Fine structural aspects of the development and aging of the tracheal epithelium of mice.

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Abstract
Fine structure of the mouse tracheal epithelium from the fetal period to the stage of old age was examined with EM and light microscopy. Tracheal epithelium of the adult mouse is ciliated and single columnar in shape, consisting of 3 kinds of cells: ciliated, nonciliated and basal. Neither goblet nor brush cells were found. Earliest signs of the cytodifferentiation of ciliated and nonciliated cells appear at 15 and 17 days of gestation, respectively. Secretory granules in nonciliated cells appear at 17 days of gestation, and increase in number from 18 days of gestation (1 day before birth) to 1 day after birth and decrease in number rapidly, while elements of smooth endoplasmic reticulum appear in this cell. Nonciliated cells constitute the major component of the tracheal epithelium in the adult mouse. They have 2 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 the epithelial cells or the basal lamina. These invaginations, here named cyst-like structures, are found in all mice over 4 mo. 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 1st tracheal ring to the tracheal bifurcation. They contain concentric circular materials with entangled filaments (15-20 nm in diameter) and/or destroyed cell debris mainly derived from granular leukocytes.