A study of the posterior esophagus in the winter flounder, Pleuronectes americanus, and the yellowtail flounder, Pleuronectes ferruginea: morphological evidence for pregastric digestion?

The morphology and the histochemistry from the mucus of the posterior esophagus of the winter flounder and the yellowtail flounder were examined using light and electron microscopy. The esophageal mucosa of both species was organized into elaborate branching folds. The epithelium consisted of a stratified layer of cuboidal cells interspersed with mucus-producing goblet cells. The cuboidal cells in the surface layer also exhibited a secretory function and were characterized by Golgi-associated granules and apical micoridges. These cells were termed esophageal surface secreting cells (ESSCs). The granules of the winter flounder ESSCs were ultrastructurally similar to mucous granules, whereas those of the yellowtail flounder were reminiscent of serous granules. Both types were analogous to those associated with salivary glands in mammals. Both goblet cells and ESSCs from the winter flounder stained positive for sulphated acid mucins, whereas in the yellowtail flounder goblet cells stained positive for sulphated and nonsulphated acid mucin combinations and ESSCs stained only for nonsulphated acid mucins. A pregastric digestive function is proposed, based upon the thick muscularis externa composed of striated circular muscle, the increased surface area due to mucosal folding, the complex histochemistry of the mucus, and the secretory nature of the ESSCs.