Progesterone hormone. Polarised light micrograph of crystals of the female sex steroid hormone progesterone. Progesterone is secreted by the ovary during the menstrual cycle. This hormone causes the lining (endometrium) of the uterus to become thicker and richly supplied with blood in preparation for pregnancy. If conception occurs, the ovaries continue to produce progesterone for four months, after which it is secreted by the placenta. It is derived from pregnenolone, which is itself made by the action of an enzyme on cholesterol. Magnification: x50 at 4x5 inch size.

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