Oestrogen positive feedback and the LH surge
Late in the first half of the cycle, sustained high oestrogen switches to positive feedback, producing the LH surge (a sharp rise in luteinising hormone) that triggers ovulation.
Remember
Sustained high oestrogen flips negative feedback to positive feedback, and the LH surge that follows is what triggers ovulation. A rise in progesterone or a fall in oestrogen is not the trigger.
Memory aid
low to moderate oestrogen is a brake, sustained high oestrogen is an accelerator
Think of oestrogen as a brake while it is low to moderate, and as an accelerator once it is sustained at a high level.
In the follicular phase (the first half of the cycle), a growing follicle, the fluid-filled sac that holds an egg, makes steadily more oestrogen. In the middle of this phase, oestrogen mainly acts as negative feedback, a brake on the hypothalamus and the pituitary gland (the hormone-releasing gland under the brain). While this brake is on, LH stays low.
Once oestrogen has stayed high for long enough, the feedback switches to positive feedback, an accelerator. The LH-producing cells of the pituitary become more sensitive to each pulse of GnRH (gonadotrophin-releasing hormone, the signal from the hypothalamus that tells the pituitary to release LH and follicle-stimulating hormone, FSH), so more LH is produced per GnRH pulse.
GnRH has to arrive in pulses, because pituitary receptors that are constantly exposed to it become desensitised (they stop responding). Continuous GnRH would suppress LH release rather than increase it. In the follicular phase GnRH stays pulsatile, and it is the pituitary's sensitivity to each pulse that increases. Oestrogen also acts on the brain to boost GnRH release, so the surge is a joint effect.
The result is the LH surge, a sharp rise in luteinising hormone, with a smaller surge of follicle-stimulating hormone (FSH). The LH surge prompts ovulation, when the mature egg is released from the ovary. After ovulation the empty follicle becomes the corpus luteum (the structure left in the ovary after the egg is released), and its progesterone suppresses LH again by negative feedback.
Key facts
- First days of the cycle
- Ovarian hormones are very low, so there is little negative feedback and FSH rises, helping the follicle to grow.
- Mid-follicular phase
- Rising oestrogen mainly gives negative feedback on the hypothalamus and the pituitary gland.
- Late follicular phase
- Sustained high oestrogen switches to positive feedback, giving a marked LH surge with a smaller FSH surge.
- Why the surge happens
- At a high oestrogen concentration the pituitary releases more LH per GnRH pulse, and oestrogen also acts on the brain to boost GnRH release, so the surge is a joint effect.
- Immediate trigger
- The LH surge breaks down the follicle wall and releases the egg.
- Timing
- In a classical 28-day cycle ovulation comes at about day 14, but cycle length varies between people.
Exam traps
- A fall in oestrogen does not trigger ovulation, because the fall comes around ovulation, after the LH surge has begun.
- Continuous GnRH would suppress LH release rather than increase it, because continuous exposure desensitises the pituitary receptors.
- The feedback switch is right, but the hormone is wrong: high oestrogen causes a marked LH surge with only a smaller FSH rise, and it is the LH surge that breaks down the follicle wall.
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