CorpusHuman Anatomy
ReproductiveCell process

Oogenesis

Also known as: oogonia, meiotic arrest

The production of female gametes, which begins before birth, pauses in prophase I for years to decades, resumes at each ovulation and completes only at fertilisation, yielding one oocyte and polar bodies from each primary oocyte.

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Overview

Peak germ cells≈6–7 million at ≈20 weeks of gestation
At birth≈1–2 million
At menarche≈300 000–400 000

Fetal life: primordial germ cells reach the gonadal ridges by week 6 and become oogonia, which multiply by mitosis to a peak of ≈6–7 million at about 20 weeks. From ≈11–12 weeks they enter meiosis to become primary oocytes, each surrounded by pregranulosa cells (primordial follicles), and arrest in diplotene of prophase I. Massive apoptosis (atresia) leaves ≈1–2 million at birth; no oogonia remain, so the pool is finite.

Childhood to menopause: atresia continues (≈300 000–400 000 at menarche). From puberty, in each cycle the oocyte of the dominant follicle completes meiosis I in response to the LH surge, producing a secondary oocyte (n, 2C) and the first polar body; the secondary oocyte is ovulated arrested in metaphase II. Meiosis II completes only at fertilisation, producing the second polar body. Over a lifetime only ≈400–500 oocytes are ovulated; menopause occurs when ≈1000 follicles remain.

Comparison with spermatogenesis: oogenesis is discontinuous, begins prenatally, draws on a non-renewing pool, produces one gamete per meiosis (unequal cytokinesis concentrating cytoplasm in the oocyte) and takes up to five decades; spermatogenesis is continuous from puberty, renews from stem cells, produces four gametes per meiosis and takes ≈64–74 days.

Location

Ovarian cortex (fetal to menopause); completion of meiosis II in the uterine tube at fertilisation.

Parts & subdivisions

  1. Oogonia
    Mitotically active germ cells present only before birth.
  2. Primary oocyte
    Arrested in prophase I from fetal life until ovulation.
  3. Secondary oocyte
    Formed at ovulation; arrested in metaphase II until fertilisation.
  4. Polar bodies
    First (after meiosis I) and second (after meiosis II).

Function

  • Produces haploid female gametes with large cytoplasmic reserves.
  • Generates genetic diversity by recombination and independent assortment.

Clinical significance

Premature ovarian insufficiency

Loss of ovarian function before 40 (≈1 % of women): Turner syndrome (45,X — accelerated atresia, streak ovaries), fragile X premutation, autoimmune oophoritis, chemotherapy/radiotherapy, surgery. High FSH, low estradiol and AMH; hormone replacement protects bone and heart.

Compiled from standard anatomy and physiology references — see the Reproductive System sources. For education only; not medical advice.