[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro) › Oogenesis, follicles and ovulation

[Physiology](https://durgical.com/en/physiology/) · [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro)

# 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](https://durgical.com/en/anatomy/ovary/) · [Uterine tubes](https://durgical.com/en/anatomy/uterine-tube/) · [Infundibulum and fimbriae](https://durgical.com/en/anatomy/infundibulum-of-uterine-tube/) · [Ampulla of uterine tube](https://durgical.com/en/anatomy/ampulla-of-uterine-tube/)

## Key points

- Primary oocytes arrest in prophase I from fetal life until the LH surge (up to ~50 years).
- Secondary oocyte arrests in metaphase II until fertilization; polar bodies discard extra chromosomes.
- Two-cell, two-gonadotropin model: LH → theca makes androgens; FSH → granulosa aromatizes them to estradiol.
- Number of oocytes falls from ≈6–7 million (mid-gestation) to ≈1–2 million at birth and ≈300 000–400 000 at puberty; only ≈400–500 ovulate.
- Long meiotic arrest explains the rise of nondisjunction (for example trisomy 21) with maternal age.

## 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 axis **Lesion** 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 ↓

Tap the pituitary to see how LH and FSH split the work between theca and granulosa cells.

## 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.

## Memory aid

Prophase I Pauses for decades; Metaphase II waits for a Mate.

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/oogenesis-follicles-and-ovulation/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Ovaries](https://durgical.com/en/anatomy/ovary/)
- [Uterine tubes](https://durgical.com/en/anatomy/uterine-tube/)
- [Infundibulum and fimbriae](https://durgical.com/en/anatomy/infundibulum-of-uterine-tube/)
- [Ampulla of uterine tube](https://durgical.com/en/anatomy/ampulla-of-uterine-tube/)

## Related topics

- [The menstrual cycle: ovary and endometrium](https://durgical.com/en/physiology/the-menstrual-cycle-ovary-and-endometrium/)
- [Fertilization and implantation](https://durgical.com/en/physiology/fertilization-and-implantation/)
- [Menopause](https://durgical.com/en/physiology/menopause/)
- [Actions of estrogen and progesterone](https://durgical.com/en/physiology/actions-of-estrogen-and-progesterone/)

## Related conditions

- [Polycystic ovary syndrome](https://durgical.com/en/conditions/polycystic-ovary-syndrome/)
- [Ovarian cyst](https://durgical.com/en/conditions/ovarian-cyst/)
- [Ovarian torsion](https://durgical.com/en/conditions/ovarian-torsion/)

## In Biochemistry

- [Aromatase (CYP19A1)](https://durgical.com/en/biochemistry/enzymes/aromatase/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/27-2-anatomy-and-physiology-of-the-ovarian-reproductive-system/) (OpenStax (Rice University), 2022); [Embryology, Ovarian Follicle Development](https://www.ncbi.nlm.nih.gov/books/NBK532300/) (StatPearls (NCBI Bookshelf), 2025); [Ovarian Cyst](https://www.ncbi.nlm.nih.gov/books/NBK560541/) (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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/oogenesis-follicles-and-ovulation/