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Embryology · General embryology: weeks 1 to 8

Week 3: gastrulation and the trilaminar disc !

  • Process

In week three the embryo goes from two layers to three: ectoderm, mesoderm and endoderm, which build the whole body.

Updated 2026-11-01

See it in 3D: Sacrum · Coccyx

What it is

Gastrulation is the process by which the epiblast forms the three germ layers. It starts with the primitive streak, a groove on the dorsal face of the disc: epiblast cells move down through it, some replacing the hypoblast (endoderm) and others settling between the layers (mesoderm); those left on top are the ectoderm. The body axes are also set: cranial–caudal, dorsal–ventral and left–right.

Key facts

  • Day 15: the primitive streak appears at the caudal end of the epiblast, with the primitive node and pit at its cranial end.
  • All germ layers come from the epiblast.
  • Buccopharyngeal (cranial) and cloacal (caudal) membranes: ectoderm and endoderm fused, with no mesoderm.
  • Mesoderm arranges itself into paraxial, intermediate and lateral plate (somatic and splanchnic).
  • Left–right sidedness is set at the node: cilia move fluid to the left, and NODAL, LEFTY2 and PITX2 are switched on the left side.
  • The primitive streak shortens and disappears by the end of week 4.

Step by step

  1. The primitive streak appears in the caudal midline of the epiblast.
  2. Epiblast cells lose their junctions (epithelial–mesenchymal transition) and invaginate through the streak.
  3. The first ones displace the hypoblast and form the definitive endoderm.
  4. The next ones settle between epiblast and endoderm: intraembryonic mesoderm.
  5. Node cells form the notochordal process heading cranially.
  6. The epiblast left on the surface is now called ectoderm.
  7. Mesoderm spreads until it meets the extraembryonic mesoderm at the edges of the disc.

Markers

  • Nodal
  • FGF8
  • Brachyury (TBXT)
  • LEFTY
  • PITX2

Clinical notes

  • Sacrococcygeal teratoma: a remnant of the primitive streak; the commonest tumor of the newborn, more frequent in girls.
  • Caudal dysgenesis (caudal regression, sirenomelia): too little mesoderm caudally; linked to maternal diabetes.
  • Situs inversus and heterotaxy: failed left–right signaling; with primary ciliary dyskinesia (Kartagener) add bronchiectasis and infertility.
  • Early gastrulation is very sensitive: alcohol at this time can damage the anterior midline (holoprosencephaly).

Test yourself

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

Which layer gives rise to all three germ layers?

The epiblast. Its cells move through the primitive streak to form endoderm and mesoderm; those that stay form ectoderm.

Which newborn tumor arises from primitive streak remnants?

Sacrococcygeal teratoma, the commonest tumor of the newborn, with tissues from all three layers.

Which structures of the trilaminar disc have no mesoderm?

The buccopharyngeal and cloacal membranes, where ectoderm and endoderm stay fused.

What problem at the primitive node explains situs inversus?

Failure of the nodal cilia or of the left-sided gene cascade (NODAL, LEFTY, PITX2) that sets left–right asymmetry.

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See it in 3D

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

← 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. 5: Third Week of Development: Trilaminar Germ Disc (Wolters Kluwer, 2015); Caudal regression syndrome: 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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Representative model, for educational purposes only.