Female Reproductive System: Oogenesis, Follicle Development, and the Ovarian & Uterine Cycles 34

Female Reproductive System

Overview

The female reproductive system is responsible for the production of female gametes (oocytes), the secretion of sex hormones, and the support of embryonic and fetal development. This guide covers oogenesis, follicle development, the ovarian cycle, hormonal regulation, and the uterine (menstrual) cycle.

Oogenesis

Stages of Oogenesis

• Oogonia: Diploid stem cells that undergo mitosis during fetal development. All mitotic divisions are completed before birth.

• Primary Oocytes: Oogonia begin meiosis I between months 3 and 7 of fetal development but arrest at prophase I. At birth, each ovary contains 2–4 million primary oocytes, but many degenerate before puberty.

• Childhood: Primary oocytes remain arrested in prophase I until puberty.

• Puberty to Menopause: Each month, 20–30 primary oocytes resume meiosis. Only one typically completes the first meiotic division, producing a secondary oocyte and a first polar body (which degenerates). The secondary oocyte begins meiosis II but arrests at metaphase II unless fertilization occurs.

• Fertilization: If fertilization occurs, the secondary oocyte completes meiosis II, forming a second polar body and a mature ovum.

Key Numbers:

• At birth: 2–4 million primary oocytes per ovary

• At puberty: 80,000–400,000 primary oocytes remain

• Ovulated in lifetime: 400–500 secondary oocytes

Comparison: Oogenesis vs. Spermatogenesis

Characteristic	Spermatogenesis	Oogenesis
Time of onset	Begins at puberty	Begins before birth
What happens at puberty	Begins and continually produces sperm cells	Continues meiosis and releases an immature gamete (secondary oocyte) each month
Number of cells produced	Millions of small, motile sperm each day	One large oocyte each month
When process ends	Continues until death	Continues until age 45–55
Duration of meiosis	Continuous throughout life	Pauses twice: at prophase I (from prenatal development to puberty) and at metaphase II (from ovulation to fertilization)

Follicle Development

Overview

Oocytes develop within follicles in the ovarian cortex. Follicle development is closely linked to oocyte maturation and is essential for ovulation and hormone production.

Stages of Follicle Development

• Primordial Follicles: First follicles formed in the female fetus, each containing a primary oocyte surrounded by a single layer of squamous follicular cells.

• Primary Follicles: Develop from primordial follicles as primary oocytes enlarge during childhood to puberty. Follicular cells become cuboidal granulosa cells, and microvilli form the zona pellucida, increasing surface area for material transfer.

• Secondary Follicles: Develop from primary follicles. The follicle wall thickens, granulosa cells secrete follicular fluid, and thecal cells begin producing estrogen. Each contains an arrested primary oocyte.

• Vesicular (Tertiary) Follicle: About once a month, one secondary follicle matures into a vesicular follicle. Pockets of fluid merge to form the antrum. The primary oocyte completes meiosis I, forming a secondary oocyte and first polar body, and arrests at metaphase II.

Ovulation and Luteal Transformation

• Ovulation: The vesicular follicle ruptures, releasing the secondary oocyte surrounded by granulosa cells (corona radiata).

• Corpus Luteum: The ruptured follicle becomes the corpus luteum, an endocrine organ that secretes progesterone and estrogen to maintain pregnancy if fertilization occurs. If not, it degenerates in about 10 days.

• Corpus Albicans: The corpus luteum is invaded by macrophages and fibroblasts, becoming scar tissue (corpus albicans), ending the ovarian cycle.

Ovarian Cycle

Phases of the Ovarian Cycle

The ovarian cycle consists of monthly events associated with oocyte and follicle maturation. It is divided into three main phases:

1. Follicular Phase (Stages 1–4): Follicle grows and develops through primordial, primary, secondary, and vesicular stages. This phase lasts from the beginning of the cycle until ovulation.

2. Ovulation Phase (Stage 5): The mature vesicular follicle ruptures, releasing the secondary oocyte.

3. Luteal Phase (Stages 6–7): The ruptured follicle transforms into the corpus luteum, which secretes hormones. If fertilization does not occur, it degenerates into the corpus albicans.

Stages of the Ovarian Cycle (Summary Table)

Stage	Structure	Description
1	Primordial follicle	Primary oocyte with single layer of squamous follicular cells
2	Primary follicle	Granulosa cells become cuboidal; zona pellucida forms
3	Secondary follicle	Multiple layers of granulosa and thecal cells; follicular fluid accumulates
4	Vesicular follicle	Large antrum; secondary oocyte arrested in metaphase II
5	Ruptured follicle	Ovulation: secondary oocyte released
6	Corpus luteum	Secretes progesterone and estrogen
7	Corpus albicans	Scar tissue; ends cycle

Hormonal Regulation of the Ovarian Cycle

Hormonal Interactions

• First-tier control: Hypothalamus releases gonadotropin-releasing hormone (GnRH).

• Second-tier control: Anterior pituitary releases luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in response to GnRH.

• Third-tier control: Ovaries secrete estrogens, progesterone, and inhibin in response to LH and FSH.

• Feedback: Estrogens and inhibin exert negative feedback on the hypothalamus and pituitary, while high estrogen levels trigger a positive feedback loop leading to the LH surge and ovulation.

If fertilization occurs: Progesterone from the corpus luteum inhibits further release of gonadotropins, maintaining the endometrium for pregnancy. If fertilization does not occur: The corpus luteum degenerates into the corpus albicans, hormone levels fall, and the cycle restarts.

Other Effects of Female Hormones

• Estrogen and, to a lesser extent, progesterone stimulate the development of female secondary sex characteristics.

• Key effects:

  • Maturation of sex organs

  • Development of external genitalia

  • Breast development and fat accumulation

  • Reduction of osteoclast activity (bone protection)

  • Increase in HDL ("good" cholesterol) and decrease in LDL ("bad" cholesterol)

Uterine (Menstrual) Cycle

Overview

The uterine cycle describes cyclic changes in the endometrium (uterine lining) in response to ovarian hormones. The endometrium has two main layers:

• Stratum functionalis: Outer, superficial layer that undergoes cyclic changes and is shed during menstruation.

• Stratum basalis: Inner, deeper layer that does not respond to ovarian hormones and regenerates the stratum functionalis after menstruation.

Phases of the Uterine Cycle

1. Menstrual Phase (Days 1–5): Stratum functionalis is shed, resulting in menstrual bleeding (35–50 mL blood loss).

2. Proliferative (Pre-ovulatory) Phase (Days 6–14): New stratum functionalis develops, endometrial glands enlarge, and spiral arteries and veins increase. Cervical mucus thins to facilitate sperm entry. Ovulation occurs at the end of this phase (day 14).

3. Secretory (Post-ovulatory) Phase (Days 15–28): Stratum functionalis becomes secretory mucosa. Endometrial glands secrete glycogen-rich "uterine milk" to nourish a potential embryo. If no pregnancy occurs, the stratum functionalis dies and the cycle restarts.

Hormonal Regulation of the Uterine Cycle

• FSH and LH rise at the start of the menstrual phase; ovarian hormones are lowest until day 5.

• Estrogen in the proliferative phase induces progesterone receptors in the endometrium and triggers further FSH and LH release, peaking at ovulation.

• Progesterone from the corpus luteum in the secretory phase inhibits the anterior pituitary, lowering FSH and LH.

• If fertilization occurs, the embryo secretes human chorionic gonadotropin (hCG), maintaining the corpus luteum and progesterone production (basis for pregnancy tests).

• If no fertilization, corpus luteum degenerates, progesterone drops, and menstruation begins.

Summary Table: Ovarian and Uterine Cycles

Ovarian Cycle Phase	Uterine Cycle Phase	Main Events
Follicular (Days 1–14)	Menstrual & Proliferative	Follicle development, endometrial growth
Ovulation (Day 14)	End of Proliferative	Release of secondary oocyte
Luteal (Days 15–28)	Secretory	Corpus luteum activity, endometrial secretion

Key Terms

• Oogonia: Female stem cells that give rise to oocytes.

• Oocyte: Immature female gamete.

• Follicle: Structure in the ovary containing an oocyte and supporting cells.

• Corpus luteum: Endocrine structure formed from the ruptured follicle after ovulation.

• Corpus albicans: Scar tissue formed from the degenerated corpus luteum.

• GnRH: Gonadotropin-releasing hormone, stimulates release of FSH and LH.

• FSH: Follicle-stimulating hormone, promotes follicle development.

• LH: Luteinizing hormone, triggers ovulation and corpus luteum formation.

• Estrogen: Hormone promoting development of female secondary sex characteristics and endometrial growth.

• Progesterone: Hormone maintaining the endometrium for pregnancy.

• hCG: Human chorionic gonadotropin, maintains corpus luteum during early pregnancy.

Additional info: The notes above expand on the original slides by providing definitions, context, and summary tables for clarity and exam preparation.