The menstrual cycle can be divided into the follicular phase, ovulation and the luteal phase. Negative feedback and in some cases positive feedback play important roles in coordination of the cycle.
~ THE FOLLICULAR PHASE
The follicular phase begins with menstruation, the shedding of the superficial layer of endometrium triggered by decreased progesterone. The low levels of progesterone and estrogen and increased levels of gonadotrophin releasing hormone (from hypothalamus) result in increased follicle stimulating hormone and luteinizing hormone secretion which act on ovaries to initially produce a Graafian Follicle and subsequently stimulate it to secrete of the estrogen. Estrogen causes proliferation of the endometrial lining of the uterus, resulting in increased thickness and vascularity. Estrogen also primes tissues for progesterone by increasing the expression of progesterone receptors in the uterus, vagina, anterior pituitary and hypothalamus.
~ THE OVULATION PHASE
Ovulation involves a mid-cycle surge of luteinizing hormone causing rupture of the Graafian Follicle and release of an ovum. The luteinizing hormone surge is due to positive feedback of high estrogen concentrations sensitizing the pituitary gland to gonadotrophin releasing hormone. Normally estrogen has a negative feedback effect on luteinizing hormone secretion (which is used in oral contraception) and this is an atypical effect that is not completely understood. The high levels of estrogen cause the cervical mucus to become thinner to facilitate sperm entering the cervix.
~ THE LUTEAL PHASE
In the luteal phase luteinizing hormone stimulates the ruptured Graafian Follicle to develop into the corpus luteum and subsequently to secrete progresterone (and estrogen to a lesser extent). Whereas estrogen causes proliferation of the endometrium progesterone causes the maturation of the endometrium to become suitable for implantation of a fertilized ovum (secretory endometrium). Progesterone also decreases the effect of estrogen on the reproductive tract by decreasing expression of the estrogen receptor. It also makes cervical mucus viscous and hence difficult for sperm to penetrate.
If implantation does not occur, the corpus luteum stops secreting progesterone resulting in menstruation. If implantation does occur, progesterone secretion continues, initially by the corpus luteum and eventually by the placenta. This is important for maintaining pregnancy as it prevents further ovulation by negative feedback on hypothalamus and pituitary and suppresses uterine contractility. As pregnancy proceeds estrogen and progesterone act together to develop the mammary glands for lactation.