Female Reproductive Cycle | Ovulation

Main Ideas

The lecture explains the ovulatory cycle and its close connection to the menstrual cycle. It traces how hypothalamic and pituitary hormones regulate follicle development, estrogen production, ovulation, and formation of the corpus luteum. The effects of estrogen and progesterone on the uterus are reserved for a subsequent lecture.

1. Hormonal Control: The Hypothalamic–Pituitary–Ovarian Pathway

• The hypothalamus releases gonadotropin-releasing hormone (GnRH).
• GnRH stimulates gonadotrope cells in the anterior pituitary to release:
  • FSH (follicle-stimulating hormone)
  • LH (luteinizing hormone)
• These hormones act on ovarian follicles:
  • LH stimulates theca cells to produce androgens from cholesterol.
  • FSH acts on granulosa cells, which convert androgens into estrogen using enzymes such as aromatase. FSH also promotes granulosa-cell growth and follicular development.

2. Oocyte and Follicle Development

Oogonia are diploid precursor cells. The oocyte remains diploid during the early follicular stages and is arrested in prophase I of meiosis through the primordial, primary, early secondary, and late secondary follicle stages.

At puberty, follicle development proceeds through the following stages, supported by FSH and LH:

1. Primordial follicle
2. Primary follicle — Contains a primary oocyte surrounded by a single layer of follicular cells.
3. Early secondary follicle — FSH promotes granulosa-cell multiplication and formation of the zona pellucida, a glycoprotein layer around the oocyte.
4. Late secondary follicle — Granulosa cells continue to proliferate and produce pockets of follicular fluid.
5. Graafian (tertiary) follicle — The fluid pockets merge into a large cavity called the antrum.

During development, the oocyte completes meiosis I, producing a secondary oocyte and a polar body. The secondary oocyte begins meiosis II but pauses in metaphase II. The granulosa cells immediately surrounding it are called the corona radiata.

3. The Follicular Phase and Estrogen Feedback

The follicular phase lasts roughly from cycle days 1–14.

• Follicle growth is driven primarily by FSH, while LH supports androgen production by theca cells.
• Developing follicles produce increasing amounts of estrogen.
• During the middle of the follicular phase, rising estrogen generally exerts negative feedback on the hypothalamus and anterior pituitary, reducing GnRH, FSH, and LH release.
• Near the end of the phase, sustained high estrogen shifts to positive feedback:
  • It stimulates increased GnRH release and a substantial rise in LH.
  • The Graafian follicle also produces inhibin B, which suppresses pituitary FSH release.

The sharp rise in LH is called the LH surge and is the key trigger for ovulation.

4. Ovulation and Movement of the Oocyte

The LH surge acts on the mature follicle by:

• Increasing blood flow and permeability around part of the follicle.
• Increasing follicular-fluid accumulation.
• Activating proteolytic enzymes that weaken tissue at the follicle’s surface.

The follicle ruptures and releases the secondary oocyte, still arrested in metaphase II and surrounded by the zona pellucida and corona radiata. This release is ovulation, which usually occurs around cycle day 14 or 15 in the lecture’s example of a typical cycle.

The fallopian tube’s fimbriae help guide the released oocyte into the tube. Cilia lining the tube help move it toward the ampulla, where fertilization may occur.

5. The Luteal Phase and Corpus Luteum

After ovulation, the remaining follicular structure first becomes a blood-filled corpus hemorrhagicum. Under the influence of LH, cells in the ruptured follicle differentiate and accumulate lipids, forming the corpus luteum. LH also stimulates the corpus luteum to produce progesterone.

The luteal phase lasts roughly from cycle days 15–28. Its central events are the formation of the corpus luteum and progesterone production.

Speaker and Source

Professor Zach Murphy, presenting for Ninja Nerd.