FRC is the combination of endocrine, ovarian and uterine cycles. Its observation and understanding is important.
Menstrual bleeding is the only clinical sign of the cyclical ovarian activity. Therefore it is generally accepted that the FRC begins with the onset of the menstruation, which is true for all the practical purposes but not precisely.
FRC starts in the late luteal phase of preceding Cycle, i.e., three four days before the onset of menstruation and continues into the follicular phase of the current cycle. Corpus luteum degenerates, its hormone production stops. Internal cohort releases the primordial follicles, few of them are recruited to proliferate. Follicles start growing more rapidly in the last days of luteal phase (premenstrual) and continue into the early follicular phase of the cycle. Preantral phase is independent of Gonadotrophins stimulation.
There is FSH selective rise during luteal-follicular transition phase as pituitary inhibition is abolished after the drop in the ovarian hormones. FSH rise is potent stimulus for follicle recruitment. Several follicles begin to enlarge because of action of FSH. One follicle is selected to ovulate others become atretic by intrinsic paracrine mechanism.
FSH & LH act on ovarian cells to select dominant follicle. Follicle most responsive to FSH are first to secrete Estrogen. Estrogen feeds back through Hypothalmo-Pituitary-Ovarian axis and begin to suppress FSH. Low FSH insufficient to sustain growth of higher threshold follicles leads to their atresia. Dominant follicle continues to mature and secrete Estrogen.
FSH alone is capable of stimulating ovarian Estrogen synthesis. Paracrine signaling activated by FSH & LH sustains growth and estrogen secretion by preovulatory follicle until ovulation.
The estrogens act on the uterine lining to make it proliferate. With increasing level of estrogen the thickness of endometrium increases, the numbers and size of the glands also increases. Estrogen also acts on cervical glands to facilitate secretion of increasing amounts of fertile mucus.
Follicle growth may not require LH which is important for the endocrine mechanisms of the ovary and is required in small amounts. LH is inhibitory in high doses. Midcycle LH surge induces oocyte maturation, causes follicle rupture and release of ovum (ovulation) and induces granulosa cells luteinization.
Follicular responsiveness to FSH and LH is developmentally regulated. FSH is crucial in recruitment, selection & Dominance of leading follicle. LH contributes to Dominance, final maturation and ovulation.