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Teresa Woodruff at Northwestern University in Chicago, Illinois, and her colleagues designed cell-culture systems that use microfluidic pumps to move fluid between cultured tissues (pictured), mimicking normal hormonal fluctuations. In one system, the researchers grew mouse ovarian tissue, including the fluid-filled sacs, or follicles, that release eggs. By varying the levels of two key hormones according to patterns seen in the human menstrual cycle, the team replicated normal follicle development and ovulation. In response to the hormones, the cultured tissue also released molecules such as oestrogen and progesterone, in line with a 28-day cycle.