Surfactant Production in the Lungs

Surfactant is produced by type II alveolar epithelial cells in the lungs. 1

Composition and Function of Surfactant

Pulmonary surfactant is a complex mixture of lipids and proteins that plays critical roles in lung function:

Surfactant is composed of approximately 75% phospholipids and 25% proteins 1
The main components include phosphatidylcholine, phosphatidylglycerol, and four surfactant-associated proteins (SP-A, SP-B, SP-C, and SP-D) 2
It reduces surface tension in the alveoli, preventing alveolar collapse during respiration 3
Surfactant helps maintain bronchiolar patency during normal and forced breathing 4
It also has immunological functions, protecting the lungs from inhaled particles and microorganisms 5

The production of surfactant involves several steps within type II alveolar cells:

Type II pneumocytes (alveolar epithelial cells) are the specialized cells responsible for surfactant production 1
The lipid and protein components are synthesized separately within these cells 2
These components are then packaged into specialized organelles called lamellar bodies 2
Lamellar bodies are the main organelle for the synthesis and metabolism of surfactants 2
After secretion into the alveolar space, lamellar bodies transform into tubular myelin, which then forms the surface monolayer - the functionally active form of surfactant 6

Type II pneumocytes producing surfactant protein C (SP-C) are detected at gestational day 16.5-17.5 in mice 1
In humans, surfactant production begins during the canalicular period of lung development, which extends to about week 20 of gestation 1
Surfactant production increases significantly during the alveolar period of lung development 1

Deficiencies or abnormalities in surfactant production are associated with several respiratory disorders:

Neonatal respiratory distress syndrome occurs due to surfactant deficiency in premature infants 3, 4
Pulmonary alveolar proteinosis is characterized by accumulation of surfactant in alveoli due to impaired clearance 1
Genetic mutations in surfactant-related genes (SFTPA1, SFTPA2, SFTPB, SFTPC, ABCA3, NKX2-1) can cause various forms of interstitial lung disease 1
Surfactant dysfunction is also observed in conditions like ARDS, pneumonia, and pulmonary fibrosis 4

After functioning at the air-liquid interface, surfactant components are recycled back into type II cells or cleared by alveolar macrophages 2
This recycling process is essential for maintaining proper surfactant homeostasis 2
Disruption of surfactant clearance, as seen in pulmonary alveolar proteinosis, can lead to surfactant accumulation and respiratory impairment 1