The projecting papilla of the duodenum is covered with a circumscribed zone of mucous membrane that differs sharply from that of the surrounding duodenum. The mucosa of the papilla has few or no villi, and often the mucosal glands are large and appear hyperplastic. Goblet cells are conspicuous, and the cells lining the glands are larger and longer than in the epithelium of the adjacent duodenal glands. Frierson Jr  noted that glands near the surface of the papilla may be distended with mucus, and described them as distended glands. Such a change has also been termed 'adenomatoid hyperplasia' , and a similar
change with excess mucus secretion, even in the full term fetus, has been noted . This type of mucous membrane ends abruptly at the base of the papilla.
The distended glands of the ampullary mucosa are frequently found along the outer surface of the papilla of Vater at incidences of 59% in the resected materials of pancreatobiliary diseases, 22% in infants and 77% in adult autopsies, as shown in table 1. Normally, such glands replace a portion of the surrounding duodenal mucosa and measure on average 1,532 |xm in length (fig. 5a) . There is no particular border between these distended glands and the ampullary and duodenal mucosae.
The distended glands are to be found situated on the Oddi's sphincter muscle and not on the muscularis mucosae of the duodenum as follows: under normal circumstances, with no distended glands, the Oddi's sphincter muscle and the muscularis mucosae of the duodenum merge in an end-to-end, acute-angled manner on the outer edge of the papilla of Vater. However, in cases with distended glands the muscularis mucosae of the duodenum is joined with the Oddi's sphincter muscle in an end-to-side, less acute, rather right-angled' manner at the latter side portion, away from the outer edge (fig. 6). Thus, the a
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