Secretion of surfactant by rat alveolar type II cells: morphometric analysis and three-dimensional reconstruction

Secretion of surfactant by alveolar type II cells occurs by exocytosis and is stimulated by several mechanical and hormonal mechanisms, including beta-agonists. We examined the morphological correlates of this process by electron microscopy and by three dimensional reconstructions of rat lung alveolar epithelial type II cells before and after isoproterenol stimulation. Male rats were given a dose of isoproterenol 2 hours prior to anesthesia. The lungs were fixed by intratracheal instillation and lung tissue was processed by methods designed to enhance the retention of saturated phospholipids. The type II cells were evaluated by transmission and scanning electron microscopy and morphometry of the type II cells was performed. Three dimensional reconstruction of secretory events at the alveolar air border was also done. We observed a dose dependent increase in elliptical cell-surface pores averaging 0.2 X 0.4 micron in size on the alveolar luminal side of type II cells. Most often these pores were surrounded by an elevated area lacking microvilli and overlying an area of granular cytoplasm. The pores observed were about half the average diameter of the lamellar bodies and a deformation of that spherical secretory organelle was seen frequently. We conclude these cell-surface pores represent the structures through which surfactant is released by type II cells. During the secretion of lamellar bodies, deformation may be imposed at the restriction of the surface pores. This deformation could be important in the transformation of the lamellar material from an intracellular storage site to its active role on the alveolar surface.