Endometrium

The endometrium consists of a simple columnar surface epithelium (secretory and ciliated cells) that dips down as simple tubular uterine glands into a highly cellular connective tissue stroma. It has two layers. The stratum functionalis (superficial ≈two-thirds) proliferates, transforms and is shed each cycle; it is supplied by coiled spiral arteries. The stratum basalis (deep layer, adjoining the myometrium) persists through menstruation and contains the gland bases and stem cells that regenerate the functionalis; it is supplied by short straight (basal) arteries.

In the proliferative phase (estrogen), glands are straight and narrow, mitoses are frequent and the lining thickens. After ovulation (secretory phase, progesterone), glands become coiled and saw-toothed and fill with glycogen-rich secretion (subnuclear vacuoles appear ≈2 days after ovulation — the earliest histological evidence of ovulation), the stroma becomes oedematous, spiral arteries lengthen and coil, and stromal cells begin decidualisation. The window of implantation opens ≈6–10 days after the LH surge. If progesterone falls, spiral arteries constrict intermittently, the functionalis becomes ischaemic, and prostaglandins and matrix metalloproteinases cause its shedding (menstruation).

In pregnancy the endometrium becomes the decidua: stromal cells enlarge with glycogen and lipid, regulate trophoblast invasion and immune tolerance. The decidua basalis lies beneath the implantation site and forms the maternal part of the placenta; the decidua capsularis covers the conceptus; the decidua parietalis lines the rest of the cavity.