Uterus at oestrus. Coloured Scanning Electron Micrograph (SEM) of the surface of the uterine endometrium at oestrus. The endometrium is the inner layer of the uterus which undergoes changes during the menstrual cycle. Here, at oestrus (time of ovulation or egg release) the endometrium has built up into a thick glandular layer. It has a honeycomb appearance with numerous dark glandular openings. After ovulation the glands become active and secrete substances such as glycogen, mucus and lipids which provide nutrients for a fertilised egg. The honeycomb is made of a collagen fibrillar network. Magnification: x100 at 6x7cm size. Magnification: x130 at 4x5 inch size.