Semithin (1 μm) sections were cut with a glass knife using a Sorvall Porter-Blum MT2-B ultramicrotome and stained with 1% toluidine blue in 1% Na-borate. Ultrathin sections (70 nm thick) were stained with 5% uranyl acetate in 70% ethanol and lead citrate (Reynolds) and observed using a Philips CM100 transmission electron microscope (Microscopy Center, University of L’Aquila, Italy).