1. School of Life Sciences, University of Warwick, Coventry CV4 7AL, U.K.
2. JEOL (UK) Ltd, Welwyn Garden City AL7 1LT, U.K.
3. Centre For Ultrastructural Imaging, King's College London, New Hunts House, Guy's Campus, London SE1 1UL, U.K.
4. School of Biosciences, University of Kent, Canterbury CT2 7NJ, U.K.