FIGURE 2.

The human menstrual cycle and rat estrous cycle. The human menstrual cycle (left) occurs over 28 days and is comprised of fluctuating levels of E2 (E2); (top) and progesterone (P4; bottom). Levels of E2 rise to a peak during the peri-ovulatory phase. E2 levels then drop, rise, and plateau between during the mid-luteal phase (approximately Days 14–28). Progesterone (P4) levels begin to at the end of the ovulation (Days 0–14) and reach their peak during the mid-luteal phase of the cycle (approximately Day 7). The rat estrous cycle (right) is similar to the human menstrual cycle but occurs over a 4/5-day period. Four phases; metestrus, diestrus, proestrus, and estrus comprise the rat estrous cycle. Estrogen levels (top) peak toward the end of diestrus and beginning of proestrus. Progesterone levels (bottom) reach a peak during mid-proestrus.