Uterus (mid-secretory stage). Coloured scanning electron micrograph (SEM) of the surface lining or endometrium of the human uterus during the mid- secretory stage of the menstrual cycle. The lining builds up (proliferation) in the first 14 days of the cycle to prepare for pregnancy, an egg is released, and then the lining becomes secretory to nourish the embryo, If no pregnancy occurs, it breaks down at menstruation after 28 days. Here, in mid-secretory stage about day 22, medium- sized secretory cells are seen with tiny microvilli projections; secretory cells enlarge through the secretory stage. A ciliated cell lies centerfield. Magnification: x3000 when printed at 10 centimetres wide.