The innervation of the trachea and extrapulmonary bronchi of the mouse.

In the mouse, nerves were located throughout the trachea and extrapulmonary bronchi in both the smooth muscle and the connective tissue. However, no nerves were found within the epithelium. In the smooth muscle there were large numbers of nonmyelinated nerves. These were usually 'en passant' elements but varicosities containing small mitochondria and vesicles were also seen; these axons sometimes appeared to be efferent to the muscle. Unilateral cervical vagotomy reduced the numbers of nerves in the muscle of the trachea and ipsilateral primary bronchus, suggesting that they were afferent. The intramuscular nerves were characterized in terms of their complement of cytoplasmic organelles; in particular nerves containing many mitochondria disappeared following vagotomy. Pretreatment of mice with 5-hydroxydopamine to accentuate the electron-opacity of catecholamine-containing granules resulted in 3.5% of the nerves within tracheal muscle showing such granules. The afferent nerves of the smooth muscle may be complex branching structures with many varicosities. The absence of epithelial nerves may be related to the absence of the cough reflex in the mouse.

The cells of the tracheobronchial epithelium of the mouse: a quantitative light and electron microscope study.

The epithelium of the conducting airways of the mouse consists of a single layer of cells. The number, type and form of these cells have been investigated at five airway levels from the trachea to the distal conducting bronchi with both light and electron microscopes. Contrary to what is found in other species, the majority (50-60%) of cells in the murine airway epithelium are Clara cells. Mucus-producing tissue was infrequent throughout the airways, though epithelial mucous cells occurred in increased numbers at the carina and in the primary bronchus. No mucous or serous cells or submucosal glands were seen in intralobular airways. On a morphological basis, three distinct forms of Clara cell were recognized. On occasion, cells were observed which were apparently transitional types between these and also between Clara cells and mucous or ciliated cells. It is suggested that the 'transforming' cells may indicate a role for the Clara cell as a developmental cell involved in the epithelial cell turnover. Evidence is also provided that Clara cells may undergo both apocrine and merocrine secretion and, it is argued that the latter may be of a PAS + ve material. Free nerve endings were not seen in the epithelium. This may be related to athe restricted ability of mice to cough. It is suggested that the lack of mucus-producing tissue and of cough reflex may be due to the small diameter of the mouse airways.

The distribution and structure of cells in the tracheal epithelium of the mouse.

The tracheal epithelium of the mouse is a single layer of columnar cells resting on a basement membrane. Many of the cell types resemble those of other species. However, goblet cells are rare and ciliated cells occur only in scattered patches. Submucosal glands are absent from all but the highest reaches of the airway. The major proportion of the epithelial cells are non-ciliated. These usually project into the lumen of the trachea. Large amounts of smooth endoplasmic reticulum and many secretory vesicles occur within the cytoplasm. Secretory activity of these cells may be either apocrine or merocrine and these cells may transform into other cell types. It is suggested that these non-ciliated cells are Clara cells and that the mouse tracheal epithelium may make a useful model for the study of this type of cell.