Part of the XSPAN Project has been development of a Cell Type Ontology. This has proved to be particularly useful for tissue mappings.

Tissues in one organism are mapped to a tissue type, and then mappings between that organism's tissue is automatically mapped to all other organisms' tissues that are also mapped to that tissue type.

Screenshot of cell type ontology viewed using COBrA.

A public version of the Cell Type Ontology is available from the Open Biological Ontologies (OBO) web site.