Endometrium. Light micrograph of a section through a healthy human endometrium, the mucosal lining of the uterus. This is the menses stage (the period) of menstruation, which occurs from the first to about the 5th day of the menstrual cycle. If, as in this case, a fertilised ovum is not implanted, the endometrium degenerates and is shed. During this degeneration a high infiltration of white blood cells (leucocytes, purple) occurs. Under hormonal influences the endometrium undergoes regular cyclical changes so as to offer a suitable environment for implantation of a fertilised ovum. Magnification unknown.