ABSTRACT
Cell types, traditionally recognized based on morphology and function, are frequently characterized today based on patterns of gene expression—more specifically, of activation of alternative gene regulatory networks. However, there is no way to obtain good agreement between classifications based on morphology vs. function or molecular vs. morphofunctional types. Additionally, cells undergo ontogenetic changes, especially during final differentiation, if any is present Therefore, exclusive focus on terminally differentiated states fails to capture the dynamic diversity of cell states. Cell types are often described as independent evolutionary units, their history being eventually identified with the evolution of their specific gene regulatory networks. However, any definition of cell types depends on the choice of the phenotypic aspects on which our interest is focused. This degree of fuzziness is a common feature of classification units. In this respect, cell types are not dissimilar from biological species, where the so-called ‘species problem’ is not a problem of fact but one of representation. As in biological systematics, universal generalizations of taxonomic criteria may well be impossible. This does not imply that we must abandon any classification of cell types but rather that we must accept some degree of taxonomic pluralism. This may require adjustments and flexibility in nomenclature.
