This project is focused on two intestinal epthelial cells that are specialized for uptake of luminal macromolecules--the absorptive cell of neonatalileum, and the M cell of Peyers's patch. Although they differ dramatically in structure and function, both cell types provide valuable models for study of epithelial membrane traffic, intracellular sorting, and transepithelial transport. In neonatal ileum, tracer proteins and immunocytochemistry will be used to detect sorting of physiologic and foreign adsorptive ligands and receptors at luminal cell surfaces and in apical endosomal tubules, and selective transepithelial transport. Subcellular fractions enriched in tubules will be analysed biochemically and used to raise organelle-specific antibodies, that in turn will serve as membrane markers. The independent recycling of apical and basolateral endocytic membranes and the convergence of these polarized pathways in the central lysosomal system will be demonstrated using tracer ligands and lysosome-specific antibodies. Transport of antigens across rat and rabbit M cells will be assessed in quantitative studies using ultrastructural and fluorescent tracers and radiolabeled probes. Relative efficiencies of transport of soluble vs. adherant molecules and particles, and specific types of adherent monomeric or polymeric molecules will be systematically compared. M cell membrane domains and the transepithelial transport pathway will be further explored using domain-specific antibodies and probes specific for acidic compartments, along with tracer proteins. New information on membrane traffic and selective and nonselective protein transport in these cells will clarify their roles in epithelial barrier function and mucosal immunity.