31.24 : Proliferative Phase

The proliferative phase is the second phase of the uterine menstrual cycle. It begins after menstruation and lasts approximately 6 to 13 days in a typical 28-day cycle.

During this phase, the uterine endometrium rebuilds itself under the influence of estrogen released by the growing follicles in the ovaries.

The basal layer of the endometrium, known as the stratum basale, including the basal parts of the uterine glands, survives menstruation intact.

The cells in the stratum basale rapidly divide, creating a new stratum functionalis layer.

As the endometrium thickens, endometrial glands develop, and arterioles coil and lengthen, penetrating the newly formed stratum functionalis.

As a result, the endometrium becomes velvety, thick, and well-vascularized, roughly doubling in thickness to about 4–10 mm.

In the meantime, events in the ovaries result in the maturation of the dominant follicle into a Graafian follicle.

As a result, the proliferative phase concludes with ovulation, triggered by a surge in luteinizing hormone levels released by the anterior pituitary gland.