Department of Ultrastructure, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium. ivwalle@vub.ac.be|Department of Ultrastructure, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium. ivwalle@vub.ac.be|Department of Ultrastructure, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium. ivwalle@vub.ac.be