V O LU M E T WO S I X T Y F I V E INTERNATIONAL REVIEW OF CYTOLOGY A Survey of Cell Biology EDITED BY KWANG W. JEON Department of Biochemistry University of Tennessee Knoxville, Tennessee AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Academic Press is an imprint of Elsevier