Background:
Regeneration
Aidan Owens
Period 7
October 1st, 2010
(omgitsaidan@gmail.com)
Regeneration is the process by which an organism regrows a part of their body that
was somehow severed or lost. Many species in the animal kingdom have the ability to
regenerate lost limbs, and other body parts. However, some do not. It is not known why
only some animals have this ability. (Brockes, 2001) Humans, for example, are not able
to regenerate to that extent. However, humans aren’t completely without regenerative
skills. There are parts of the human body that can be regrown. For example, when a
human suffers from a wound, the healing process is regeneration. Humans are always
creating new blood, skin, and linings for the stomach, intestines, and lungs. Human livers
also have the ability to regenerate, to a certain extent. (Exploratorium.edu)
Regeneration is a very important topic of study because if it can be understood
how the process works, more research can be done on how to speed up and advance the
regeneration and wound healing processes in humans. In humans, the healing of wounds
involves the activity of a specialized network of tissues, blood cells, cytokines, and
growth factors. This results in increased cellular activity, which, in turn, leads to the
healing of the wound, or, in this case, regeneration. (MacKay, 2003)
In order for human wounds to heal, they must undergo a four phase process. The
phases of wound healing are hemostasis, inflammation, proliferation or granulation, and
remodeling or maturation. Hemostasis is basically the stopping of bleeding. The
inflammation phase presents as erythema, swelling, and warmth that often accompanies
pain. This stage usually lasts up to 4 days post injury. The proliferative phase starts
around four days after wounding and usually lasts until day 21, depending on the size of
the wound. It is characterized by the presence of pebbled red tissue in the wound base
and involves replacement of dermal tissues and sometimes subdermal tissues in deeper
wounds as well as contraction of the wound. The remodeling or maturation phase is when
the dermal tissues are repaired. This process can sometimes take up to two years. (Keast,
year unpublished)
The phenomenon of regeneration has been known to scientists for over 250 years.
One very good model of regeneration in humans is the planaria. When planaria are
cut in half, they are able to regenerate the part that was severed within a few weeks. For
example, if a planaria is cut in half across its center, it will be able to regenerate either its
head or its tail, depending on where it was severed. Once severed, planaria heal their
wound within 30 minutes post-amputation and regenerate by forming a blastema. A head,
complete with cephalic ganglia (brain), photoreceptors and all other sensory components
is fully regenerated by seven days. Pigmentation of the new tissue will occur within two
to three weeks post-amputation. (Alvarado, 2000) Planarian regeneration involves the
regeneration of new tissue at the wound site via blastema formation. A blastema is
composed of an outer cell layer, called an ectoderm, and cells that grow under the wound
epidermis. These cells eventually grow into the lost body parts. The makeup of both
planarian and vertebrates’ blastema is the same, however in planaria the wound
epithelium forms by shape modification, instead of cell proliferation, and the
mesenchymal cells form from pre-existing neoblasts, unlike in vertebrates. Because of
the relative simplicity of the blastema formation, combined with the planarian’s basic
body plan, developmental plasticity and evolutionary position, these invertebrates are a
very attractive model in which to study regeneration and wound healing. (Newmark,
2001)
When the planaria is cut, it has many reactions that a human would have if they
had just received an injury. The planaria will immediately pull away from the wounding
agent. This is so because the stimulus for regeneration is injury. This reaction reflects a
predator avoidance reflex. (Needham, 1952) Next, a strong muscular contraction at the
site of wounding occurs within seconds and minimizes the surface area of the wound. A
head fragment containing the brain will continue to locomote, possibly to escape the fate
its body might have befallen to a hungry predator. This shows that not only the processes
that occur in planaria regeneration are similar to vertebrates, but also their reactions to the
wounding agent. (Reddien, 2004)
By using planaria to study regeneration, much can be learned about human wound
healing, as well. Because the processes are so similar, planaria make perfect regeneration
models. Regeneration studies are very important because people face injuries and
debilitating wounds every day. If something is found to speed up the wound healing
process, it would greatly improve many aspects of medical care, as well as peoples’ lives
in general.
Works Cited
Alvarado, Alejandro. "Regeneration in the Metazoans: Why Does It Happen?" BioEssays
22 (2000): 578-90. Print.
Brockes, J.P., Kumar, A., and Velloso, C.P. (2001). J. Anat. 199, 3–11.
Exploartorium.edu. "Planaria:A Window on Regeneration."
Www.exploratorium.edu/imaging_station. Web.
<http://www.exploratorium.edu/imaging_station/research/planaria/story_planaria.pdf>.
Keast, David. "THE BASIC PRINCIPLES OF WOUND HEALING."www.pilonidal.org.
MacKay, Douglas. "Nutritional Support for Wound Healing." Alternative Medicine
Review 8.4 (2003): 359-77. Print.
Needham AE. 1952. Regeneration and Wound Healing. New York: Wiley & Sons
Newmark, Phillip A. "Regeneration in Planaria." ENCYCLOPEDIA OF LIFE
SCIENCES: 1-7. Print.
Reddien, Peter W. "FUNDAMENTALS OF PLANARIAN REGENERATION." Annu.
Rev. Cell Dev. Biol. 20 (2004): 725-57. Print.
Sanchez. "Regeneration." The Sánchez Lab Home Page. Web. 7 Oct. 2010.
<http://planaria.neuro.utah.edu/regeneration.php>.