Uterine wall. Light micrograph of a section through the wall of the uterus, showing the lumen (white) & the upper layers of the endometrium (the mucosal lining of the uterus). The endometrium consists of a simple columnar epithelium (red outer layer), supported by a thick layer of connective tissue (blue). During the menstrual cycle, the endometrium becomes progressively thicker, more glandular & more vascular, in preparation for the implantation of an embryo. If this does not occur, much of the endometrium breaks down and is lost during menstruation. Magnification: x150 when printed at 10 centimetres wide