6.6.3 Annotate a graph showing hormone levels in the menstrual cycle, illustrating the relationship between changes in hormone levels and ovulation, menstruation and thickening of the endometrium We begin with the hormone FSH (Follicle Stimulating hormone). 1) FSH increasing causing the ripening of the Graafian follicles and stimulating estrogen production (an example of positive feedback). 2) Increases in estrogen levels cause the repair of the endometrium and stimulate LH (Luteinizing hormone) production — this is another example of positive feedback. 3) A peak in LH levels cause ovulation (the release of the ovum from the ovary to the oviduct) and the subsequent development of the corpus luteum. The corpus luteum is a temporary endocrine structure formed after the release of the ovum from the graafian follicles. Its role is to secrete progesterone and therefore levels of progesterone increase. 4) Increasing levels of progesterone maintain the endometrium in preparation for fertilization. a. If fertilization does occur the corpus luteum continues to secrete progesterone and the endometrium lining is maintained. b. If fertilization does not occur, the corpus luteum disintegrates, therefore decreasing progesterone levels. This means the endometrium is no longer maintained and so menstruation (the loss of the endometrium) takes place. c. Since there is a negative feedback relationship between progesterone and FSH, decreasing levels of progesterone result in increasing levels of FSH and this brings us back to the beginning of the cycle where we started from.