Ovarian Cycle and Hormones

Overview

This lecture covers the three phases of the ovarian cycle, the roles of key hormones, and their interactions throughout the menstrual cycle.

Phases of the Ovarian Cycle

• The ovarian cycle consists of three phases: follicular, ovulatory, and luteal (secretory).
• The main goal of the ovarian cycle is to release a mature egg for potential fertilization.

Follicular Phase

• The follicular phase focuses on the maturation of the follicle containing the ovum (egg).
• Follicles produce estrogen, which is important for endometrial (uterine lining) growth.
• Follicle Stimulating Hormone (FSH) predominates, stimulating follicle and ovum maturation.
• Feedback loops with the anterior pituitary and hypothalamus regulate hormone production.

Ovulatory Phase

• Approximately 24–36 hours before ovulation, a surge of Luteinizing Hormone (LH) triggers egg release.
• LH is produced by the anterior pituitary and initiates ovulation.

Luteal Phase

• After ovulation, the ruptured follicle becomes the corpus luteum, which is yellow in color.
• The corpus luteum produces progesterone, stabilizing the endometrium for possible implantation.
• If pregnancy occurs, human chorionic gonadotropin (hCG) maintains the corpus luteum and progesterone production.
• Without pregnancy, the corpus luteum degenerates, progesterone drops, and the cycle restarts.

Hormonal Changes Throughout the Cycle

• LH and FSH are low at the start, rise as the follicle matures, surge at ovulation, then decrease after ovulation.
• Progesterone is low during the follicular phase and rises after ovulation, peaking during the luteal phase.
• Estradiol (a form of estrogen) rises leading up to ovulation, drops after, and then fluctuates to regulate cycle progression.
• Estrogen predominates in the first half, while progesterone predominates in the second half of the cycle.

Key Terms & Definitions

• Follicular Phase — First phase where the follicle matures and estrogen is produced.
• Ovulatory Phase — Middle phase, marked by the LH surge and egg release.
• Luteal Phase — Last phase, corpus luteum forms and secretes progesterone.
• FSH (Follicle Stimulating Hormone) — Stimulates follicle and egg maturation.
• LH (Luteinizing Hormone) — Triggers ovulation.
• Corpus Luteum — Structure that remains after ovulation, produces progesterone.
• Progesterone — Hormone stabilizing the endometrial lining post-ovulation.
• Estradiol — A type of estrogen, promotes endometrial growth.
• hCG (Human Chorionic Gonadotropin) — Hormone that maintains the corpus luteum in early pregnancy.

Action Items / Next Steps

• Review diagrams or graphics showing hormone levels over the menstrual cycle.
• Study the feedback mechanisms between the hypothalamus, pituitary, and ovaries.