Ultrastructure

Ultrastructure (or ultra-structure) is the nanostructure of a biological specimen, such as a cell, tissue, or organ, at scales smaller than can be viewed with light microscopy. It is viewed with ultramicroscopy or electron microscopy. Such cellular structures as organelles, which allow the cell to function properly within its specified environment, can be examined at the ultrastructural level.

Ultrastructure, along with molecular phylogeny, has often been a reliable (that is, phylogenetic) way of classifying organisms.