Effect of fixation on the light microscopical visualization of mast cells in the mucosa and connective tissue of the human duodenum.

Mast cells have been studied in the mucosa and connective tissue of 28 biopsies of human duodenum. They were collected from individuals who for various reasons had to undergo upper intestinal endoscopy. Tissues were fixed in both standard formalin and a formalin-acetic acid (FA) fixative developed for visualization of mucosal mast cells. With both fixatives connective tissue mast cells (CTMC) were equally well recognized. Only with the FA fixative, an additional population of mast cells was observed both in the mucosa and the connective tissue. The population consisted of round cells, always smaller than CTMC, with a number of granules staining violet with toluidine blue. These mast cells comprised two cell types, differing in size, the larger being present in both the mucosa and the connective tissue, the smaller ones in a subepithelial position in the crypts of Lieberkühn and in the adjacent submucosa. It was suggested that this first mentioned additional population was similar to the mucosal mast cells (MMC) observed in rats and mice. The subepithelially located metachromatic cells differed in morphology from the globule leukocytes found in rats and mice. It was concluded that in the human duodenum a population of cells with MMC staining characteristics is present, which can only be visualized under special fixation conditions. Their further characterization and role in various disease conditions remains to be elucidated.