UNDER REVIEW (September 2016)
Action of Mechanism:
Progesterone, a female sex hormone, is secreted by the corpus luteum in the ovary after the rupture of the follicle during ovulation (mid-cycle). It fertilization occurs, the placenta takes over the secretion of progesterone after around 8 weeks, whilst the corpus luteum degenerates. If fertilization does not occur, the corpus luteum undergoes luteolysis after 2 weeks, causing progesterone levels to fall. Progesterone is important for vascularisation and final growth of the endometrium, which increases in size and number of secretory glands. These preparations make the endometrium suitable for implantation by the blastocyst (fertilized egg). If implantation does occur, progesterone is essential for maintenance of pregnancy, preventing spontaneous abortion. Otherwise, progesterone levels fall, causing menstruation (shedding of endometrium). This is because progesterone withdrawal is linked with the entry of leukocytes into endometrium which produces inflammatory mediators eg prostaglandins and cytokines. Endometrial blood vessel constrict, causing reduced oxygen and blood supply (ischaemia) which results in tissue death and endometrium shedding. Progesterone exerts its effects by acting on intracellular receptors and changing gene expression. It is also made from cholesterol in the adrenal cortex, where it is a precursor of testosterone synthesis. Progesterone has a role in milk production by mammary glands. Synthetic progesterones include: progesterone derivatives – dydrogesterone, medroxyprogesterone, testosterone derivatives – norethisterone, norgestrel, desogestrel, norgestimate, gestodene.