The Menstrual Cycle:
- On average begins at age of 12-14 years old and it starts from the first day of menses and ends just before the start of the new menstrual cycle. The average length of the menstrual cycle ranges from 21 to 35 days.
- During the prepubertal stage, the primary oocytes are nurtured by the granulosa cells with nutrients and steroid hormones to develop from primordial follicle to a primary follicle.
- During each menstrual cycle, few follicles enter a stage where a fluid containing steroid hormones secreted by the granulosa cells as well as some proteins and mucopolysaccharides and FSH accumulate in the central area of the follicle. At this stage the follicles are called graafian follicles.
Follicular Phase ( days 0-14) :
- The hypothalamus secretes gonadotropin-releasinghormone (GnRH) causing the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
- FSH causes a single primordial follicle to develop into a graafian follicle while the neighboring follicles regress and become atretic . after they degenerate, the remaining follicle is called dominant follicle
- FSH and LH interact with the ovaries’ theca and granulosa’s cells.
- Theca cells: synthesize progesterone and are stimulated by LH to produce testosterone which is the precursor for estradiol
- Granulosa Cells: are stimulated by FSH to convert the androgens that diffuses from the neighboring thecal cells to testosterone which is then converted to estradiol.
- Estradiol stimulates the proliferation of the Uterus ‘s endometrium and acts as a negative feedback on the anterior pituitary, suppressing the LH and FSH levels.
Ovulation Phase (day 14) :
- Ovulation occurs 14 days prior to menses, regardless of the cycle length.
- Ovulation starts after the burst of estradiol at the end of the follicular phase. After a certain level is reached, estradiol has a positive feedback effect on the anterior pituitary by up-regulating GnRH receptors. These receptors make the anterior pituitary more responsive to GnRH and the anterior pituitary secretes more FSH and LH. This sharp rise of LH triggers ovulation.
- Cervical mucus increase and becomes watery to allow penetration by sperm
- At the end of ovulation , estradiol levels decrease.
Luteal (secretory) Phase ( days 14-28)
- The corpus luteum (which is formed from the residual elements of the ruptured follicle) develops and synthesizes estradiol and progesterone.
- High progesterone levels increases the secretory activity and vascularity of the endometrium to prepare it to receive the fertilized ovum
- The basal body temperature increases because the progesterone increases the hypothalamic thermoregulatory center.
- Then the cervical mucous becomes less and thicker since it is ” too late” for sperm to fertilize the ovum
- The corpus luteum regresses if fertilization has not occured, and as a result estradiol and progesterone levels decrease abruptly.
- due to the regression of the corpus luteum and the sudden loss of the estradiol and progesterone, the endometrium lining is sloughed leading to menstrual bleeding or menses
- During menses, primordial follicle for the next cycle is recruited.
- Estrogen and progesterone levels increase steadily for:
1) Endometrium maintenance
2) Development of breasts for lactation after delivery
3) To suppress development of a new follicle.
- The source of steroid hormones is the corpus luteum during 1st trimester
- The source of steroid hormones is the placenta in 2nd and 3rd trimesters