The mechanism by which basal cells play a role in attachment of airway epithelium to the basal lamina has not been determined. Our hypothesis is that basal cells form a structural bridge between columnar cells and the basal lamina via hemidesmosomes, the cytoskeleton, and desmosomes. To evaluate this hypothesis, we determined the percentage of the columnar cell surface area associated with attachment to the basal lamina and the basal cell in tracheal epithelia of different heights. Tracheas from mice, hamsters, rats, bonnet monkeys, cats, and sheep were prepared for electron microscopy by standard techniques. The height of the epithelia ranged from 8.6 microns in the hamster to 56.8 microns in the sheep. The number of basal cells/100 microns ranged from 3.4 in the hamster to 21.4 in the sheep. The percentage of the basal lamina covered by basal cells increased from 32.6 in the hamster to 94.7 in the sheep. In the shorter epithelia of the hamster, 32% of the columnar cell attachment to the basal lamina was indirect through basal cells, and in the taller epithelia of the sheep, 92% of the columnar cell attachment was by this means. Conversely, the percentage of columnar cell surface in contact with the basal lamina decreased from 67.4% in the hamster to 5.3% in the sheep. These data demonstrate that basal cells play a role in attachment of columnar epithelium to the basal lamina by presenting a surface area for cell-to-cell attachment, thus acting as a bridge between columnar cells and the basal lamina.

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The morphology of basal cells varies according to the height of the epithelium they are associated with. In taller epithelium, basal cells appear to have more tonofilaments (keratin filaments) than basal cells in shorter epithelium. We hypothesized that the changes in basal cell structure represent differentiation of junctional adhesion mechanisms related to the attachment strength necessary for the various-height epithelium. To evaluate this hypothesis, we used light- and electron-microscopic morphometry to quantitate junctional adhesion structures in basal cells from mice, rats, cats, rabbits, and sheep. The height of the tracheal columnar epithelium ranged from 12.0 microns in the mouse to 56.8 microns in the sheep. The volume density of basal cell keratin filaments ranged from 0.012 in the rat to 0.261 in the sheep and total desmosome length/basal cell profile (BCP) ranged from 0.08 microns to 1.77 microns, respectively. Total hemidesmosome length/BCP was similar in each airway sample. A close correlation was obtained between the height of the epithelium and the volume fraction of keratin filaments (r = 0.96) and total desmosome length/BCP (r = 0.94) in all airways studied. Total hemidesmosome length/BCP was not closely related to the height of the epithelium (r = 0.31). Based on these observations, we conclude that the basal cell is a differentiated cell with respect to junctional adhesion and a primary function of the airway basal cell is for attachment of columnar cells to the basal lamina.

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