They were then post-fixed in 1% OsO4 for 1 h, dehydrated in a graded acetone series, dried by critical point method in a Polaron Jumbo (Quorum Technologies Ltd., Lewes, UK) apparatus and coated with gold in an Edwards S150A Sputter Coater unit (Perkin-Elmer, Santa Clara CA, USA). The specimens were examined with a Zeiss DSM 962 conventional scanning electron microscope.