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Embryology · General embryology: weeks 1 to 8

Gametogenesis: spermatogenesis and oogenesis !

  • Process

How eggs and sperm are made: cells with half the chromosomes, ready to join at fertilization.

Updated 2026-11-01

See it in 3D: Ovaries · Testes · Epididymis · Ductus deferens

What it is

Gametogenesis turns primordial germ cells into haploid gametes (23 chromosomes) through mitosis, meiosis and maturation. In males it runs continuously from puberty; in females it starts before birth and pauses in prophase of meiosis I for years. That difference explains why meiotic errors rise with maternal age.

Key facts

  • Week 4: primordial germ cells appear in the wall of the yolk sac, near the allantois.
  • Weeks 4–6: they migrate along the dorsal mesentery of the hindgut to the gonadal ridges.
  • Fetal month 5: the ovary holds its peak number of germ cells (several million); hundreds of thousands remain at birth.
  • At birth every primary oocyte is arrested in prophase of meiosis I.
  • Puberty: spermatogenesis begins; one full cycle takes about two and a half months.
  • Sperm gain motility in the epididymis but can fertilize only after capacitation in the female tract.

Structure

  • Sertoli cells: support and feed germ cells, form the blood–testis barrier and respond to FSH.
  • Leydig cells: make testosterone in response to LH.
  • Follicular (granulosa) cells surround the oocyte; the primordial follicle is the ovary's reserve unit.

Step by step

  1. Spermatogonium (mitosis in the seminiferous tubule) → primary spermatocyte.
  2. Meiosis I → two secondary spermatocytes; meiosis II → four haploid spermatids.
  3. Spermiogenesis: the spermatid builds an acrosome, a compact nucleus and a tail, and sheds cytoplasm.
  4. Oogonium (fetal mitosis) → primary oocyte, which enters meiosis I and stops in prophase.
  5. LH surge before ovulation: meiosis I finishes → secondary oocyte + first polar body.
  6. The secondary oocyte stops in metaphase II and completes it only if fertilized (second polar body).

Markers

  • KIT
  • FSH
  • LH
  • AMH

Clinical notes

  • Advanced maternal age: oocytes have waited decades in meiosis I and nondisjunction is more common (trisomy 21).
  • Teratoma: a germ cell tumor containing tissues from all three germ layers.
  • A binucleate oocyte or an abnormal sperm can explain triploidy and some partial moles.
  • Male infertility: abnormal sperm count, motility or shape.

Test yourself

Think of the answer, then tap the question to see it.

In which phase of meiosis is the oocyte released at ovulation arrested?

Metaphase II. It completes meiosis II only if fertilized, extruding the second polar body.

From where do primordial germ cells migrate to the gonad?

From the wall of the yolk sac near the allantois, along the dorsal mesentery of the hindgut, to the gonadal ridges.

How many functional gametes come from one primary oocyte, and how many from one primary spermatocyte?

A primary oocyte gives one egg (plus polar bodies); a primary spermatocyte gives four spermatids.

Which testicular cell responds to FSH and forms the blood–testis barrier?

The Sertoli cell. The Leydig cell responds to LH and makes testosterone.

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← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 2: Gametogenesis: Conversion of Germ Cells into Male and Female Gametes (Wolters Kluwer, 2015); How do cells divide?: MedlinePlus Genetics (MedlinePlus (U.S. National Library of Medicine), 2026); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026).

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