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HPG Axis: Hormonal Control of the Ovarian Cycle

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  • 0:05 Female HPG Axis
  • 3:33 GnRH Patterns
  • 5:04 FSH
  • 7:10 LH
  • 8:53 HPG Signaling
  • 11:31 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.

Everything in the female reproductive system is connected. See how the brain interacts with the ovaries during the female's ovarian cycle in this lesson covering the female HPG axis.

Female HPG Axis

Here we are, girls - the age of puberty! Okay, now everybody get in line. Who's first? Are you ready to leave your childhood behind and enter the world of womanhood? It may be a bit awkward at first, but don't worry, you'll get used to it. Now remember, don't squeeze your pimples; you'll just leave scars. Don't wear too much makeup, and don't forget about PMS - try not to let the mood changes get the best of you!

Do you ever wonder where all those changes you go through in puberty come from? Well, you see, you have this pathway in your body called the HPG axis. That stands for hypothalamic-pituitary-gonadal axis. It starts in your brain, where the hypothalamus and the pituitary are located, and it allows your brain to communicate with your ovaries using molecules called hormones.

The HPG axis uses hormones to communicate with the ovaries
Female HPG Axis

Hormones are like messengers. They carry signals or instructions from one structure to the next via the body's bloodstream. And they usually work with other hormones in the body to pass along those instructions. And puberty, well, that's the time of life when the HPG axis gets activated. You see, before that, the pathway is there, but it hasn't really been turned on yet. Once you hit puberty, though, your HPG axis is now active and ready to go.

Go where, you ask? Well, it helps propel you from the realm of childhood through puberty and into womanhood! Basically, it prepares your body for reproduction. Without an HPG axis, you wouldn't be able to produce gametes, or eggs, that are necessary for reproduction.

So, how does it all work? Well, let's start at the top, the head honcho of the group - that's the brain! Within your brain, you have an area called the hypothalamus. It's an area of your brain important for many things, including regulating reproduction. The hypothalamus is located at the bottom, middle portion of your brain, directly above another structure known as the pituitary. The pituitary is another portion of the brain important in hormone regulation and numerous signal pathways within the body.

The two main functions of the hypothalamus (with regards to reproduction) both involve the same neurohormone (that's just another way of saying a hormone that's made in the brain). That hormone is called gonadotropin-releasing hormone, or GnRH for short. GnRH from the hypothalamus is responsible for both the initiation of puberty and for the regulation of hormones involved in female reproduction.

Now, remember how we said hormones don't usually work alone? Well, GnRH is just the first hormone in a series in the HPG pathway. You see, GnRH is made in neurons in the hypothalamus, and once produced, GnRH is released into a series of capillaries in the brain called the hypophyseal-portal system. This is a network of blood vessels that connects the hypothalamus to the anterior pituitary, allowing them to communicate with each other.

GnRH Patterns

Once GnRH gets to the pituitary, it diffuses out of the blood and into endocrine cells located in the anterior pituitary. The anterior pituitary is the front half of the pituitary, and it contains a number of endocrine cells. Endocrine cells secrete hormones into the body's bloodstream. These hormones can travel all over the body, but we're just going to focus on those that travel to the ovaries.

GnRH communicates with endocrine cells that release hormones called gonadotropins. Okay, let's take a moment to think back to what GnRH stands for: gonadotropin-releasing hormone, right? The function of GnRH is right there in its name. It releases gonadotropin hormones from the anterior pituitary.

GnRH releases gonadotropins from the anterior pituitary
Gonadotropins

So, now that we understand what GnRH does, let's look at how. Importantly, GnRH levels fluctuate as the female progresses through her monthly ovarian and menstrual cycles. That's because the gonadotropins - called LH and FSH - are only needed during certain parts of the ovarian cycle (that's the cycle that ovaries go through). So, as GnRH levels change, so do the levels of gonadotropins that it triggers. When GnRH is high, then gonadotropin release is high, and when GnRH is low, gonadotropin release is also low.

FSH

So, what are these gonadotropins I keep talking about? There are two of them, actually - luteinizing hormone (or LH for short) and follicle-stimulating hormone (or FSH for short). Each of these hormones has a specific role in female reproduction.

When GnRH diffuses out of the capillary system and into the anterior pituitary, it causes both LH and FSH to be released into the body's bloodstream. Both of these hormones travel down to the ovaries, where they control different parts of the ovarian cycle. The ovarian cycle is just a series of changes in the ovaries that occur on a monthly cycle basis. This cycle is split into two halves, the follicular phase and the luteal phase, with ovulation (which is the release of a mature egg) being right in the middle of the two.

Let's start with FSH first. FSH has a couple of functions:

  1. FSH is important in the follicular phase of the ovarian cycle, where it stimulates the growth and maturation of follicles in the ovary. A follicle is a group of cells surrounding an immature egg.
  2. FSH also stimulates the production of the hormone inhibin, whose job is to regulate FSH production.
  3. FSH indirectly stimulates the production of the steroid hormone estrogen.

Estrogen is released by cells in the developing follicle and has many functions, some of which include:

  • Aiding in egg development and maturity
  • Stimulating breast development
  • Regulating female sexual behavior
  • Regulating signaling in the HPG axis

Now, before we can move on to the next step in our HPG pathway, we have to talk about our other gonadotropin. So, file this FSH information away in the back of your mind while we talk a little bit about LH. Then we'll see how everything fits together.

Ovulation occurs between the follicular and luteal phases in the ovarian cycle
Ovarian Cycle

LH

LH is also released from the anterior pituitary in response to GnRH, just like FSH is. However, LH isn't needed until the last stages of the follicular phase of the ovarian cycle. For that reason, most of the LH release is inhibited by low levels of estrogen until the egg has finished maturing.

Once the egg has reached maturity, it produces high levels of estrogen. These high levels now travel up to the hypothalamus, where they stimulate a surge of GnRH. GnRH then causes LH to be released from the pituitary, allowing it to travel to the ovaries, where it triggers ovulation (or release of the mature egg from the ovary).

Here are a few functions of LH:

  1. LH triggers the completion of meiosis 1 in the developing oocyte, allowing it to reach maturity.
  2. A surge in LH at the end of the follicular phase triggers ovulation, the rupturing of the follicle wall allowing the mature egg to be released from the ovary.
  3. Following ovulation, LH aids in the formation of the corpus luteum, a structure that forms from the now-empty follicle and secretes the hormone progesterone.

Progesterone is what helps prepare the uterus for pregnancy. It also feeds back to the hypothalamus to stop, or block, the production of GnRH and GnRH release.

Okay, so now that we have the functions of each gonadotropin and the hormones that they release down, let's look at how all these different hormones interact.

HPG Signaling

Let's start back at the beginning. First up, GnRH from the hypothalamus travels down to the anterior pituitary. This is where it triggers the release of both LH and FSH from the pituitary. Okay, that's the easy part. The tricky part is what comes next.

See, FSH causes the release of two hormones: estrogen and inhibin. Each of these hormones has a job in our pathway as well. Estrogen is released when FSH stimulates follicle development, and inhibin is released a little later on when FSH is no longer needed.

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