fixed with 2.5% glutaraldehyde, dehydrated in a series of ascending ethanol concentrations (50e100%) (v/v), dried in desiccator, and coated with gold in a sputter-coater (SCD 005; BAL-TEC,
Switzerland); then, it was examined with a scanning electron microscope (Quanta 200FEG; FEI Company, Hillsboro, Oregon, USA).