Did you know that it is actually your brain that initiates your period? Yes, what happen to your ovaries and uterus is actually determined not only in that local area but also by your brain, thyroid, inflammation etc.
I like to break the menstrual cycle into three parts, the first is the development phase (follicular) and ovulation, then the nurturing phase (luteal) and finally the shedding phase (menstruation). Today we are going to dive deep into the first part of the cycle. When it works well, and why it might not!
Follicular phase Overview
The follicular phase starts at the initial day of your period (flow) and continues until ovulation. During this phase, the innermost layer of the endometrium (uterine lining) becomes thickened due to the hormonal changes. The two important characteristics of this phase are the lowering of basal body temperature and the development of ovarian follicles (your soon to be eggs!). The maturation of the ovarian follicles, known as folliculogenesis, initiates just a few days prior to the period. The development of follicles in follicular phase depends on secretion of the Follicle-stimulating hormone (FSH), secreted from pituitary gland in the brain. That is why doctors often measure FSH in the blood, to ensure that your body is properly stimulating egg development.
On the outside of a mature follicle there is a layer of cells, known as granulosa cells. In the follicular phase, these fascinating little things are responsible for the production of estradiol, one of our primary sex hormones. Estradiol plays a significant role in the proliferation of the endometrial lining of the uterus, and other tissues.
When the development of the ovarian follicle is at its peak and when estradiol levels are just at the perfect level, your brain will send a signal to promote ovulation of that egg. This signal comes in the form of luteinizing hormone (LH). LH increases can be measure in the blood, but also in the urine. In fact, the urine ovulation predictor kits that can be found in the drug store actually measure LH1.
Follicular phase- detailed and nerdy section!
I have been mentioning the brain continuously throughout this article. And that is because your brain controls every single system in your body! Even the FSH and LH that come from the brain are controlled by other systems within the brain (we always knew were complex, right?!)
Why do I tell you all of this? It is important for women to understand that throwing one more supplement at their uterus will not always solve their menstrual issues! This is especially true with issues such as hypothalamic amenorrhea, PCOS and any timing issues with menses.
Here we need to understand the underlying issues and work to fix those! (NOTE: if you are undergoing fertility treatments then some of the following words, such as gonadotrophin etc, will sound familiar. This is because fertility clinics use pharmaceutical medications to change these systems)
Gonadotrophin Releasing Hormone
So before we get to ovulation, and before we get follicular stimulation, we will have a signal from the hypothalamus (and area of our brain), known as Gonadotropin-releasing hormone (GnRH). GnRH is classified into three types- GnRH-I, GnRH-II, and GnRH-III.But GnRH-I, GnRH II are mainly participating in menstrual cycle and reproduction. GnRH-I stimulates FSH and LH secretion from the anterior pituitary.
This gets tricky though, as pulsatile GnRH production stimulates FSH and LH secretion, but the continuous release of GnRH suppresses FSH and LH secretion. Therefore, females with a risk of early puberty can be controlled by administering GnRH agonists, like leuprolide (Lupron). Lupron is also used to control ovulation in fertility treatments as it can suppress the Gonadotropin release. GnRH-II discharges from the placenta (which develops with pregnancy) maintaining a pulsatile fashion. This pulsatile nature change within a menstrual cycle, change the uterine and ovarian environment2.
Gonadotropes responsible for releasing of FSH and LH, and Lactotropes responsible for releasing of prolactin (our milk signal), are different types of anterior pituitary cells and play a vital role in the development of the reproductive system. Apart from these hormones, the anterior pituitary secretes activin, inhibin, and follistatin, which play a role in menstrual cycle regulation2.
These signals from our brain both create a menstrual cycle and regulate our hormone production, but they are also regulated by our hormones levels. Don’t get too caught up in this aspect of the cycle, but do keep in mind that we NEED to work on our brain health, and therefore our mindsets to have the best menstrual cycle possible.
What does FSH do?
Activation of Gonadotropin-releasing hormone stimulates the secretion of two gonadotropins, which include FSH and LH. The FSH level increases few days before the start of menses. FSH influences conversion of androgens (male sex hormones) to estrogens by inducing granulosa cell growth and activation of aromatase activity. In the Follicular phase, increased FSH level initiate a cohort of follicle development and this results in the selection of the dominant follicle, that ideally will become an ovulated egg. Subsequently, FSH level lowers as the growing follicular granulosa cells starts to produce estrogen and inhibin B. FSH will give one final burst as progesterone increases along with the LH burst during ovulation of the dominant follicle. After this the FSH levels decrease for the remainder of the cycle2.
What does LH do?
LH is another important gonadotropin that maintains a concentrated level throughout the follicular phase. In the late follicular phase, the dominant follicle increases the production of estradiol (a type of estrogen present in non-pregnant female) which stimulates LH to rise rapidly. This quick rise and fall is what can be detected on an ovulation predictor kit.
Stimulation of LH causes maturation of ovarian follicles and full-fledged discharge of estrogens for promoting ovulation, development of the corpus luteum and induces the production of estrogens, progesterone, inhibin and other hormones by the corpus luteum to maintain the hormonal homeostasis2.
The Uterine System
What many people don’t realize is that our ovaries run the show when it comes to the control of your uterus. Yes, these little tiny grape like organs will provide enough hormonal stimulation to increase the lining of your uterus, which is required for pregnancy, or even to have a period!
In the follicular phase, developing ovarian follicles contain granulosa cells, which have an important role in endometrial lining development by producing estradiol. The theca, granulosa or interstitial cells of ovarian follicles raise the progesterone levels prior to ovulation. The proper signaling between ovary and uterus is very essential for normal endometrial development, which is important for fertility and childbirth3.
Ovulation
The oocytes (eggs) produced in the ovaries are grown in tiny fluid-filled sacs known as follicles. Hormonal influences lead to oocyte maturation, the production of immature ova (egg cells) and further maturation converts ovum (singular of ova) to an egg, which is ready to fertilize and travel into the uterus through the fallopian tube. This whole process is termed as ovulation. Follicles become dilated and the diameter of the follicle is approximately 20 mm on average, prior to the ovulation. As discussed above, the LH surge is primarily responsible for ovulation. To produce the LH surge, estradiol level rise to 200 pg/mL and continue at this levels for approximately 50 hours. After a 12 hour duration of elevated LH ovulation takes place. The egg then moves towards, and through the fallopian tubes2.
Overview & Summary
Clearly John Mayer was on to something! Your body IS INDEED a wonderland! J
In all seriousness though, it takes an immensely complex system for you to have a period, launch an egg, and fertilize that egg. Your brain needs to talk to your ovaries, which need to start to develop a bunch of eggs, then one drives forward and proclaims itself the winner (or dominant follicle). Then that follicle needs to continue to develop, and the cells around it to produce estrogen and other regulators to build up the uterine environment and prepare it. Then when all of that is just peachy your brain senses it is time, it sends the signals needed for ovulation, your hormones change, the egg is released and then…. In a very anticlimactic fashion, we wait… wait for the egg to travel down the fallopian tube, wait to see if it finds the perfect sperm etc…
And then that takes us to the luteal phase, the second half of your menstrual cycle. Which we will address next week!
Until then, awe in the amazingness that is the female body!
References
- Reed, B. G. & Carr, B. R. The Normal Menstrual Cycle and the Control of Ovulation. Endotext (MDText.com, Inc., 2000).
- Beshay, V. E. & Carr, B. R. in Clinical Reproductive Medicine and Surgery 31–42 (Springer New York, 2013). doi:10.1007/978-1-4614-6837-0_2
- BAERWALD, A. R. & PIERSON, R. A. Endometrial development in association with ovarian follicular waves during the menstrual cycle. Ultrasound Obstet. Gynecol. 24, 453–460 (2004).
