The ovaries are the female gonads, located on either side of the uterus in the pelvic cavity. Like testes, they have dual functions: producing eggs (exocrine) and producing hormones (endocrine).

Key Difference from Males: Women are born with all the eggs they will ever have — approximately 1-2 million primordial follicles at birth. Unlike men, who continuously produce new sperm throughout life, women cannot make new eggs. The supply only decreases over time.

Follicle Structure: Each egg is contained within a follicle — a fluid-filled sac that supports the egg and produces hormones. Follicles go through stages of development:

• Primordial follicle: Dormant egg with flat follicular cells — the "savings account" of eggs
• Primary follicle: Egg begins to grow; follicular cells become cube-shaped
• Secondary follicle: Multiple layers of granulosa cells; theca cells form outside
• Tertiary (antral) follicle: Fluid-filled cavity (antrum) develops
• Graafian follicle: Mature, pre-ovulatory follicle ready to release its egg

The Numbers Game: By puberty, only about 400,000 follicles remain (most have died through atresia). Each cycle, about 20 follicles start developing, but usually only one becomes dominant and ovulates. The rest degenerate. By menopause (average age 51), the supply is essentially exhausted.

Oogenesis is the process of egg development, and it's dramatically different from spermatogenesis in both timing and outcome.

Before Birth: In the fetus, oogonia (egg stem cells) divide by mitosis and then begin meiosis. By about 5 months of fetal development, all oogonia have transformed into primary oocytes and entered meiosis I — where they stop. They remain frozen in this state for years.

At Puberty and Beyond: Each menstrual cycle, some primary oocytes resume meiosis. But here's the unique part: meiosis I completes only just before ovulation — that's potentially 40+ years after it started! The result is two cells of very unequal size:

• Secondary oocyte: Gets almost all the cytoplasm — this is what will become the egg
• First polar body: Gets almost no cytoplasm — essentially discards the extra chromosomes

At Fertilization: The secondary oocyte is arrested in meiosis II. Meiosis II only completes if a sperm fertilizes the egg. If fertilization doesn't occur, the egg never completes meiosis.

Key Differences from Spermatogenesis:

• Starts before birth (vs. continuously after puberty in males)
• Results in one egg per meiosis (vs. four sperm in males)
• Completes only if fertilized (vs. always completing in males)
• Cytoplasm divided unequally (vs. equally in males)

The menstrual cycle is a roughly 28-day cycle that prepares a woman's body for possible pregnancy. It involves coordinated changes in the ovaries (ovarian cycle) and uterus (uterine cycle).

Phase 1: Follicular Phase (Days 1-14)

Begins with menstruation on Day 1. FSH from the pituitary stimulates several follicles to develop. As follicles grow, they produce estrogen, which:

• Causes the uterine lining (endometrium) to thicken and become vascularized
• Initially suppresses FSH (negative feedback)
• At very high levels, triggers the LH surge (positive feedback)

Phase 2: Ovulation (Day 14)

The estrogen peak triggers a massive release of LH — the LH surge. This surge causes the dominant follicle to rupture and release its egg. The egg is swept into the fallopian tube where fertilization can occur.

Phase 3: Luteal Phase (Days 15-28)

After ovulation, the ruptured follicle transforms into the corpus luteum ("yellow body"), which produces both progesterone and estrogen. Progesterone:

• Prepares the endometrium for possible implantation
• Thickens cervical mucus to block further sperm entry
• Maintains the uterine lining

If No Pregnancy: After about 14 days, the corpus luteum degenerates. Progesterone and estrogen drop, causing the endometrium to shed — menstruation begins, and the cycle starts over.

If Pregnancy Occurs: The embryo produces hCG (human chorionic gonadotropin), which maintains the corpus luteum. Progesterone stays high, and menstruation doesn't occur.

The female reproductive cycle is controlled by the same HPG axis as in males, but with a crucial difference: cyclical variation and positive feedback.

The Hypothalamic-Pituitary-Ovarian Axis:

1. Hypothalamus releases GnRH in pulses (frequency varies throughout the cycle)
2. Anterior Pituitary releases FSH and LH in response
3. Ovaries produce estrogen (from developing follicles) and progesterone (from corpus luteum)

The Feedback Loops:

• Negative feedback (most of the cycle): Estrogen and progesterone inhibit GnRH, FSH, and LH
• Positive feedback (mid-cycle): When estrogen reaches a critical high level, it switches from inhibiting to stimulating LH release — this causes the LH surge

Why Positive Feedback Matters: This is the only example of positive feedback in the endocrine system. It ensures that ovulation occurs as a distinct, sudden event rather than a gradual process. The LH surge is like a starting gun that triggers ovulation within about 36 hours.

Hormonal Contraceptives: Birth control pills work primarily by providing steady estrogen and/or progesterone, which maintains negative feedback and prevents the LH surge. No LH surge = no ovulation = no pregnancy.

Female reproductive capacity changes dramatically with age, primarily due to declining ovarian reserve — the number and quality of remaining follicles.

The Timeline:

• Birth: ~1-2 million primordial follicles
• Puberty: ~400,000 remain
• Age 35: Accelerated loss begins
• Age 37-38: About 25,000 remain
• Menopause (average 51): Few follicles remain; cycles stop

Fertility Decline: Female fertility starts declining in the late 20s, with more rapid decline after age 35. At age 30, a woman has about a 20% chance of conceiving each month. By age 40, it's about 5%.

Menopause is defined as 12 consecutive months without menstruation. The years leading up to it (perimenopause) involve:

• Irregular cycles
• Fluctuating hormone levels
• Hot flashes and night sweats
• Mood changes
• Vaginal dryness

Hormone Replacement Therapy (HRT): Can relieve menopausal symptoms but has risks (increased blood clots, stroke, certain cancers). The decision to use HRT involves weighing benefits against risks.

Male vs Female Reproductive Aging: Men can produce sperm throughout life (though quality declines with age). Women have a finite egg supply that eventually runs out. This fundamental difference has major implications for fertility timing.