Ultrastructural study of the alveolar-capillary membrane and the tubular endoplasmic reticulum in alveolar type I cells of the caprine lung

The ultrastructural features of alveolar type I cells of the goat lung were studied by using vascular perfusion and direct airway instillation of fixatives. The morphological features of smooth endoplasmic reticulum (SER) were characterized by measuring the diameter of individual SER tubules which conformed in size and appearance to the tubular endoplasmic reticulum (TER) already described in various types of epithelium. The TER appeared as large tubular aggregates in a palisade arrangement; these aggregates ramified into various areas of the extended cytoplasm of alveolar type I cells. The TER also existed as a mixture of short cisternae and vesicles, and glycogen alpha particles were present in the non-perikaryonic portion of the cell. The different forms of TER had varying relationships to the plasmalemma. The interchangable configurations seen in the structure of TER indicated the functional modalities of the cells and were comparable to similar structural modifications in electrolyte-secreting cells. The role of TER, microtubules, and large populations of endocytic vesicles in the alveolar type I cells in the goat lung is examined in the context of physiological eructation of rumenal gases and the absorption of electrolyte-rich fluids which escape into the lung at each eructation in ruminants.