Learning aims

At the end of this module you should be able to:

• know the various prenatal stages of lung development.
• be able to list and localize the various cells that are typical for lung tissue.
• know the components of the blood-air barrier.
• be able to describe the development of the various somatic cavities.
• know where the pericardio-peritoneal duct lies.
• know the difference between the vasa publica and privata in the lungs.
• be able to explain the occurrence of fistulas between the esophagus and trachea based on your knowledge of the development of the two structures.
• know the various mechanisms in charge of the switch of the circulation systems at birth.

What you should already know

• Embryogenesis
• Cardiovascular system

Introduction

The respiratory system starts to develop with an inconspicuous laryngo-tracheal groove (stage 10, ca. 28 days) that forms in the ventral midline at the lower end of the pharynx. The caudal part of this groove develops into the lung anlage in that it divides into right and left lung buds (stage 12, ca. 30 days). The upper part of this groove remains undivided and becomes the trachea.

The two lung buds extend and form the main bronchus (stage 14, ca. 33 days). These subdivide further and form 2 (on the left side) and 3 (on the right) lobular buds and thus the lobular bronchi. The whole lung development takes place in five main phases that partially overlap; followed by a maturation stage:

• Embryonic phase (3rd - 8th week)
• Pseudoglandular phase (5th - 17th week)
• Canalicular phase (16th - 26th week)
• Saccular phase (24th week up to birth)
• Alveolar phase (36th week up to the 18th month post partum)
• Stage of the microvascular maturation (from birth to 2 - 3 years)

Adult lungs have an inner surface area of 140 m². his extent can only be achieved through an enormous increase in the inner complexity of the pulmonary parenchyma. This is described in the following chapters.

Delving deeper

• Why does a laryngo-tracheal groove form in the ventral part of the foregut?
• What are the mechanisms underlying the dichotomous division of the bronchi?
• Why does further branching normally occur distally in the bronchial tree and not in the trachea?