Uterine lining. Coloured scanning electron micrograph (SEM) of the lining of a uterus. This is the endometrium, a highly-folded mucous membrane that is covered with ciliated epithelial cells (not clearly seen). The endometrium undergoes cyclical changes under the influence of hormones secreted by the ovary, in order to offer a suitable environment for the implantation of a fertilised egg. Towards the later stages of the menstrual cycle it becomes thicker, with an increased blood supply. If no egg is implanted in the uterus, the endometrium is shed during menstruation.