1. FSH: Follicle-stimulating hormone; and LH: luteinizing hormone Organ of orgin: Gonadotropin that is secreted from the anterior pituitary after receiving signal from the hypothalamus (GnRH – gonadotropin-releasing hormone) Target organ: ovaries (gonads) (testes in males) Effect: regulate gametogenesis directly by targeting gonad tissue, and also indirectly via regulation of sex hormones

Estrogen: Organ of Origin: Ovary – secreted by growing follicle in increasing amounts until immediately before ovulation Target Organ: Hypothalamus, blood Effect: Regulate LH and FSH (when estrogen low, LH and FSH not secreted) Progesterone: Organ of Origin: corpus luteum Target organ: Uterus – build up lining, and negative feedback to prevent LH and FSH secretion Effect: prevent further egg maturation, prepare uterus for implantation

2. Estrogen levels Estrogen levels are the highest immediately before ovulation – due to the follicle releasing estrogen. The high levels of estrogen cause LH and FSH to be released, which finalizes follicular maturation, and ovulation occurs. This marks the end of the follicular phase, and the luteal phase begins. The estrogen levels are high as the Corpus Luteum develops, secreting estrogen and progesterone. The combination of estrogen and progesterone exhibits negative feedback on the hypothalamus, preventing LH and FSH secretion, thus preventing ovulation of a second egg.

3. LH Levels LH levels are highest immediately before ovulation in the follicular phase. The developing follicle has slowly released estrogen, and the high levels of estrogen invoked positive feedback on the hypothalamus to release GnRH, which signaled the anterior pituitary to release FSH and LH. The high levels of LH cause maturation to finalize, and then a day after the peak, the high LH signals ovulation – releasing the secondary oocyte from the ovary. The LH peak then triggers the remaining follicular tissue to develop into corpus luteum.

4. Estrogen – endometrium As estrogen level increases, the endometrium starts to redevelop (having immediately followed menstruation). The combined effect of high progesterone and high estradiol promote further thickening of endometrium, thus preparing the uterus for implantation.

5. Progesterone – Endometrium Progesteron is produced by the Corpus luteum, and its function (in the luteal/post-ovulatory phase) is to exert netagive feedback to prevent the secretion of LH and FSH, and also to stimulate development and maintenance of the uterine lining – causing arteries to enlarge and endometrial glands to grow. Thus, the endometrium thickens and develops greatly with the presence of progesterone.

6. Events are triggered by the peak in LH because the peak in LH causes ovulation (release of secondary oocyte (egg) from one ovary) to occur. The ovulation leaves follicular tissue behind in the ovary – and LH stimulates the follicular tissue, which becomes a mass of secretory cells releasing estrogen and progesterone, which means the endometrium will develop.

FSH is Follicle-Stimulating hormone, which means it promotes development and maturation of the egg cell and peaks near ovulation. FSH falls greatly after ovulation because there is possibility of fertilization, which means a fertilized egg would implant in the uterine lining. It would be bad for another egg to mature if pregnancy is already under way – would mess with the hormone cycle. The great drop in FSH levels prevents a second egg from maturing.

The combined effect of estrogen and progesterone inhibit the release of GnRH by the hypothalamus which inhibits the release of LH and FSH; the combined sex hormones also building up the endometrium. It makes sense for the levels to be low during menstruation, because during this time the endometrium is disintegrating and the uterus is contracting. It would be counterproductive to build-up the endometrium lining at the same time it is shedding.

Feedback Loop 1: low levels of Estrogen in Follicular phase causes negative feedback on anterior pituitary, which keeps LH and FSH levels low Feedback loop 2: Increased levels of Estrogen released by developing follicle causes positive feedback on the hypothalamus to release GnRH so the anterior pituitary release LH and FSH – which the surge will then cause ovulation Feedback loop 3: The combined effect of high estrogen and high progesterone (released by the corpus luteum) causes negative feedback on the hypothalamus, preventing the release of GnRH, thus preventing the release of LH and FSH.

10. Compare and Contrast Most animals exhibit a seasonal increase in reproductivity – they reproduce only when energy sources and environmental conditions are favorable Late fall is common Typically only 1x/year Similar – controlled by hormones – shed of uterine lining if pregnancy doesn’t occur