Uterus (early proliferation). Coloured scanning electron micrograph (SEM) of the endometrium (lining) of the human uterus during the early proliferative stage of the menstrual cycle. The cells of the endometrium are not clearly defined, but their cilia (light green) and secretory microvilli (blue & dark green) can be seen. The uterine lining builds up during early/late proliferation (the first 14 days of the cycle) to prepare for pregnancy. Then an egg is released and the lining becomes secretory to nourish the embryo. If no pregnancy occurs, the endometrium breaks down at menstruation after 28 days. Magnification unknown.