Primordial follicle formed during embroyonic development
- Follicular phase: At the beginning of the ovarian cycle , an increase of GnRH from the hypothalamus slowly increases the secretion of FSH and LH by the anterior pituitary. FSH and LH stimulate the follicle to grow, enlarge and change forms.
- (at puberty)
→Primary follicle→secondary Follicle
FSH and LH
also stimulates the follicle and surrounding ovarian cells to secrete estrogen
. Several days before the midpoint of the ovarian cycle, rising levels of estrogen stimulate the anterior pituitary to release a surge of LH
: An LH surge triggers ovulation at the midpoint of the cycle (day 14)
The mature follicle ruptures and releases the secondary oocyte
into the peritoneal cavity, the secondary oocyte then enters the uterine tube (for potential fertilization)
At this point the secondary oocyte is surrounded by a protective covering called the zona pellucida
, and a layer of cells derived from the follicle, the corona radiata
. More importantly, the secondary oocyte has started, but not yet completed, meiosis ll. meiosis ll will only be completed, if fertilization occurs.
(Yellow)- The Ruptured follicle transforms into the corpus luteum, which secretes large amounts of progesterone(
and estrogen) Progesterone prepares the endometrium of the uterus for pregnancy. v
High levels of progesterone (and estrogen) also signal the anterior pituitary to inhibit FSH secretion, temporary preventing other follicles from developing.
If fertilization does not occur, the corpus luteum degenerates (12 post ovulation) which causes progesterone ( and estrogen) levels to rapidly fall, consequently, the ovarian cycle starts again.
If fertilized the secondary oocyte quickly completes meiosis ll to produce a mature egg, or ovum, which units with a sperm.