Female Reproductive System

Female Reproductive System & Cycle

Overview

• The female reproductive cycle is a cyclical series of events occurring in the ovaries and uterus of females during their reproductive years (puberty to menopause), excluding pregnancy.
• The cycle approximates a month in length but can vary.
• It comprises the ovarian cycle (events in the ovary) and the uterine cycle (events in the uterus).
• Pregnancy interrupts the cycle, with the placenta assuming hormonal control.

Components of the Female Reproductive Cycle

• Ovarian Cycle:
  • Pre-ovulation (follicular phase).
  • Ovulation.
  • Post-ovulation (luteal phase).
• Uterine Cycle:
  • Menstrual phase.
  • Proliferative phase.
  • Secretory phase.
• The female reproductive cycle includes:
  • Oogenesis (oocyte production in the ovarian cycle).
  • Preparation of the uterus to receive a fertilized egg (in the uterine cycle).
• Hormonal control involves the hypothalamus, anterior pituitary gland, and ovaries.
  • Hypothalamus releases hormones stimulating the anterior pituitary.
  • Anterior pituitary hormones stimulate the ovaries to produce more hormones.

Hormonal Regulation and Cycle Phases

Pre-Ovulatory Phase (Ovarian) / Proliferative Phase (Uterine)

• Lasts approximately 10-14 days.
• The hypothalamus releases Gonadotropin-Releasing Hormone (GnRH).
• GnRH stimulates the anterior pituitary to release Follicle-Stimulating Hormone (FSH).
• FSH stimulates follicle growth and development in the ovary.
  • Primordial follicles develop into primary follicles, then secondary, and finally tertiary follicles.
  • Follicular cells support the oocyte and produce estrogen.
• Estrogen stimulates the stratum basalis in the uterus to regenerate and form a new stratum functionalis layer.
• The endometrium thickens due to estrogen influence.
• Endometrial glands are present but dormant during this phase.

Positive Feedback and Ovulation

• High estrogen levels trigger positive feedback on GnRH release.
  • This is an unusual occurrence.
• A surge of GnRH causes a surge of Luteinizing Hormone (LH) from the anterior pituitary.
• LH surge triggers ovulation, causing the follicle to rupture and release the oocyte.

Post-Ovulatory Phase (Ovarian) / Secretory Phase (Uterine)

• Following ovulation, LH maintains the corpus luteum.
• The corpus luteum, formed from the ruptured follicle, produces estrogen and progesterone.
• Progesterone stimulates endometrial glands to secrete glycogen-rich mucus, nourishing a fertilized egg.
• The body tolerates high estrogen and progesterone for ~10 days to allow fertilization and implantation.

Negative Feedback and Menstruation

• If fertilization/implantation doesn't occur, negative feedback is triggered.
• High estrogen and progesterone levels inhibit GnRH release from the hypothalamus.
• Reduced GnRH decreases LH production, causing the corpus luteum to degenerate into the corpus albicans.
• Estrogen and progesterone levels decline.
• Spiral arteries in the stratum basalis constrict, reducing blood flow to the stratum functionalis.
• The stratum functionalis dies and is shed, resulting in menstruation.

Cycle Restart

• The block on GnRH is removed after menstruation.
• GnRH is produced again, stimulating FSH release.
• FSH triggers follicle development, and the cycle repeats.

The Pill (Oral Contraceptive)

• Most pills contain synthetic estrogen and/or progestin (synthetic progesterone).
• The pill suppresses GnRH release, preventing FSH and LH production.
• This inhibits follicle development and ovulation, preventing pregnancy.
• Typically, 21 hormone pills (estrogen & progesterone) are followed by 7 placebo pills.
• Placebo pills lead to a drop in hormones, triggering menstruation.
• Long-term use:
  • Slightly increases the chance of breast cancer.
  • Slightly decreases the chance of ovarian cancer.
  • These increased or decreased chances wear off after about ten years.

Male Contraceptive Pill

• There are difficulties in blocking the development of hundreds of millions of sperm as opposed to only one oocyte, therefore, it is harder to develop.

Fertilization and Implantation

• If fertilization and implantation occur, menstruation must be prevented.
• The early embryo produces Human Chorionic Gonadotropin (HCG).
• HCG mimics LH, maintaining the corpus luteum and progesterone/estrogen production.
• This keeps the endometrium thick and prevents menstruation.
• Pregnancy tests detect HCG in urine, indicating pregnancy because it is only made by an embryo.