JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Menstruation. Illustration of the uterus during the time of menstruation: at secretory phase. The lining of the uterus (endometrium), seen here, has developed to prepare for pregnancy. It is part of the female menstrual cycle, under control of female hormones. After a proliferative phase in which the endometrium wall (light pink) deepens with the formation of tubular glands (brown) and many blood vessels (blue, red), an egg is released from an ovary. The uterus enters a secretory phase with secretions produced from uterine glands, and if the egg is not fertilized, this uterine layer is shed as the monthly bleed in women (top right). This bleed is known as menstruation.
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