Cycle Ovarien et Ovulation

Ovulation

• Ovulation occurs on day 14 of the menstrual cycle.
• GnRH is produced by the hypothalamus.
• LH and FSH are produced by the pituitary gland.
• Estrogens are produced by the follicle.
• Progesterone is produced by the corpus luteum.

Hormonal Regulation of Ovarian Cycle

• GnRH stimulates the release of LH and FSH.
• FSH stimulates the growth and maturation of follicles.
• LH triggers ovulation and the development of the corpus luteum.
• Estrogens and progesterone play a role in the feedback mechanisms regulating the cycle.

Positive and Negative Feedback

• Negative feedback occurs when estrogen and progesterone levels are high, reducing the release of GnRH, LH, and FSH.
• Positive feedback occurs when estrogen levels reach a threshold, stimulating a surge in LH, triggering ovulation.

Follicular Phase

• Follicular growth is stimulated by FSH.
• Theca cells produce androgens.
• Granulosa cells convert androgens to estrogens.
• Estrogen levels increase.
• Estrogen exerts positive feedback on LH release.
• A surge in LH triggers ovulation.

Ovulation Process

• The follicle ruptures and releases the ovum.
• A surge in LH is crucial in the process.
• Enzymes are released, mediating the release of the ovum.
• Changes in blood flow affect blood vessels.
• The cells of the ruptured follicle form the corpus luteum.

Luteal Phase

• The corpus luteum produces progesterone.
• Progesterone maintains the uterine lining for potential pregnancy.
• If no fertilization takes place, the corpus luteum degenerates, and progesterone levels decrease resulting in menstruation.

Effect of LH surge on Follicle

• The surge in LH triggers enzymatic reactions.
• Vasodilation/vasoconstriction occurs due to changes in blood flow.
• A fluid build-up leads to the rupture in the follicle.
• Hydrolases (e.g., PGF2α) are released, affecting follicular cells.
• Release of the oocyte happens.

Different Stages of Follicular Rupture

• AMPc plays a role through various cellular reactions related to the rupture of the follicle.
• Acidic hyaluronic acid contributes to the follicular fluid build-up.
• Lysosomes and/or cellular degradation of follicular cells occurs within the follicle around the time of ovulation.
• Vasoconstriction/vasodilation of blood vessels occurs, leading to follicle rupture.

Maturation of Ovum

• LH surge triggers the resumption of meiosis I in the oocyte.
• Oocyte maturation includes formation of polar bodies and the expelling of polar bodies.
• The oocyte exits the follicle as a secondary oocyte.
• The nucleus of the ovum undergoes meiotic division.

Ovum Structure

• Oocyte size is around 120 μm.
• It has a rich cytoplasm with an outer membrane.
• The zone pellucida is a protective layer around the ovum.
• The corona radiata is a layer of cells surrounding the zone pellucida.
• Polar body is expelled from the oocyte during meiosis