Thin sections were examined at 80 kV using a model H7500 transmission electron microscope (Hitachi High Technologies, Inc.). Digital images were captured using a model HR-100 camera system (Advanced Microscopy Techniques, Inc.) and processed with PhotoShop (Adobe Systems, Inc.). Semi-thick sections were examined at 120 kV using a G2 Spirit BioTwin model TEM (FEI, Inc.) equipped with a tilt stage, Xplore3D acquisition system, and model Ultrascan 1000 CCD system (Gatan, Inc.). Tilt series images were collected at 0 and +/268 degrees with 1 degree intervals. Images were aligned and reconstructed into tomogram volumes using Inspect3D software (FEI, Inc.) and rendered using Amira (Visage Imaging, Inc.).