Respiratory Development

Lung Development

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Vascular development | Main Anatomy Index | Gastrointestinal development

Last updated 30 March 2006

This page was contributed by David Boshell

Lung Development

On day 22, the respiratory diverticulum or lung bud develops ventrally from the foregut endoderm and then undergoes a series of branching:

On days 26-28 it bifurcates into 2 primary bronchial buds that will become the future lungs.

In the 5^th week, the secondary bronchial buds are formed that will become future lung lobes (3 on the right, 2 on the left).

In the 6^th week, the tertiary bronchi are produced that will become future bronchopulmonary segments.

By the 16^th week, at about 16 generations of branching, the small terminal bronchioles appear.

By the 28^th week, the terminal sacs (primitive alveoli) appear, and additional primitive alveoli will continue to develop until about 8 years of age.

After the 36^th – 38^th weeks, the 1^st generation of terminal sacs have matured into alveoli, with thin epithelium and production of alveolar surfactant.

The developing respiratory tree is enclosed in a jacket of perioesophageal splanchnopleuric mesenchyme that will differentiate into the cartilage, connective tissue, vessels and smooth muscle of the bronchi and lungs.

Factors controlling pulmonary branching

The mesoderm surrounding the respiratory endoderm ensures non-budding of the tracheal primordium, and frequent budding of the bronchial tree.

Factors controlling pulmonary branching include:

Epidermal growth factor (EGF) and transforming growth factor beta (TGFbeta), associated with growth and branching of lung buds.

TFGbeta is co-localised with collagen I and III, fibronectin and proteoglycans, which are concentrated along ducts and in branching areas, thus suggesting involvement in stabilisation of formed elements.
TFGbeta is not found in rapidly expanding portions.

The protein epimorphin, important in the formation of epithelial tubes; it is located in mesenchyme.
The proteoglycal syndecan, which maintains the stability of epithelial sheets along tubules or ducts.

Stages of human lung development
Stage	Period	Events
Embryonic	26 days – 6 weeks	Lung bud arises from foregut endoderm; undergoes 3 initial rounds of branching, producing lungs, lung lobes and bronchopulmonary segments
Pseudoglandular	6 – 16 weeks	Respiratory trees of lungs have 16 generations of branching, producing terminal bronchioles
Canalicular	16 – 28 weeks	Terminal bronchioles divide into respiratory bronchioles; respiratory vasculature begins to form
Saccular	28 – 36 weeks	Respiratory bronchioles divide into terminal sacs (primitive alveoli), process lasting until 8 years
Alveolar	36 weeks - term	Alveoli mature

Larson, W. Human Embryology 2^nd ed., Churchill Livingstone, 1997, p138: Table 6-1.