Uterine lining. Light micrograph of a section through human uterine lining (endometrium) during the proliferative stage of the menstrual cycle. The endometrium thickens during this stage, which lasts for 14 days, in preparation of embryo implantation. Tubular glands (purple with white centres) in the endometrium secrete mucus during this stage to moisten the uterus. After an egg (ovum) is released (ovulation), the glands start to release glycogen to nourish the embryo, if implantation occurs. If implantation does not occur, the endometrium breaks down and menstruation occurs. Mag: x23 when 10 cm.