<i>In Vivo</i> incorporation of choline‐³H, leucine‐³H and galactose‐³H in alveolar type II pneumocytes in relation to surfactant synthesis. A quantitative radioautographic study in mouse by electron microscopy

TLDR

The study quantified the temporal incorporation of choline‑3H, leucine‑3H and galactose‑3H into alveolar type II pneumocytes in vivo using electron‑microscopic autoradiography. Type II pneumocytes were selectively labeled with choline‑3H, a phosphatidylcholine precursor, and the intracellular distribution of the labeled precursors was traced through the endoplasmic reticulum, Golgi apparatus, and lamellar bodies. Choline and leucine were first localized in the ER, then rapidly transferred via the Golgi to lamellar bodies—leucine via fusion of multivesicular bodies—while galactose entered lamellar bodies modestly; newly formed lamellar bodies, including previously undescribed small carriers, were ultimately excreted as surfactant into the alveolar cavity, demonstrating that surfactant synthesis in type II pneumocytes involves coordinated phospholipid, protein, and polysaccharide precursors.

Abstract

Abstract A quantitative time study of the incorporation of choline‐ 3 H, leucine‐ 3 H and galactose‐ 3 H in lung epithelial cells, was undertaken in vivo with electron microscopic autoradiography. Type II pneumocytes were selectively labeled with choline‐ 3 H, a specific precursor of phosphatidylcholine, which is the main component of pulmonary surfactant. Choline was initially localized in endoplasmic reticulum, then was rapidly transferred through the Golgi complex and stored in lamellar bodies. Previously undescribed small lamellar bodies are suggested as phospholipid carriers between Golgi complex and lamellar bodies. After initial incorporation in the endoplasmic reticulum, the leucine label migrated through the Golgi complex into lamellar bodies by fusion of multivesicular bodies, which are the carriers between the two structures. Galactose was modestly incorporated into lamellar bodies via the Golgi complex. Intra‐alveolar myelin figures, recognized as excreted surfactant, were labeled 120 minutes after injection with the three precursors. These findings indicate that the synthesis of a secretory product by type II pneumocytes involves phospholipid, protein and polysaccharide precursors; the secretory product is segregated as lamellar bodies which are destined to be excreted into the alveolar cavity to become part of the lining layer.