6. Reproductive system - Ovarian cycle

Focus on the female reproductive system.
Important concepts:
- Oogenesis: Creation or production of egg cells (oocytes) released by women.
- Folliculogenesis: Formation or growth of the follicle, which is the structure housing the developing egg cell.

Oogenesis: Refers to the process of egg cell production in females.
Folliculogenesis: Refers to the development and maturation of ovarian follicles, which are the protective structures surrounding the developing egg.

General structure of the ovarian cycle: It consists of various stages of folliculogenesis and is intimately linked with the changes in hormone levels that regulate the reproductive system.
Key phases:
- Follicular Phase: The period during which ovarian follicles mature, leading to ovulation.
- Luteal Phase: The post-ovulation phase where remnants of the follicle transform into the corpus luteum, which produces hormones.
- Cycle Duration: Typically lasts 28 days, with approximately 14 days in each phase.

Resting Follicle: The initial state of follicles present at birth.
Primary Follicle: After puberty, some resting follicles develop into primary follicles, undergoing initial maturation.
Secondary Follicle: Characterized by further growth and development where antrum (fluid-filled space) starts to form.
Tertiary Follicle (Graafian Follicle): Developed from the secondary follicle; at this stage, only a few follicles will continue maturing, with typically one becoming the dominant follicle that ruptures to release the egg.
Rupture of Follicle: Described as the follicle exploding, akin to the Kool-Aid man analogy, marking the release of the mature egg (oocyte).
Corpus Luteum Formation: After the follicle ruptures, the surrounding cells transform into the corpus luteum, which is essential for hormone production post-ovulation.

Key Hormones Involved:
- FSH (Follicle Stimulating Hormone): Released by the anterior pituitary; it is crucial for follicle development and stimulates the growth of follicles and egg cells.
- LH (Luteinizing Hormone): Also released by the anterior pituitary; it plays a key role in triggering ovulation and stimulating the production of estrogen.
- Estrogen: Produced by developing follicles; it plays various roles, including the regulation of the ovarian cycle and promoting the thickening of the uterine lining.
- Progesterone: Released by the corpus luteum; it helps maintain the uterine lining for potential implantation of a fertilized egg.
- Inhibin: Secreted by the growing follicles; it inhibits the further release of FSH to prevent the maturation of multiple follicles simultaneously.

Follicular Phase:
- Triggered by increased levels of FSH and LH; follicles begin to grow.
- Estrogen levels rise due to follicular development.
- Results in high concentrations of estrogen, leading to a surge in LH, initiating ovulation.
Luteal Phase:
- Begins post-ovulation when the ruptured follicle forms the corpus luteum.
- The corpus luteum secretes both progesterone and estrogen.
- Functions to prepare the uterus for a potential fertilized egg and provides negative feedback to inhibit further ovulation.
- If fertilization does not occur, the corpus luteum degrades, leading to decreased hormone levels, which results in menstruation.

Hormonal regulation throughout the ovarian cycle directly impacts the uterine cycle, which involves the thickening and subsequent breakdown of the uterine lining in response to hormonal changes.
The cycles are interlinked: the preparation in the ovarian cycle coincides with the changes observed in the uterine cycle regarding ovulation and menstruation.

Negative Feedback Mechanisms:
- FSH and LH regulation to prevent the maturation of multiple follicles.
- Use of inhibin to provide a check on FSH levels.
Importance of Hormonal Balance:
- A delicate balance of estrogen and progesterone is essential for the success of the reproductive cycle.
- Implication: Failure of these feedback mechanisms can lead to reproductive health issues, such as irregular cycles or anovulation.