ABSTRACT
In Metazoa, epithelia are defined based on observations of bilaterian tissues as sheets of polarized and cohesive cells relying on a basement membrane. Whatever the animal group, the differentiation of the epithelial cell type is a crucial step of the early embryological development. Nevertheless, epithelia can harbor diverse features: presence or absence of a basement membrane, different types of junctions, multilayering, and more. This chapter shows, beyond the consensual definition of animal epithelium found in any textbook, the diversity of features found in animal epithelial cells and in particular in non-bilaterians. This often overlooked or ignored diversity raises questions concerning the emergence and evolution of epithelial features. This chapter argues that epithelia were already present in the last common ancestor of all extant animals but explains why the features of this ancestral epithelium still remain uncertain. The efforts of the recent years to characterize the molecular toolkit involved in non-bilaterian epithelia offer new clues and promising perspectives to answer this key question of animal evolution.
