Concentrically arranged endoplasmic reticulum containing some lamellae (bar-like structure) in alveolar type II cells of rat lung

We examinedin vivo the effect of pilocarpine (a cholinergic agent) and cycloheximide (an inhibitor of protein synthesis) on the “bar-like structures” in alveolar type II cells of rat lung to clarify their origin and significance in pulmonary surfactant production and secretion. Lungs were examined with an electron microscope using ultrathin sectioning, freeze-fracture technique, and morphometry. The bar-like structures in type II cells consisted of a concentrically arrangeed endoplasmic reticulum containing some amount of osmiophilic periodic material similar to the lamellae of lamellar bodies. Pilocarpine induced the accumulation of lamellar bodies of normal size which paralleled the increase in the number of bar-like structures in the cytoplasm of the type II cells. Cycloheximide induced a decrease in size of the lamellar bodies and an enlargement of the bar-like structures. Our morphological findings suggest that: (1) The phospholipid that would normally be incorporated into the lamellar bodies might be sequested instead in the concentrically arranged endoplasmic reticulum, forming the bar-like structures, and (2) The enlargement and the increased number of bar-like structures may be responsible in part for the changed metabolic process of surfactant production by alveolar type II cells.