The fine structure of the mouse tracheal epithelium from the fetal period to the stage of old age was examined with electron and light microscopes. The tracheal epithelium of the adult mouse is ciliated and single columnar in shape, consisting of three kinds of cells: ciliated, non-ciliated and basal. Neither goblet nor brush cells were found. The earliest signs of the cytodifferentiation of ciliated and non-ciliated cells appear at 15 and 17 days of gestation respectively. The secretory granules in non-ciliated cells appear at 17 days of gestation, and increase in number from 18 days of gestation (one day before birth) to 1 day after birth and decrease in number rapidly, while elements of smooth endoplasmic reticulum appear in this cell. Non-ciliated cells constitute the major component of the tracheal epithelium in the adult mouse. They have two kinds of granules: one a secretory granule surrounded by a single limiting membrane and released by exocytosis, and the other surrounded by double limiting membranes and considered to be derived from mitochondria.
With the advance of age, some parts of the epithelium invaginate into the lamina propria without any noticeable changes in either the epithelial cells or the basal lamina. These invaginations, which the present authors named “cyst-like structures, ” are found in all mice over four months of age examined, and increase in number and size with age. They seem to be physiological structures and are distributed in all parts of the tracheal epithelium from the first tracheal ring to the tracheal bifurcation. They contain concentric circular materials with entangled filaments (15-20nm in diameter) and/or destructed cell debris mainly derived from granular leucocytes.