Ovarian Cycle and Follicular Phase

Overview of the Ovarian Cycle

  • The ovarian cycle lasts approximately 28 days on average, but can vary significantly among women.

  • It involves the maturation of an egg in two main phases:

    • Follicular Phase (Days 1-14)

    • Luteal Phase (Days 14-28)

Phases of the Ovarian Cycle

  • Follicular Phase (Days 1-14)

    • Begins on day 1 of menstruation and ends at ovulation.

    • The key event is ovulation, which occurs around day 14.

    • Involves the maturation of the follicle, which houses the oocyte (egg).

  • Luteal Phase (Days 14-28)

    • Begins after ovulation and continues until the start of menstruation.

    • More details on this phase will be discussed in future modules.

Hormonal Regulation of the Follicular Phase

  • The cycle is governed by hormones, which include:

    • GnRH (Gonadotropin-Releasing Hormone) from the hypothalamus triggers the release of:

    • FSH (Follicle Stimulating Hormone)

    • LH (Luteinizing Hormone)

  • FSH encourages follicle development and estrogen production by granulosa cells.

  • LH promotes the secretion of androgens from thecal cells, which are then converted to estrogen by granulosa cells.

Variability in the Cycle

  • The follicular phase can vary in length, ranging from 1 to 10 days depending on individual differences, while the luteal phase is generally consistent at about 14 days.

  • This variability can explain differences in cycle lengths reported by women (e.g., 21, 25, 30 days).

Follicle Development Stages

  1. Primordial Follicle

    • Present at birth, contains a primary oocyte stalled in prophase I of meiosis.

    • Surrounded by a single layer of flattened granulosa cells.

  2. Primary Follicle

    • Develops from primordial follicles during childhood.

    • Features multiple layers of granulosa cells and the cal cells.

  3. Secondary Follicle

    • Distinct pockets of fluid begin to form within the layers of granulosa cells.

    • Oocyte is still primary, developing further but not yet ovulated.

  4. Vesicular (Graafian) Follicle

    • Characterized by a large cavity called the antrum filled with follicular fluid.

    • This follicle is ready for ovulation.

    • Contains a secondary oocyte.

Important Functions of Follicular Cells

  • Granulosa Cells

    • Secrete hormones, mainly estrogen, as they mature further under FSH stimulation.

  • Thecal Cells

    • Work closely with granulosa cells to produce androgens, which are converted to estrogens by granulosa cells.

Feedback Mechanisms

  • Rising estrogen levels create a positive feedback mechanism that intensifies the output of LH and FSH from the anterior pituitary, culminating in a surge of LH that stimulates ovulation.

Summary of the Follicular Phase

  • Starts/End: Begins on day 1 and culminates at ovulation around day 14.

  • Hormonal Control: Driven primarily by FSH and regulated by LH.

  • Oocyte Development: Transitions from primary oocyte in primordial follicles through to secondary and vesicular follicles.

Next Steps

  • The next module will cover the luteal phase and further details on the ovarian cycle, including how these phases interact with the menstrual cycle at large.