Uterus during menstruation. Coloured scanning electron micrograph (SEM) of the lining of the uterus being shed during menstruation. The upper layers of the epithelium (yellow) are being shed and the breakdown of the underlying blood vessels releases red blood cells (red dots). The sub mucosa is purple. Menstruation occurs for a few days during a woman's menstrual cycle. Prior to menstruation, the uterine lining (endometrium) thickens to prepare it for the reception of a fertilised egg. If a fertilised egg enters the uterus, it implants in the wall and develops into an embryo. If the released egg is not fertilised, the thickened wall is shed. Magnification: x500 when printed at 10 centimetres wide.