Hypertrophic smooth muscle in the partially obstructed opossum esophagus. The model: histological and ultrastructural observations. Obstruction is a complication of many esophageal diseases, but the morphological changes occurring in the obstructed esophagus are poorly understood. We developed a model of esophageal obstruction in the American opossum, Didelphis virginiana. A nonconstricting band around the gastroesophageal junction led to esophageal distention and tortuosity in the weeks following its placement. Despite a marked increase of the esophageal circumference, the esophageal wall was not thinned, and the circular muscle layer had actually increased its thickness. This was due to an increase in the size of individual smooth muscle cells with proportional increases in the cell surface area and volume. The electron density of hypertrophic smooth muscle cells varied much more than that of normal esophageal smooth muscle cells. As cell size increased, the tissue became more compact and the size of the extracellular space decreased. Also, the extracellular space was filled by an amorphous electron-dense material. Additional changes in the structure of hypertrophic smooth muscle cells included prominent intermediate filaments in the vicinity of thick filaments. There was no difference in the structure of the hypertrophic smooth muscle at 4 weeks and at 8 weeks after placement of the band. The morphological features described here resemble those seen in human esophageal spasm and achalasia of humans and could affect esophageal smooth muscle function. 