Light and electron microscopic study of the thoracic respiratory air sacs of the fowl

There are conflicting reports in the existing literature on the nature of the epithelial lining and the content of the supporting connective tissue of the respiratory air sacs of birds. The present study describes the light and electron microscopic structure of the thoracic air sacs of the fowl. A simple squamous epithelium lined the greater part of the thoracic air sacs. The squamous cells characteristically contained vesicles filled with lamellar or myelinoid material. Localized areas of cuboidal to columnar ciliated epithelium were randomly distributed and often associated with underlying blood vessels. Isolated ciliated cells first appeared on squamous or low cuboidal cells and increased in frequency as the cells became taller. Occasional basal and goblet cells were seen between the ciliated columnar cells. A fibrous connective tissue stroma supported the epithelium. Fine elastic fibres were particularly prevalent immediately below the epithelium. Isolated smooth muscle myocytes were present in the connective tissue stroma. A sheet of smooth muscle extended some distance into the membrane from the attachment of the latter to the body wall. Numerous small blood vessels, lymphatics and occasional nerve bundles were observed in the stroma.