Ultrastructure of the abdominal air sac of the fowl.

Abstract

Summary Electron microscopy of the abdominal air sac of fowls revealed that the tissue is composed of 4 distinct cell types: flat mesothelial serosal cells on the peritoneal surface, flattened epithelial cells, and basal cells lining the air sac cavity, and fibroblasts and collagen between the two surfaces. The epithelial cell cytoplasm contains dense osmiophilic lamellar inclusions, and is covered with numerous microvilli. The inclusions appear similar to the structures believed to be the source of the alveolar lining layer in the lungs of several species. The thinner portions of the tissue are avascular. However, the mesothelial serosal cells reveal numerous pinocytic vesicles, suggesting this may be the source of essential tissue nutrients. The basal cells present may account for the metaplasia to ciliated and goblet cells observed when air sacs are subjected to prolonged insult.