Hormonal Control of the Reproductive Cycle

Hi everyone! Welcome to your study notes on the hormonal control of the reproductive cycle. Don't worry if this topic seems tricky at first – we're going to break it down into simple, manageable parts. Understanding this cycle is super important, not just for exams, but for understanding human health, fertility, and how contraceptives work. Think of it as a fascinating monthly story directed by chemical messengers called hormones. Let's get started!


Meet the Conductors: The Hormones and Their Sources

To understand the menstrual cycle, we first need to meet the four main characters – the hormones! They are produced in two key locations: the pituitary gland (a tiny gland at the base of your brain, like a 'master control centre') and the ovaries.

The Hormone Team:

1. Follicle-Stimulating Hormone (FSH)
Produced in: Pituitary Gland
Main Job: It travels to the ovaries and tells them to start preparing an egg for release. It stimulates the growth of a structure called a follicle, which contains the developing egg.

2. Luteinising Hormone (LH)
Produced in: Pituitary Gland
Main Job: Its big moment is triggering ovulation – the release of the mature egg from the follicle. It also helps the leftover follicle transform into a new structure.

3. Oestrogen
Produced in: The growing follicles in the ovaries.
Main Jobs:

  • Repairs and thickens the lining of the uterus (the endometrium) after menstruation, getting it ready for a potential pregnancy.
  • Controls the pituitary gland hormones (we'll see how soon!).

4. Progesterone
Produced in: The corpus luteum (what's left of the follicle after ovulation).
Main Jobs:

  • Maintains the uterine lining, keeping it thick and rich in blood vessels. Think of it as the 'pregnancy promoting' hormone.
  • Strongly inhibits the pituitary gland from releasing more FSH and LH.

Quick Review Box

From the Brain (Pituitary): FSH & LH (They stimulate the ovaries)
From the Ovaries: Oestrogen & Progesterone (They prepare the uterus and control the pituitary)


The 28-Day Story: The Menstrual Cycle Step-by-Step

The average menstrual cycle is about 28 days. It's not just one event, but two interconnected cycles happening at the same time: what's happening in the ovaries (Ovarian Cycle) and what's happening in the uterus (Uterine Cycle). Let's walk through the timeline.

Part 1: The Follicular Phase (Roughly Days 1-14)

This is the 'building up' phase.

1. Day 1 (Start of Menstruation): The cycle begins with menstruation (the period). This happens because hormone levels from the previous cycle are very low, causing the uterine lining to break down.

2. FSH Kicks In: The low hormone levels signal the pituitary gland to release FSH.

3. Follicle Growth: FSH travels to the ovary and stimulates several follicles to grow. Usually, only one will become the dominant one.

4. Oestrogen Rises: As the follicle grows, it produces more and more oestrogen.

5. Oestrogen's Effects:

  • It tells the uterus lining (endometrium) to rebuild and thicken.
  • It tells the pituitary gland to release less FSH. (This is negative feedback – it stops too many follicles from growing).

Part 2: Ovulation (Around Day 14)

This is the main event!

1. The Oestrogen Peak: When oestrogen levels get really high, something amazing happens. It flips from inhibiting the pituitary to stimulating it!

2. The LH Surge: This high oestrogen level causes the pituitary gland to release a massive amount of LH. This is called the LH surge.

3. Egg is Released: The LH surge is the direct trigger for ovulation. About 24-36 hours after the surge, the mature follicle ruptures, releasing the egg from the ovary.

Part 3: The Luteal Phase (Roughly Days 14-28)

This is the 'waiting' phase.

1. Corpus Luteum Forms: The LH surge also causes the leftover empty follicle to transform into a yellow, hormone-producing structure called the corpus luteum. (Did you know? "Corpus luteum" is Latin for "yellow body"!)

2. Progesterone Rises: The corpus luteum is a progesterone factory! It pumps out high levels of progesterone (and some oestrogen).

3. Progesterone's Effects:

  • It makes the uterine lining even thicker, more stable, and ready for a fertilised egg to implant.
  • It strongly tells the pituitary gland to stop releasing both FSH and LH. (This is strong negative feedback. It's like putting a 'Do Not Disturb' sign up, preventing a new cycle from starting while the body waits to see if pregnancy occurs).

The Fork in the Road: What Happens Next?

Scenario A: No Fertilisation
If the egg isn't fertilised, the corpus luteum starts to break down after about 10-12 days. As it degenerates, levels of progesterone and oestrogen drop sharply. Without these hormones to support it, the uterine lining breaks down, leading to menstruation. The drop in hormones also removes the 'Do Not Disturb' sign, so the pituitary starts releasing FSH again, and a new cycle begins on Day 1.

Scenario B: Fertilisation Occurs
If the egg is fertilised and implants in the uterus, the developing embryo produces a hormone that keeps the corpus luteum alive. The corpus luteum continues to produce progesterone, which maintains the uterine lining and prevents menstruation, supporting the early pregnancy.

Key Takeaway

The menstrual cycle is a carefully timed dance between four hormones. FSH grows the follicle. The follicle makes oestrogen, which builds the uterine lining. A peak in oestrogen causes an LH surge, which triggers ovulation. LH then creates the corpus luteum, which makes progesterone to maintain the lining. If there's no pregnancy, all levels drop, and the cycle restarts.


Visualising the Cycle: Interpreting the Graph

In your exam, you'll almost certainly see a graph showing the hormone levels and uterine lining changes over 28 days. Let's learn how to read it.

Imagine a graph with Days 1-28 along the bottom (x-axis).

  • FSH Level: Starts with a small peak (to grow the follicle), then drops (due to oestrogen), and has another tiny rise around ovulation.
  • LH Level: Stays low for the first half, then has a dramatic, sharp SPIKE around Day 14 (the LH surge!), then falls back to low.
  • Oestrogen Level: Rises steadily during the first half, peaking just before Day 14 (this peak causes the LH surge). It dips after ovulation, then has a smaller rise in the second half.
  • Progesterone Level: Is very low for the whole first half. After ovulation (Day 14), it rises dramatically and stays high until the last few days of the cycle, when it crashes down.
  • Uterine Lining (Endometrium): Starts thin during menstruation (Days 1-5), then thickens steadily until Day 14 (due to oestrogen), and then is maintained at its thickest during the second half (due to progesterone), before breaking down again if hormone levels fall.
Common Mistakes to Avoid

A common mistake is mixing up which hormone causes what. Remember this simple trick:
- The Oestrogen peak CAUSES the LH surge.
- The LH surge CAUSES ovulation.


Practical Applications: Using Our Knowledge

Understanding this hormonal control allows us to develop technologies for family planning and fertility treatment.

Hormonal Contraceptives (e.g., The Birth Control Pill)

How does it work? The pill contains synthetic (man-made) oestrogen and progesterone. A woman takes one pill every day.

The Biological Principle: Taking the pill keeps the levels of oestrogen and progesterone in the blood consistently high. The body is "tricked" into thinking it is in the luteal phase (or even pregnant).

This high level of progesterone constantly sends a strong negative feedback signal to the pituitary gland, inhibiting the secretion of FSH and LH.

  • Without FSH, no follicles develop in the ovaries.
  • Without the LH surge, ovulation does not occur.
No egg released = No chance of fertilisation.

Treating Infertility

The Problem: Some women may have difficulty becoming pregnant because their bodies do not produce enough FSH and LH, so ovulation doesn't happen regularly.

The Solution: Doctors can prescribe fertility drugs which contain hormones like FSH and LH.

The Biological Principle: These injected hormones directly stimulate the ovaries. The FSH helps one or more follicles to mature, and the LH can be used to trigger ovulation at the right time. This increases the chance of releasing a mature egg for fertilisation.

A possible side effect is that the drugs might overstimulate the ovaries, leading to the release of multiple eggs at once. This increases the chances of multiple births (e.g., twins or triplets).

Key Takeaway

We can manipulate the reproductive cycle! - To prevent pregnancy (contraception), we use progesterone/oestrogen to inhibit FSH/LH and stop ovulation. - To promote pregnancy (infertility treatment), we use FSH/LH to stimulate follicle growth and ovulation.