Further ultrastructural observations on the alveolar epithelium of the newborn rat

Abstract

Typical osmiophilic lamellated bodies of the type II alveolar pneumonocyte of the neonate rat contain parallel, straight lamellae. They often enclose membrane-bounded regions of cytoplasm resembling the sequestered areas in autolysosomes. In other lamellated bodies, the lamellae are less electron dense and are arranged in concentric circles. The concentrically ordered lamellae are eliminated into the alveolar cavity. The osmiophilic lamellated bodies are interpreted as advanced lysosomes.Mitochondria of the type II alveolar cell are often indented by lamellar masses. Mitochondria enclosing lamellae or dense masses persist for a time after the type I alveolar cell morphology has been attained. Lamellar masses in mitochondria arc believed to be related to lysosomal activity in the surrounding cytoplasm, and examples of such inclusions in mitochrondria of other tissues arc cited. An alveolar cell mitochondrion indented by lamellated material may continue to operate, the mitochondrion proper and the pocket of lysosomederived lamellae combining as a functional unit.