Physiology · Reproduction, pregnancy and the fetus
Oogenesis, follicles and ovulation !
- Mechanism
- Exam priority: medium
Girls are born with all their eggs, paused halfway through division; each month one of them finishes maturing and is released.
Updated 2026-11-01
Also called: Folliculogenesis, Ovarian reserve
See it in 3D: Ovaries · Uterine tubes · Infundibulum and fimbriae · Ampulla of uterine tube
What it is
Oogonia multiply only before birth; by birth they have become primary oocytes paused in prophase I, each inside a primordial follicle. From puberty, each cycle a group of follicles grows, one becomes dominant and the LH surge restarts meiosis just before ovulation. The oocyte pauses again in metaphase II and finishes meiosis only if a sperm enters.
How it works, step by step
- Primordial follicles (oocyte plus one layer of flat cells) leave the resting pool continuously, independent of FSH at first.
- Primary and secondary follicles form a zona pellucida, cuboidal granulosa layers and surrounding theca.
- Antral follicles become FSH-dependent; the follicle with most FSH receptors and aromatase keeps growing as FSH falls.
- The dominant (Graafian) follicle acquires LH receptors on its granulosa and makes large amounts of estradiol.
- The LH surge completes meiosis I (first polar body), loosens the cumulus and activates proteases and prostaglandins in the follicle wall.
- The follicle ruptures and releases the secondary oocyte with its cumulus; the tube's fimbriae pick it up.
- The remaining granulosa and theca cells luteinize into the corpus luteum.
Hormone axes and feedback
Hypothalamic–pituitary–ovarian axisLesion None (normal)
Tap a box to see the hormone, or choose a lesion to see the lab pattern.
- Stimulates (+)
- Inhibits / negative feedback (−)
- Positive feedback (high E₂ at mid-cycle)
- Level ↑
- Level ↓
Typical values
| Parameter | Value | Note |
|---|---|---|
| Oocytes at 20 weeks of gestation | ≈6–7 million | peak |
| Oocytes at birth | ≈1–2 million | |
| Oocytes at puberty | ≈300 000–400 000 | |
| Diameter of the dominant follicle before ovulation | ≈18–25 mm | on ultrasound |
| Oocyte viability after ovulation | ≈12–24 h |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Polycystic ovary syndrome: many small antral follicles stall because high LH and androgens with relatively low FSH prevent dominance; no ovulation.
- Functional ovarian cysts arise from a follicle that fails to rupture or a corpus luteum that fills with fluid; a large cyst can twist (torsion).
- Anti-Müllerian hormone, made by small growing follicles, is used as a marker of ovarian reserve.
Test yourself
Think of the answer, then tap the question to see it.
At which stage of meiosis is the oocyte released at ovulation, and when does it finish?
It is a secondary oocyte arrested in metaphase II; it completes meiosis II, releasing the second polar body, only when a sperm penetrates it.
Why are theca and granulosa cells both needed to make estradiol?
Theca cells have LH receptors and 17α-hydroxylase to make androgens but little aromatase; granulosa cells have FSH-induced aromatase but cannot make androgens. Together they produce estradiol.
Why does the risk of trisomies rise with maternal age?
Oocytes stay arrested in prophase I for decades; the cohesion holding homologous chromosomes weakens, so nondisjunction at the end of meiosis I becomes more likely.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The menstrual cycle: ovary and endometrium
- Fertilization and implantation
- Menopause
- Actions of estrogen and progesterone
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 27.2 Anatomy and Physiology of the Ovarian Reproductive System (OpenStax (Rice University), 2022); Embryology, Ovarian Follicle Development (StatPearls (NCBI Bookshelf), 2025); Ovarian Cyst (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 82). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.