells her?
In the Ovary, when the follicles just begin maturing, they produce a little bit of estrogen. As they grow and grow, the follicle produces more and more estrogen. The vice president uses the amount of estrogen that’s there to figure out what stage of follicle growth the ovary is in. If there is low estrogen, then Ms. Pituitary knows that she needs to send more FSH to stimulate more follicle growth. If there is a lot of estrogen then she knows that its now time to stop stimulating the follicle, and that she should give the command for Ovulation. This is when she switches her command to LH or Lutenizing hormone! This command tells the ovary to pick the most mature follicle, and let it “ovulate” or release its egg.
Let’s recall that earlier we said that the wrapper that remains is called the Corpus Luteum. Although its pretty much a dead follicle, it serves a very important function of producing hormones. These hormones are Estrogen, Inhibin, and lots of Progesterone. As we know, hormones are the language of your body, so let’s try to understand what the corpus luteum is saying. Lets start with the hormone that sounds the least intimidating… Estrogen. As most people know this is a very important hormone that makes a woman, womanly so to speak. Having many effects all over the body, estrogen contributes to the growth of the cushion known as the endometrium of the uterus, it maintains the female gender specific characteristics, and has other functions like supporting muscle and bone growth. Now moving on to Inhibin. Sounds like the word inhibit right? Believe it or not, it does exactly that. Inhibin tells Ms. Pituitary to stop or inhibit the release of the FSH command. Mainly because there is no need to stimulate any more follicles as an egg has already been released. Progesterone is the most important hormone that is produced by the Corpus Luteum and it serves many functions. To understand what this hormone is trying to say let’s break down the word. Gestation means pregnancy development. Pro-gestation hormone or progesterone, promotes the pregnancy by creating the cushion for the baby in your uterus. It also tells your CEO to “Stop giving any commands to the pituitary. There is a chance that we can have a baby this month, so don’t stimulate any other follicles unless its necessary!”
So we end up with the CEO giving no GnRH commands, which results in the VP not giving any FSH or LH commands. All the hormones needed right now are released by the corpus luteum, and the body is geared up to having a baby this month. Let us assume that no fertilization occurs, and that this is a normal menstrual cycle. The corpus luteum degenerates, and the hormones it produces disappear along with it. This removes the inhibition command from the CEO, who then gives the GnRH command to the VP, who can immediately restart the follicle growth phase and a New Cycle will begin. In addition, the decrease in hormones needed for the baby’s cushion in the uterus will result in the inability to maintain it. Hence the cushion sheds off, and women have their period!