Reaumur appears to be the first
scientist to perform a serious study of regeneration. In 1712, he presented
his paper on regeneration of crayfish limbs and claws to the French
Academy. He noted that crayfish would regenerate only the lost part of the
limb or claw, but often amputations toward the tip of the limb would induce
the animal to cast off the rest of the limb at a defined point near its
base. This loss is known as autotomy and is caused by the contraction of
specialized muscles at the breakage site. Reaumur not only studied
regeneration but also invented the alcohol thermometer and developed a
formula for the production of steel. He was considered one of the
preeminent natural philosophers of the 18th century.
The Swiss scientist, Abraham
Trembley, was influenced by Reaumur and began studying the freshwater polyp
(now known as hydra) in the 1740s. This work led to the remarkable
discovery that hydra could regenerate their heads and feet and if cut into
a few pieces, all of them would regenerate to form new individuals. He was
able to split the head of the polyp longitudinally and allow two heads to
regenerate. By repeatedly splitting the new heads, he was able to generate
a multiheaded animal that he named Hydra in reference to the mythological
creature. Later, Linnaeus renamed the polyp hydra in honor of
Trembley. Trembley studied this creature in detail and performed numerous
experiments, initially with the intent of determining whether the creature
was a plant or animal. He eventually became convinced (correctly so) that
it indeed was an animal. By studying hydra, Trembley was one of the first
scientists to demonstrate that animals could reproduce asexually.
The Swiss scientist, Charles
Bonnet, was greatly influenced by the work of his cousin, Abraham Trembley,
and Rene-Antoine Reaumur. He began studying regeneration in earthworms and
published his work in 1744. His scientific career was cut short by deafness
and blindness, so he resorted to more philosophical pursuits, including a
defense of preformationist philosophy, while attacking epigenesis. At the
time, epigenesis was linked to the theory of spontaneous generation and
Bonnet was adamantly opposed to such a view. He thought that the evidence
gleaned from work on regeneration provided support for the preformationist
philosophy. We now know that Bonnet was wrong about epigenesis (both
reproduction and regeneration are epigenetic phenomenon) but correct in his
opposition to spontaneous generation. Charles Bonnet was also one of the
first scientists to demonstrate that some animals (in his case, aphids)
could reproduce parthenogenetically (reproduction by the development of an
unfertilized egg) as was predicted by Reaumur. He also was the first person
to recognize the syndrome characterized by hallucinations that occur in the
absence of mental illness. This syndrome is still recognized today and is
known as Charles Bonnet Syndrome.
The Italian scientist, Lazzaro
Spallanzani, began studying regeneration in vertebrates as well as
invertebrates in the 1760s. In 1768, he published An Essay on Animal
Reproductions in Italian, which was translated a year later into English
by M. Maty. In this work, Spallanzani briefly described his experiments
demonstrating that salamanders can regenerate their limbs, tails, and jaws.
He also discussed his work showing that frog and toad tadpoles can
regenerate their tails, that snails and slugs can regenerate heads and
horns, respectively, and he confirms and extends Bonnet’s observation on
earthworm regeneration. Spallanzani was a great admirer of Charles Bonnet
and was deeply influenced by the Swiss scientist’s philosophy. He
interpreted his regeneration data in a way that supported his
preformationist philosophy. Even though we now know that this latter
interpretation was incorrect, his experimental work on the remarkable
regenerative abilities of these animals has been confirmed repeatedly over
the last two centuries. Spallanzani was also instrumental in debunking the
theory of spontaneous generation (he shares this honor with Francesco Redi
and Louis Pasteur) and was the first scientist to perform a successful
artificial insemination (these experiments were performed using dogs).
Thomas Hunt
Morgan, the Nobel Prize winning geneticist/developmental biologist, began
his research career studying regeneration. He coined the terms morphallaxis
and epimorphosis to describe the two major types of regeneration.
Morphallaxis refers to the
type of regeneration that occurs in the absence of cellular proliferation,
while epimorphosis refers
to regeneration that requires active cellular proliferation. It has been
reported that Morgan once said “that since he had been unable to solve the
problem of regeneration . . . he had decided to try something easier such as
the problem of heredity.” (Lenhoff, S. G. and Lenhoff, H. M., 1986, in
Hydra and the Experimental Birth of Experimental Biology--1744. Boxwood
Press, Pacific Grove, CA. Quoting from Morphogenesis by John Tyler
Bonner,
1952).
