The epithelium of normal human alveolar mucosa originating from the anterior vestibulum was subjected to stereologic analysis. Eight biopsies were collected half-way between the muco gingival junction and the vestibular fornix from 20 to 50 year-old females, and processed for light and electron microscopy. At two levels of magnification, electron micrographs were sampled from four artificially selected strata in regions of epithelial ridges. Stereologic point counting based on a computer-aided system for analyzing stratified epithelia served for examining a total of about 860 electron micrographs. The alveolar epithelium was 0.26 mm thick, occasionally interdigitated by short, slender connective tissue papillae, and consisted of (1) a narrow basal and suprabasal, and (2) a broad spinous and surface compartment. It displayed a differentiation pattern which, in most subjects studied, was similar to that of normal human buccal epithelium, however, on the average, produced less mature surface cells. This pattern was expressed mainly by a density increase of cytoplasmic filaments (98 Å in diameter), a concomitant decrease of the cytoplasmic ground substance, the formation of dark-cored membrane coating granules, and individually variable amounts of glycogen deposition. In some subjects, a mixed differentiation pattern was found. The structural organization of alveolar epithelium, in analogy to cheek epithelium, was compatible with the function of distensibility
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