The author acknowledges the French Agence Nationale de la
Recherche (ANR) for financial support, under grant NAPOLECO
(ANR- 2010-INTB-910-01), and the Centre Lyonnais de Microscopie
(CLYM) for the access to the JEOL 2010F and Nanoscope 5 (VEECO)
microscopes and the Centre Technologique des Microstructures for
the access to the cryo-ultramicrotome. KMV would also like to
thank B. Vastenhout and F. Gilleron for fruitful discussions
regarding staining methods.