[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Respiratory system, diaphragm and body cavities](https://durgical.com/en/embryology/#a-respiratory) › Lung development: the five stages

[Embryology](https://durgical.com/en/embryology/) · [Respiratory system, diaphragm and body cavities](https://durgical.com/en/embryology/#a-respiratory)

# Lung development: the five stages

- Process

The lung starts as a bud from the foregut in week 4 and matures in five stages that go on long after birth.

Updated 2026-11-01

**See it in 3D:** [Trachea](https://durgical.com/en/anatomy/trachea/) · [Main bronchi](https://durgical.com/en/anatomy/right-main-bronchus/) · [Lobar bronchi](https://durgical.com/en/anatomy/lobar-bronchus/) · [Segmental bronchi](https://durgical.com/en/anatomy/segmental-bronchus/) · [Lung fissures](https://durgical.com/en/anatomy/oblique-fissure-of-right-lung/)

## Key points

- Week 4: the respiratory diverticulum (lung bud) grows from the ventral wall of the foregut.
- The epithelium of larynx, trachea, bronchi and alveoli comes from endoderm; cartilage, muscle and connective tissue from splanchnic mesoderm.
- The tracheoesophageal septum separates trachea and esophagus; if it fails, tracheoesophageal fistula results.
- Week 5: the right main bronchus gives three lobar bronchi and the left two.
- Pseudoglandular (weeks 5–17): conducting airways only; no gas exchange possible.
- At birth only about one sixth of the adult number of alveoli exist; most form in childhood.

## What it is

The respiratory system appears in week 4 as a diverticulum from the ventral wall of the foregut. It branches and matures in five stages: embryonic, pseudoglandular, canalicular, saccular and alveolar. Knowing which stage the lung is in explains why a 24-week preterm baby can barely breathe and why the causes of pulmonary hypoplasia act in the second trimester.

## Key facts

- The lung bud depends on retinoic acid from nearby mesoderm, which switches on TBX4.
- Branching depends on signals between endoderm and the surrounding mesoderm (for example FGF10).
- By the end of month 6 there are about 17 generations of bronchial branches; more form after birth.
- Before birth the lung is full of fluid; fetal breathing movements are needed for it to grow.

## Step by step

- Embryonic (weeks 4–7): lung bud, trachea, main, lobar and segmental bronchi.
- Pseudoglandular (weeks 5–17): the bronchial tree branches down to the terminal bronchioles; the tissue looks like a gland.
- Canalicular (weeks 16–26): respiratory bronchioles appear, capillaries grow and pneumocytes start to differentiate.
- Saccular (week 26 to birth): thin-walled terminal sacs form against capillaries; surfactant rises.
- Alveolar (late pregnancy into childhood): sacs mature into alveoli and their number keeps rising for about ten years.

## Stages of lung development

| Stage | Weeks | What forms |
| --- | --- | --- |
| Embryonic | 4–7 | Bud, trachea and bronchi |
| Pseudoglandular | 5–17 | Conducting airways |
| Canalicular | 16–26 | Respiratory bronchioles and capillaries |
| [Saccular](https://durgical.com/en/conditions/neonatal-respiratory-distress-syndrome/) | 26 to birth | Terminal sacs and surfactant |
| Alveolar | Late pregnancy into childhood | Mature alveoli |

## Markers

- NKX2-1
- TBX4
- FGF10
- Retinoic acid
- SHH

## Clinical notes

- A preterm baby in the late canalicular stage (around week 24) can survive with support, because capillaries now lie near the airway.
- Pulmonary hypoplasia: oligohydramnios or diaphragmatic hernia during the pseudoglandular and canalicular stages.
- Fetal airway malformations (CPAM, sequestration) arise from abnormal buds or branching.

## Memory aid

Each stage builds what the air will travel next: first the tubes, then the channels, then the sacs and finally the alveoli.

## Test yourself

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

## From which embryonic structure does the respiratory system bud, and in which week?

From the ventral wall of the foregut, as the respiratory diverticulum, in week 4.

## Why could a fetus in the pseudoglandular stage not breathe outside the uterus?

Because at that stage only conducting airways exist; there are no respiratory bronchioles or capillaries next to the airway for gas exchange yet.

## Which germ layer forms the respiratory epithelium, and which forms the tracheal cartilage?

The epithelium comes from endoderm; cartilage, muscle and connective tissue come from splanchnic mesoderm.

## What share of the adult number of alveoli is present at birth?

About one sixth; the rest form during childhood, mostly in the first years.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/lung-development-the-five-stages/)

## See it in 3D

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

- [Trachea](https://durgical.com/en/anatomy/trachea/)
- [Main bronchi](https://durgical.com/en/anatomy/right-main-bronchus/)
- [Lobar bronchi](https://durgical.com/en/anatomy/lobar-bronchus/)
- [Segmental bronchi](https://durgical.com/en/anatomy/segmental-bronchus/)
- [Lung fissures](https://durgical.com/en/anatomy/oblique-fissure-of-right-lung/)

## Related

- [Surfactant and fetal lung maturity](https://durgical.com/en/embryology/surfactant-and-fetal-lung-maturity/)
- [Esophageal atresia and tracheoesophageal fistula](https://durgical.com/en/embryology/esophageal-atresia-and-tracheoesophageal-fistula/)
- [Pulmonary hypoplasia](https://durgical.com/en/embryology/pulmonary-hypoplasia/)
- [Congenital pulmonary airway malformation, sequestration and bronchogenic cyst](https://durgical.com/en/embryology/congenital-pulmonary-airway-malformation-sequestration-and-bronchogenic-cyst/)
- [Foregut: esophagus, stomach rotation and duodenum](https://durgical.com/en/embryology/foregut-esophagus-stomach-rotation-and-duodenum/)
- [Intraembryonic coelom: pericardial, pleural and peritoneal cavities](https://durgical.com/en/embryology/intraembryonic-coelom-pericardial-pleural-and-peritoneal-cavities/)
- [Fetal lung, kidney and brain maturation: what preterm birth interrupts](https://durgical.com/en/embryology/fetal-lung-kidney-and-brain-maturation-what-preterm-birth-interrupts/)

## Related conditions

- [Neonatal respiratory distress syndrome (hyaline membrane disease)](https://durgical.com/en/conditions/neonatal-respiratory-distress-syndrome/)
- [Prematurity and its complications](https://durgical.com/en/conditions/prematurity-and-its-complications/)

[← 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: [Newborn Breathing Conditions - Respiratory Distress Syndrome (RDS) | NHLBI, NIH](https://www.nhlbi.nih.gov/health/respiratory-distress-syndrome) (National Heart, Lung, and Blood Institute, 2022); [Fetal development: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002398.htm) (MedlinePlus, U.S. National Library of Medicine, 2025); Langman's Medical Embryology, 13th ed., ch. 14: Respiratory System (Wolters Kluwer, 2015).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/embryology/lung-development-the-five-stages/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/lung-development-the-five-stages/