Using morphologic and ultrastructural criteria, the present study in the rat was undertaken to distinguish lymphoepithelial modules from lymphoid aggregates and lymph modes, and to demonstrate the uptake and transport of ferritin and latex particles, instilled in the trachea, by the lymphoepithelium. Unlike lymph nodes or lymphoid aggregates, lymphoepithelial nodules can be distinguished by the following characteristics of the lymphoepithelium: it is flattened and devoid of goblet cells; it contains intracellular and "intercellular" vacuoles; occasional macrophage-like cells infiltrates and surrounding lymphocytes clusters can be observed. At any time, no latex particle was found inside or under the lymphoepithelium, but until 1 hour after local instillation, ferritin particles were localized close to the cilia, at the luminal surface of the epithelium, within vesicles at the surface of some cells, and in the intercellular space. Moreover, ferritin particles were consistently present on both sides of the basement membrane after 1 hour. These results suggest that lymphoepithelial nodules may be considered as original structures capable of uptake and transport across the basement membrane of certain foreign materials.

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