Embryology of the Respiratory System

Question 1

What is the folding of the embryo and formation of the gut tube?

Answer 1

Embryo folding occurs during the 4th week of development, when the lateral edges of the embryonic disc fold toward the midline, creating a cylindrical shape.

The cephalocaudal folding forms the head and tail ends, and lateral folding brings the lateral body walls together.

Gut tube formation: As the embryo folds, a portion of the yolk sac is incorporated into the developing body, forming the primitive gut tube, which extends from the mouth (oropharyngeal membrane) to the anus (cloacal membrane). The gut tube differentiates into the foregut, midgut, and hindgut, which will eventually form the gastrointestinal tract.

Question 2

What is the origin of the lung buds and pleurae?

Answer 2

The lung buds develop from the ventral aspect of the foregut during the 4th week. The foregut endoderm forms the lining of the trachea, bronchi, and lungs.

The tracheoesophageal septum divides the foregut into the dorsal (esophagus) and ventral (respiratory tract) portions.

Pleurae formation: The pleural cavities develop from the lateral mesoderm (somatic and splanchnic layers). The splanchnic mesoderm surrounds the lung buds to form the visceral pleura, while the somatic mesoderm forms the parietal pleura.

Question 3

How does the bronchial tree develop and how are the alveolar sacs formed?

Answer 3

The bronchial tree begins to form in the 5th week, when the lung buds divide into the main bronchi.

Branching of the bronchi occurs, forming secondary and tertiary bronchi, which further divide into smaller bronchioles.

Alveolar sacs begin to form in the late fetal period (around 24 weeks) when the terminal bronchioles differentiate into respiratory bronchioles, leading to the development of alveolar ducts and ultimately alveolar sacs.

Alveolar development continues after birth, with the majority of alveolar sacs developing postnatally, expanding surface area for gas exchange.

Question 4

How does the diaphragm develop?

Answer 4

The diaphragm develops from several embryological structures:
- Septum transversum (a mesodermal tissue) forms the central part of the diaphragm.

• Pleuroperitoneal membranes contribute to the formation of the diaphragm’s posterior part.
• Muscle migration from the body wall forms the diaphragm’s peripheral muscular components.
  > By the 7th week, the diaphragm starts to separate the thoracic and abdominal cavities.
  > The diaphragm is innervated by the phrenic nerves, which originate from the C3-C5 spinal segments.