Proliferative under oestrogen, secretory under progesterone
Expansion
Two layers: the functional layer, which is shed each month, and the basal layer, which is retained and regenerates it.
Phases
- Menstrual, days 1 to 4: shedding of the functional layer as progesterone falls and the spiral arteries constrict
- Proliferative, days 5 to 14, driven by oestrogen: the epithelium regenerates from the basal layer, glands are straight and narrow, stroma is compact, and mitoses are frequent
- Secretory, days 15 to 28, driven by progesterone: glands become tortuous and saw-toothed with subnuclear vacuoles then luminal secretion, the stroma becomes oedematous and decidualised, and spiral arteries coil
“Straight glands mean oestrogen; coiled secretory glands mean progesterone, and therefore ovulation.” That is why an endometrial biopsy can confirm ovulation.
Unopposed oestrogen, from anovulation, obesity, polycystic ovary syndrome, oestrogen only HRT or a granulosa cell tumour, produces continued proliferation without secretory change, leading to endometrial hyperplasia and then carcinoma. This is precisely why a progestogen is added to HRT in a woman with a uterus.
Implantation requires a receptive secretory endometrium; if it occurs, hCG rescues the corpus luteum and progesterone is maintained.