Passive diffusion in small intestinal mucosa in childhood.

The passive permeability of the small intestinal mucosa in childhood was investigated by studying the penetration of two light- and electron-dense tracer molecules (ruthenium red mol. wt = 1000 and horseradish peroxidase mol. wt = 40 000) into fixed biopsy specimens. Ruthenium red confirmed the presence of damaged and extruding epithelial cells in normal mucosa and showed a significant increase in such cells in abnormal mucosa. Horseradish peroxidase demonstrated that antigenically-sized molecules could enter a proportion of these cells and significantly more cells were penetrated in abnormal mucosa, reflecting the ruthenium red results. Thus, anatomical pathways for passive diffusion of antigen through damaged or extruding cells exist in normal and, to a greater extent, in abnormal small intestinal mucosae in childhood. Further study using living tissue may reveal the importance of this phenomenon in relation to the active uptake of antigen.