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The Female Reproductive Axis: Coordination of the Brain, Ovaries & Uterus

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  • 0:05 Reproductive Cycle…
  • 2:35 Days 1-14
  • 6:07 Days 15-28
  • 8:32 As One Cycle Ends, the…
  • 9:26 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

Without proper communication between the brain, ovaries and uterus, reproduction might not be as efficient. Learn about how these organs work together to ensure the body is ready for reproduction.

Reproductive Cycle Communication

Communication: It's the key to everything. From the complex, like your relationships at home or work, to the simple, like ordering a meal at a restaurant. It can be as complex as explaining a biological process or as simple as just telling your dog to sit. But whether complex or simple, you need some way to communicate information to the other party. And just like we humans need communication to thrive in the world around us, our body needs a way to communicate in the world within us.

One way it does this is through hormones, chemical messengers that the body uses to relay signals from one location to another. One of the systems that uses hormones the most is the reproductive system. In females, this ability for one structure to communicate to another is key in regulating the reproductive cycles of the ovaries and the uterus. In addition to the ovaries and the uterus working together, we can't forget about the head honcho of the group, the one in charge of it all: the brain. Specifically, the areas of the brain known as the hypothalamus and the pituitary.

The ovaries, uterus, hypothalamus and pituitary use hormones to communicate with one another
Reproductive Cycle Communication

Each of these structures needs to be able to communicate to the others so that they can coordinate all of the events leading up to reproduction. Think about it this way: the job of the ovaries is to recruit and mature an oocyte each month, preparing it for fertilization. The job of the uterus is to prepare a welcoming environment to nourish and protect the fertilized oocyte as it develops into a baby. So, what would happen if the ovary released an oocyte all ready for fertilization, but after fertilization when it got to the uterus, there was nowhere for it to develop? Or what if the brain sent signals down to the ovary telling it to release an oocyte to be fertilized, but the ovary hadn't finished the maturation process and released an immature egg not ready for fertilization? The reproductive process wouldn't be very successful then, would it? That's why the ovaries need a way to tell both the brain and the uterus when they are ready, and in turn, the brain and the uterus need to communicate to the ovaries. All of this is done through the use of hormones.

Days 1-14

Let's take a closer look at how this happens. Every month, the female's body goes through the reproductive cycle. This cycle involves two parts:

  1. The ovarian cycle, which takes place in the ovaries. It recruits an oocyte each month, triggering growth and maturation in preparation for fertilization.
  2. The uterine cycle prepares the uterus to nourish and protect the fertilized oocyte should pregnancy occur.

Both events occur concurrently - that means they occur at the same time - and that coordination is under the control of the brain. The hypothalamus and the pituitary parts of the brain are kind of like the control centers for the reproductive system. They help integrate information and coordinate events so that the ovarian and uterine cycles work together.

The uterine and ovarian cycles occur simultaneously under control of the brain
Uterine and Ovarian Cycles Occur Simultaneously

To help you better understand, let's break down the monthly cycle into days. On average, these cycles take about 28 days in human females. During the first half of the cycle, the hypothalamus triggers the pituitary to release low levels of the hormones FSH and LH. FSH travels to the ovaries, where it helps control the growth and maturation of the developing oocyte. This part of the cycle takes place during the first 14 days and is called the follicular phase of the ovarian cycle.

During the first week, while the developing oocyte is still in the early stages of development, the uterus is undergoing changes of its own. The first five to seven days of the uterine cycle is when the uterus sheds its old tissue from the previous month. This occurs in the relative absence of hormones from the ovaries. Just as this process is being completed, the follicle cells surrounding the developing oocyte in the ovary start producing low levels of the hormone estrogen. At these lower levels, estrogen travels to the uterus, where it promotes the initial rebuilding of uterine tissue during the proliferative phase of the uterine cycle. This includes cell replication and the beginning stages of uterine gland growth and vascularization (that's just a fancy way of saying the growth of arteries and veins).

Both growth of the oocyte and growth of the uterine tissue continue as the cycle progresses. But around the midpoint of the cycle, things begin to change. You see, as the oocyte reaches its final stages of maturation, the estrogen levels begin to increase. High levels of estrogen then travel back to the hypothalamus, where they trigger a surge in the hormone GnRH. This surge in GnRH triggers a subsequent surge in FSH and an even larger surge in LH. It is this surge in LH that travels all the way down to the ovary and triggers the process called ovulation. Ovulation occurs around day 14 and is considered to be the end of the follicular phase. It causes the follicle cells surrounding the oocyte to rupture, releasing the oocyte from the ovary. At the same time, the uterine cycle transitions from the proliferative phase to the secretory phase.

Ovulation occurs around day 14 and the oocyte is released from the ovary
Ovulation

Days 15-28

Both the ovary and the uterus are now in the second half of their cycles, days 15-28. Once ovulated, the oocyte begins to make its way down the female's uterine tubes, while the follicle it left behind breaks down into a structure called the corpus luteum. During this time, the uterine tissue continues to rebuild under the influence of hormones secreted by the corpus luteum. The corpus luteum secretes high levels of progesterone and median levels of estrogen. Progesterone travels back to the hypothalamus, where it blocks, or inhibits, the release of GnRH. This is important because we don't want the ovary to recruit another oocyte until the current oocyte's fate has been determined. Additionally, both progesterone and estrogen travel to the uterus, where they help complete the rebuilding of the uterine tissue during the secretory phase.

During the secretory phase, ovarian hormones:

  • promote the thickening of the uterine tissue
  • promote uterine gland growth and increases in secretions produced by the glands
  • promote increased vascularization of the uterine tissue

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