Esophageal mucin: An adherent mucus gel barrier is absent in the normal esophagus but present in columnar-lined Barrett's esophagus

OBJECTIVES: The presence of a protective adherent mucus gel barrier against gastric reflux in the healthy esophagus is uncertain. The aim was to chara cterize the surface mucin composition and determine the extent of any adher ent mucus gel layer on the normal esophagus, and compare this with that in Barrett's esophagus. METHODS: Isolated surface mucins were characterized by density centrifugati on, gel filtration chromatography and chemical composition. Adherent surfac e mucus was visualized in situ on unfixed and cryostat sections of mucosa a nd biopsies using a method that preserves mucus layer thickness. RESULTS: There was a complete absence of adherent mucus gel layers on norma l human, pig, and rat esophagi. This was in contrast to the thick adherent mucous layer (median thickness = 100-200 mum) seen on the corresponding gas tric mucosa. Small quantities of glycoprotein with a composition characteri stic of a secretory mucin were isolated from the pig esophagus surface. The mucin, density range between 1.44 and 1.48 g.ml(-1), contained 80% carbohy drate and was rich in serine, threonine, and proline. The mucin fragmented into smaller glycoprotein units on proteolysis and partially on reduction. Cryostat sections from columnar-lined esophageal. biopsies had a substantia l adherent surface mucous layer (median thickness = 90 mum, interquartile r ange = 84-94 mum) staining for neutral mucins (gastric-type epithelium) and acidic mucins (intestinal metaplasia). CONCLUSIONS: A secretory mucin, with an analysis distinct from that of gast ric or salivary mucin, is present in very small quantities on the esophagea l mucosa and in amounts insufficient to form an adherent gel layer. It is u nlikely that mucus has a role in protecting the normal esophagus against re flux. However, an adherent mucous layer was observed over columnar-lined es ophagus, and this may protect against reflux. (Am J Gastroenterol 2001;96:2 575-2583. (C) 2001 by Am. Coll. of Gastroenterology).