Embryology · Respiratory system, diaphragm and body cavities

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 · Main bronchi · Lobar bronchi · Segmental bronchi · Lung fissures

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	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.

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.

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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: Newborn Breathing Conditions - Respiratory Distress Syndrome (RDS) | NHLBI, NIH (National Heart, Lung, and Blood Institute, 2022); Fetal development: MedlinePlus Medical Encyclopedia (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.