This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The neonatal Fc receptor (FcRn) transports immunoglobulin G (IgG) across epithelia, binding IgG into acidic vesicles (pH<6.5) that form on the apical surface of the cell and releasing IgG into the blood at pH7.4. Structural features of this process are being studied by EM tomography. In particular we have conjugated nanogold particles to the constant fragment (Fc) of IgG and have both administered the conjugates at low pH to cultured monolayers of MDCK cells and given them as food to neonatal rats. Following preparation of the relevant cells or tissues by rapid freezing and freeze substitution, we have looked for evidence of gold internalization in the vesicular compartments at and near the apical side of the cells. Preliminary tomograms of these samples show some internalization of gold, however it is difficult to visualize.