So what is the menstrual cycle?
The menstrual cycle refers to the monthly(ish!) cycle of physiological changes lasting approximately 28 days, though this length can vary from person to person and cycle to cycle. Each person’s menstrual cycle is unique and can change with age (1). Menstrual cycles can also change through exposure to hormones, such as hormonal contraception, hormone replacement therapy and gender-affirming hormone therapy, or with menstrual conditions, such as polycystic ovarian syndrome (2-4). Diet and life stress (relationships, job, etc.) can also cause irregularities in your menstrual cycle, in which high stress can pause or shorten your cycle and period (5-7).
Phases of the menstrual cycle
1. Follicular phase
The Follicular phase begins with the first day of the period until ovulation. A part of the brain called the hypothalamus produces and releases Gonadotropin Releasing Hormone (GnRH) to the anterior pituitary gland located at the base of the brain (6). The anterior pituitary gland is often called the “master gland” as it produces a lot of essential hormones for us, such as the growth hormone (regulates muscles, bones and fat distribution), adrenocorticotropic hormone (ACTH; part of the stress system), luteinizing hormone (LH) and follicle stimulating hormone (FSH) (7).
LH and FSH stimulate the physiological changes in the menstrual cycle. As its name suggests, FSH stimulates the follicles in the ovaries to develop and mature, producing a gradual increase in estrogen. Each menstrual cycle, some follicles receive FSH and mature but only one becomes the dominant follicle (aka “The One Follicle to Rule Them All”), which will be released into the fallopian tubes through ovulation to meet up with hypothetical sperm. This process is called folliculogenesis (8), which I will explain using The Bachelor as an analogy (6, 8-10).
The First Rose Ceremony
Out of the many applicants for this season of The Bachelor: Ovary Edition, a subset of contestants (follicles) are selected. These are called the primary follicles. The first rose (FSH) stimulates the follicles to mature and develop granulosa cells, which have FSH-receptors and produce estrogen (11). The more granulosa cells, and therefore FSH-receptors, the more FSH the follicle can gobble up. In the Bachelor analogy, this might look like contestants building a romantic connection with the Bachelor, fueling more and more granulosa cell proliferation.
The more a contestant develops their relationship with the lead, the less likely other contestants will capture the lead’s heart. Therefore, these other contestants do not get a rose and are sent home. In the ovary, this is called follicular atresia, where excess follicles not receiving sufficient FSH will degenerate.
With more FSH, primary follicles develop into secondary follicles. Maybe at this point, the Bachelor contestants develop deeper conversations with the Bachelor. This depth is reflected in our follicles developing theca cells along with the granulosa cells. Briefly, theca cells express LH-receptors which stimulate them to produce androstenedione (a cousin of testosterone), which is then transferred to granulosa cells to make more estrogen.
As we progress through this season of The Bachelor: Ovary Edition, fewer and fewer follicles are left. Tertiary follicles develop more granulosa and theca cells, therefore more hormones (estrogen).
The Final Rose
Systemically, we see a gradual rise in estrogen until the late-follicular phase. Once estrogen hits a certain threshold (like the contestant saying “I’m falling in love with you”), there is a switch in the system. The estrogen stimulates the hypothalamus to produce more GnRH and the anterior pituitary to produce an LH surge (6).
The LH Surge marks the end of the season and it is time for the final rose: ovulation.
During ovulation, the dominant follicle is broken open and the oocyte (egg) is released from the follicle into the fallopian tube.
2. Luteal phase
The Luteal phase of the menstrual cycle is more consistent than the Follicular phase, lasting around 14 days from ovulation until the next period. After ovulation and the release of the oocyte into the fallopian tube, the remaining tissue from the dominant follicle forms the corpus luteum (6). Continuing The Bachelor analogy, the corpus luteum is like the expensive engagement ring and Instagram influencer sponsorship deals… it keeps the ~love~ alive. The corpus luteum produces the hormones, namely progesterone, which is required to renovate the uterus lining to prepare for implantation of a fertilized oocyte (6, 8-10).
Throughout the Follicular phase, the uterine tissue was thickening and there was an increase in cervical mucous to decrease vaginal pH to facilitate sperm entry and survival (6). This is like the construction of The Bachelor couple’s house (The Bachelor meets HGTV analogies). In the Luteal phase, there is a relative decrease in uterine thickening and the tissue gets more blood supply, making it a suitable environment for implantation (6, 8, 10). This is the interior design stage, adding in the custom cabinets, rugs and couches - ready for The Bachelor baby.
However, The Bachelor sponsorships eventually run out and the number of Instagram followers plummet. Without fertilization and implantation of the oocyte, the corpus luteum undergoes luteolysis and degenerates (8). The corpus luteum hormones (namely, progesterone) decrease and the luteal phase comes to an end. It is the end of one epic Bachelor couple, but Bachelor Nation demands another season. The production and release of GnRH, FSH, and LH by the hypothalamus and anterior pituitary rises once again, kickstarting the next menstrual cycle.
As the corpus luteum degenerates and there is a decrease in progesterone production, there is also reduced blood supply to the now-thickened uterine lining (6). The uterine tissue also releases prostaglandins, which stimulate bleeding and smooth muscle contractions… boom, your period (8). This is officially day one of your next menstrual cycle! You may also notice that day one and two of your period (and the new follicular phase) are the heaviest for uterine bleeding. By day three, estrogen starts being produced by the primary follicles, stimulating the regeneration and thickening of the uterine lining, as well as clotting to stop menstruation (8). And voila, we have the next season The Bachelor: Ovary Edition.
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