The epithelial basal lamina of the isolated colonic mucosa: scanning and transmission electron microscopy.

The morphology of the rat colonic basement membrane was studied under control conditions and after induction of secretion by distension in the Ussing chamber. The basement membrane was visualized by removing the epithelium with the aid of Ca2+ chelation combined with vibration. Specimens with and without metallic coating were studied with SEM and TEM, respectively. The surface of the basement membrane in the surface region of the epithelium, i.e. between the mouths of the crypts, is rough. In contrast, the deeper parts of the crypts have a rather smooth basal lamina, which is finely pleated. Fenestrations of the basal lamina with a diameter of 0.5-1 microns were found frequently at the surface region, but less frequently in the crypts. Fenestrations with diameters larger than 2 microns were never observed. After the distension experiments and long-time incubation in vitro (5 h), the size of the fenestrations remained unchanged, however, their number increased compared to the controls.