A freeze fracture and thin section study of intestinal cell membranes and intercellular junctions of a nematode, Ascaris.

The microvillar and lumenal plasma membrane P-face of Ascaris intestinal cells is shown to be covered by relatively large (13 nm) particles at a fairly high density (1000/microm2), while the E-face has virtually none. The P-face of the lateral cell membranes, those separating the cells, have fewer and smaller (8 nm) particles. The intestinal cells are also shown to be connected by an apical complex of smooth septate and tricellular junctions similar to those found between some insect midgut cells. A periodic layer of tannic acid staining material is found on the cytoplasmic sides of the smooth septate junction, and when the intercellular space is filled with lanthanum, smoothly curved, 10 nm wide septal walls can be seen. Below the belt of septate junctions are a large number of gap junctions. These have closely packed arrays of particles on the P-face with some particle aggregates adhering to the closely packed pit arrays on the E-face.