DAVID PARKER / SCIENCE PHOTO LIBRARY DAVID PARKER / SCIENCE PHOTO LIBRARY
Polarised light micrograph of crystals of progesterone, a female sex hormone which is secreted monthly in the ovary and which stimulates development of the uterus to receive a fertilised egg and to nourish the embryo. In pregnancy, progesterone prevents menstruation and the release of more eggs; this characteristic of progesterone is the basis of the contraceptive pill: Magnification x40 at 35 mm.
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