1. Contribution from the Centre for Drug Delivery Research and Electron Microscopy Unit, The School of Pharmacy, University of London, London WC1N 1AX, United Kingdom, Immunologie et Chimie Thérapeutiques, Institut de Biologie Moléculaire et Cellulaire, UPR9021 CNRS, 67084 Strasbourg, France, and Dipartimento di Scienze Farmaceutiche, Università di Trieste, 34127 Trieste, Italy