TBL10 - Development of the Lungs

Question 1

What is the lung bud (respiratory diverticulum) derived from? What is epithelium of the trachea and its descendent airways therefore derived from?

Answer 1

1) The lung bud (respiratory diverticulum) is an outgrowth from the anterior wall of the distal foregut
2) Thus, epithelium of the trachea and its descendent airways is derived from endoderm

Question 2

What types of tissue are found in the airways and where are they derived from?

Answer 2

Connective tissue, cartilage, and smooth muscle of the airways are derived from mesenchymal cells of the visceral mesoderm

Question 3

What separates the distal foregut into posterior and anterior portions? What function does this serve?

Answer 3

While the lung bud expands caudally, the tracheo-esophageal septum divides the distal foregut into posterior and anterior portions occupied by the esophagus and trachea, respectively

Question 4

What is the most common type of tracheoesophageal fistula and why can it be associated with polyhydramnios and pneumonia?

Answer 4

1) Abnormalities in partitioning of the esophagus and trachea by the tracheoesophageal septum result in esophageal atresia with or without tracheoesophageal fistulas (TEFs)
2) 90% of these defects result in the upper portion of the esophagus ending in a blind pouch and the lower segment forming a fistula with the trachea
3) A complication of some TEFs is polyhydramnios, since in some types of TEF, amniotic fluid, when swallowed does not pass to the stomach and intestines
4) Also, gastric contents and/or amniotic fluid at birth may enter the trachea through a fistula, causing pneumonitis and pneumonia

Question 5

What begins to occur as the lung buds progressively branch? What does this action cause in relation to blood supply?

Answer 5

1) As the lung buds progressively branch, differentiation of the lobar and segmental bronchi occurs
2) The differentiation of bronchioles at distal ends of the segmental bronchi is associated with angiogenic proliferation of pulmonary capillaries from the segmental pulmonary arteries

Question 6

What happens to the pulmonary capillary endothelium once alveolar sacs are formed?

Answer 6

When clustering of the pulmonary alveoli forms the alveolar sacs, the pulmonary capillary endothelium is placed in close opposition to the simple squamous epithelium that lines the alveoli

Question 7

Before birth, what is important about fetal breathing? What initiates neonatal respiration?

Answer 7

1) Before birth, fetal breathing movements and consequent aspiration of amniotic fluid are important to development
2) Fetal breathing movements begin before birth and cause aspiration of amniotic fluid. These movements are important for stimulating lung development and conditioning respiratory muscles
3) When respiration begins at birth, most of the lung fluid is rapidly resorbed by the blood and lymph capillaries, and a small amount is probably expelled via the trachea and bronchi during delivery. When the fluid is resorbed from alveolar sacs, surfactant remains deposited as a thin phospholipid coat on alveolar cell membranes
4) With air entering alveoli during the first breath, the surfactant coat prevents development of an air-water (blood) interface with high surface tension. Without the fatty surfactant layer, the alveoli would collapse during expiration (atelectasis)
5) Fluid removal from the lungs coincides with the onset of neonatal respiration