Secretory phase: progesterone and the endometrium
After ovulation, progesterone from the corpus luteum (the gland left when the egg is released) makes the oestrogen-primed endometrium (the womb lining) thick, with coiled secreting glands: the secretory phase.
Remember
After ovulation, progesterone from the corpus luteum acts on the oestrogen-primed lining and makes it secretory, with coiled glands, and it is a fall in progesterone, not a rise, that brings on the period.
Memory aid
oestrogen is the builder who puts the house up, and progesterone is the person who furnishes it ready for the guest
Use it to hold the order: oestrogen builds the lining first, then progesterone makes it ready for an embryo.
The womb lining is called the endometrium. Its monthly changes are the uterine cycle, which has three phases: menses (the period), the proliferative phase (growth under oestrogen) and the secretory phase.
Before ovulation, oestrogen thickens the lining, and its glands lengthen and begin to coil. After ovulation, the follicle (the fluid-filled sac in the ovary that held the egg) becomes the corpus luteum. This temporary hormone-producing structure releases large amounts of progesterone, which acts on a lining that oestrogen has already primed.
That is the secretory phase. It begins with the endometrium at its maximum thickness, with very pronounced coiled glands (small secreting structures). It runs alongside the luteal phase of the ovarian cycle (the cycle that describes what happens in the ovary). The luteal phase is the time between ovulation and the next period, and it usually lasts about 14 days.
As the phase goes on, the glands swell with secretions. In the late secretory phase they take on a saw-tooth outline under the microscope, so a thick lining with saw-tooth glands shortly before a period points to the late secretory phase.
The secretory lining is ready for implantation, and it is maximally receptive to an embryo in the mid-secretory phase. If there is no pregnancy, the corpus luteum shrinks, progesterone and oestrogen fall, and the functional layer (the upper layer of the lining that thickens each cycle) is shed as the period. The basal layer (the deeper layer that stays behind) then regrows the lining in the next cycle.
Key facts
- Driver
- progesterone from the corpus luteum, acting on a lining that oestrogen has already primed.
- Timing
- the secretory phase follows ovulation and runs alongside the luteal phase, which usually lasts about 14 days.
- Start of the phase
- the endometrium is at its maximum thickness, with very pronounced coiled glands.
- Late appearance
- in the late secretory phase the glands have a characteristic saw-tooth appearance.
- Phase before it
- oestrogen drives the proliferative phase before ovulation, regrowing the lining after menses.
- Sign of ovulation
- very coiled, secreting, saw-toothed glands show that progesterone has acted, which happens after ovulation.
- If no pregnancy
- progesterone and oestrogen fall and the lining is shed as the period.
Exam traps
- Oestrogen is not the main driver of the secretory phase, because it is the main driver of the proliferative phase before ovulation, whereas progesterone dominates the change after ovulation.
- A fall in progesterone does not build the glandular lining, because it happens when the corpus luteum shrinks at the end of the cycle without pregnancy, and it triggers the breakdown and shedding of the lining.
- Menstrual shedding does not follow a rise in progesterone, because it follows a fall when the corpus luteum shrinks, while rising progesterone after ovulation builds and ripens the lining instead.
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