Endometrial Cycle and Menstrual Physiology
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Direct answer
Endometrial cycle and menstruation are the monthly changes in the uterine lining driven by ovarian oestradiol and progesterone, with the proliferative phase building the endometrium and the secretory phase preparing it for implantation. Menstruation follows when the corpus luteum regresses, progesterone and oestradiol fall, and spiral arteriole vasospasm and prostaglandin release cause ischaemic breakdown of the functional layer. The basal layer survives the shedding and regenerates the lining within about four to five days.
What you must remember
- Menstrual phase: days one to four or five, when the functional layer is shed after hormonal withdrawal; typical blood loss is around 30 to 40 mL.
- Proliferative phase: days five to 14, dominated by oestradiol, with stromal mitosis and straight glands that lengthen as the lining thickens.
- Ovulation: around day 14 in a 28-day cycle, marking the transition between the two phases.
- Early secretory phase: days 15 to 18, with progesterone causing subnuclear vacuolation of glandular epithelium.
- Mid to late secretory phase: days 20 to 28, with glycogen-rich glands, stromal oedema and predecidualisation, followed by coiling of spiral arterioles.
- Withdrawal trigger: falling progesterone and oestradiol after corpus luteum regression lead to spiral artery vasospasm, prostaglandin F2 alpha release and ischaemic necrosis of the functional layer.
- Regeneration: the basalis survives and the surface epithelium regenerates the lining within about four to five days of bleeding.
A day-by-day reading of a biopsy
A 34-year-old woman undergoes an endometrial biopsy on day 26 of her 28-day cycle to assess for infertility. The pathologist sees tortuous, saw-toothed glands with intraluminal secretion, and an oedematous stroma with predecidual change.
- Read the glands. Tortuous glands with intraluminal secretion place the sample in the secretory phase rather than the proliferative phase.
- Read the stroma. The predecidual stroma places it in the late secretory phase, consistent with the day 26 sampling.
- Judge ovulation. Fully mature secretory change in this sample suggests the cycle is ovulatory, because the progesterone of a corpus luteum produced the changes.
- Consider the alternative. If the same biopsy had shown straight glands and a dense mitotic stroma, the reasoning would have pointed to the proliferative phase, which in turn suggests an anovulatory cycle or inappropriate timing of sampling.
The biopsy is therefore useful not only for the day of the cycle but also for judging whether ovulation has happened, which is the practical link between histology and physiology.
Where the exam tries to catch you
Endometrial questions test timing, mechanism and which layer survives.
- Trap: assuming the secretory phase starts on day 15 in every woman. Correct: the day of ovulation varies, so the secretory phase length is more stable than the proliferative phase.
- Trap: blaming absolute tissue loss for menstrual bleeding. Correct: the fall in progesterone is the trigger, and bleeding is produced by spiral arteriole vasospasm and prostaglandin release, not by an absolute loss of tissue.
- Trap: expecting the functional layer to survive. Correct: the basalis survives, and the endometrium is thin at menses because the functional layer is shed.
- Trap: confusing typical blood loss with heavy menstrual bleeding. Correct: typical loss is 30 to 40 mL, and the threshold for heavy menstrual bleeding is set well above that value.
Frequently asked questions
What causes menstruation?
Menstruation is caused by withdrawal of progesterone and oestradiol after the corpus luteum regresses. The falling hormones trigger spiral arteriole vasospasm, prostaglandin F2 alpha release and ischaemic breakdown of the functional endometrial layer.
Which layer of the endometrium survives menstruation?
The basalis survives menstruation and regenerates the functional layer within about four to five days. The functional layer is the part that is shed during the menstrual phase.
What histological features mark the secretory phase?
The secretory phase shows subnuclear vacuolation of glandular epithelium in the early part, glycogen-rich tortuous glands in the middle, and stromal oedema with predecidualisation in the late part. These features are driven by progesterone from the corpus luteum.
How much blood is normally lost during menstruation?
Typical menstrual blood loss is around 30 to 40 mL per cycle. Losses above roughly 80 mL per cycle are classed as heavy menstrual bleeding and need evaluation.