1. [3] This paper was supported in part by USPHS Grant K23-DE016324.
2. [2] The authors greatly acknowledge Drs. S. Honigberg and D. Guo for permission to use their specimens in preparation of this paper.
3. [1] All micrographs presented in this paper were obtained with Field Emission SEM XL30 (FEI, Hillsboro, OR).