Conceptual foundations of metamorphosis and regeneration: from historical links to common mechanisms.

Metamorphosis and regeneration represent two familiar examples of developmental processes. In Western thought, their conceptual link with broader issues of generation or animal reproduction, including "spontaneous generation," began with the ancient Greeks. Their ideas dominated scientific discourse on these topics until William Harvey, in the mid-17th century, redefined and distinguished between epigenesis and metamorphosis as fundamentally different modes of reproduction. However, preformation theories, formulated in response to perceived threats of epigenetic concepts to religious doctrine, eclipsed epigenesis for much of the 18th century. Abraham Trembley's 1740 discovery of hydra regeneration helped to fuel the "epigenesis-preformation debate" at that time. During that period, an exchange between C.F. Wolff, a young epigenesist, and A. von Haller, the reformed preformationist, saw robust models erected to comprehend generation, metamorphosis, and regeneration under each world view. Subsequent elaboration of experimental zoology and the incremental advance of biological knowledge saw the birth of divergent research traditions in pursuit of specialized problems. Embryology, regeneration, and metamorphosis, like genetics, identified independent study topics and elaborated new techniques, thereby creating their own research trajectories. Conceptual reintegration has slowly emerged with the evolution of current biotechnology. We have now begun to discern common molecular mechanisms underlying and reuniting metamorphosis and regeneration, as revealed in the accompanying symposium papers.