Development of the Respiratory System

1. Anatomical Divisions
2. Weeks 4-8
   
   • Tracheoesophageal Fistula
3. Weeks 8-16
4. Weeks 16-26
5. Weeks 26 to Term
   
   • Respiratory Distress Syndrome
6. Quiz

• The respiratory system can be divided by anatomical location into the upper and lower airways, or by function into the conducting and respiratory portions.
• The respiratory system starts developing around week 4 of gestation as an outpouching of the primitive gut tube, the respiratory diverticulum. It then becomes a distinct structure when a tracheoesophageal septum forms to separate them. If formation of this septum fails, a tracheoesophageal fistula is left.
• The next stage of development is the Pseudoglandular stage (weeks 8-16). This involves the formation of the bronchioles.
• The next stage is the canalicular stage (weeks 16-26), where the respiratory bronchioles develop.
• The final stage is the terminal sac stage (weeks 26 to term) and involves the formation of primitive alveoli from the endodermal epithelium of the airways, with surrounding mesoderm contributing supporting tissues.
  • From the endoderm two cell lines develop: type I pneumocytes, which are epithelial cells involved in gas exchange, and type II pneumocytes, which produce surfactant.
  • Surfactant is a substance which is required to reduce the surface tension in the lungs. Insufficient amounts of surfactant can lead to respiratory distress syndrome in premature infants. Surfactant production can be induced by administering antenatal glucocorticoids to the mother when preterm birth is anticipated.

Anatomical Divisions

The respiratory system can be divided based on anatomical location into the upper and lower systems:

• The upper airway is made up of the nasal cavity, oral cavity, pharynx and larynx.
• The lower airway is made up of the trachea, bronchi, bronchioles and alveoli.

Opinion differs on where the upper airway ends, and the lower airway begins with some sources placing the larynx and trachea in different portions as listed here.

More helpfully for understanding physiology, the airway can also be divided based on function into the conducting portion and the respiratory portion:

• The conducting portion includes the trachea, bronchi and bronchioles up to and including the terminal bronchioles. It warms, moistens and conducts air to the respiratory portion. Respiratory bronchioles belong to the respiratory portion.
• The respiratory portion is made up of the respiratory bronchioles, alveolar ducts and alveoli, and is the site of gas exchange.

Weeks 4-8

The respiratory system begins development from an outpouching of the primitive gut tube known as the respiratory diverticulum. This structure is derived from the endoderm of the primitive gut tube, which is created by folding of the embryo and will go on to develop into the gastrointestinal tract.

The outpouching occurs from the foregut (the cranial end of the gut tube) and outer surface is covered with mesoderm.

As it is an outpouching, the respiratory diverticulum is continuous with the gut tube at the start of the development. To allow the formation of two separate systems, a tracheoesophageal septum forms.

By the end of week 4, the respiratory diverticulum splits into two bronchial buds. These give rise to the branching airways. The airway and alveolar epithelial lining develops from endoderm; the surrounding mesoderm contributes supporting tissues, including connective tissue, smooth muscle and pulmonary vessels.

Diagram - The development of the lungs from the beginning of week 4 to week 8 of gestation

SimpleMed original by Dr. Maddie Swannack

Tracheoesophageal Fistulae

A tracheoesophageal fistula is an abnormal connection between the trachea and the oesophagus.

Embryologically, this occurs because the respiratory diverticulum and the foregut are continuous, and if there is a defect in the formation of the tracheoesophageal septum this connection persists.

There are multiple different types, which are shown in the simplified diagram below.

Diagram - The 5 types of tracheoesophageal fistula. Type C is the most common by far

SimpleMed original by Dr. Maddie Swannack

Weeks 8-16

This stage is known as the Pseudoglandular Stage.

The conducting portion of the airways continue to develop, forming bronchioles from the tertiary bronchi formed by the end of week 8.

During the pseudoglandular stage, branching epithelial tubes are surrounded by mesenchyme, but gas-exchanging alveolar structures have not yet formed. The respiratory epithelial lineage is endodermal, with surrounding mesodermal support; the lungs are not yet capable of gas exchange.

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