FREEZE-FRACTURE STUDY OF THE RAT LUNG Ⅱ. COMPARTMENTALIZATION OF LAMELLAR INCLUSION BODIES IN RAT ALVEOLAR TYPE Ⅱ CELL
Differences in cleaving behavior of alveolar type Ⅱ cell lamellar bodies examined by the freeze-fracture technique were observed in specimens processed by various preparatory methods. All methods preserved the general appearance of rat type Ⅱ cells; however, in tissue frozen after glycerinization at 38°C without chemical fixation, the lamellar bodies were fractured more superficially, so that the outermost layer of the lamellar bodies could be observed. Results of this study suggest that some lamellar bodies are composed of parallel stacks of discoid compartments which consist of multilayered lamellae. I believe that glycerinization at higher temperatures enhances the mutual adhesiveness of lamellae during subsequent freeze-fracturing. Based on these observations, I propose that disc-like compartments represent lamellar aggregates of surfactant phospholipids synthesized by enzymes at the limiting membrane, and that the linear zone of intersection of these discs with the membrane represents the site of active formation for these structures.