Planarian organizers.

An organizer is defined as a group of cells that secrete specific factors and can change the fate of adjacent cells and instruct a specific pattern. Spemann and Mangold were the first to use the term, when in 1938 they discovered that the dorsal blastopore lip of a salamander embryo induced a secondary axis after transplantation. Since then, several such regions have been identified in the embryos of many animal species. However, little is known about the presence of organizers at the adult stage, although some organizing activity must be required during regenerative processes to pattern the new tissue. In this study we review the current knowledge on planarians, flatworms that can regenerate any lost body parts, including their heads, within a few days. We will summarize the current data that made it possible to identify planarian anterior and posterior tips as regenerative organizers. We will present the current knowledge about the molecular networks that define each organizer, and we will discuss the presence of organizers in planarians during normal homeostasis. We will propose some unanswered questions concerning both planarian regeneration and regenerative medicine, and examine future research prospects in this field.