[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general) › Week 3: gastrulation and the trilaminar disc

[Embryology](https://durgical.com/en/embryology/) · [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general)

# 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](https://durgical.com/en/anatomy/sacrum/) · [Coccyx](https://durgical.com/en/anatomy/coccyx/)

## Key points

- Week 3 = 'week of threes': three germ layers, notochord, the start of neurulation and somites.
- The epiblast gives all three layers; the hypoblast is replaced.
- Primitive streak remnants: sacrococcygeal teratoma.
- Laterality defects: nodal cilia and the NODAL–PITX2 cascade.
- Major structural defects become possible from week 3.

## 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

- The primitive streak appears in the caudal midline of the epiblast.
- Epiblast cells lose their junctions (epithelial–mesenchymal transition) and invaginate through the streak.
- The first ones displace the hypoblast and form the definitive endoderm.
- The next ones settle between epiblast and endoderm: intraembryonic mesoderm.
- Node cells form the notochordal process heading cranially.
- The epiblast left on the surface is now called ectoderm.
- 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).

## Memory aid

The primitive streak is the epiblast's 'boarding gate': the first through take the floor (endoderm), the next the middle (mesoderm), and those who stay behind are the roof (ectoderm).

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/week-3-gastrulation-and-the-trilaminar-disc/)

## See it in 3D

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

- [Sacrum](https://durgical.com/en/anatomy/sacrum/)
- [Coccyx](https://durgical.com/en/anatomy/coccyx/)

## Related

- [Week 2: the bilaminar disc ('the week of twos')](https://durgical.com/en/embryology/week-2-the-bilaminar-disc/)
- [Primitive streak and notochord (nucleus pulposus, sacrococcygeal teratoma)](https://durgical.com/en/embryology/primitive-streak-and-notochord/)
- [Germ-layer derivatives: ectoderm, mesoderm, endoderm](https://durgical.com/en/embryology/germ-layer-derivatives-ectoderm-mesoderm-endoderm/)
- [Neurulation and closure of the neural tube](https://durgical.com/en/embryology/neurulation-and-closure-of-the-neural-tube/)
- [Paraxial mesoderm and somites: sclerotome, myotome, dermatome](https://durgical.com/en/embryology/paraxial-mesoderm-and-somites-sclerotome-myotome-dermatome/)
- [Molecular regulation: Shh, Wnt, FGF, BMP, Hox genes and retinoic acid](https://durgical.com/en/embryology/molecular-regulation-shh-wnt-fgf-bmp-hox-genes-and-retinoic-acid/)
- [Weeks 3–8: the embryonic period and organogenesis](https://durgical.com/en/embryology/weeks-3-8-the-embryonic-period-and-organogenesis/)

[← All of embryology](https://durgical.com/en/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](https://medlineplus.gov/genetics/condition/caudal-regression-syndrome/) (MedlinePlus (U.S. National Library of Medicine), 2026); [Fetal development: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002398.htm) (MedlinePlus (U.S. National Library of Medicine), 2026).

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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/embryology/week-3-gastrulation-and-the-trilaminar-disc/