Hydra as a unique model for the study of regenerative mechanisms in metazoans

Scientific study of regeneration as an aspect of development began in the third century BC with Aristotle who studied development of the chick embryo with a focus on the anatomical origin of organs such as the cardiovascular system.1 Regeneration as a separate area of study began in earnest with Trembley2 who focused his experiments on the freshwater invertebrate, Hydra. Trembley’s approach involved experiments in which he would wound adult polyps or graft a segment of Hydra (e.g. the head pole) to adult polyps and observe the subsequent regenerative process the occurred. The strength of Hydra for such studies is its high regenerative capacity, simplified body structure, and state of constant renewal in which all cells of the organism are in constant turnover. Since Trembley, additional organisms have been incorporated as models for the study of regeneration to include such organisms as: planaria, the red spotted newt, the axolotl, and most recently the freshwater fish, zebrafish (Danio renio).3, 4 As stated though, Hydra’s is unique among these models because of its high regenerative capacity, simplified body structure, and state of constant renewal. Because of these attributes, studies have been able to target the complete lineage of the epithelial and stem cell populations using advanced techniques as will be discussed.