1. Section on Tissue Biophysics and Biomimetics, Laboratory of Integrative and Medical Biophysics, National Institutes of Health, Bethesda, Maryland 20892, and Laboratoire de Spectrométrie Physique CNRS UMR 5588, Université J. Fourier de Grenoble, B.P. 87, 38402 St. Martin d'Hères, France