Scanning electron microscopy.Samples were fixed in formaldehyde, rinsed in phosphate-buffered saline, and freeze-dried in an Edwards-Pearse freeze-drier (BOC Edwards, Crawley, United Kingdom). The samples were then mounted on aluminum planchettes by using adhesive carbon tabs, sputter coated with gold, and viewed in a JEOL JSM6310 scanning electron microscope at 10 kV (JEOL, Tokyo, Japan).