Planarian embryology in the era of comparative developmental biology

During the last decade, the field of evolutionary developmental biology (evo-devo) has emerged as a major research discipline in modern biology and an essential approach to understanding evolutionary relationships in the animal kingdom. At the same time, planarians have become a useful and important model with which to address basic questions regarding the molecular and cellular basis of regeneration, tissue repair and stem cells in adult organisms. Nevertheless, little attention has been paid to their embryonic development, even though this provides a unique opportunity for studying how molecular developmental mechanisms are re-deployed during adult regeneration or the independent losses of spiral cleavage that took place in different lophotrochozoan lineages. In this paper, we review the most relevant works on planarian embryos from a historical point of view. In doing so, we highlight the questions that have recurrently intrigued researchers, most of which remain unanswered. Finally, we present a comprehensive scenario for planarian embryogenesis in an attempt to provide a testable hypothesis that will help to bridge the gap between this divergent mode of development, the ancestral canonical spiral cleavage, and adult planarian regeneration.