Distribution of collagen fibers in the aggregated lymphoid follicles of swine terminal ileum.

The arrangement of the collagen bundles was studied in the Peyer's patches of swine terminal ileum, by means of light microscopy (using silver-impregnation technique and picrosirius F3BA staining) and scanning electron microscopy (after NaOH-maceration). The lymphoid tissue forms a large and continuous patch along the antimesenteric border. The follicles are disposed mainly in the tela submucosa and sometimes they reached in the tunica mucosa surface (follicle/dome structures). Some follicles are located in the lamina propria of the tunica mucosa. Light microscopy showed black and brown-stained fibers, and yellow and red, and green-stained fibers, respectively by silver impregnation technique and picrosirius red staining, in the tela submucosa. In this tela, by scanning electron microscopy, the collagen fibers appeared as thick bundles forming a network of parallel layers. This network was denser in the interfollicular than in the follicular area, and formed a capsule surrounding the lymphoid follicles. Our results pointed out that a clear correspondence exists between the findings of currently used light microscopy techniques and the scanning electron microscopy after alkali-water maceration method. The arrangement of the collagen fibers in the antimesenteric border of the tela submucosa suggested a functional compartmentalization within the aggregated lymphoid follicles. This could facilitate the antigen-to-cell and cell-to-cell interaction during the immune response and thus create a suitable microenvironment for an active cell metabolism. The tunica mucosa showed a porous structure and its frequent gaps were likely the sites through which lymphocytes and other cells could freely migrate thus participating in the immunological activities of these structures.