Scanning electron microscopic studies on the subepithelial tissue of the gastrointestinal mucosa of the rat.

The three-dimensional architecture of the subepithelial tissue of the gastric, the small and the large intestinal mucosa of the rat was observed by scanning electron microscopy (SEM) after removal of cellular elements through prolonged osmication followed by ultrasonication (the method by Highison and Low, 1982), or by cell-maceration with a low temperature NaOH solution (the method by Ohtani, 1987). The basal lamina was exposed by the former method, and the collagenous fibrous sheet immediately under the basal lamina was disclosed by the latter. The surface of the subepithelial tissue is grossly smooth in the pyloric gland and crypts of the small and the large intestine. However, in the fundic glands of the stomach, the surface structure of the subepithelial tissue greatly differs according to glandular location. In the pit of the fundic glands, the surface of both the basal lamina and the sub-basal laminar fibrous sheet is smooth. In the neck region, however, shallow round depressions are seen. The most striking feature of the subepithelial tissue of the fundic glands is the presence of numerous hemispherical concavities in the middle and basal regions of the glands which harbor parietal cells. On the surface of the small intestinal villi, the basal lamina is elevated by the underlying capillary network and forms a meshwork of ridges that surround shallow basins in which numerous round fenestrations 1-5 microns in diameter are seen. The fibrous sheath of the marginal arteriole is observed as a cord-like protuberance on the apical margin of the villi; this suggests its role in the maintenance of the structural integrity of the villi. In the large intestine, well defined round fenestrations are clearly seen, mainly distributed on the upper third of the crypts, and continuing to the lamina propria.