The development of the epithelium and its innervation in the avian extra-pulmonary respiratory tract.

The development of the epithelium and its innervation was investigated with the electron microscope in the larynx and trachea of embryo domestic fowls between the 10th and 21st days of incubation. From days 10 to 16 the airways were lined by undifferentiated cells which on day 15 or 16 became arranged in two distinct layers. Differentiation of the epithelial cells commenced on day 17, so that at hatching on day 21 fully differentiated ciliated cells, goblet cells, non-ciliated columnar cells, granular cells and basal cells were arranged into a simple or pseudostratified columnar epithelium. Comparisons are made with the differentiation of the respiratory epithelium in other vertebrates. Intra-epithelial axons were first seen on day 15 and increased rapidly in number. The varicose axon profiles contained both granular and agranular vesicles. The axons lay close to most types of epithelial cell, and some approached near to the surface of the epithelium. A synaptic complex, possibly efferent, involving a granular epithelial cell is described.