2. Hormonal Regulation 1.On day 1, rising GnRH levels (released from the hypothalamus) cause increased synthesis and release of FSH and LH from the anterior pituitary gland. 2.FSH and LH stimulate follicle growth and maturation and estrogen secretion. 3.Rising plasma estrogen level feeds back on the anterior pituitary inhibiting the release of LH and FSH but at the same time stimulating it to synthesize and accumulate these gonadotropins. HypothalamusGnRH Anterior Pituitary FSH LH Follicles Estrogen

4. Uterine Cycle – Menstrual Phase A series of cyclic changes that the uterine endometrium goes thru each month as it responds to changing plasma levels of ovarian hormones. Days 1-5 is the menstrual phase. In this stage, the uterus sheds all but the deepest portion of the endometrium. Stratum functionalis is detached from stratum basalis & sloughed off thru the vagina. On day 1, FSH levels are just starting to rise. On day 5, estrogen production is beginning.

5. Uterine Cycle – Proliferative Phase Days 6-14 is the proliferative phase. Under the influence of the rising estrogen levels, the stratum basalis generates a new stratum functionalis. As the new layer thickens, its glands enlarge and its spiral arteries increase in number.

6. Uterine Cycle – Secretory Phase Progesterone from the corpus luteum acts on the endometrium causing the spiral arteries to elaborate and coil more tightly and converts the stratum functionalis to a secretory mucosa. The uterine glands enlarge, coil, and begin to secrete nutritious glycoproteins into the uterine cavity. This is also stimulated by progesterone. What’s the purpose of these changes?

7. Uterine Cycle – Secretory Phase When progesterone levels fall (i.e., if implantation does not occur), spiral arteries kink and spasm. This denies the endometrial cells O 2 and nutrients causing them to die. Spiral arteries constrict one last time and then suddenly relax and open wide. Blood rushes into the capillary beds and bursts them causing them to slough off and the uterine cycle is back to day 1.