Regeneration

Binet, in The Psychic Life of Micro-organisms (1888), brought experiments on the nucleus to bear on regeneration. He wrote that the nucleus can regenerate the protoplasm but the protoplasm cannot regenerate the nucleus, so a nuclear element has to divide for reproduction to occur.¹ Morgan, in Regeneration (1901), argued over terminology. He found Delage's terms "regular" and "accidental" regeneration partly useful, though the process is entirely regular even when it follows an accident. He proposed "restorative regeneration" for what others called pathological, and kept "physiological regeneration" because it was in current use.²

Loeb, in The Organism as a Whole (1916), tested the claim that pieces regenerate faster the nearer they lie to the oral pole. On a Tubularian near Oakland, California, he observed that both oral ends of a bisected stem piece regenerate equally fast.³ Child (1924) reported that Lund found no difference in rate along the stem of Obelia, only in the delay before regeneration began. Child noted that Lund counted regeneration only from visible hydranth development, whereas earlier authors included the cell activity that starts soon after section.⁴ Thompson, in On Growth and Form, wrote that regeneration tends to fall short of full restoration. He noted that Trembley and Voltaire knew this, and he quoted Dalyell that each successive regeneration is smaller and more imperfect.⁵

Mondia and colleagues (2011) studied Xenopus tadpole tails. Amputation leads to regrowth of about the amount removed, so they argued that some mechanism must match regeneration to the level of cutting. They proposed that each tail segment keeps a marker of its position, and they noted that long-range signals regulating regeneration are still poorly understood.⁶ Lobo, Beane and Levin (2012) reviewed planarian work. Wounding alone raises neoblast mitoses throughout the animal. Only loss of tissue sends neoblasts to the wound and produces a second mitotic peak there, which forms the blastema.⁷

Lobo and colleagues (2012) described classical electric-field experiments on planarian trunk fragments. Regeneration was normal when the anterior cut faced the cathode. Double-headed worms resulted when it faced the anode.⁸ They also reported that depolarization of the blastema is required for head regeneration, and that the data suggest a voltage-driven calcium influx triggers anterior gene expression.⁸ Six hours after amputation the membrane potential in the bud-region blastema was depolarized five-fold compared with other regions and the uncut tail.⁹