• The cycle commences with menses, during which the superficial portion of the endometrium, referred to as the functionalis, is shed. • The proliferative phase is marked by rapid growth of glands and stroma arising from the deeper portion of the endometrium (basalis). During the proliferative phase, the glands are straight, tubular structures lined by regular, tall, pseudostratified columnar cells. Mitotic figures are numerous, and there is no evidence of mucus secretion or vacuolation. The endometrial stroma is composed of spindle cells with scant cytoplasm that are also actively proliferating (see Fig. 22.19A). • At ovulation, endometrial proliferation ceases and dif- ferentiation commences in response to the effects of progesterone made by the corpus luteum in the ovary. • Postovulation is initially marked by the appearance of secretory vacuoles beneath the nuclei in the glandular epithelium (see Fig. 22.19B). Secretory activity is most prominent during the third week of the menstrual cycle, when the basal vacuoles progressively move to the apical surface. By the fourth week, the glands are tortuous, producing a serrated appearance. This serrated or “sawtooth” appearance is accentuated by secretory exhaustion and shrinkage of the glands. • Stromal changes in the late secretory phase, due predomi- nantly to progesterone, are the most significant features. Prominent spiral arterioles appear accompanied by an