Ovarian cycle

Ovarian Cycle Overview

  • The ovarian cycle refers to the growth and maturation of an oocyte for fertilization and reproduction.

  • It repeats every 28 days from puberty until menopause unless the female is pregnant.

  • The ovarian cycle consists of three main phases:

    • Follicular phase

    • Ovulation

    • Luteal phase

Phases of the Ovarian Cycle

1. Follicular Phase (Days 1-14)

  • Involves the recruitment and preparation of oocyte follicles for ovulation.

  • Lasts about 12-14 days.

2. Ovulation (Day 14)

  • The release of a mature egg occurs on this day, marking the midpoint of the cycle.

3. Luteal Phase (Days 15-28)

  • Development of the corpus luteum which produces estrogen and progesterone, critical for preparing the body for potential pregnancy.

  • Lasts about 14-16 days.

Hypothalamic-Pituitary-Ovarian Axis

  • The hypothalamus regulates hormonal signals critical for the ovarian cycle through:

    • Kiss1 neurons control the LH surge, influencing ovulation.

    • Gonadotropin-releasing hormone (GnRH) is released from the hypothalamus.

    • Anterior pituitary gland releases FSH (Follicle-stimulating hormone) and LH (Luteinizing hormone) into the bloodstream.

Granulosa and Germ Cells

  • Key cells involved in the ovarian cycle:

    • Granulosa Cells: Line the follicle, essential for the maturation of oocytes.

    • Primary Oocyte: Immature egg cell prior to maturation.

    • Secondary Oocyte: Mature egg cell ready for ovulation.

  • Antrum: Fluid-filled space that develops within the follicle containing the oocyte.

Relationship Between Ovarian and Menstrual Cycles

  • The ovarian cycle governs endocrine preparation and egg release, while the menstrual cycle prepares and maintains the uterine lining.

  • Both cycles are coordinated over a 22-32 day span (averaging 28 days).

  • Events of the menstrual cycle occurring in the ovaries contribute to the ovarian cycle, specifically:

    • Follicular phase: follicle maturation due to FSH.

    • Ovulation: egg release triggered by estrogen rise and LH surge.

    • Luteal phase: transformation of the follicle into a corpus luteum producing progesterone.

  • The uterine cycle involves menstruation, occurring from day 1 to 5, including the shedding of endometrial tissue.