Abstract
Lungs from marsupials, bats and rodents were studied by light and electron microscopy. In all three groups, the great alveolar cells exhibit similar morphologic and cytochemical characteristics. Cytoplasmic vacuoles seen in these cells by light microscopy correspond to cytosomes that are demonstrable in them by electron microscopy. Such cytosomes are osmiophilic, periodic acid-Schiff-positive and stainable with Sudan black after acetone extraction. After fixation in a mixture of aldehydes, followed by extraction in chloroform-methanol and postfixation in osmium tetroxide, cytosomes lose their osmiophilia. The cytoplasm of the great alveolar cell is notable for a loosely ordered granular endoplasmic reticulum, an extensive Golgi apparatus and numerous multivesicular bodies. Many forms transitional in appearance between multivesicular bodies and cytosomes are present. In these, osmiophilic matter occupies the intervesicular space. It is proposed that these bodies are the precursors of cytosomes. The cytosomes are interpreted to be products of the "lysosomal" system in this cell. Ultimately they are secreted onto the alveolar surface.
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