AEO is an ontology of anatomical structures that expands CARO, the Common Anatomy Reference Ontology, to about 200 classes using the is_a relationship; it thus provides a detailed type classification for tissues. The new classes were chosen for their use in categorizing the major vertebrate and invertebrate anatomy ontologies at a granularity adequate for tissues of a single cell type. The ontology should be useful in increasing the amount of knowledge in anatomy ontologies, facilitating annotation and enabling interoperability across anatomy ontologies