A new cytoplasmic bar-like structure seen in type II alveolar cells of rat lungs.

Type II alveolar cells from healthy (control) and from pilocarpine-treated rats were studied using transmission electron microscopy of ultra-thin sections and freeze fracture replicas. In freeze-fracture preparations, a bar-like laminated structure about 0.4 micrometer in diameter and 1 similar to or approximately 6 micrometer in length was found in type II alveolar cells; it seemed to be identical to the membrane-bound slits containing electron dense materials which were also observed in ultra-thin sections. The frequency of this structure in the cells increased together with an increase in the number of lamellar bodies after injection of pilocarpine, suggesting a close connection between the bar-like structure and the formation of pulmonary surfactant.