The Menstrual Cycle: Hormones

Welcome to Part 2 of the Menstrual Cycle Series:

The Hormones. It's amazing how such small chemical substances can have such a large impact. Hormones run our bodies and our brains, from when we eat, when we sleep, how we grow, how we feel, how we bond, how we learn--the list goes on. We will focus on the main hormones involved in the female menstrual cycle.

Let's recap what we learned in Part 1:

• Hypothalamus: Director and coordinator, sends messages to the pituitary gland, receives feedback messages.

• Pituitary: Receives messages from from Hypothalamus. Sends messages to the ovaries. Receives feedback messages.

• Ovaries: Receive messages from the pituitary. Send feedback messages.

• Uterus: Receives messages.

These structures exist in the feedback loop that is the HPO Axis. They talk to each other through hormone messengers, with some telling each other to turn on or release certain hormones, and others sending feedback that its time to halt hormone production. Let's start from the top, in the hypothalamus.

Hypothalamus:

• Gonadotropin-releasing hormone (GnRH): Released by the hypothalamus. Sent to the pituitary (anterior), telling it to release Luteinizing hormone and Follicle-stimulating hormone.

Pituitary:

• Follicle-stimulating hormone (FSH): Exactly what it sounds like-tells the ovaries to grow and mature follicles containing eggs, in preparation for ovulation. Growth of follicles causes estrogen levels to rise.

• Luteinizing hormone (LH): From Latin Luteum, meaning "egg yolk". Tells the ovary to release an egg from one of the follicles at ovulation. Does this due to feedback messaging from high levels of estrogen.

Ovaries:

• Estrogen: Produced by the ovaries following follicle growth; Spike sends a message to the pituitary to send LH for ovulation. Promotes growth of the uterine lining in preparation for pregnancy.

• Inhibin: Exactly what it sounds like: Released after FSH tells the ovarian cells to mature follicles. Tells the pituitary to "inhibit" FSH--we don't need more follicles--we just need one now, in order to release an egg for ovulation.

• Progesterone: Produced by the "corpus luteum", the ruptured follicle from which an egg emerges. Produced in order to change the environment of the uterus and vagina to support pregnancy. Stimulates the uterine lining (endometrium) to become nice and coiled and twisted, and secrete substances that will help an embryo implant.

Are you confused yet? It's okay if you are--I'm a midwife, and it took even me a while to really wrap my head around it. It's not just a top--down process. It's top-down, then back up, then back down, and so on. This is of course a more simplified rendition, or as simplified as I can make it. Now that we've got the hormones, we're going to put it together to ride through the whole menstrual cycle. Strap in!

Pictured below is another example of the HPO Axis, it's messengers, and hormones. Photo from: https://dsdgenetics.org/index.php?id=48.