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.