Oogenesis, Folliculogenesis, and the Ovarian Cycle

Oogenesis

the production of female gametes and egg cells

Folliculogenesis

growth and development of ovarian follicles and mature eggs

Folliculogenesis Stages

1. resting follicle (most common and pool of follicles present before puberty)

2. primary follicle (resting follicle in the initial stages of developments)

3. cells within follicle divide to form secondary follicle

4. oocyte within follicle releases fluid (antrum) and follicle becomes a tertiary follicle (most die)

5. tertiary follicles that fully mature will rupture to release oocyte

6. remaining cells in the follicle form corpus luteum (produces estrogen)

Ovarian Cycle Phases

1. Follicular Phase 1-14 day (increase in estrogen)

2. Ovulation

3. Luteal Phase 14-28 day (increase in progesterone and inhibin)

Pathway to Ovulation

1. GnRH is released from the hypothalamus (stimulates the release of FSH and LH from anterior pituitary)

2. FSH and LH stimulate the follicle (FSH causes the release of inhibin that inhibits the release of FSH) negative feedback

3. at the end of the follicular phase, there are high concentrations of estrogen that cause a surge in LH (positive feedback) and triggers ovulation

Luteal Phase

1. ruptured follicle turns into corpus luteum

2. corpus luteum secretes progesterone and estrogen (helps prepare the uterus for possible implementation and provides negative feedback on the hypothalamus and decreases GnRH, provides negative feedback on hypothalamus and a decrease in GnRH corpus also releases inhibin, and present to release of FSH)

3. if fertilization or implantation does not occur, the corpus luteum degrades

4. progesterone and estrogen decrease which leads to menstruation

Uterine Cycle

the uterine and ovarian cycle occurs at the same time (menstrual: 1-5, proliferative: 6-14, ovulation, secretary: 15-28)

Menstrual Phase

occurs in the conjunction with the ovarian cycle (menstrual phase occurs in the first half of the follicular phase) and begins on the first day and lasts ~5 days (includes cramping and blood loss of ~10 to 80ml)

Proliferative Phase

starts when menstrual phase ends (during the 2nd half of the follicular phase), endometrium grows back thicker and more vascularized (more blood vessels), occurs due to estrogen release, prepares for possible implementation (days 6-14)

Secretary Phase

occurs during luteal phase, endometrium continues to prepare for implementation (due to release to progesterone and estrogen from corpus luteum), increased blood supply to support potential embryo, endometrium starts to degrade toward the end of the phase (due to reduced release of progesterone and estrogen from degrading corpus luteum (days 15-28)

Correlation

menstrual phase occurs during the early follicular phase of the ovarian cycle (hormone levels are low), proliferative phase in the mid to late follicular phase (increases estrogen), secretory phase corresponds with luteal phase (driven by the secretion of progesterone and estrogen from the corpus luteum)